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+title,body,top_comment,vote,subreddit,upvote_percentage,date,comments,original_poster,post_karma,comment_karma,media_link
+AskScience Panel of Scientists XVII,"Please read this entire post carefully and format your application appropriately. This post is for new panelist recruitment! The previous one is here. The panel is an informal group of redditors who are either professional scientists or those in training to become so. All panelists have at least a graduate-level familiarity within their declared field of expertise and answer questions from related areas of study. A panelist's expertise is summarized in a color-coded AskScience flair. Membership in the panel comes with access to a panelist subreddit. It is a place for panelists to interact with each other, voice concerns to the moderators, and where the moderators make announcements to the whole panel. It's a good place to network with people who share your interests!  You are eligible to join the panel if you:  Are studying for at least an MSc. or equivalent degree in the sciences, AND, Are able to communicate your knowledge of your field at a level accessible to various audiences.   Instructions for formatting your panelist application:  Choose exactly one general field from the side-bar (Physics, Engineering, Social Sciences, etc.). State your specific field in one word or phrase (Neuropathology, Quantum Chemistry, etc.) Succinctly describe your particular area of research in a few words (carbon nanotube dielectric properties, myelin sheath degradation in Parkinsons patients, etc.) Give us a brief synopsis of your education: are you a research scientist for three decades, or a first-year Ph.D. student? Provide links to comments you've made in AskScience which you feel are indicative of your scholarship.  Applications will not be approved without several comments made in /r/AskScience itself.   Ideally, these comments should clearly indicate your fluency in the fundamentals of your discipline as well as your expertise. We favor comments that contain citations so we can assess its correctness without specific domain knowledge. Here's an example application:    Username: /u/foretopsail    General field: Anthropology    Specific field: Maritime Archaeology    Particular areas of research include historical archaeology, archaeometry, and ship construction.     Education: MA in archaeology, researcher for several years.    Comments: 1, 2, 3, 4.  Please do not give us personally identifiable information and please follow the template. We're not going to do real-life background checks - we're just asking for reddit's best behavior. However, several moderators are tasked with monitoring panelist activity, and your credentials will be checked against the academic content of your posts on a continuing basis. You can submit your application by replying to this post. ","Username: /u/angrygofer General Field: Biology Specific Field: Human Biology Particular areas of research include virology, immunology, physiology, molecular biology. Education: Human Biology B.S. and current Human Biology M.S. student.  Comments: https://www.reddit.com/r/askscience/comments/6xpk7u/what_determines_the_boiling_and_freezing_points/dmhywbb/ https://www.reddit.com/r/askscience/comments/6xtr3s/if_the_symptoms_of_an_illness_are_typically_the/dmjc0f0/ https://www.reddit.com/r/askscience/comments/6x4c74/do_our_skin_cells_or_liver_cells_contain_the/dmhzn7w/ ",181,askscience,96% ,Mon Aug 7 20:16:55 2017 UTC,34,AskScienceModerator,124420,857,
+Does the heating of the earths surface during summer months affect plate tectonics?,Is there more or less movement in summer months due to the surface expanding with heat. Or the opposite with shrinking during the colder months? ,[removed] ,5,askscience,86% ,Tue Sep 19 01:23:50 2017 UTC,4,Delittle3,2,10,
+Do the radioactive elements in nuclear weapons emit detectable radiation?,"I am curious to know whether nuclear weapons emit detectable radiation at any stage of their lifecycle.  For the sake of argument, let's say we are considering a thermonuclear hydrogen bomb, but if someone feels like sharing any expertise on fission weapons too, great. Intuitively, I'd think that there would have to be a certain amount of radiation released during construction of these weapons just by passive decay of any uranium or plutonium involved in the construction...but what about after they are completed?  Do they emit radiation during transport, storage, and/or once in-flight towards a target?  How would such radiation be mitigated - lead-shielded transport containers, perhaps? Is there any hope of us ever developing some kind of ""early warning system"" to detect such radiation that would allow us to say, ""That missile/bomb/etc. that is on a ballistic trajectory is/is not armed with a nuclear warhead"" - ? Thank you in advance to anyone who cares to comment. "," I am curious to know whether nuclear weapons emit detectable radiation at any stage of their lifecycle. For the sake of argument, let's say we are considering a thermonuclear hydrogen bomb, but if someone feels like sharing any expertise on fission weapons too, great.  Thermonuclear weapons contain fission warheads in them as well as fuel for the fusion reactions. Any fissile material used for the fission part of the explosion is radioactive, and can be detected in principle.  Intuitively, I'd think that there would have to be a certain amount of radiation released during construction of these weapons just by passive decay of any uranium or plutonium involved in the construction...but what about after they are completed? Do they emit radiation during transport, storage, and/or once in-flight towards a target? How would such radiation be mitigated - lead-shielded transport containers, perhaps?  The nuclei inside the warhead don't care whether you're still assembling the bomb, transporting it, or storing it, they'll continue to decay all the same. You can always detect their radioactivity in principle, although the people with the bomb can shield it to try to mask its presence.  Is there any hope of us ever developing some kind of ""early warning system"" to detect such radiation that would allow us to say, ""That missile/bomb/etc. that is on a ballistic trajectory is/is not armed with a nuclear warhead"" - ?  To do something like that in-flight, I don't know. But we do have ways of looking for nuclear weapons at various stages of their lives. We can try to go to the root of it, where the fission fuel is first being enriched or diverted. We can monitor the antineutrino flux from nuclear reactors to look for unexpected changes in the power of the reactor, which could signify that it's been shut down to add or remove material from the core which could end up being diverted for the purpose of making weapons. We can use neutron or gamma ray detectors to monitor for nuclear weapons tests, or RDDs (dirty bombs). Detecting and tracking down improper usage of radiological materials is obviously a very important task, and many people have thought very hard about how to do these things. There are lots of ways to monitor for these kinds of activities, and new ideas are constantly coming about within the nuclear nonproliferation/applied nuclear physics community. ",7,askscience,89% ,Mon Sep 18 22:35:47 2017 UTC,3,RaspersProgress,212,990,
+"For a reaction with a given reaction mechanism, is the activation energy a constant?","I know that catalysts use a different reaction mechanism with a lower activation energy, but for a given mechanism, does the activation energy change with respect to temperature, pressure, or reactant concentration? ","In practice the energy required to overcome a barrier is the same for the same reaction path.  Most of the factors you named will change the entropy of the system, change the average kinetic energy, or change the reaction path entirely.  Those factors will have an impact on the rate of the reaction, however this affects the free energy and not the activation energy itself.  ",22,askscience,74% ,Mon Sep 18 17:59:51 2017 UTC,2,vitcaffeine,64,12,
+What is gravity like at the center of the Earth?,"So say you were able to reach the center of the Earth, what would gravity be like? Would there be a sense of getting pulled in a direction (down?), or would you be weightless like in space? ","The gravitational field at the exact center of Earth is 0. (Strictly speaking, this doesn't occur exactly at the center because of the nonuniformity of Earth, but any compact mass distribution has a point where the field is 0. So somewhere near/at the center, the field is 0.) So, assuming you were some magical being that could survive the heat and pressure and molten core and also move about without really interacting with anything other than via gravity, you would feel no net force at the center. (You would also feel weightness, but this is because weightlessness is a sensation we get when we don't feel the reaction force to gravity, e.g., the normal force from your chair or ground.) If you displace yourself a small distance from the center, you will feel pulled back toward the center, then overshoot it because of your non-zero speed, then move out to some maximum, come back to the center, and repeat. Your motion is similar to that of a pendulum or other simple harmonic oscillator. In fact, for an exactly uniform and spherical Earth, any object placed at rest anywhere inside Earth will experience SHO with period T = 2π√(Rearth3/(GMearth)), which is about 85 minutes. ",18,askscience,75% ,Mon Sep 18 18:59:40 2017 UTC,18,GoodGuyPolarBear,204,37,
+"Since the surface of the Sun is only ~14 times hotter than my oven when I'm baking a pizza, why am I able to feel the Sun's heat from 93 million miles away?",,"For one, the Stefan-Boltzmann law says that for a black body, the total power radiated per unit area scales with the fourth power of temperature. So the surface of the Sun may only be 14 times the absolute temperature, but the power radiated per unit time per unit area is multiplied by about 38,000. In fact, this law is what was used to first estimate the temperature of the Sun. Suppose we know a certain object at a known temperature has some heat energy flux. We can then also measure the heat energy flux from the Sun across a region with known area. Comparing the two and using the SB-law then gives an estimate for the temperature of the Sun. (Stefan originally made an estimate of about 5700 K, which is very close to the actual value of 5778 K.) Note that for this method you also need some estimate of how much energy is absorbed by the atmosphere since only the energy flux at the surface of Earth was feasibly measurably at the time. ",11,askscience,92% ,Mon Sep 18 21:01:08 2017 UTC,12,DownvoteIfYoureHorny,964,20141,
+What Does Sagittarius A* Look Like?,Suppose we were able to exist at a safe distance from Sagittarius A*.  Perhaps we are in Carl Sagan's Ship of the Imagination.  What would we see?  What about various distances?  Do the stars that orbit it move fast enough for us to see with the naked eye?  Is there visible matter surrounding it?  What would such a hole in space even look like?  How would it bend light and spaetime?  Etc... etc... etc... I am an author writing a story about a man who has visions of Sagittarius A* and I want his visions to be accurate despite the fact that he is insane. ,"Well - most things are really a matter of perspective, and it all depends on where you are when you're looking. The black hole itself isn't that far across, at least in cosmological terms. Estimates vary, but it might be as small as 24 million km across: on the range of a very large star, and within the orbit of Mercury. The star closest to the black hole does move very quickly: up to 1% of the speed of light at periapsis! But periapsis is still 40 billion km away. It takes 11 years to complete an orbit, even though it's so fast, so it'd still take 5.5 years to cross half the sky. Probably too slow to watch. However, there's some hope, because it's an elliptical orbit, and it's much faster as it gets in closer. Much better, but it'd still be almost three years per orbit, so too slow to watch. Most of the motion you see in the night sky is, after all, based on the Earth's rotation. Of course, there are probably many objects in much closer than that star, and they could be moving at all sorts of speeds. So a smaller object might look like something zipping through the sky, like a satellite. As far as those smaller objects go, though, we haven't been able to detect anything specific; it's below the resolution of our instruments.  But we do believe there to be a large cloud of gas and dust, superheated because of the black hole's pull. We're blocked from detecting this in visible light, but there's evidence it's extremely bright. So your observer might not be able to be too close, if he's still able to see at all. ",24,askscience,75% ,Mon Sep 18 16:32:19 2017 UTC,6,JoshuaGoudreau,30,22,
+Why Does Distilled Water Evaporating Slower Than Tap Water?,"Sorry if this has been asked before, I searched around didn't see anything but assumed this would've come up before. Anyways, so my wife uses distilled water to clean baby bottles and stuff like that in one of those steamers. Well she pointed out to me the other day that, when she sets the bottle down on the drying rack, the little droplets of distilled water don't evaporate very quickly.  They'll sit there for a few days without evaporating she said.  I didn't really believe her so we did an experiment where we had identical bottles. Both cleaned with distilled water in the steamer but we rinsed one with tap water, then set them both in the same position next to each other on the drying rack. By the evening, the tap-water-rinsed bottle was completely dry but the distilled only bottle was still covered with those droplets on the inside. What's going on here? ","It shouldn't. Dissolving salts into water raises the boiling point, or in other words it increases the amount of energy required to evaporate the liquid, since when evaporating you have to 'give back' the energy given off when the salt dissolved. (if it didn't give off energy the salt would be insoluble and then the boiling point wouldn't change - in other words, you can only raise the boiling point of water by adding impurities)  So salt water (and tap water) normally evaporates more slowly than salt-free distilled water. I'm a bit doubtful whether you had the exact same amount of water in both bottles. As for why the drying is slow in general, that's most likely because there isn't very good air circulation in the bottle. Once the air in the bottle is saturated with moisture, further evaporation can only occur once the air is replaced. A bottle of water with a long narrow neck will dry out slower than a drinking glass containing the same amount of water, and both will dry out slower than the same water in a wide, shallow pan. ",22,askscience,63% ,Mon Sep 18 18:39:40 2017 UTC,13,Throwaway----4,44,13349,
+"How accurate is it theoretically possible for weather forecasting to be? For example, with a powerful enough supercomputer, will we eventually be able to predict the exact track of a hurricane weeks in advance? Are there any other factors limiting prediction accuracy aside from computing power?",,"The practical limit of a weather forecasts is about 10 days, I believe the theoretical limit is around two weeks (sorry I can't find a reference, on mobile). The limit is because the weather is a chaotic system. Chaos in this context doesn't mean random, it has a specific meaning relating to the growth of tiny errors in the initial conditions of the forecast. Imagine you had two identical worlds (i.e. a perfect simulation) but in one a butterfly decided to fly in a different direction (I guess I'm assuming butterflies have free will!). The air flow in the two worlds will start to diverge, the tiny difference will grow and after about two weeks the weather patterns will look completely different.  There are other types of forecasts that extend beyond the current 10 day limit, for example I work on seasonal forecasting. But that requires a slow moving component of the climate system - e.g. an El Niño - that exerts a force on the weather that pushes it in a certain direction. Then we can't tell you if it will rain on January 12th, but we can predict a higher than average monthly temperature. Similar thing with climate forecasts, that's about calculating the forcing of the system to determine mean temperature changes. ",16,askscience,78% ,Mon Sep 18 20:28:32 2017 UTC,9,poozemusings,127,5,
+What is the gold and silver foil they put on satellites and why is it important?,"I was looking at a picture of a satellite in the news the other day and noticed that every time I've seen some kind of space-related piece of equipment, it is wrapped in gold/silver foil. Is this real gold and real silver? What is it? Why is it used?  Thank you!  ","Despite the common knowledge that space is ""cold,"" it's actually difficult to get rid of heat in a vacuum. Spacecraft without a highly reflective surface tend to absorb the sun's energy and heat up to the point of failure. Putting ""foil"" (which is actually a more advanced insulation) around satellites makes sure they can maintain a good operating temperature. https://www.nesdis.noaa.gov/content/good-gold-are-satellites-covered-gold-foil ",13375,askscience,90% ,Sun Sep 17 11:32:56 2017 UTC,670,BarSeraph,6014,2533,
+"In physics, why is the formula to calculate force F=ma instead of F=mv (mass x velocity)?","For example, if a car that weighs 2 kilograms his traveling at a constant speed of 31 meters a second as the car hits you. It would give you a fair amount of force right?  But the formula states that since the car is not accelerating and is at a constant speed, a=0, since 2,000 x 0 = 0 the formula says that the car hits you with a force of 0 newtons (which I don't think is accurate) Even if the car started at 0 m/s and started to accelerate at 2m/s/s wouldn't the calculations still be wrong and have an incorrect amount of force calculated? ","The problem is that you are not calculating an impact force correctly. The equation F = ma tells you the net force on an object. But the equation does not tell you directly the impact force of one object on another when they crash. For instance, if an object of mass M traveling at speed V just before impact crashes into a rigid wall and comes to rest over a time period T, then Newton's second law tells you that the average impact force felt by the object was F = MV/T because its average acceleration was just V/T. So, indeed, the impact force increases with impact velocity, as expected. ",21,askscience,70% ,Tue Sep 19 04:47:58 2017 UTC,21,kool_kolumbine_kid,2193,348,
+Could we railgun the Moon?,Is the reach of the US Navy railgun long enough to reach the Moon? ,"No. The US Navy railgun can accelerate projectiles to Mach 6. This is merely 2 km/s. You need 7.6 km/s to get into orbit and significantly more to reach the Moon (haven't done the math but it must be slightly less than 11.2 km/s which is escape speed). Even if we had a railgun powerful enough to reach orbital speed, at such great speeds and with the high density of air at sea level the projectile would burn up like a meteor and probably never leave the atmosphere. Some enthusiasts of the rail gun idea affirm that a heat shield could resist this effect and that launching fast enough could account for speed loss to air drag. Honestly I haven't found reputable sources confirming this or showing the math, and I seriously doubt it because of the very high air density at low altitudes. Spacecraft reentering the atmosphere usually burn up at altitudes between 80km and 120km. ",18,askscience,68% ,Tue Sep 19 08:59:49 2017 UTC,10,Borsippian,78,549,
+What is the Theoretical Limit on the Energy Density of Ultracapacitors?,"For example the specific energy of uranium 235 is about 700k MJ/kg, and supercapacitors top out at 30 MJ/kg. What would be the theoretical limit for the ultracapacitors that are being researched today? I've heard several orders of magnitude so I'd guess that means 3k+ MJ/kg. What's the maximum theoretical limit for specific energy for a capacitor with known laws of physics? What formulas are used to determine the theoretical limits here? ","Sorry, I'm on mobule. With the bare laws of physics, the limit is given by  E/m = c2 = 90 billion MJ/kg And now you are going to say: ""That's not helpful!"" But it is. You see, take a spherical capacitor formula, C = constant x radius Now, capacitance grows with radius, so you can just take a thin spherical shell with little mass and make it big. Very big. As big as you want. So by a spherical capacitor, there is no limit besides E=mc2 and the one-atom thickness shell. But mass is proportional to R2, capacitance grows with R, electric field decreases with R2, and the maximum outward pressure the sphere supports decreases with R. So we can increase the voltage by R2 /R = R Energy is CV2, so maximum stored energy increases with R3, which is faster than mass. What does this teach us? That the question has a problem: It assumes that maximum stored energy only depends on mass, and that's not the case: volume is important too. In the way the question was asked, the limit is E=mc2 ",3,askscience,100% ,Tue Sep 19 09:48:26 2017 UTC,2,Radiatin,3004,14200,
+How do rainbows that curve away from the sun work?,"As I walked to work today I saw a iridescent cloud above, and it wasn't just that but a full rainbow (or two if you look closely enough). After some walking I notice that the rainbow curved away from the sun, and was not centered around it (as seen in the image). Normal rainbows result from by water-droplets acting like a really bad lens/mirror and sending a chromatic aberration at the observer. No matter if it's mirror or lens, it's always centered around the sun (or without clear curve in the case of iridescent clouds). Yet I don't know how these work, as they're clearly curved, and that away from the sun. ","Normal rainbows are on the opposite side of the sky to the sun, centered around the anti-solar point, not around the sun, mind. What you captured is the circumzenithal arc. Like most halos, and unlike rainbows, it is formed by sunlight interacting with ice crystals in cirrus clouds. A segment of the upper tangent arc also appears in the photo, about halfway between the CZA and the sun. ",60,askscience,79% ,Mon Sep 18 16:52:38 2017 UTC,2,GregTheMad,1932,133915,
+How is online gaming possible if there must be some delay?,"Online communication always has a delay, especially when people communicating are on opposite sides of the US. If that's the case, then how are fast FPS video games, like Call of Duty, possible to play online?  ","Modern games tend to employ two different techniques simultaneously in order to compensate for lag, but players with lower latency will still have a small advantage.  Server state rewind: You're playing an FPS at home and you pull the trigger.  At that moment, the stuff you see on your screen is already out of date by 20-50ms.  And the command to fire your gun doesn't reach the server for another 20-50ms.  But the server knows when you fired the gun, so it just rewinds the game to that exact moment to find out what you hit. Client-side prediction: Certain actions that you perform in the game (firing, moving, jumping, whatever) don't actually execute until the server receives the command.  But the game would feel terribly sluggish if your client were to wait for official confirmation.  So your client simulates the command locally under the assumption that the server will allow the action.  You pull the trigger, and your client immediately plays a gunshot sound and draws tracer rounds on the screen.  You feel like it executed immediately... but other players in the game actually don't see your shot until 100ms later.  These techniques allow games to feel responsive and accurate, but they can still cause conflicts.  Sometimes you pull the trigger, but you're already dead on the server (somebody shot you and you don't know it yet) so the action isn't actually performed.  You feel like the game robbed you of a kill because that's what it looked like on your screen. ",9,askscience,67% ,Tue Sep 19 09:26:33 2017 UTC,13,Master_Vicen,2270,783,
+[Chemistry] How do you know how many isotopes of atom exist?,"I'm trying to understand. I know that an an element is determined by the protons in it's nucleus. If Hydrogen gains a proton then it is now Helium. But how do you know how many isotopes there can be? Carbon 12 has 6p and 6n,  but could I go as low as having 6 protons and 1 neutron or is that not possible and is it something that I will just have to memorize?  ",You have to discover each individual isotope until you find the limit. The limits of existence for isotopes of a given element are called the driplines. They're called driplines because the outermost nucleons in the nucleus just drop off on very short timescales. ,5,askscience,86% ,Mon Sep 18 03:00:52 2017 UTC,1,YearnsForStarfleet,13,2891,
+Would it be possible to slow light down enough for the naked eye to see it moving?,"Light moves 66% of c in water. Would it be possible to create a liquid(other states of matter also count) in which light moves so slowly so that it's visible with the naked eye? An example: Let's say that we have a curtain of said liquid. If I stand on one side of it, and quickly am to walk to the other side, and looked through the curtain, would I then see a past reflection of myself, one which stands on the other side of the curtain? ","As has been said, light in a medium can be slowed down dramatically, with the most famous examples being in Bose-Einstsein Condensates (BECs).  I just want to add a little niggling issue, which is that you can't see light at all take a path that isn't into your eye.  Hollywood has us primed, for example, to think we can see laser beams, but in reality the only way you can see laser beams is to be directly in its beam or if the room is so filled with crap (i.e. dust, chalk, mist, etc.) for the laser light to hit that enough light is being scattered OUT of the beam and some of it scattering out into your eye. So we think lasers look like this: https://www.scienceabc.com/wp-content/uploads/2016/10/Green-Red-laser-beam.jpg which really only happens in a smoke filled room.  And, for example, military grade ""laser-based"" weapons (skip ahead to about 1:13): https://www.youtube.com/watch?v=XLwqWBtmUEc don't look like anything.  The ship just bursts into flames. The reason I bring this up is because the goal of ""seeing something interesting while not physically sticking your head into the Bose-Einstein Condensate/laser path"" is at odds with the goal of ""keeping light within the fancy medium"".  Which is not to say no light escapes such experiments (it does and can be detected with finely tuned instruments), but rather that a ""visually interesting"" show to the human eye, means you're doing the experiment REALLY poorly (and, in fact, it may not even work).  Let's say that we have a curtain of said liquid. If I stand on one side of it, and quickly am to walk to the other side, and looked through the curtain, would I then see a past reflection of myself, one which stands on the other side of the curtain?  Yes, this can and does happen.  It's really not a whole lot different than hearing a radio echo of a radio transmission. ",187,askscience,87% ,Sun Sep 17 08:07:58 2017 UTC,34,blackcarpet2000,4630,1003,
+What would happen if only your arm and nothing else was exposed to a vacuum?,Let's assume you had a vacuum chamber with a hole somewhere in the wall you put your arm through which sealed it perfectly ,"You'd be ok. As opposed to popular belief the human body does not explode in a vacuum, blood does not boil. The question implies you're still breathing in a pressurized environment so you don't have to worry about asphyxia and won't feel your saliva boiling. As air presses your arm into the vacuum chamber you may feel a very strong force. According to this page the average guy has a 13'' biceps circumference unflexed, so if my math is right that would be a force of 870N - comparable to the weight of an average adult. It might hurt in the point where skin touches the borders of the hole and you might not be able to pull it out without repressurizing the chamber. ",5,askscience,74% ,Mon Sep 18 02:37:55 2017 UTC,12,Raknarg,1647,32566,
+"Would a stream of fluid which responds to magnetism, like ferrofluid, flow faster or more efficiently through a pipe if a magnet is placed at the opposite end of the stream?","I recently watched this gif on r/blackmagicfuckery, in which magnetic sand falling through an hourglass is responding to a magnet on the base of the glass. This made me wonder if that magnet is causing the sand to flow faster through the hourglass, but I assumed that because the sand molecules were not in constant enough contact with one another, the magnetic field could not permeate up the sand stream and contribute to the flow rate.  But if we were to use ferrofluid, or some other fluid which responds to magnetism, in which the molecules would be in a more constant contact, would that contribute to a faster flow rate? If it does I'm assuming the distance between the magnet/ length of the stream plays a major role. Thinking of potential applications, could this be used to increase flow rates through pipes? like if I needed to increase the flow rate between two tanks of fluid, could I add some ferrofluid, or ironfilings, etc, to one of the tanks and a magnet to the other? Would this get the total amount of liquid to flow faster from one tank to the other? Heres the Gif, https://www.reddit.com/r/blackmagicfuckery/comments/70mrfz/magnetic_sand_hourglass/ Any insight is appreciated, thanks. ","Well since the fluid is attracted to the magnet. It will feel a force in the direction of the magnet. In this case the force due to gravity and the force due to the magnet are aligned with each other. So yes, the fluid will accelerate downward more quickly than if the magnet was not there. ",1,askscience,57% ,Sun Sep 17 22:47:07 2017 UTC,3,TheBlackLagooner,3431,176,
+Eagle Vision Vs. Physics: How can they see a mouse from 2 miles away?,"Hello folks. I have a question that falls at the intersection between neuroscience and physics that has been bugging me recently (I only have a decent background in the former, so physics people, please help). So I recently watched Planet Earth II (which, btw, is absolutely stunning in 4K), and there was a bit on eagles, and how they can spot prey from up to two miles away. I've heard this fact in several other places as well, and I have no reason to distrust it, except... The physics of this seems impossible. For something to be processed and recognized by the visual system, a sufficient number of photons that bounced off the object must fall incident upon the retina. Eagles eat mice. Mice are what, two inches long at the most? The square area of a mouse from two miles must represent less than a trillionth of the photons that impact the retina of the eagle.  I understand how movement detection works with center-surround cells and lateral inhibition, and I'm assuming this must be the mechanism, such that it isn't about acuity of perceiving the mouse, but rather noticing its movement. But even then, I simply don't see how enough photons can hit the eagle's retina to sufficiently drive a conscious response (or fixed-action pattern for you ""consciousness is an exclusively human phenomena"" people) that enables the eagle to see the mouse and go for it. And if we accept this premise as true, what does that say about thresholds of activation for eagle retinas? Single photon detection? ","Let's start with some calculations regarding the stimulus: I think 2 inches is a bit small for a mouse, let's say 4 inches. At a distance of two miles, we can describe the size of the mouse in terms of visual angle as: 2*arc tan ((4 inch / 2) / (2 miles * 63360 inches / mile) = 0.0018 degrees of visual angle ~= 0.11 arcmin.  Just to give a point of reference: on a standard eye chart for the 6/6 or 20/20 row, each character subtends 5 arcmin and you need to have a sensitivity of 1 arcmin to tell some letters apart.  Now there are some physical limitations to the resolution of the eye as determined by photoreceptor size and spacing. However, it turns out that we exhibit hyperacuity so that the resolution for telling apart whether two lines are aligned or offset is actually 5-10 times higher than visual acuity limits set by the eye. For a discussion of this, see Fahle & Poggio (1981) (<- pdf!) and in addition to the first demo, see also here. The idea is roughly something like: the eye collects several discrete samples at a certain resolution and later cells (e.g. in the thalamus) interpolate in both space and time intermediate locations. So our functional resolution, if we can call it that, is actually closer to 0.1-0.5 arcmin = 6-30 arcsec. Which isn't so far off from our initial calculation about the size of the retinal projection of a mouse at a 2 mile distance. In fact, in some cases, sensitivity can even reach below 1 arcsec (0.02 arcmin) (Klein and Levi 1985 <- pdf!).  However, it should be noted that there is a difference, both in terms of acuity and in terms of underlying physiological mechanisms, between vernier acuity which is measured with static stimuli and motion sensitivity. And for an eagle, it's probably not enough to notice a little splotch in a field, but also that it is moving. Perhaps surprisingly, we can actually experience motion without any object actually moving whatsoever. Here is an example. Other examples are motion aftereffects where we experience something in the world as moving even though everything is stationary (like after walking and staring at your feet, you look up and the sidewalk seems to recede away from you). Here is an example. Motion processing is somewhat dissociated from object processing, so really what we want to know is whether the eagle could detect motion at that distance. (Just as an aside to point out the separability of motion mechanisms: for individuals with cataracts who have 20/100 vision, they could still have displacement thresholds (see below) of as low as 50 arc seconds = 0.83 arcmin (Bueno & Hurst 1995 <- pdf!).  Typically what is measured in motion sensitivity is a displacement threshold: what's the minimum distance something needs to move for the motion to be detected. This varies as a function of speed, but for relative motion (motion of an object relative to another / on a non-uniform background), thresholds can be as low as 0.3 arcmin = 18 arc sec (Legge & Campbell 1981). For detection of oscillatory motion in a sinusoidal grating, perhaps only 14 arc sec = 0.23 arcmin (Wright & Johnston 1985 <-pdf!), and for detecting shear motion in random dot patterns, only 5 arc sec = 0.083 arcmin (Nakayama & Tyler 1981 <- pdf!). Now perhaps those aren't quite fair comparisons because it's not one, low contrast thing moving very quickly against a fairly uniform background, and we're talking about humans not eagles, but even so it doesn't seem implausible. I'm not familiar with eagle vision or experiments done on eagles to know in detail where the 2 mile number comes from. If the estimates come solely from receptor size/spacing and do not account for hyperacuity, then they may even have better vision than that. tl;dr based on what we know about the human visual system, this doesn't seem implausible  ",7,askscience,72% ,Sun Sep 17 12:09:18 2017 UTC,7,Mhroberts007,22,15,
+"Why do simple words like ""Me"", ""Sun"", ""Water"" etc'... vary between languages?",I'm probably looking at it in a limited time span. But I cannot comprehend how such simple words change over time. I mean we all share common ancestry. different languages should have common core vocabulary. ,"It is generally the case that different languages share a common core vocabulary, assuming that they're related. They're not identical because sound changes are completely regular and exceptionless throughout a language, though they are generally more resistant to being replaced by entirely new words. First, let's talk about sound changes. English, like all Germanic languages, shows a chain of sound changes known as Grimm's law where old *p *t *k became /f θ x/1, old *b *d *g became /p t k/, and old *bʰ *dʰ *gʰ became /b d g/. These sound changes happened everywhere. We see them in words like fish and father, (cf. Latin piscis and pater), three and foot (Latin tri and pes, pedes), hound and what (Latin canis and quod) and a great many others. The point is, wherever Grimm's law could operate, it did. This includes even words that we might think would be resistant to sound change, like pronouns. Most English speakers don't use it anymore, but Grimm's law is also evident in the pronoun thou (Latin tu). Intuitive or not, simple words aren't resistant to sound change. They are, however, generally more resistant to lexical replacement, where a word gets shoved out by another. Pronouns, lower numerals, and nouns for body parts and features of the natural world are often good places to look for related words: for instance, per Buck's A Dictionary of Selected Synonyms in the Principal Indo-European Languages, the words for 'night' in Ancient and Modern Greek, Latin, Italian, French, Spanish, Romanian, Welsh, Breton, Gothic, Old Norse, Danish, Swedish, Old, Middle, and Modern English, Dutch, Old, Middle, and Modern High German, Lithuanian, Latvian, Old Church Slavonic, Bosnian-Serbian-Croatian, Czech, Polish, and Russian all come from a single source. Contrast this with a much less basic word like 'purse', where among about 30 languages there are perhaps a dozen totally unrelated forms. 1 /θ/ as in three, /x/ as in German Bach. ",1,askscience,53% ,Sun Sep 17 19:39:55 2017 UTC,10,stormdead_,7,47,
+Why can humans make such a wide range of vocal sounds compared to other animals?,"To the human ear, it seems like a person can do many more things with their voices than animals can (we can talk/sing/growl/scream/moan/do impersonations/etc. whereas they seem to have a smaller range of things).  Is this because we're adapted to hear differences between the timbres of human sound, or because we have evolved to be able to produce a wider range of timbres? Or maybe animals and humans are both physically capable of producing a wide range of timbers, but animals just make less ""weird"" or unusual noises than humans? Or perhaps there's a different reason I haven't thought of? ","It's almost completely due to physiology. Do to evolution, the way our mouth, throat, vocal chords, etc. are sized and shaped allow us to make a larger set of distinct sounds. It also has to do with just how cognitively complex we are as humans compared to other species. This allows us to have a much broader understanding of language and how it can be used. A lot of research has been done on language in extinct hominids and while some would suggest that some (ex. Homo neanderthalensis) had as complex languages as us, most accept that they used a combination of basic sounds like grunts in combination with body language to communicate. This of course was limited because of their underdeveloped vocal system and lower brain capacity. ",0,askscience,43% ,Mon Sep 18 00:46:36 2017 UTC,4,pentaSEC,254,3611,
+If muscles are made of proteins how can additional protein not assist with muscle growth ?,Looking @ whey protein but can't get access to these studies. If someone can explain the mechanism as to why additional protein won't help you grow I would appreciate it. I would also be interested to have the same questions answered regarding recovery. ,"Eating more protein than necessary does not signal to your muscles to grow. Only increased demand on the muscles, exercise, will do that. After you start exercising, then your muscles adapt and grow to meet the new requirements. To meet those new requirements, they need protein. If there isn't enough protein, they physically cannot grow. ",0,askscience,42% ,Mon Sep 18 02:17:10 2017 UTC,8,boredandmiddleaged,10,-14,
+"I know about some cool chemical reactions, but what are some interesting physical reactions?",,Recrystallisations from supersaturated solutions can be pretty spectacular. Sodium acetate crystalisation is an example and is sometimes called hot ice because the physical process releases a lot of heat. As such it's used in heat packs. With some persistence (it's tricky) it can also be done in your kitchen. The triple point can be quite easily reached for some compounds like cyclohexane. It's pretty cool seeing something transition from solid liquid to gas all in the same container. ,7,askscience,82% ,Sun Sep 17 16:25:40 2017 UTC,12,Selliulus,35,1538,
+How does adding heat to a liquid allow it to dissolve more substance? What effect does heat have on whatever IMFs are at work.,"If a solid is clumping to itself while surrounded by liquid, it probably means the solid molecules have greater IMFs between themselves than with h20 (what about if they're just pressed together? E.g a pill. I'm not sure). Heat means more kinetic energy in the water built that's where I can't bridge the understanding gap I haven't had a fun chem question like this in a while, brings me back to muh roots ","Like almost always in chemistry, temperature affects both the kinetics, i.e. your solid is dissolved more quickly, and the equilibrium by favouring entropy over energy. Since a liquid solution has higher entropy than separated solid and liquid, hot liquids tend to dissolve larger amounts of solids. ",9,askscience,81% ,Sun Sep 17 10:29:54 2017 UTC,4,thetrapjesus,78,1825,
+What would hurt more saltwater or freshwater?,"If you were to jump into a freshwater lake vs jumping into salt water, will the impact be harder on your body for one or the same for both?  Hypothetically if you could replicate both jumps from the exact same height, landing the exact same way and both bodies of water were the exact same temperature ","As I see it, there are two considerations: surface tension; and, density. Surface tension will be a minor element, but for completeness, the surface tension of salt water as measured in a lab over various concentrations averaged around 0.22 mN/m.  Whereas, for pure water, is around 73 mN/m. So, fresh water is ""harder"". However, since the force you experience would be from mass X acceleration, the density of the fluid you are displacing would dominate over the surface effect. Seawater, as an example of salt water, is 2.5 - 3% more dense than fresh water.  I can't calculate the force one would generate, due to too many unknown variables, mass, height (velocity) and surface are of impact, and associated deceleration. However, if someone wants to make some assumptions, and calculate how many Newtons of force such jumps would generate to demonstrate the difference would not be in mN, then that would be worth bonus points on the quiz. ",12,askscience,71% ,Sun Sep 17 13:11:17 2017 UTC,3,joshbvl,134,6,
+What is the reference frame of virtual particle pair production?,"I was watching a YouTube video which talks about virtual particle pair production and Hawking radiation.  Around 4'30"", there's mention of virtual particles and a spaceship.  They move on from this idea quickly, but it got me curious. Is there a reference frame for virtual particle production?  It seems to me that pair production generally is relatively low energy particles (the video suggests it is less common at higher energy), but the energy of a particle is relative to the observer.  Virtual particles which are low energy in one reference frame might be thought of as extremely high energy to someone traveling close to the speed of light relative to that frame.  Are virtual particles always in some particular reference frame? I suppose another way to imagine this is to look at the Casimir effect.  If I had two close plates in a very fast rocket, oriented perpendicular to the direction of travel, the plates would be different distances apart depending on whether I am on the rocket or stationary relative to the rocket (due to Lorenz contraction).  But it seems paradoxical that there would be different forces measured between the plates. Is there a better way of thinking of this, or am I missing something entirely? ","Virtual particles do not literally exist; they are just intermediate states in a perturbation series expansion. Terms in these expansions are represented pictorially by Feynman diagrams. Feynman figured out how to do these kinds of perturbation theory calculations in a way that is invariant under change of inertial reference frame. In other words, this diagram represents a mathematical expression which does not depend on your frame of reference. Each individual particle will have an energy and momentum which depends on your frame of reference, but when you combine everything in the way Feynman discovered, the result is something which is the same in each frame. ",12,askscience,84% ,Sun Sep 17 15:26:23 2017 UTC,7,hansn,19111,395171,
+"Why are vitamins often sold in ""International Units"" instead of milligrams?",,"International units are a measure of activity. Paraphrasing Wikipedia and the cites WHO paper, the goal in developing international units was to develop a common measure of activity (some biological response chosen by the group  tasked with defining an International Unit for that particular vitamin) so that different preparations can be compared.  https://en.m.wikipedia.org/wiki/International_unit https://en.m.wikipedia.org/wiki/WHO_Expert_Committee_on_Biological_Standardization ",4,askscience,83% ,Sun Sep 17 18:55:02 2017 UTC,1,Rosindust89,3954,33086,
+How is a species declared extinct?,"Pretty much the title. Who declares it officially and how is it determined with any certainty that's it's gone and not just difficult to find, especially smaller species in large ranges? ","It used to be easy. The convention was that if a species hadn't been sighted in 50 years it was considered extinct, but as of the 90's, according to the International Union for Conservation of Nature: ""A taxon is Extinct when there is no reasonable doubt that the last individual has died. A taxon is presumed Extinct when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the taxon's life cycle and life form.""* Hth *http://www.iucnredlist.org/static/categories_criteria_3_1 ",9,askscience,81% ,Sun Sep 17 16:38:46 2017 UTC,2,tag_malone,4,1,
+"Why do we classify different breeds of dogs as the same species even though they looks so different, when other animals species look closer to each other but are classified as two different species?","For example, if I had never seen a Chihuahua and a great dane, there's no way I would think they are the same species. Finches and Sparrows look almost the same to me, but they are different species. ","A species is a distinct group of animals or plants that have common characteristics and can breed with each other.  A Chihuahua is able to mate and produce fertile offspring with a Great Dane.  Finches and sparrows are not able to mate.  Horses and donkeys are able to mate and produce offspring, but the mules are sterile. ",1,askscience,67% ,Mon Sep 18 02:24:59 2017 UTC,9,lastfire123,414,593,
+"With aneutronic fusion, how would you determine that fusion was occurring?","My understanding is that the prime means of detecting fusion is measuring the release of neutrons, but with aneutronic fusion, how would you detect for it? ",You don't need to detect neutrons to know that fusion reactions are occurring. Here is a list of candidate reactions for aneutronic fusion reactors. You can detect any of these reactions simply by detecting the particles in the final state. ,5,askscience,78% ,Sat Sep 16 17:44:45 2017 UTC,2,Rideron150,1678,4012,
+What happens if a white dwarf is accreting mass slowly until he reaches the tipping point to become a neutron star?,"So i am not talking about a merger of two white dwarfs for example, which i know are violent events. More like a slow procress like syphoning matter from a binary partner.  How violently would such a process happen? Is there some kind of supernova happening when the dwarf collapses? Bonus question: what happens if a neutron star amasses matter slowly and turns into a blackhole? ","A white dwarf that accumulates matter from a partner star causes what is called a nova. Matter collects on the surface until a certain point where the star ""explodes"" in a nova. However the star usually survives this. ",9,askscience,72% ,Sat Sep 16 14:14:58 2017 UTC,6,katzmarek,115,658,
+What's the amplitude of a electromagnetic wave ?,"Last day, in physics class, I learnt that mechanical waves have an ""Amplitude"". I wondered if an electromagnetic has one, and then what could be its value. (sry for my English ) ","An electromagnetic wave consists of a wave in the E-field and a wave in the B-field. These waves are not independent of each other; their amplitudes, frequencies, and directions are related. (For instance, in a vacuum, the electric field, magnetic field, and wave velocity are all mutually orthogonal. The amplitudes of the electric magnetic waves satisfy the equation E = Bc, and their frequencies are the same.) By convention, when we generally talk about ""the amplitude"" of the electromagnetic wave, we usually mean the amplitude of the electric wave. Like any other wave, the amplitude is a measure of the maximum deviation of the wave from the equilibrium position. In this case, the amplitude measures the deviation of the electric field E from equilibrium. ",9,askscience,69% ,Sat Sep 16 12:58:38 2017 UTC,12,Ezatrixx,42,110,
+"Quantum Physics, How many states can matter be the superposition of at once?",Schrodinger's Cat was Dead/Alive.  Could it have also been Dead/Alive/Maimed in a similar scenario? is there a limit to the superposition? ,"The limit is the number of basis states, in other words the dimension of the Hilbert space in which the states exist. This can be infinite. ",8,askscience,73% ,Sat Sep 16 18:11:30 2017 UTC,5,bubsd,392,667,
+"Cassini was launched in 1997, how come the photos it send back are HD on par with 2017?",,"So the imaging sensors on Cassini are honestly not all that amazing by today's standards - the CCD chip is only 1024 x 1024 pixels, the equivalent of a 1 megapixel camera. These days, most phones far surpass that technology. However, it does have the benefit of extremely sensitive pointing correction and image geometry compared typical camera (most notable with the use of the SPICE system), which means that making an image mosaic produces a much more seamless final image; this is similar to the panorama mode on your phone, but far more accurate. For example, this 9000 x 3500 pixel image was made from a mosaic of 141 separate images. ",29,askscience,92% ,Sat Sep 16 07:28:12 2017 UTC,5,DonutMerchant,2053,53,
+Would an Argon-Oxygen atmosphere be breathable?,Could humans breath an Argon-Oxygen atmosphere? For bonus points: what about a Neon-Oxygen atmosphere? ,"Yes, although the act of breathing would require more effort due to density differences.  The partial pressure of oxygen is the critical factor, and at an appropriate % of oxygen, the partial pressure is the same.   Deep sea divers actually use oxygen-helium mix to avoid complications at higher pressures with nitrogen.   Noble gases at normal pressures would not be expected to have complications.   Related topic, that scene in the movie ""The Abyss"" in which a rat breathes a liquid isn't fake, the stuff exists: https://en.wikipedia.org/wiki/Liquid_breathing It's the viscosity that's the issue, our lungs aren't made to move fluids.  ",23,askscience,77% ,Sat Sep 16 12:01:03 2017 UTC,14,VirtualArmsDealer,49,292,
+Does nuclear waste produce energy?,"Might be a stupid question, but I was thinking that if it could produce energy, we could use it as an RTG and blast it off into space. ","Nuclear waste contains many radioactive nuclides, and it does produce a little bit of energy. RTGs generally need special nuclides which decay via the right mode (generally alpha decay), and have half-lives within some suitable range (if it's too short, it's not practical to work with and can't act as fuel for long enough; if it's too long, the specific activity of the material is too small to fuel the craft). Some of these ""special"" nuclides will undoubtedly exist in nuclear waste, but to separate them out and use them as RTG fuel would be costly and potentially hazardous for the people involved. Rather than going through all the effort to recover a little bit of radioactive material to power an RTG, we can use nuclear waste to power certain types of nuclear reactors, and produce much more energy. Fast breeder reactors on a U/Pu fuel cycle can burn up waste from the standard LWR types of reactors which currently make up the majority of power reactors in the world. ",12,askscience,76% ,Sat Sep 16 15:12:06 2017 UTC,10,thosedamnpiggles,1328,787,
+"Why is it that when you try to focus on 1 star it seems to slowly disappear, and when you focus your eyes just next to the star you see it sharply again?",,"The very center of your vision has a much higher density of color sensing cells (cones) than the rest of your eye. Unfortunately for your night vision, it's got less rods, which can't differentiate colors but are much more responsive to low light. See figure 11.10 here for the distribution of each type relative to angle off of center of vision.  I you're looking directly at something, there are practically no rods at all. But by looking a little bit to the side, you've put the star's light on a part of your retina that's much more sensitive to dim light. ",6,askscience,62% ,Fri Sep 15 17:26:31 2017 UTC,12,Dadikoe,126,54,
+Can you analytically find the probability for a sequence of numbers to have at least one digit in the correct order?,"Can you analytically find the probability of a sequence of numbers that has at least one digit in the correct order? self.askscience submitted 2 days ago  (*last edited 22 hours ago) by chunkybeefbombs For example, with a 3-number sequence, the possible sequences are 123, 132, 213, 231, 312, and 321. 123 and 321 have the 2 in the second spot, 123 and 132 have the 1 in the first spot, and 213 and 123 have the 3 in the third spot. That means the probability for a sequence to have one number in the right place for three terms is 0.67. Is there a way to calculate the probability for larger sequences that at least one digit will be in its correct spot? ","A permutation of the numbers {1, 2, 3, ..., n} such that no number appears in its original position is called a derangement. Let dn denote the number of derangements of a set with n elements. (Of course, you are asking for n!-dn, the number of permutations with at least one fixed point, but it's a bit easier to calculate dn directly.) There is a clever way of calculating a recurrence for dn. Think of the n elements as envelopes that will be matched with N letters. The number dn counts the number of ways no envelope gets matched with its letter if we do the matching in a completely random way.  Suppose we attempt to match envelope #1 first. There are precisely n-1 outcomes in which this envelope is not matched correctly. So suppose envelope #1 is not matched and ends up getting matched with letter #k (with k not equal to 1). Now let's attempt to match envelope #k. If this envelope gets matched to letter #1, then the entire problem has been reduced to finding the derangements of n-2 envelopes. (It's as if we envelope/letter #1 and #k were never there to begin with.) If envelope #k gets matched to some other letter, then the entire problem has been reduced instead to finding the derangements of n-1 envelopes. (Now, it's a bit more subtle to see that, but the point is that after envelope #1 is matched to letter #k, the remaining envelopes have only one letter they cannot be matched with if we want a derangement. Since we have included the case where envelope #k gets matched with letter #1 in Step #2, we can equivalently say that letter #1 is the ""match"" for envelope #k in Step #3.) Putting this all together gives the recurrence  dn = (N-1)(dn-2 + dn-1)  That is, the total number of derangements of the original set first branch into N-1 possibilities when envelope #1 is matched to letter #k. But for each of these possibilities, we have the D(N-2) derangements from Step #2 and the D(N-1) derangements from Step #3.  The recurrence  dn = (n-1)dn-2 + (n-1)dn-1  is a second-order, linear recurrence relation, and must be supplemented by two ""initial conditions"". A set with 1 element has precisely 0 derangements since any permutation of 1 element always puts that element in the correct position. A set with 2 elements has precisely 1 derangement since of the 2 possible permutations, only 1 of them is an actual derangement. Hence we have the conditions  d1 = 0   and   d2 = 1  This is a second-order, linear recurrence relation with non-constant coefficients. Such relations are generally very difficult, maybe impossible, to solve analytically. This pdf of lecture notes contains some examples of such relations and how to solve them.  Luckily, the relation for dn can be solved exactly. The solution is  dn = n!*(1 - 1/2! + 1/3! - 1/4! + ... + (-1)n/n!)  In other words, the number  dn/n!,  which is the ratio of derangements to total permutations, is equal to the terms of the Taylor series for e-x (evaluated at x = 1) truncated at the xn-term. It immediately follows that if n is very large, then dn/n! ~ e-1. So for large n, the approximate proportion of derangements is 37%. Equivalently, the number of permutations with at least one correct match is about 63% for large n. ",0,askscience,50% ,Sat Sep 16 00:43:27 2017 UTC,7,chunkybeefbombs,1,0,
+"When two neutron stars merge and cause a ripple of gravitational waves, what happens to the objects (planets, asteroids) near the merger?","Do objects get morphed as the fabric of spacetime stretches and shrinks? If a human was close enough, would they feel the gravitational wave? ","I answered the question about what would happen if we were certain distances to the black hole merger we detected. Gravity waves produced by two neutron stars merging would be extremely weak. Here's what would happen if closer to the black hole merger we detected.  ""The waves we detected as they passed are extremely ""weak"". To give you an idea of how weak, at ten light years you wouldn't have actually personally noticed the waves we detected. At a little over one light year the gravitational waves would be a billion times stronger than we detected yet this would only be a shift in one of the LIGO arms of 4 nanometers. That's only about half the width of a hydrogen atom. The diameter of the Earth would shift by about a hundredth of a millimetre. This might cause some earthquakes that were on the verge of happening anyways but not the end of the world. Let’s now assume we’re at least 10,000 kilometers from ground zero and let's go ahead and factor out the gravity of the two initial orbiting black holes because if you were actually that close that's the real danger and we only want to feel the waves created by them combining. At that distance the shift caused by the gravitational waves would be about one part in a thousand. If you were floating in space you would likely feel that, since a person would experience a shift of a millimeter or two. Would it hurt, or possibly harm you? That’s hard to say. It would really depend on how resilient humans are to gravitational wave distortion, and we don’t have any experimental data on that."" ",37,askscience,84% ,Fri Sep 15 00:16:11 2017 UTC,13,Duke_Koch,2691,7951,
+Does a Field in Physics always exist (as an expample: the electric field) even when no Particles of that field interact with eachother?,,"Yes it does.  It might seem like a philosophical question, but it's not, it has real observable effects.  1) Even if a quantum field has a zero ""vacuum expectation value"" (VEV), which is to say, that it is ""empty"" of excitations (i.e. particles)., the vacuum state itself still has a rich structure of quantum fluctuations about its ""average zero value"" that can cause real effects like the Casimir effect: https://en.wikipedia.org/wiki/Casimir_effect 2) Most particle fields have zero VEVs (i.e. when there's no ""particles"" their average value is zero).  However, a famous example of a field where this isn't true is the Higgs field.  What is special about the Higgs field is that even without any excitation, its VEV is non-zero.  Thus, even when the field is ""empty"" it still has a constant value.  It is some particle's coupling to this non-zero vacuum value that essentially gives them mass (being vague here). You may have heard of the Higgs Boson, and you may have asked: ""If this particle only occurs at such crazy high energies such that you need a particle accelerator to make it, why do we care?"" , well that's basically right.  Most people don't care about the boson itself, it doesn't matter much.  But rather, its existence proves that the FIELD exists and that was key.  We'd assumed for a very long time that there was this field with a non-zero VEV that was responsible for this clearly observed effect of mass in some things that ought not, naively, to have mass, but we could never experimentally probe the field itself.  It wasn't until we could ""smack"" the field hard enough to produce an excitation (a particle) that we could detect that we could prove the field itself was there and that this Anderson-Higgs mechanism was real. So, basically, even if a field is empty of particles it does things. ",18,askscience,92% ,Fri Sep 15 08:13:37 2017 UTC,22,Kingclonegr,14,1,
+What is the science behind why dogs are so friendly?,"I've always wondered why the domesticated canine  as a whole, are generally very friendly and very willing to form bonds with those outside their species! Therapy dogs are used not only for humans, but also big cats and other animals! What causes this behavior, and why is it not seen as often (or as strongly)  in other animals? (Also not sure if my flair is appropriate, please let me know if it needs to be adjusted!)  ",Episode 2 of cosmos explains it really well. Some wolves survived by hanging around humans and getting scraps. Over many years humans tamed them and selectively bred wolves that were more friendly and dogs came about  ,1,askscience,60% ,Sat Sep 16 01:15:27 2017 UTC,4,icebear1298,330,82,
+Could someone help fix my understanding of action potentials and the Na+/K+ pump?,"The simplified explanation I was taught goes like this: Non-gated sodium channels open following a stimulus. Once enough Na+ ions diffuse through these channels (as well as through the membrane to a lesser extent), the electrical charge reaches a threshold of -55mV, voltage-gated sodium channels also open, and the membrane depolarizes until it reaches around 30 mV. At this point, voltage-gated sodium channels close, voltage-gated potassium channels open, and K+ ion efflux repolarizes the membrane to a resting potential of -70 mV, at which point Na+/K+ pumps start to function. However, non-gated potassium channels are still open, and K+ ions are still diffusing out of the membrane through these leak channels, creating hyper-polarization. Na+ is stuck in the axon at this point, as well. The closing of the K+ channels, plus Na+/K+ pump activity, restores membrane charge to threshold, with Na+ ions on the outside and K+ ions on the inside. This explanation seems pretty flawed to me. The Na+/K+ pump transports 3 Na+ ions out for every 2 K+ ions it transports in. If it kicks in during the refractory period, which is caused by leak channels allowing the escape of positively-charged potassium ions, wouldn't it decrease the membrane's positive charge even further by forcing 3 positively charged ions out for every 2 it pushes in? How does the membrane return to resting potential? ","I didn't mention a nucleus, I mentioned a neuron. And although the action potential originates in the axon hillock, the actual ions flow in through the dendrites, making the soma of the neuron change its charge and so have the charge originate in the nucleus.  Then those ions create the potential difference and the action potential begins at the axon hillock.  The refractory period is caused by the interesting structure of the voltage-gated sodium channels. They have a ball and chain protein attachment which literally swings up and blocks the channel, like an old tub with the chain and plug. Then, the potential starts to be more negative as potassium ions flow out.  The ball and chain releases, however, the voltage is now different, so the sodium channels are closed because of that reason.  The thing is, there is lots more potassium inside the neuron than out, so because of the concentration gradient a lot of the potassium ions flow out. However, as the potential becomes more negative because of the potassium flowing out and the sodium potassium pump, the potassium ions(as they are positive) are being drawn in because of the charge difference, and the rate of flow out(because of concentration gradient) and in(because of charge gradient) is equal. The potassium channels and the sodium potassium pumps work in tandem to keep the resting potential stable.  ",1,askscience,67% ,Sat Sep 16 01:26:11 2017 UTC,6,MemeticMonstrosity,33,1,
+"When riding my bicycle, I can stop pedalling, make a 180-degree turn, and the bike still continues moving (though at much lower speed) in the opposite direction. Why?","Momentum is directional and as far as I can see there is no other input energy being applied to the bike (or me), so how can it move in the opposite direction? ","When you turn the front wheel by turning the steering bar, the front wheel is turned to be out of line with the direction of motion. Yet the momentum of the bike keeps pushing it forward. This causes the front wheel (and by extension, the bike) to exert a force on the ground (perpendicular to the direction of the wheel). Due to Newton's Third Law, this force on the ground is met by an equal and opposite force from the ground working on the bike. It is this force that alters the momentum of the bike, rotating it in the direction that you're steering. For this process to work, the wheel must be able to exert a force on the ground. Therefore, it must have grip. Trying to turn on a very slippery surface (e.g. ice) will have very different effects. edit: The force exerted by the bike also slightly alters the momentum of the Earth. The total momentum of the combined Earth+bike system is still conserved. But due to the non-trivial difference in mass between the Earth and the bike with its rider, the effect on the Earth is completely negligible. ",21,askscience,68% ,Fri Sep 15 06:29:38 2017 UTC,26,stupid2017,3712,269,
+"If electricity from wall outlets is AC, how come some plugs have a left-right orientation?","As in, some plugs have one side bigger, which forces a specific orientation when plugging it into the outlet. ",[removed] ,222,askscience,86% ,Thu Sep 14 15:07:22 2017 UTC,54,linearly-independent,263,296,
+Are bananas more radioactive then other foods?,Banana equivalent dose?  Can somebody explain this to me?  Are they just using bananas as an example or are they more radioactive than other healthy foods?  I am paranoid to eat bananas lol. ,"Potassium-40 is a common naturally-occurring radionuclide, present in many foods. Bananas are a stereotypical food which is high in potassium. Potassium-40 is not the only naturally-occurring radionuclide in food, and bananas are not the only food which is slightly radioactive (everything is). Eating a banana is not dangerous. ",1,askscience,52% ,Sat Sep 16 01:56:19 2017 UTC,19,pencilpens,12,1,
+Do non-humans exhibit signs of autism/autism spectrum disorders?,,"I can't answer your question in full. But what I do know is that we regularly create ASD type symptoms in animals for testing/research purposes. So broadly, yes, artificially altered animals can exhibit signs of autism. If you are asking if it is observed to occur naturally, then I am unsure. As a neurologist I can see no reason why not.  ",239,askscience,89% ,Fri Sep 15 01:31:39 2017 UTC,30,mypunkrock,370,10236,
+How do spacecraft like Cassini avoid being ripped to shreds by space dust?,,"There's simply so little of it. A couple of dust impacts over a whole mission, maybe. I'd be interested in seeing what happens to voyager in a billion years, maybe it would run into some occasional bits in interstellar space and become a cloud of dust heading in one direction. More likely it won't run into much and will eventually get stuck in a huge orbit around a black hole after being swung around a few it was too fast for. Maybe align close enough to something like a star or black hole and get sucked in. A couple billion years from now. Would love to see the condition of it before that happens, though.  ",4181,askscience,92% ,Wed Sep 13 23:06:54 2017 UTC,373,HeatAndHonor,3325,1360,
+How do we see reflections on roads on hot days or from far away? How do they have reflective properties?,,It's a type of inferior mirage. The light from the sky is refracted by the hot layer of air near the ground. Light travels faster through the hot air than through the cooler air above it. This causes you to see a shimmering image of the sky instead of the ground.  ,2,askscience,75% ,Fri Sep 15 00:00:45 2017 UTC,3,AssumeTheFetal,4211,236861,
+Why can't humans consume raw meat but most other animals can?,,"We can, really, if it's fresh and clean and parasite free. People eat sushi and beef tartar and eggs Benedict and live oysters typically without I'll effect. It is riskier though, proper handling is critical. Any contamination by pathogens or parasites is more likely to be passed on. ",2,askscience,58% ,Fri Sep 15 00:56:56 2017 UTC,13,Madman1010,45,39,
+Could an creature have both an endo and exoskeleton? Do any?,,"As far as i understand it, no creatures have both as it is simply not required. One of the basic functions of a skeleton (exo or endo) is simply to provide structural support for the creature, so it can move. If it has a exoskeleton it has no need of an endoskeleton, and vice versa.  However i'm not sure if you would classify a creature which has an endoskeleton, but also has some kind of bonelike armour (not connected to the endoskeleton) under this category. ",274,askscience,89% ,Wed Sep 13 22:22:58 2017 UTC,61,Xbxbxb123,3209,10074,
+Why aren't we using gravitational potential batteries instead of electro-chemical ones for large scale power storage?,I have heard of an idea that we could dig holes and put weights on vertically moving platforms. When there is excess power generation motors lift the weights. When the power is needed we simply let the weights descend turning electric generators. It seems to me that this way of power storage can be build on a huge scale and would be much simpler to make than enormous batteries. With power storage for cities solar panels and wind turbines can store power for cloudy windless days. Why aren't we using this technology? Is it even possible? ,"We do, there are many power stations with two pools, top and bottom, at night, they pump bottom to top with excess power, during peak they run top to bottom through turbines.  So these would be gravitational batteries using water as the working fluid. ",48,askscience,78% ,Thu Sep 14 07:58:29 2017 UTC,39,Captain_Fatbeard,422,22,
+Why do we need quantum theories to explain what happens to light with multiple polarization filters ?,"Hi, I just watched a video from YouTube channel minutephysics. It's this video. The first thing that crossed my mind was - ""wait, are you joking me?"". Now, don't get me wrong, I'm not here to joke around and I don't want to sound arrogant, but that's exactly what my thought was when I saw them putting a filter between filters like a magician trying to do a trick. I even asked myself - ""But why didn't you put a filter over a 90 degree rotated filter and then tell me that it got brighter? Huh? Gotcha!"". Then I watched it until the end, and I realized that they went too far in quantum state of particles and the determinism logic which isn't intuitive to anyone, and that my assumptions were totally based on classical physics or don't need no quantum theories like the entanglement in order to explain to me what happened in the video. I also jumped to the comments section and I could see a few people asking the same thing, but always getting redirected to how the entanglement doesn't work that way, even though they didn't mention any entanglement at all. So, this is what I think happens. The logic is very simple. First, we have 2 filters. The filter A, and filter C. The filter C is rotated 45 degrees, which lets 50% of light, or in other words, blocks 50% of it. We also happen to know that a filter rotated 22.5 degrees (half of 45 degrees), lets 85% of light, or again, blocks 15% of light, which magically is not 25%, but I'll get to it. Now, when we introduce a filter B, which we put in between filters A and C, we see the light getting brighter. But, if we concluded that filters rotate the wave of light, the filter C is now ALSO rotated 22.5 degrees to the filter B, which blocks only 15%. The math is 15% + 15% = 30%. Of course that it is brighter. Now, the tricky and the second part is about the thing I said I'll get to it, and what I also wanted to ask reddit. What if the light wave (or filters) are simply not distributed as a standard gaussian distribution model, but they have a slightly smaller variance (the deviation from a standard mean), so that the half from a mean angle gives probability density of 15% and not 25% ? Wouldn't that explain everything? Why would we need entangled particles, realism and locality to explain this? What am I missing? ","QM has nothing to do with this- it's a purely classical effect. You seem pretty confused about this whole setup, so I'm going to start from the beginning.  The fundamental laws of classical electromagnetism are Maxwell's equations. They are, in differential form and natural units:  Div B = 0 Div E = Rho Curl E = -dB/dt Curl B = J + dE/dt  Where B is the magnetic field, E is the electric field, rho is the charge density, and J is the current density.  For the sake of convenience, we'll say that Rho and J are both zero- the net charge at any point is zero, and there's no net flow of charges through any point.  Combining 3 and 4, we get Curl Curl E = - Curl dB/dt = -d/dt Curl B = -d2 E/dt2  Applying some vector calculus identities, we get the wave equation- d2 E/dt2 = d2 E /dx2 + d2 E/dy2 + d2 E/dz2 . As the name suggests, solutions to this differential equation are waves- the electromagnetic field has waves, and in natural units, they propagate at speed 1 (i.e., the speed of light).  Now, this is a linear differential equation, so the sum of any two solutions is also a solution. The physical implications of this are that, when two light waves are passing through the same point, the field there is just the sum of the contributions of either wave.  Now, we need to talk about polarization. We can see from the wave equation that the electric field has to propagate as a sine wave along every axis- but nothing fixes the phases of its components. If E_x and E_y are completely in phase, then when one is at its largest value, so is the other- this is linear polarization. Given any two sinusoids with different linear polarizations, you can express every other possible sinusoid along the same axis as a linear combination of those two. In mathematical terminology, sinusoids along an axis form a two dimensional vector space.  Now, let's say we have a filter that only allows light polarized at a 45 degree angle- which I'm going to call (1,1) for convenience. We pass vertically polarized light - (0,1) - through it. Well, (0,1) = 1/sqrt(2)(1,1) + 1/sqrt(2)(-1,1), so what we get out is 1/sqrt(2)(1,1)- our light has been dimmed by a factor of sqrt(2). The important thing to note here is that, while we've rotated halfway to horizontal, our intensity has decreased by a lot less than half. If we do the same thing again, but with a filter that only lets through (1,0) light, we'll end up with half our original intensity.  Now, let's try the same thing again, but with three filters. The first only lets through (1,sqrt(3)) light, the second only lets through (sqrt(3),1) light, and the third only lets through (1,0) light.  (0,1) = 1/(2sqrt(3))(3(1,sqrt(3)) - sqrt(3)(sqrt(3),-1))), so we get out sqrt(3)/2 of our original intensity. Repeat three times, and we're at about 65% of the original intensity when we get to horizontal.  This is getting long, so I'm not going to work through the math here, but this holds in general- as you add more evenly spaced filters, the total loss in intensity goes down, and in the limit of infinitely many filters, light comes out the other side just as bright as it went in.  ",3,askscience,59% ,Fri Sep 15 00:14:58 2017 UTC,7,flackjap,76,428,
+How do magnets magnetize other objects?,I left a magnet in a bucket of screws and when I took the magnet out I noticed some of the screws were magnetized and clung to others. How does this happen? ,"In a ferromagnetic material, the atoms electron clouds have a magnetic dipole. Usually, these dipoles point in all different directions, so there is no net magnetic field. However, if you expose the material to a strong magnetic field, the dipoles align themselves in such a way to minimize their energy. Once the magnet is taken away, the alignment remains, and there's a net magnetic field. Wikipedia talks about this on the page about ferromagnetism. ",1,askscience,60% ,Fri Sep 15 05:38:05 2017 UTC,4,Bay_Visions,896,128,
+Does the formation of moonlets in Saturn's rings resemble the formation of planets in our own solar system?,"As the title says, does the formation of moonlets in the rings of Saturn in any way resemble the formation of planets in our own solar system? ",no comments,2,askscience,60% ,Thu Sep 14 13:53:33 2017 UTC,0,kayakguy429,5401,7325,
+Why don't positrons appear in the standard model but electrons do?,"Positrons are just electrons with the same mass but positive charge right? So why are they not as ""fundamental"" as electrons? I understand that other anti-matter particles are just different combinations of particles on the standard model, but how can that be the case for positrons? ","Positrons are included in the SM. When you see the popular diagrams of Standard Model particles, each box represents the particle and its antiparticle (assuming they're distinct). If you see ""muon"", that refers to both the positive muon and the negative muon. ",6,askscience,66% ,Thu Sep 14 02:39:59 2017 UTC,9,Mad_Max_The_Axe,198,149,
+"Aside from propulsion, could we send a probe to Alpha Centauri?","Well, I say ""aside from propulsion"" - I know we could launch something like Voyager and it would get there eventually. But let's say we come up with a box that can accelerate a probe to .5c and decelerate it again.  The question is - could we reliably calculate the orbital mechanics to get a probe into orbit around one of the stars in the Centauri system?  ","There are other issues, e.g. cosmic radiation in interstellar space must be worse than in the Solar System, and it could damage electronics. But let's focus on your question specificallly:  The question is - could we reliably calculate the orbital mechanics to get a probe into orbit around one of the stars in the Centauri system?  I'd say yes. Sure there is some margin of error in our knowledge of the masses of both stars, but it's ok, we have a 41% margin of error between a circular orbit and an escape trajectory. Sure the uncertainty in the star's mass is a lot lower than that. However, probes are usually designed specifically for the environment they will have to operate in and for very specific scientific goals, usually achieved in a very specific orbit. It doesn't sound realistic to send a probe there without more detailed knowledge of the system. Who knows if in the future we discover some exoplanets there and get to know more about it thanks to them... ",1,askscience,60% ,Thu Sep 14 12:34:10 2017 UTC,9,DonLaFontainesGhost,11294,128604,
+How are space probes controlled?,I don't know anything at all but it seems they use radio waves to control space probes and I don't see how they can get radio waves out and back to earth in time. ,"Yes, it's all radio communications. Since they are so far away the signal will be terribly weakened proportionally to the square of the distance due to free space loss. In order to be able to distinguish the weak signal from background noise, they use high-gain (parabolic) antennas and a very low bandwidth, so that the energy per bit remains high enough.  get radio waves out and back to earth in time.  So you're talking about the signal propagation time? You are correct, they are limited by the speed of light. It takes 20 minutes to get a signal from Mars and much more from the outer planets. Space probes are designed to operate autonomously during this time or stay idle waiting for commands. That was a very general question; feel free to followup if you're interested in the details of a particular point. ",1,askscience,100% ,Thu Sep 14 12:55:54 2017 UTC,2,Black-Marlin,25,18,
+Are humans born with all the veins and arteries they'll ever have or do more grow as the older?,If they gain more how does this growth process take place with disrupting blood flow? ,"If I can add to this question, when a vein collapses or is damaged, or the same thing with an artery, how do the sell separate once service by that vein or artery (or capillary) get blood flow restored? Many drug addicts have collapsed the veins in their arms; do new veins grow to replace them? ",14,askscience,66% ,Wed Sep 13 21:56:22 2017 UTC,5,READERmii,3524,6688,
+Is there a giant rocky planet ?,Why almost every giant planet ( Jovian ones ) are gas planet ? Is there a possibility of a rocky giant planet with the size of Jupiter ? ,"If a rocky planet is sufficiently large, its gravity will draw in hydrogen and helium from the interplanetary medium and it will build up an atmosphere and end up like a gas giant. If it is close enough to its star, the solar wind can strip this hydrogen proto-atmosphere away as it forms, so larger rocky planets can exist closer to stars. It isn't exactly known where the rocky/gassy cutoff is, from what I've red it's about five times the mass of Earth. ",12,askscience,76% ,Thu Sep 14 01:11:05 2017 UTC,10,CreativeThienohazard,270,453,
+What is dust and where does it come from?,,"Outside it is made up of rocks getting ground up against each other when cars drive over them(one possible source) and if inside It's more or less dead skin cells floating around and covering everything.You also get Atmospheric dust that is produced by saltation and sandblasting of sand-sized grains, and it is transported through the troposphere.  Middle Eastern dust is causing havok with the locals at 50 times normal levels in the middle east.Cosmic dust is widely present in space, where gas and dust clouds are primary precursors for planetary systems. Vast dust storms occur on Mars that cover almost the entire planet.So dust is any small particle that can become an aerosol and be picked up and deposited in a new or different location or even back down where it came from. ",13,askscience,66% ,Thu Sep 14 00:31:01 2017 UTC,8,peterthefatman,228,144,
+"When did the habit of writing st, nd, rd and th in superscript after numerals become a thing in English, and is writing ""10th"", for example, actually correct (especially in typed documents or on the Internet), or should it also be superscripted, strictly speaking?",,no comments,8,askscience,72% ,Thu Sep 14 03:43:31 2017 UTC,1,NBerthiaume,144,3092,
+Why are the different gas giants different colors?,Presumably the material they were made of was all the same gas cloud. Why do the different planets then have such dramatically different colors? ,"The primary reason is temperature. Jupiter, the warmest of the giant planets in our Solar System, is covered in whites and browns. Note that almost everything you see when you look at Jupiter is ammonia clouds, which on their own are bright white. Some latitudes are regions of upwelling (zones), and have high ammonia cloud-tops, while other latitudes are regions of downwelling (belts), and have low ammonia cloud-tops, as shown in this diagram. In between these high and low heights sits a thick brown hydrocarbon haze (not shown in the diagram), very chemically similar to smog. The cloud-tops in the zones are sticking up above most of the haze and thus appear fairly white. The cloud-tops in the belts, though, lie below the haze layer, and thus appear colored brown by the overlying haze. Moving on to Saturn, where it's a good deal colder, that hydrocarbon haze can now reach much higher heights above the cloud tops. It's still all ammonia clouds under there, but the overlying haze now means the entire planet is covered in a brownish-tan hue. Getting out to Uranus & Neptune, which are now cold enough to form methane haze, there's a sudden change in color. It turns out methane is very good at absorbing red light, meaning the only light left to reflect is bluish-green light, and thus gives both planets their characteristic color. Every now and then you'll also see bright white methane clouds peaking out over the methane haze on these planets, though more prominently on Neptune - most likely because it has a much greater internal heat source than Uranus (for reasons we don't really understand yet). ",8,askscience,66% ,Thu Sep 14 01:26:12 2017 UTC,5,samcobra,10532,10046,
+What liquid is the wettest?,I'm sure wettest is the wrong word. I'm trying to ask what liquid would soak through the most layers of say paper towels or whatever. Like per tablespoon which liquid soaks the most.  ,"""Wettest"" or ""wetting"" are perfectly fine words to use. Predicting the wettest liquid for a given material requires consideration of the liquid's and material's surface energies as well as the interfacial tension between the two phases. Essentially, the wettest liquid is one that gives the lowest interfacial tension as well as having a low surface energy itself. Definitively describing this, however, is tricky as the three surface/ interfacial energies are not independent of each other and all values must be determined to fully describe/ predict outcomes in the system. For capillary action, as you have described, the height of liquid uptake into capillaries of a given diameter can be determine from Jurin's law. The gamma X cosine term is equal to (solid surface energy) - (interfacial energy). So to maximise this term, and in turn the height of uptake, the surface energy of the solid must be higher than that of the interface, i.e. the interface must be more energetically favourable to form. This is why most porous solids soak up most liquids - solid surface energies are nearly always higher than liquid-solid interfaces. An interesting example of the opposite effect is liquid mercury - you cant use a paper towel to clear up a mercury spill as the mercury will not be taken up by the capillaries. Instead it's expelled from them as the interfacial tension is too high or, in other words, the mercury doesn't wet the cellulose towel. ",5,askscience,65% ,Thu Sep 14 05:15:16 2017 UTC,4,laptopquesting,58,135,
+When did we know space debris and dust was such a big problem?,Did we think of it before a major incident or was an early space craft unexpectedly ripped to shreds? ,"At the beginning of the space era they supposed it could happen. We've always known micrometeoroids exist because we have several meteor showers per year on Earth. But there was not enough data to estimate their flux or size distribution and the probability of an impact (estimates from observations of meteor showers must have been very rough, if they existed at all). Fun fact: when they heard a loud bang in the Apollo 13 mission, at first they thought they had been hit by a meteoroid, it was later confirmed to be the rupture of an oxygen tank, most likely not meteoroid-related. It was much later that we had precise estimates. Most of the data we have today comes from NASA's LDEF (Long-Duration Exposure Facility) and ESA's Eureca (European Retrievable Carrier). Both of them stayed in orbit for several years, containing samples of different materials and exposing them to all the environmental hazards of space. They were later retrieved by a Space Shuttle and brought back to Earth for detailed analysis. Crater counts and size distribution gave us most of the data we have about meteoroids and debris. We also have some data from inspections of the Hubble Space Telescope and from the ISS's MPLM modules which were brought back to Earth. Must also mention that in the early years of the space era they only thought about meteors and never cared much about debris. Only in the 90's they started to realize it was becoming a problem and that we should avoid contaminating orbits. It's also true that most of the debris in Low Earth Orbit comes from two major events: the Iridium-Kosmos collision in 2009, and the intentional destruction that China did on one of their own satellites (FengYun 1C) in 2007 to test their anti-satellite weapon. Both events are quite recent and visible as clear spikes in debris-altitude charts. https://upload.wikimedia.org/wikipedia/commons/b/be/Spacedebris_upd_2011.jpg ",2,askscience,100% ,Thu Sep 14 15:40:13 2017 UTC,1,sinderling,73,12285,
+What was the first empirical evidence of the existence of black holes?,,"With the advent of rocket-borne x-ray telescopes in the 1960s, a bright source called Cygnus X-1 was discovered that was emitting a lot of energy from a very small area of the sky. The source was also in orbit around another star, and from the star's orbit they could figure out the companion's mass, which was over ten times the mass of sun and too big to be a neutron star. It was surmised that the x-ray and gravity source was an accretion disk around a black hole. ",10,askscience,86% ,Thu Sep 14 05:05:07 2017 UTC,3,screen317,47309,151774,
+What is the likelihood of the LIGO Gravity Wave Detection Results being errant?,"I have been lightly paying attention to the LIGO events the last couple of years and I understood that their original detection and publication and subsequent detections to have all been published and vetted with a high degree of significance. However when I brought up the LIGO detection tonight at a local astronomy club meeting as observational evidence of BH merger, the physics teacher in the club adamantly denied the papers/detectors validity, saying that the amount of error in the study was too high and the initial paper had been widely discredited.  I'd never heard of that, and I thought the studies were still being celebrated for their success.  Is there a problem with the detection or their publications? "," However when I brought up the LIGO detection tonight at a local astronomy club meeting as observational evidence of BH merger, the physics teacher in the club adamantly denied the papers/detectors validity, saying that the amount of error in the study was too high and the initial paper had been widely discredited.   That person is mistaken. The LIGO results are held in high esteem in the academic community. To give you a sense of this, the project leaders are widely seen as next-in-line for a Nobel prize.  The person speaking has likely mistaken LIGO for the BICEP2 results, which was a high profile polarization measurement claiming the detection of gravitational waves through analysis of the CMB. However, it has since been countered by arguments that the polarization they observed was not due to gravitational waves in the CMB, but due to foreground dust. Here is a longer article about BICEP2 if you're interested. ",8,askscience,68% ,Thu Sep 14 06:03:16 2017 UTC,7,jorshrod,5847,10459,
+"Why does all visible light travel at the same speed through a vacuum, but not through a medium?","Additionally, does this apply for all other types of radiation in the electromagnetic spectrum? E.g. radio waves travel slower through a medium than UV waves? ","There is really no good reason ""why"" massless particles travel at a constant speed through vacuum. We just found empirically that they do.  I suppose one could answer, that massless particles do not interact with the Higgs field. Alternatively, one could state that the Maxwell Equations of electrodynamic say, that electromagnetic radiation must travel at a constant speed.    A third ""explanation"" could be, that the Lorentz Transformations of special relativity involve a constant with the dimension of speed, which ends up being identified with the speed of light.  But at the end of the day, it is really just an empirical fact.   Additionally, does this apply for all other types of radiation in the electromagnetic spectrum? E.g. radio waves travel slower through a medium than UV waves?  There is no fundamental distinction between visible light and any other frequency of light. They are all electromagnetic radiation.   Why does light not move at a constant speed though media?  Electromagnetic waves interact with the charged particles in any medium. This interaction creates a second electromagnetic wave. The velocity of the superposition of those two waves depends on the velocity of the medium.  ",8,askscience,85% ,Thu Sep 14 00:49:32 2017 UTC,5,Cassiopeia_June,2530,515,
+How do cells transfer from the fetus to the mother (Microchimerism)?,"To my understanding, the body would reject foreign cells, so how and why would the body allow cells permanently? ","This is entirely an off the cuff answer, but perhaps the fetus does not reject them because the immune system is not fully developed at that stage. My recollection from immunology was that immunity (recognition of what is ""foreign"") develops in early childhood.  ",13,askscience,71% ,Tue Sep 12 21:13:11 2017 UTC,4,OverweightRoshan,318,4082,
+Do butterflies remember being caterpillars?,,"Yes and no. Butterflies don’t have a complex memories that humans have, so no they don’t remember being caterpillars. However, caterpillars can be trained to respond to stimuli and they will remember how to react to that stimuli when they’re butterflies. So no they can’t remember being caterpillars, but they can remember stuff they learned while they where caterpillars  ",23,askscience,78% ,Tue Sep 12 17:45:14 2017 UTC,18,z1000j3wz,138,42,
+"What, if any, are the differences between a man-made vaccum and the vaccum of space?",,"Not many, but there are a few. First of all, it's never a perfect vacuum, just extremely low pressure. Humans can't breathe, water can't be held in a liquid state, but a few air molecules are still bouncing around. The vacuum of space is exposed to UV rays from sunlight. These have a ionizing effect that can break molecular bonds. As a result, oxygen molecules are broken into lone atoms (often called ""atomic oxygen""). These aren't stable, so they will try to recombine with the first thing they find along their path to make a molecule. Engineers designing satellites must be aware of this and make an appropriate choice of materials that won't degrade when exposed to atomic oxygen. Plastics are particularly vulnerable. Glass fiber, which is naturally an oxide, and aluminium, which forms a thin layer of oxide in the surface that protects the rest inside, resist quite well. This isn't a problem in a vacuum chamber on Earth. The other difference is thermal control through radiated heat. This isn't a property of the vacuum, but of the chamber you're in. The walls of the vacuum chamber will radiate heat to the inside, from all directions, at a moderate power determined by their temperature. In space, you'll receive a lot of power from sunlight, some power from Earth or whatever planetary body you're close to, and almost nothing from deep space. Satellites and probes must be properly designed to maintain a controlled temperature in this environment. ",4,askscience,76% ,Mon Sep 11 22:48:57 2017 UTC,3,that_one_salty_guy,1574,53,
+How does boiling water clean it? What can it NOT clean?,"I remember reading about plastic microfibers in our water, can boiling clean that? ","If you boil water, you'll kill most pathogens living in it. Dissolved chemicals or particulates will remain, so if you boil brown water it'll still be brown. If whatever is making the water brown happens to be a toxin drinking the boiled water is still inadvisable. However, you can use boiling the water to clean it. Catch the steam, let it condense and have some clean water. We don't do that generally because it's a rather arduous process and it consumes a lot of energy. ",8700,askscience,82% ,Mon Sep 11 13:16:52 2017 UTC,954,JustaLackey,5776,4657,
+Are there elements elsewhere in space that don't appear on earth?,"My history teacher was talking today about how Galileo determined that heavenly bodies are composed of the same materials as what appears on earth, not ether, as was previously believed. It just got me thinking about how, definitely the planets/stars are made of the same elements that occur on earth, but are there any elements that are exclusive to them that don't appear here? And if not, how did we discover that fact? Thanks in advance!  ",The elements are the elements. It's a list of atoms with one proton and two protons and three protons and so on. There couldn't really be one not on the list. There isn't one hiding between 23 and 24 protons or anything. The ones with atomic numbers higher than about 95 don't happen naturally on earth and are created mostly in labs. They could occur naturally somewhere in space from some high energy process but they all last very small amounts of time so they wouldn't make up planets or anything.  There is some theory that if you got to a certain number of protons that was really big you'd get back to making stable elements and no one has tested that. Even that would probably only last a few minutes but sci-fi likes the idea that that could be the location of a really truely unique element we have never seen.  ,7,askscience,72% ,Mon Sep 11 19:38:46 2017 UTC,13,clevername-here,6,34,
+I get really confused thinking about the galaxy we live in and had some question regarding it and was hoping for some help understanding some of these confusions..," What is (or could be) the oldest thing in our galaxy? Does being the oldest define the age of our galaxy or does its size define the age of our galaxy?  Or are the they same? Do older or younger things within our galaxy potentially have more complexity and/or more or less intelligence?  What I mean by this, is that if a black hole is one of the oldest things and has been around for billions of years, does that mean it has more information or knowledge compared to an AI system developed on Earth today?  I'm not sure if this question even makes sense or if I'm comparing things which are not comparable. Why does it seem that the older things within the galaxy, the bigger they are, and the newer things are the smaller they are? I remember back in high school science it (may) was mentioned that they couldn't explain the rotation of spiral galaxies, in the sense that it all rotated at the same speed, from the inside to the outer arm.  Is this understanding still true or not?  If not, what is the current understanding?  Thank you very much. "," What is (or could be) the oldest thing in our galaxy?  At the moment I believe the object discussed in this paper is the oldest known object in the Milky Way. It is informally known as the Methuselah star. Age determination for stars is not as precise as we would like, but it is thought to have formed very shortly after the Big Bang.   Does being the oldest define the age of our galaxy or does its size define the age of our galaxy? Or are the they same?  Not exactly. Some of the oldest structures in the Milky Way (globular clusters) provide some limits on the age of the galaxy. It is very likely that the Milky Way began forming almost as soon as possible in cosmological history. Galaxies are dynamic, and often during their lifetime capture smaller collections of stars, so it is not easy to define a singular age, unlike a star.   if a black hole is one of the oldest things and has been around for billions of years, does that mean it has more information or knowledge compared to an AI system developed on Earth today?  No. There is no reason to suspect that black holes, or any astrophysical objects, are intelligent. Information, in the context of black holes, is almost certainly not in a form useful for intelligent life.   Why does it seem that the older things within the galaxy, the bigger they are, and the newer things are the smaller they are?  This is incorrect. In the case of stars, the largest stars evolve and die the fastest. a 100 solar mass star is gone after a few million years. A star like the sun will last billions. So it is extremely likely that a large star is actually the younger object. Nevertheless there is not a simple relation between age and size; you have to consider the physics of the object in question.   couldn't explain the rotation of spiral galaxies, in the sense that it all rotated at the same speed, from the inside to the outer arm.   The galaxy rotation curve tells you how fast stars are rotating around the center of a galaxy, as a function of the radius from the center. The speeds of the stars depend on the gravitational potential of the galaxy. You can make a prediction of what this potential is, and hence what the curve would look like, by measuring the matter you can see in the galaxy with a telescope. Measurements of this curve, made by directly measuring the speeds of the stars in a galaxy, conflict with this prediction. This is the origin of why we think dark matter exists. It is merely matter we cannot see, which modifies the galactic potential so that the measured rotation curves make sense. The other hypothesis, modified Newtonian dynamics (MOND), is to invoke a scale dependence of gravity on distance, which accounts for the rotation curve.  At the moment, there are some really good reasons to believe that dark matter exists. (MOND has much less support behind it.) What, exactly, it is remains unknown. Perhaps it is just ordinary matter too cold for us to see. Perhaps dark matter is small black holes. Perhaps it is a gas of weakly-interacting particles that don't interact electromagnetically. There are arguments for or against these ideas; perhaps dark matter is a mixture of these things, or something else we haven't thought of. All we know is that, short of modifying gravity, that matter has to be there.  ",4,askscience,61% ,Wed Sep 13 05:05:53 2017 UTC,8,xow,208,126,
+The dark side of the moon ?,Have we ever had pictures of the other side of the moon and if not why? Surely a probe could have gotten pics with the sun facing the other side.  ,"Yes, we have lots of pictures, it was one of the first places to be explored at the beginning of the space era (there was a lot of curiosity to let a probe see what ground-based telescopes can't). https://upload.wikimedia.org/wikipedia/commons/4/4f/Back_side_of_the_Moon_AS16-3021.jpg And a small correction, it's the far side. It's not ""dark"" because it does receive sunlight when we have a new Moon as seen from Earth. ",1,askscience,52% ,Tue Sep 12 06:04:08 2017 UTC,3,The-sauce_boss,5,0,
+Is it possible that life on earth has originated on more than one occasion?,"Today, I started wondering if its possible that life has originated multiple times. We often say that all life evolved has evolved from one common ancestor, but is it possible that there is more than one? For example, there is a common ancestor for all archaea, and a different one for eukaryotes, etc. ","Furthermore, it's conceivable that there were multiple origins of life early on, but the others didn't ""make it."" Perhaps, for whatever reason, they were less efficient at metabolism or reproduction than life-as-we-know-it, so they weren't able to get a foothold. ",8,askscience,74% ,Mon Sep 11 19:13:28 2017 UTC,15,jaredaddy,64,153,
+"How can the size of a jigsaw puzzle be estimated when pulling pieces one at a time out of a bag at random, and on average how many pieces will need to be pulled before the exact size of the puzzle can be determined?","If the pieces from a rectangular jigsaw puzzle of an unknown piece count are emptied into a bag and then pulled out one at a time at random, with an estimate on the total number of pieces in the puzzle being made after each piece is revealed, how should that estimate be calculated using the following information?  Whether the piece pulled out is an edge, corner, or field piece. whether the piece pulled out connects to any piece(s) already pulled out.  On average, how many puzzle pieces will it take before we can say with certainty the size of the puzzle? Assume the puzzle has no picture on it to help place the pieces, and that the pieces are laid out in the typical jigsaw puzzle format, with each piece being roughly the same size as all the others and the pieces arranged in a grid pattern with no duplicate pieces. Also, I'm sure the aspect ratio of the puzzle makes a difference, but I'm not sure how it would affect the process so I'll leave that open. ","A great problem, and probably something you can write a master thesis on. Some assumptions:  Edge and corner pieces are recognizable as such If two pieces fit together, we always know this. We cannot use any sort of pattern on the pieces. Apart from the previous two bullet points we have no idea where a piece belongs to.  Some initial thoughts: You can make estimates based on the relative number of center (M) and edge (E) pieces, but different length to height ratios will lead to different E to M ratios. All you get that way is a lower limit on the size (corresponding to square puzzles, asymmetric puzzles will have the same ratio at a larger overall size). Corner pieces (C) help: There are just 4 of them, if you draw the first one it doesn't tell you much, but with the second one you can be reasonably confident that the puzzle is not too much larger than what you have already. The third and fourth will refine these estimates even more. You know the length or height once you have a continuous connection between the corresponding edges (you don't need to have them in a straight line). This is a problem in percolation theory. In the limit of infinite puzzle size, you need on average half the puzzle pieces for this if I remember correctly. There is another heuristic estimate, and one that will lead to a reliable (but not exact) estimate the fastest: Count the number of connections you found. I don't have an exact formula, but in a puzzle of N pieces (N>>1), the probability that two random pieces are next to each other is approximately 4/N. With sqrt(N) pieces drawn your expected number of connections is 2, while your expected number of corner pieces is 4. With 2sqrt(N) pieces drawn you expect 8 connections and 8 corner pieces. With 4sqrt(N) pieces drawn you expect 32 connections and 16 corner pieces. The number of connections grows much faster, with its inevitable sqrt(observed) scaling it gives a more reliable estimate than the corner pieces. In addition, its dependence on the overall puzzle shape is much smaller. ",17,askscience,85% ,Mon Sep 11 23:45:03 2017 UTC,10,Senno_Ecto_Gammat,16738,159501,
+Why can't nuclear waste be reused?,"From my understanding, uranium in a reactor decays into thorium, so why can't that process be redone until you get a stable element like lead (the end of the actinium chain) ","It can be, in certain types of reactors (breeder reactors). However there aren't many breeder reactors around the world for commercial power generation at this time. In the United States, we're still on a ""once-through"" fuel cycle, which means that we don't reprocess or reuse any fuel. Once it's gone through a reactor once, it's considered waste, and just gets put into storage. It's a very inefficient system, but it's stuck around because of the potential proliferation hazards of reprocessing fuel. ",18,askscience,87% ,Mon Sep 11 21:15:21 2017 UTC,8,TheHuffinPuffin,400,-2,
+"In Rutherford's gold foil experiment, how we're the alpha particles fired at the foil?",,"I'm on mobile and can't copy and paste from the pdf, but ""The Scattering of the α-Particles by Matter"" PDF! starting at the end of the first page outlines the setup of one of the experiments and describes the source for the beam.  Look at some of the other sources in the Wikipedia article on the experiment.  ",7,askscience,74% ,Tue Sep 12 02:37:28 2017 UTC,4,oscarthegrouch15,64,4,
+"Has anyone ever made a star chart not from Earth's perspective, Alpha Centauri A for example? How much would it differ from Earth's chart?",,"Yes. Here's one example: http://www.astronexus.com/endeavour/chart It starts at looking from Earth toward Orion. Replace ""Earth"" with ""Alpha Centauri"". As you might expect, the sky looks mostly the same, but a few of the nearby stars have moved. For example, Sirius is right next to Betelgeuse, and there is a first magnitude star in Cassiopeia: the Sun. Also try Space Engine. ",17,askscience,91% ,Mon Sep 11 15:14:47 2017 UTC,8,slimboy88,13,1159,
+Were cyclones more powerful when the Earth was covered in superoceans?,"Are there simulations? Did they leave any geological record as the supermonsoon did? Are there limiting factors after a certain ocean size/cyclone size or did more warm ocean equal more energy to the storms? How long did they last? Can we compare them to known cyclones on other planets? EDITS:  1) I categorized this twice but I don't see it working, is this planetary science more than earth science?? 2) I'd really like some links to theoretical simulations, even just on paper, if anyone has any references, so that I could play with them and do actual computer simulations. 3) Thanks to everyone, I'll need some time to reply but answers are really interesting so far! ","TL;DR yes, but not by virtue of superoceans themselves. I am not sure of the effects of supercontinents on their own, but I can answer this question in the context of Earth's history, specifically the end Permian mass extinction event, which took place in the time of Pangaea. I am a graduate student in geology and currently studying mass extinction events. /u/Neolavitz is right in that the biggest limiting factor for tropical storm growth is ocean water temperature. To elaborate... When certain conditions are met, the oceans can become very warm. One such warming event (called a Hothouse state) took place at the end of the Permian, when the Great Dying occurred. It is thought that this Hothouse state was triggered by a massive eruption at the Siberian Traps, which released enormous amounts of CO2 and other nasty compounds onto the surface of the planet. One of the consequences of this was dramatically slowed ocean circulation in a haline mode. A haline mode ""generates warm saline bottom water that heats the ocean"" (166), which transfers heat from the equator to the poles. This is in contrast to our present cycle, where deep ocean currents transport cold water to low latitudes, creating a gradient of heat and overall cooler oceans worldwide. In the Hothouse state, cyclones, which are restricted to about 40 degrees of latitude N or S in our current climatic regime, may traverse the entire globe (90N and 90S) thanks to worldwide elevated ocean temperatures. They would also create a positive feedback situation:  As storms reached to higher latitudes, they would help deliver more heat to those regions. That would, in turn, further warm higher latitude surface waters, making it more likely that subsequent storms would have an ever-greater poleward reach. Polar storms would also lead to increased polar cloudiness, which would impede surface heat radiation to space, thus warming the poles even more.  Magntitude of storms would increase. Modern cyclones are limited in their size by colder, deeper waters. The bases of their waves reach the colder deep waters and lose heat and energy. In a warmed ocean, this restraint would no longer exist. Kidder and Worsley specifically say, ""the cyclone-magnitude governor would be completely removed in a Hothouse..."" (emphasis mine). So to answer your third question, no, there are theoretically no limiting factors in a very warm, humid situation. To answer your fourth question, the vast, dry deserts of Pangaea were the most likely stopping zones for these storms, as they would be deprived of moisture in the deserts. Source:  Kidder, D.L., and Worsley, T.R., 2010, Phanerozoic Large Igneous Provinces (LIPs), HEATT (Haline Euxinic Acidic Thermal Transgression) episodes, and mass extinctions: Paleogeography, Paleoclimatology, Paleoecology, v. 295, p.162-191.  ",6567,askscience,91% ,Sun Sep 10 13:47:22 2017 UTC,416,luxux3,4209,341,
+"Uncertainty in position allows a particle to tunnel from one position in space to another, could uncertainty in time (Energy-Time uncertainty) allow a particle to tunnel from one instance in time to another?","If it could happen in some way analogous to standard quantum tunneling, there'd have to be some sort of confinement in time, though I'm not sure what could even mean. ","The energy-time uncertainty principle is different from the others. Usually, you have observables in quantum mechanics which have some sort of probabilistic distribution, and the normal uncertainty relation is given as σA σB ≥ something where σA is the standard deviation of the distribution describing A, and ""something"" depends on the properties of the observables A and B. (""something"" can be zero, where the uncertainty principle just tells you that standard deviations are positive; it can't be negative). But time is not an ""observable"" in the technical sense I gave above, because there is no probability distribution for the time of the system, so there's no standard deviation. The time is just a parameter which the probability distributions of all the observables depend on. This basically answers your question: time cannot ""tunnel,"" it doesn't have a distribution and can't ""collapse"" etc. So what is the correct statement about the energy-time uncertainty principle? It is the following: Consider any observable (once again, in the sense I have in the first paragraph) B. Now consider the energy, E. The following is true: σE σB ≥ (ħ/2)|d<B>/dt|  Here, <B> is the average value (mean) of the observable B. If the average value of B doesn't change with time, then there's nothing interesting to say here. But if B is changing with time, then there will always be an uncertainty relation between E and B. Let's think about what this means physically. If you have a system where some observable B is changing, then the energy of that system must not be well-defined - instead there is some spread. This must be true, in particular, for any unstable system, since an unstable system is defined as one which is changing w.r.t. some observable. So unstable systems have some distribution of energies which must satisfy σE Δt ≥ ħ/2 where Δt = σB/|d<B>/dt| is sometimes called the ""time uncertainty."" You can think of Δt as the approximate time it takes for <B> to change by an amount σB. If this amount of time is large (which happens for stable systems with long lifetimes), then the energy uncertainty is small. Vice-versa, if this amount of time is small (e.g. in unstable systems), the energy will have a very large spread.  ",22,askscience,81% ,Tue Sep 12 01:22:04 2017 UTC,6,iaswob,8791,20536,
+Do cows produce a significant amount of greenhouse gases ?,"Was arguing with a vegan about being a vegan and she brought up the emissions from the agricultural industry more specifically the meat industry (cows).  Is the emissions from just the cows actually a significant amount both on a globl scale and different countries? Sources would be nice Edit: wow thanks for all the informative responses this really opened my eyes although not in the way that would make any vegans happy Edit 2: this is my first ever ""big"" post so i thought ill ask here do i still get notifications for deleted comments? ","It's significant, but not the main source of global warming. CO2 is now at about 405ppm, while methane is at 1.8 ppm. Even taking the highest multiplier for methane only gets you to 144 equivalent ppm. And there are many other sources for methane besides animal agriculture, including leakage from oil and natural gas wells.  ",304,askscience,81% ,Mon Sep 11 11:15:47 2017 UTC,132,LIONEL_RICHIE1910,317,15,
+How do alambric headphones work without a battery?,,"""Alambric"" sounds like a literal translation into English, I assume you meant ""wired"". Signals do carry a little bit of power. Headphones are basically very small speakers, just like any other kind of speakers they work when electricity traverses a coil, producing a magnetic field, which interacts with a fixed magnet. As electricity changes intensity/direction, the coil moves closer or farther from the fixed magnet, pushing the speaker's membrane to generate the vibration we perceive as sound. Since the speakers in your headphones are so small and require so little power, the signal in the wire is more than enough. (This would also work in theory if you connect bigger speakers to your phone, but in practice the volume of sound will be so low that you will only hear it barely and in a very quiet environment. If you want more volume, you'll need an amplifier and it will use an external power source.) It also depends on the fact that it's an analog signal. A digital signal would need a converter, and this would consume power. ",1,askscience,100% ,Mon Sep 11 14:15:30 2017 UTC,3,Skyices,11,17,
+can disorders such as those on the autism spectrum go away completely over time?,,"To answer this question, we need to define ""go away"". Autism spectrum disorders are neurodevelopmental disorders, so those who have them have brains that function differently than most people's. However, there is no easily identifiable ""autistic brain structure"", no gene or cluster of genes that can be said to cause autism. For this reason, autism spectrum disorders are diagnosed by observing and interviewing an individual and their close ones. The basis for a diagnosis are things that are observable to others. That is why some people, especially those diagnosed as children, seem to ""grow out of"" their diagnoses: they learn to mask those symptoms that made other people recognise them as autistic. For some people, many or all symptoms abate to a degree that's they would no longer be diagnosable (sub-clinical) in adulthood. Other people may start out less noticeable,  but end up getting diagnosed in adulthood. As far as I know, there is no clear understanding of how and why it turns out this way. So the answer is ""depends on what you mean by 'go away'"". ",3,askscience,62% ,Mon Sep 11 07:16:21 2017 UTC,5,DarkSpark22,138,99,
+How did chemical elements other than H and He come to exist?,,Stellar nucleosynthesis and supernova nucleosynthesis predominantly. In the hot cores of stars there is enough heat and pressure for lighter elements to fuse into heavier ones. In the sun the major process the proton-proton chain which forms helium. The Carbon-Nitrogen-Oxygen cycle is another process present in a lesser extent in the sun. Larger stars than the sun are hotter and have more pressure to eventually fuse elements up to around iron/nickel. At this point fusion becomes energetically unfavorable and once the iron core becomes large enough it can collapse and form a supernova. The intense heat and enormous amounts of neutrons released in the supernova can fuse much heavier elements like uranium. There is also evidence that neutron star mergers or similar events are responsible for these heavier elements. Another route for the synthesis of heavy elements in slow neutron capture in giant stars. ,8,askscience,90% ,Mon Sep 11 03:34:51 2017 UTC,4,htraos,785,10838,
+Why the exciter power rating in large generators is so small compared to the generators power rating?,As I learned the generators in power plants don't use permanent magnets but electro-magnets instead. This is quite interesting - you need electricity for electric generator. Why you only need let's say 2.5MW of (electric) power for powering the exciter/electromagnets although the output of generator is 100MW? Isn't the magnetic field in generator (maintained by exciter) opposed if you put a load on the generator? I still tend to think that if you draw 1MW from generator its magnetic field is opposed by 1MW. I know the extra power comes from let's say falling water (in case of hydro) but is still can't wrap my head around all that work-balance of magnetic fields. ,"You don't lose power from the magentic field. You only need energy to setup the magnetic field, after that you technically need zero power to sustain the magnetic field. The only reason it actually takes power is loses, like the resistance in the rotor coils. Once the field is set up, it certainly feels an opposing force to its rotation, but the prime mover (turbine, engine) supplies the torque to overcome that and hence deliver electrical power.   Consider that you can use a permanent magnet instead. Energy went into making the permanent magnet, but after that it can run a generator indefintely. You don't run a generator on magnets and have to change them out as they die like batteries. Energy isn't being drained from the magentic field.       ",2,askscience,75% ,Sun Sep 10 16:59:59 2017 UTC,6,hectofun,2,0,
+"What is an ""interaction-free quantum measurement""?","This nature paper from 2 years ago and another one from the 90s with Anton Zeilinger have proven that you can have an ""interaction free measurement"". I thought all a measurement was is a particle interfering or ""touching"" its environment, and has nothing to do with an observer (or god forbid consciousness).Usually when people bring up quantum woo people say that a measurement is just an interaction. So what does it mean when you can make a measurement without having an interaction? They said something about the quantum Zeno effect in the linked paper and I know that's where you keep interacting/measuring a particle so it never changes, but what are the implications of ""interaction free measurement"" since I thought all measurement was just interaction with the environment/decoherence? ",The idea is that there is information in the knowledge that there was no interaction.  The tricky part is figuring out how that could be useful. An example of a thought experiment is the 'quantum bomb tester.'  https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester ,4,askscience,63% ,Sun Sep 10 14:37:50 2017 UTC,6,someinternetdudejoe,616,102,
+"Can someone give a complete, blow-by-blow description of exactly what happened in this video of a transformer/power-line explosion?",https://twitter.com/ataccounts/status/906812103658524672 ,"Electricity wants to flow to ground. This video does a great job of showing how that process can overwhelm the distribution system. Power is generated at a high voltage 13200 VAC in the US. (This video appears to be a Caribbean country so the actual voltage is unknown to me but the principles are the same) It is more effecient to deliver high voltage to the point of use then step it down at the source. In this case the big explosion at the end was the step down transformer. That transformer changes the voltage down to a more usable number. Some US examples are 4160VAC, 480VAC, 208/240VAC. There is no way to know what the stepped down voltage was. Suffice to say the fire on the ground at the beginning of the video was a downed live power line of an unknown stepped down voltage. At the beginning of the video the power generator station is still supplying high voltage to the step down transformer, the step down transformer is doing its job providing power down stream to the downed power lines. The fire in the streets is live voltage traveling to ground.  As it passes to ground it is overcoming resistance and converting to heat.  That process is what fuels the fire. Eventually the resistance to ground is overcome and the voltage finds an unrestricted (or rather less restricted) path to ground. At that point the amp draw increases so much that the transformer blows.  Either the internal windings inside the transformer failed and created a short or the resistance between the high voltage points feeding the air was overcome and the final explosion occurred. That final explosion was two separate out of phase legs of power meeting and shorting to each other. At that point either an upstream fuse or a the generator itself failed causing a wide spread power outage. ",7,askscience,67% ,Sun Sep 10 09:43:58 2017 UTC,4,xpostfact,288,7823,
+A while back on NPR I heard that there are different Capsaicinoids that give different heat profiles to hot peppers. Could someone elaborate on this?,"IIRC it was on an episode of Science Friday. They were specifically discussing how the various hot peppers (jalapenos, habaneros, etc.) had fundamentally different capsaicinoids which react in different ways to the heat receptors in tongues and, in turn, have different heat profiles. ","Chillies synthesise a number capsaicinoid chemicals, 5 of which are naturally occurring. Largely they differ by the length of the aliphatic tail. Capsaicin is the most abundant with a Scoville rating of around 16 million and Homocapsaicin is the least abundant with a Scoville rating about half that of Capsaicin. https://en.wikipedia.org/wiki/Capsaicin#Capsaicinoids The mode of action for capsaicinoids is to bind the TRPV1 ion channel on the surface of heat sensing nerves in the mouth, oesophagus and gut. On binding the capsaicinoids force the channel to be open and this causes the nerve to continually fire. Ultimately the different capsaicinoids display different binding kinetics for the TRVP1 receptor. That is, the rate at which they bind or unbind is slightly different for each chemical and the degree to which they can hold open the ion channel, while bound, also differs.  It is these differences that likely give the different capsaicinoids their different effects. Such as causing strong or weaker heat stimulus for shorter or longer periods depending on how strongly bound the chemical is and the degree to which it can open the TRVP1. Different peppers will have different ratios of the capsaicinoids and so they ""burn"" profile will differ. https://www.nature.com/nchembio/journal/v11/n7/full/nchembio.1835.html ",11,askscience,74% ,Sun Sep 10 09:56:42 2017 UTC,2,No_name_Johnson,1296,42773,
+Where does the light go when you turn off the lights in a room with no windows?,,"Scattered and absorbed.   Nothing is perfectly smooth so there is a lot of scattering. And anything with color will absorb some light.  So it all scatters and absorbs over and over until it's all ""gone"" A good question is what happens in a room with nothing but mirrors  (spoiler: mirrors diffract and absorb a little bit too, so same thing) ",45,askscience,74% ,Sun Sep 10 03:21:16 2017 UTC,36,Smp11301,1042,672,
+"If you ahave an ear infection and can't hear that well out of one ear, does the other compensate for that?",,no comments,2,askscience,62% ,Sun Sep 10 19:40:52 2017 UTC,4,starwolf16,30,53,
+Does a child on a swing have acceleration in the negative direction?,"I recently came across a question that asked ""Which of these has 0 acceleration?"" The one that kind of threw me off was the one stated in the title. What do you guys think? I feel like if one puts into account the friction of the chains, then yes there is deceleration.  ",The acceleration of the child is not constant and not always in the same direction. So it makes no sense to say the child has positive or negative acceleration. It is a misleading question. ,1,askscience,57% ,Sun Sep 10 23:51:05 2017 UTC,9,00jg00,22,38,
+"In python, why does print(0.1 + 0.1 + 0.1 - 0.3) return a non-zero number?",,"Humans represent non-integers as decimal fractions, a / (10 ^ b). For example, .1 = 1 / (10 ^ 1). Some non-integers, in this representation, have an infinite number of digits; for example, 1/3 = 0.33333….  Those numbers don't have a decimal fraction representation, but can only be approximated. For example, 1/3 ~= 3 / (10 ^ 1) or 33 / (10 ^ 2) etc. Obviously, if you use approximations, some errors are introduced. For example, 1/3 ~= 33 / (10 ^ 2) and 3 * 33 / (10 ^ 2) = .99. Computers, on the other hand, represent non-integers as binary fractions, a / (2 ^ b). All the same principles apply; some numbers can only be approximated that way, and it shows up as errors in almost all calculations. In your case, 0.1 = 1/16 + 1/32 + 1/256 + 1/512 + …, so a computer with 8 bits of precision might approximate 0.1 as 25/256. Meanwhile, 0.3 = 1/4 + 1/32 + 1/64 + 1/512 … so a computer with 8 bits of precision might approximate 0.3 as 19/64 = 76/256 Which would lead to .1 * 3 - .3 coming out to -1/256 A computer with more precision would have a similar, but smaller, error, which is what you are seeing. ",8,askscience,68% ,Sun Sep 10 14:32:47 2017 UTC,12,DadYak,794,1203,
+Bernoulli Question - What frame of reference to use?,"So if you're in a closed convertible traveling on a highway, relative to the car the air is moving fast, which causes low pressure and pulls the roof of the car upwards. But motion is relative - the air inside the car is moving fast relative to the air outside the car - shouldn't this cause the roof to pull downwards? Obviously both can't be true - how can you determine which frame of reference to use? ","Bernoulli's equation is actually not Galilean invariant. This means that the equation has a different form in different reference frames. The most common form of Bernoulli's principle is for steady flow of an ideal fluid, which states that the quantity  H = p/ϱ + u2/2 + χ  (where p = pressure, ϱ = fluid density, u = fluid velocity, and χ = external potential) is constant along streamlines. If the flow is also irrotational, then H is constant throughout the entire fluid. The principle can be summarized as (u · ∇)H = 0. The problem with this equation is that it does not hold in an arbitrary inertial frame. Indeed, how could it?  What is steady flow in one frame is not necessarily steady flow in another frame. Bernoulli's equation is derived from conservation of energy. Hence, in particular, for a frame in which ""obstacles"" have some non-zero velocity, the equation is wrong because we have to add a term to the equation (u · ∇)H = 0 which represents the work done on the fluid by those moving ""obstacles"". (""Obstacle"" here is a general term and can also refer to a constriction.) This is really not a surprise since it is well-known that constraint forces can do work. So when we change frames, we have to be very careful of taking account of such forces. For instance, a fairly common and accessible introductory physics question is that of a block sliding down a plane with friction. If we were to examine the scenario from a frame which moves uniformly with respect to the block+plane system, and not take account of the constraint forces, we would find some naive contradictions in energy conservation. The fundamental principle is that the change in kinetic energy of a particle is equal to the total work done on the particle. In other words, the change in kinetic energy is the line integral of F(t) along the path of the particle. This principle is frame-independent and is always true. However, if the forces are conservative and time-independent, we can introduce a scalar potential, and then we get the arguably more common form of energy conservation in the equation T + U = const. But in a frame that is moving with respect to an obstacle or constriction in a fluid, the force is not time-independent. So to find the correct form of Bernoulli's equation, we have to appeal to the fundamental work-energy theorem instead of something akin to ""T + U = constant"". In fact, there is another common form of Bernoulli's equation for irrotational, unsteady flow, which again states that (u · ∇)H = 0, but now where H is given by  H = p/ϱ + u2/2 + χ + ∂φ/∂t  where φ is the velocity potential. This added term includes any work done on the fluid by the moving obstacles. For a more general fluid, we would possibly have to add other terms to account for other constraints (particularly if the fluid is not irrotational).  edit: As for the question about air rushing into a car, let's use Bernoulli's principle in the following form. If the flow is steady and irrotational, then the quantity  H = p/ϱ + u2/2 + gh  is constant throughout the fluid. (This is the most commonly stated form of Bernoulli's principle.) Given what I explained above, if you want to appeal to this principle, we must consider the flow from the rest frame of the car. If the flow along the car is steady and irrotational, then we can very easily show that the pressure inside the car should be higher than the pressure just outside the car, and so there should be a flow of air out of the car near the windows. Indeed, this is actually what happens. But the big caveat here is that the flow has to be steady and irrotational. So if you want to verify this for yourself, you should crack open the window only a very small amount, so as to make sure the flow is as close to steady and, more important, irrotational as much as you can make it. Hold a piece of a paper near the crack and it will be pushed outward toward the window. If you open the windows too much, then there is a very strong flow separation along the car. Vortex shedding will occur at the leading edge of the windows, thus making the flow both very not irrotational and very unsteady. So Bernoulli's principle as stated above simply does not hold. ",7,askscience,89% ,Sun Sep 10 15:13:56 2017 UTC,1,max_p0wer,1115,16871,
+How do scientists measure extreme depth without sending in people/robots?,"Assuming that a person had access to equipment and resources, how would they measure the depth of a hole? Is there a limit to how deep people/drones/robots could travel into it safely? How would you measure how deep it was long past that point? ","I'd send a sound wave down and measure when the echo came back. Know the speed of sound, depth = echo time * speed of sound / 2. 2 for down and back. SoS = 340 m/s. You could probably measure the echo to somewhere around a millisecond, so accuracy on the order of a meter.  https://en.m.wikipedia.org/wiki/Echo_sounding ",4,askscience,75% ,Sun Sep 10 15:38:30 2017 UTC,9,breakfastepiphanies,201,94,
+Why is there a trail of red light lagging behind a moving red laser?,"Hey! So I was playing with my cat (of course), and noticed when I was moving his laser around quickly, there was a ""trail"" of light behind it. Why is this? ","This has to do with the eyes having a limited time resolution (a frame rate, if you will, but not literally) and not with the laser leaving a trail. Light from different locations (along the dot's path) is reaching your eyes faster than they are refreshing. ",72,askscience,80% ,Sat Sep 9 16:35:23 2017 UTC,24,imnotgrownupyet,8649,21453,
+Cold you take a picture with a smartphone on the moon ?,Like could it survive the extreme temperatures? Edit: sorry for the typo ,"There are two main issues when using a normal smartphone in space: 1) Thermal control. Heat in space is only lost through thermal radiation (infrared), which is a very inefficient method of cooling down. Smartphones were not designed for this, so the heat produced by the processor might be too much and cause it to fail or burn. See mbf-'s answer below. 2) Radiation. Cosmic rays are bombarding it constantly. If a charged particle traverses a transistor, the ionization it produces can affect it. It can flip bits in a computer's memory, or even burn it definitely. Embedded computers in satellites normally have radiation hardened memory and an operating system that stops using memory segments that have suffered permanent damage. Your smartphone was not designed to deal with this problem. ",0,askscience,50% ,Sun Sep 10 04:28:21 2017 UTC,11,galaxeblaffer,30,39,
+"If matter and Energy are two sides of the same coin, and we have real life examples of matter converting to energy all the time (chemical reactions), do we have any naturally occurring examples where energy is converted to matter?","I understand that the The Large Hadron Collider at CERN would be an example, but it's man made. Until the LHC, did we have any natural examples of such change? ",High energy showers induced by cosmic rays in the atmosphere. ,15,askscience,71% ,Sun Sep 10 14:56:13 2017 UTC,11,Hellion1982,4696,1941,
+Why do we need to boil pasta? Why isn't it enough to soak it in water?,"When I make pasta, I usually microwave it with other ingredients afterwards. Is there any reason why I should boil the pasta beforehand rather than just soak it?  ",You need the heat from the boiling to cook the starch/flour which is an ingredient of the pasta. Try this at home. Take some flour and mix it with cold water. do the same with boiling water. you'll notice the difference. only by cooking the pasta gets soft but still sticks together.   ,26,askscience,75% ,Sun Sep 10 09:41:59 2017 UTC,22,deltalessthanzero,20763,9940,
+Is anything truly random?,Like it may SEEM to be unexplainable and random but is everything patterned if you trace it far enough? ,You are basically describing Laplace's Demon. However quantum physical processes can be completely random. e.g. radioactive decay is perfectly random. You cannot predict when a particle will decay. You can only give probablities.  ,8,askscience,66% ,Sat Sep 9 18:23:11 2017 UTC,44,Bootz_Tootz,2089,16128,
+Does anyone use Candela (unit)?,"Candela is one fundamental unit in the SI system, yet it is defined from the luminosity function for an average human. Isn't it weird a fundamental unit is defined from a biological background? In physics you will never see this unit appear and it does not define any fundamental constant in nature. You can always use watt per radian or any other luminosity unit. Do biologists use candela anyway? Edit:typo ","Well, let me put it this way.  I have never once seen candelas used in the literature to report a measurement. My only experience with it has been historical or pedagogical (i.e. the text reads ""luminous intensity is measured in units of candelas or lumens"").   A quick search of the literature is inconclusive because the primary results returns papers authored by people with the name Candela, and a search of only the body of the text returns papers like those mentioned above. A search for the symbol (cd) returns a lot of results more concerned with the element Cadmium.  However, I did manage to find a few papers that did report on efficiencies of electrophosphorescent in 'devices' that used units of cd/m2 [1] [2]. So perhaps some people use the unit in some respects, but they are likely few and far between.  Other scientists may have different experience than me though, as I am not an optical physicist or astronomer, or really concerned with this sort of thing in any professional context.  ",3,askscience,100% ,Sat Sep 9 09:58:36 2017 UTC,6,MaoGo,11,34,
+"If space is expanding outwards, would part of the night sky eventually get darker?","Mainly referring to stars that are getting farther away but also other factors if relevant. If everything is moving away from a point, would that point eventually be darker/emptier than the rest of the sky? ","Our galaxy is not expanding, because its stars are gravitationally bound.  The sky will go dark eventually, but because of the decrease in available fuel for fusion reactions, not because of the expansion of space.  ",8,askscience,79% ,Sat Sep 9 06:43:06 2017 UTC,7,cynber_mankei,1708,5900,
+"If quantum entanglement can't be used to transmit information, then how is this experiment (see description for link) possible?","Been catching up on some reading, came across this experiment here. I've been told over and over again that entanglement can't be used to transmit information. So how the heck does this work? Aren't they extracting or inferring information about one photon's path from its entangled twin's behavior -- the exact sort of thing I've been told isn't possible? ","if you have an entangled pair, you cannot transmit information from one to the other without either having them interact again, or by sending a bit of classical information along. In this case, the red light that passes through the image does interact with the yellow readout light through its interactions in the second nonlinear crystal NL2 and then on BS2.  This ensures that no information is transmitted faster than the speed of light or anything.  The clever design of the experiment ensures that the photons that are read out are different than the photons that interacted with the object, but the two modes still have to interfere again before readout to produce the image. ",26,askscience,75% ,Fri Sep 8 23:41:52 2017 UTC,8,jon_stout,3074,13856,
+Does detonating two nuclear bombs side by side have the same effect as detonating one doubly-large bomb?,,"No, it's more effective to use multiple bombs. If we lived in space then things might be different, but our cities are on the surface of the Earth, and that means they tend to be mostly planar. Which means that the damage to a city from a nuclear bomb is essentially the intersection between a sphere and a plane. To maximize effect a bomb is detonated in the air. Bigger bombs need to be detonated at higher altitudes, which means that as you scale up to bigger and bigger bombs.  More and more of the bomb's energy just ends up uselessly expended into the upper atmosphere or out into space, instead of into the target, because you're having to detonate the bombs farther and farther away. For example, according to nukemap a 1 megaton bomb would have an airblast radius of 4.37 mi, destroying an area of 60 square miles. A 500 kiloton bomb, however, would have an airblast radius of 3.47 mi, destroying an area of 38 square miles. So 2x 500 kt bombs would be able to destroy 1.26x the total area that a single 1 MT bomb could. Similarly, 20x 500kt bombs would be able to destroy 2.7x the total area that a single 10 MT bomb could. ",23,askscience,81% ,Sat Sep 9 01:37:40 2017 UTC,14,BrokeDiamond,4286,27069,
+[Physics] What is the difference between the sounds of different vowels?,"I'm not asking about how they're made or the physiology in any way. I mean, if you recorded two sustained vowel sounds, chopped off the beginnings and the ends, and compared the sustained sounds in the middle, what differentiates them? What differentiates a Long A from a Short A or an E? Is it pitch? Timbre? ",Check out formants. I learned about them from dormant filters in sound synthesis which can make human vowel like sounds.  http://person2.sol.lu.se/SidneyWood/praate/whatform.html https://en.wikipedia.org/wiki/Formant So from what I can see each vowel is characterized by having certain formant frequencies be dominant.  ,12,askscience,76% ,Fri Sep 8 15:22:09 2017 UTC,4,ArMcK,2398,50774,
+"For a project like Breakthrough Starshot, how is it possible to take an image that is useful when it is moving 15-20% of C?",Wouldn't any camera able to capture images at that speed be too large to be considered for starshot?  ,"Not necessarily. First of all, speed of a camera doesn't simply correlate with size.  Secondly, even at considerable speed, space is really big.  It takes hours for light to cross our solar system.  It would take a starship probe maybe weeks, and the further away something is, the slower its angular velocity and the easier it is to image. In short, you might not be able to get the kind of up close hi res images new horizons got of Pluto, but you can get way more than you get from here.   ",3,askscience,81% ,Fri Sep 8 21:11:50 2017 UTC,1,mynameisalsomatthew,1206,1567,
+What is the speed of kinetic energy?,"This might be an odd question but is there a speed of kinetic and potential energy? I ask this because I'm sitting on a couch right now and there is a water bottle about a foot away from me. My movements and even my heartbeat is making the water in the bottle move. If I get up, the water is still. So I know it's me and not, say, air current from my fan or AC.  Now, I've been trying to figure out how fast it takes my heartbeat to transfer into the water. Is there a set limit or is it based on the force, in this example my heart, acting on the water? Or is it like sound or light where it only moves in one single speed? ","The speed of vibrations in a substance of matter is known as the speed of sound in that substance. This is what causes sounds which we hear. The speed will generically be different in air, your body, the table, water, etc. You can Google a lot of ""speed of sound in blank"" if you want specific numbers. The exact value depends on things like the material's compressibility and density. So essentially, your heart pumping your blood will cause small oscillations (sound waves) in nearby cells which can propagate outwards, though the sound waves will dissipate. It seems that when you're standing up, it dissipates too quickly to move the water bottle, but you do see the sound waves get there when you're sitting down. (Calling this the speed of kinetic energy is not really precise. Sound waves carry kinetic energy, but this is just one way that energy can be transferred. Light waves also carry kinetic energy, and they move at the speed of light.) ",3,askscience,60% ,Fri Sep 8 22:23:51 2017 UTC,13,Onyx_Initiative,671,5714,
+How vulnerable is our power grid to a powerful coronal mass ejection?,"The Carrington event of 1859 is the most powerful geomagnetic storm to hit the Earth in recent history and overloaded telegraph lines, even causing fires. I've heard it stated that an event similar in magnitude could overload the power grid and blow out the main transformers that keep our power running and that these transformers would take years to repair and millions would die from starvation and not being able to check Facebook.  I get the sense that this doesn't tell the whole story and is mostly sensationalized clickbait. How likely is this scenario? Would the transformers be repairable to some degree? Could we salvage the power grid in time to prevent us from becoming a third world country for decades? I recognize it could be very bad, but how bad exactly? ",no comments,9,askscience,100% ,Fri Sep 8 16:52:33 2017 UTC,0,jeranim8,402,2887,
+How do your sinuses drain in outer space?,,"Sinuses don't drain based on gravity. In fact, the opening of your larger sinuses (e.g maxillary) isn't at the most dependent, or lowest spot. Sinuses drain using muco-ciliary clearance. A gel layer on top of a sol layer is pushed across the surface of the sinus mucosa by cilia, this is essentially mucus. This layer isn't really a liquid or solid so generally conforms to the cavity but does not necessary take the shape of the cavity. It is pushed by the cilia in the direction of the natural opening, or the ostium. The ostium leads to the nasal cavity where additional cilia push this substance to the nasopharynx and oropharynx where you swallow it. Because this is guided by the active motion of the cilia it would not depend on gravity. ",14,askscience,85% ,Fri Sep 8 14:06:59 2017 UTC,6,RedditTrollin,4722,12160,
+What uses will Neutrinos probably have in the future? What are they used for now?,,Non-research things neutrinos could be used for: -communicating directly through the Earth or with submarines -verifying that nuclear power plants aren't being used to make bombs ,0,askscience,50% ,Sat Sep 9 04:57:58 2017 UTC,5,MirrorFeatheredCrow,11,-5,
+Is the 'radioactive days' real and do explosions on the Sun's surface affect our health?,"I've been hearing this for a while in my country. I've even experienced stuff like that. By that I mean headaches on the same days with other people, high blood pressure, etc. I've also heard from many people that it's 'radioactive days', explosions on the Sun burst out huge energy and it does affect our health.  Sorry for a bit of gibberish and thanks in advance. ","From what you're describing, no. People misunderstand the concept of radiation, but basically there are two categories of things we think of as ""radiation"":  electromagnetic waves and energetic particles. EM waves are the same thing as light, and they include radio, microwave, infrared, visible light, ultraviolet, x-rays, and gamma rays.  Of those, UV, x-rays, and esp. Gamma can be harmful to humans at relatively low intensities.  The atmosphere mostly screens them and protects us.  Some UV slips through, which is why sunscreen is a thing.  The other category of ""radiation"" is the emission of charged particles: electrons and protons, mostly.  Whether these are harmful just depends on how fast they're going. The sun sprays out charged particles, but the Earths magnetic field catches most of that before it reaches the surface. Of both kinds, if the radiation has enough energy to knock electrons off of atoms, it is called ""ionizing"" radiation.  Non ionizing radiation is literally incapable of harming you. Ionizing radiation can, but your body heals itself, so to do lasting damage, you need either long or intense exposure. The sun's output varies a bit, but not THAT much, and Earth's atmosphere and magnetic field screens most harmful radiation.  The symptoms you're describing couldn't come from solar output.  More likely the shared headaches are either the result of airborne allergens or just coincidence.  ",4,askscience,83% ,Fri Sep 8 21:37:17 2017 UTC,9,TalkToDaHand,598,1769,
+"Which is faster, light or electricity?","I know of the fallacy that, ""because I turn the switch first electricity must be faster,"" but I'm genuinely interested in which is faster. Electricity seems like it, if anything, wouldn't be bound by the law of not being able to go faster than light. That said, I don't actually know. A friend and I were discussing it, so I figured I would ask the people who would actually know. Which is faster, light or electricity, or is it something that depends on the scenario? ","Electrical signals travel at the speed of light in the medium. The speed of light, like the speed of sound, slows depending on the medium it's traveling through. So electricity will flow through a wire at the speed of light through the wire, which will be slower than the speed of light in a vacuum. However, the actual electrons in the wire move incredibly slow. In a copper wire, they'll move ~.00028m/s ",0,askscience,40% ,Sat Sep 9 05:25:37 2017 UTC,16,VanimaUruloki,43,532,
+Is it possible to tell if a photon which is linked by quantum entanglement to another photon has collapsed without collapsing it in the process?,"I know that it is not possible to make the photon collapse to a particular state, but is it possible to tell whether the photon has collapsed without directly observing it (and thus not collapse it in the process)? Because if it is, then it would be possible to transmit information faster than light through the time interval of observations. For example, if 2 photons are observed at a time interval of 5 x 10-8 s that would be a 0, while observation at a time interval of 1 x 10-7 s would be a 1. ","You cannot.  You collapse the state by observing.  This is the core of why entanglement does not enable FTL communications.  The only way to see the state is to observe it, but observing the state can't tell you anything about why it collapsed, and the result of the collapsed state is probablistic. ",4,askscience,100% ,Fri Sep 8 22:26:23 2017 UTC,3,aberki1234,5244,500,
+Why is CMB in every direction?,"The cosmic microwave background radiation (CMB) is observable in every direction. It is the radiation emitted by the opaque plasma that existed after the big bang that is now reaching us 13 billion years later. if we are still moving away from the origin of the big bang, surely after 13 billion years we would have exited the bounds of the plasma cloud that sources the CMB. Is this true? If so, why is the CMB in every direction? Wouldn't it be like looking at the earth from above the surface, filling a little or a lot less than half the sky? ", if we are still moving away from the origin of the big bang  That's your problem. We aren't moving away and there wasn't an origin point. All of the universe expanded in all directions at the same time and there is no no center of the universe. It's counter intuitive I know but this guy explains it pretty well.  https://www.youtube.com/watch?v=pAMpdiBKen8  ,8,askscience,100% ,Fri Sep 8 20:47:09 2017 UTC,9,pdeboer1987,80,339,
+How does the Hubble telescope position itself to observe different parts of the sky?,,"It uses reaction wheels. Those use conservation of momentum to rotate, precisely position,  and maintain the attitude of a spacecraft without use of reactive engines (most of the time, anyway). Hubble in particular had those fail several times and Space Shuttles were flown in to replace them. ",8,askscience,73% ,Fri Sep 8 21:14:29 2017 UTC,3,Jonnny_Yorke,99,52,
+Is there a scientific approach to creating (and maintaining) a landfill other than covering trash with giant mounds of dirt?,If so - what kind of things are taken into consideration? ,"Well, what you are describing is a dump. Dumps are much worse for the environment than landfills. Landfills are sealed from the groundwater. They use a series of increasingly impermeable or supportive layers of soil and artificial liners. They have pumps to keep the leachate from pooling and finding or creating any leaks, but it can't really be stored or processed to clean water, so they lay down a network of pipes to redistribute it back into the landfill. There are also pipes devoted to extracting methane as anaerobic bacteria digest the organic material. The extracted methane is then burned, sometimes for electricity. When a landfill is full, they invest a lot of time and money into making the final soil cap is replanted with appropriate plantings. I've heard of some former landfill sites being turned into parks. There's a lot that goes into landfills. ",2,askscience,67% ,Sat Sep 9 01:23:54 2017 UTC,11,ima_rabbit_et_cetera,1204,1468,
+Why is it we can see through nebulas and thick dust clouds in other galaxies but we only seem to be able to speculate what the inner workings of our own has giants look like?,"I watch any and all things space and universe related I can find on TV and YouTube but whenever our gas giants (Jupiter, Saturn) or even the ice giants (Neptune, Uranus) are discussed in detail, it seems as though there is relatively little known about what is beyond the veil of their outer cloud tops. I realize we generally know the gasses that make up their mass but there isn't as much certainty about what the core (if any) consists of. So my question is, why can we not use the same technology we use to pierce through the dust and gasses that make up the center of the Milky Way to expose what Jupiter is hiding in its core? Is it too dense to see past? Does it have something to do with type of elements that make up its atmosphere or its proximity to our viewing equipment? Sorry for the length here but I've wondered this for years and a simple google search proved pretty useless. Thanks.  ","The density of gas nebulae is no greater than 106 per cm3 and often much lower. By comparison the density of the air you are currently breathing is billions of times higher, about 1019 particles per cm3. If you were inside a stellar nebulae you wouldn't really know it. You'd have perfect visibility. It's only when they are millions of kilometers in diameter that we can see them. The mean density of Jupiter is billions of times higher again.  But that's not to say we can't see anything inside our gas giants. We can. The Cassini space craft used infrared and ultraviolet to get under the surface clouds. There is only so far we can go through, that thing gets pretty dense on the inside. ",6,askscience,88% ,Fri Sep 8 20:52:07 2017 UTC,5,Cloudzer223,3,312,
+"Why does the intensity of all electromagnetic waves dissipate at the same rate, 1/r^2?","I tried to find some answers about this fascinating property online, but all I got was the math behind it. Anyone have a more simple explanation as to why all electromagnetic waves behave this way? Thanks. ","if you radiate something into all directions equally, you radiate across a spherical surface. the surface area of that is ~r² so the intensity must decrease like 1/r² just by continuity (what arrives at some r-distant spherical surface must be what was radiated at the source, it can't have increased on the way). ",9,askscience,80% ,Fri Sep 8 21:00:05 2017 UTC,6,agentbobR,64,647,
+Why does my water bottle sometimes expel gas when I open it? Is this due to the chemicals in the water?,,"Most likely you're simply experiencing thermal expansion. In which the water and the air (gas) inside is warmed to ambient temperature from a cold temperature, or a warmer temperature from holding it in your hands. The increase in temperature also causes an increase in pressure, which us what is released when you open it.  ",2,askscience,63% ,Sat Sep 9 02:57:11 2017 UTC,11,YourSpeciesIsLesser,113,36,
+What is the most probable physical distance between you and a random person on the internet?,,"I imagine you'll need to provide a specific starting point before anybody can answer that question. Simplest answer would be total land area divided by population: per Google, 510.1m/7.442b = 0.0685 people per sq. km. EDIT: Ah, that's not quite right, that'd be average distance to nearest person at all. You'd need to multiply that by the percentage of people online at any given moment (best answer I could find after a brief search: https://webmasters.stackexchange.com/questions/18392/how-much-percentage-of-total-internet-users-are-online-at-any-given-moment). If we go with an even larger number to improve the odds, we'll say 80 million online at any one time. That accounts for 1.07% of the population, so we multiply 0.0685 by 0.0107 to get 0.00073295 people per sq. km. Inverting that gives us the total area, and circularizing that area means at some point within roughly 21km of your location will be another internet user. Of course, that's not accounting for areas that have zero effective population. Unfortunately, someone else will have to help with such a calculation, as the first four articles I read gave me 1%, 10%, 40%, and 83% of the surface is occupied by humans, which will all give vastly different answers. It also does not account for time zones or dates, where people on the day side are more likely to be online, although during weekdays it'll be before or after work with highest population densities. And of course, it also does not account for the local standard of living: third world countries are going to have a smaller percentage of people with access to the internet, much less actively online. ",9,askscience,74% ,Fri Sep 8 20:03:13 2017 UTC,3,GoTomArrow,9717,14375,
+What are p-values? What would it mean to go from a p-value of 0.05 to 0.005?,"A month ago, Nature made waves by publishing a commentary that the standard p-value should be changed from 0.05 to 0.005. If my intro to statistics covered p-values, I have completely forgotten, and the description in the commentary is abstract for me.    What are p-values? Is the last panel of this XKCD comic accurate? Why is the standard 0.05?  Is it related to the fact that 95% of a normal distribution is within two standard deviations from the mean? What would the new standard mean in practical terms?  Would it wreak havoc with the current social sciences?   (cross-poted to /r/explainlikeimfive/ and /r/askscience) ","EDIT for clarity:  The proposal under consideration is about changing how we interpret p-values.  So I need to go through what p-values are, then we can talk about the proposed change to a threshold called alpha that is used when we interpret p-values.   The p-value is the likelihood of getting data at least as unlikely as the data you actually got, given that the null hypothesis is true.  If you leave out any part of that definition, you are not talking about p-values and you will mess up in your reasoning (and I would ding you points on your exam, I used to teach intro statistics). Null hypothesis testing is a probabilistic version of proof by counterexample.  You assume something, you show that your data is very unlikely given that assumption, and that provides evidence that your assumption is false. For example, you might assume that your drug is no better than placebo (is equally likely to cure patients).  Then you administer the drug to a bunch of patients, and the placebo to a bunch more patients, and you compare how many patients were cured.  If more of the patients who got the drug get better, then you have evidence against the null hypothesis that they're the same.  But, if roughly similar numbers of patients were cured, then you haven't shown anything at all; maybe your drug isn't better, or maybe you just didn't have enough patients to prove anything. The bigger the difference between the two, the smaller the p-value, because it's less likely the drug and placebo are the same if they cure different numbers of people. The more people you have in the study, the smaller the p-value, because who gets better has an element of chance, and you might randomly have some placebo people get better on their own.  If there's no element of chance, you don't really need p-values; it'll just be really obvious, since one thing is always better.  P-values are for when you cannot tell by eye because there's too much data and the difference is too small, or when you want to prove the difference to someone. In practice, we want to make up our minds at some point, even though this whole thing is probabilistic.  To do that, we say that if the p-value is below a certain threshold (EDIT for clarity: this threshold is called alpha; the proposal under consideration is lowering alpha from 0.05 to 0.005), we reject the null hypothesis.  If you designed your experiment right, if the null hypothesis is false, then some other hypothesis of interest must be true, and then your experiment will show support for that other hypothesis.   Lowering that threshold is demanding more evidence before we are willing to reject the null hypothesis.  It basically says how often we are willing to reject the null hypothesis when it's actually true.  Going back to the drug example, if we insist on a p of 0.05, that means that out of every 20 drug trials where we showed the drug is better than placebo, we will be wrong one time and the study didn't really show them to be different EDIT where the drug isn't better than placebo, we'll average one study that incorrectly finds that the drug is better (a false positive). If we ask for a p-value of 0.005, then we'll only be wrong 1 in 200 drug trials.  The threshold is essentially arbitrary; the 2 standard deviations thing is probably why that particular threshold was picked, yes, it makes it really easy to calculate since we can almost always assume an approximately normal distribution when doing these tests. Given just how many drug trials are performed, that might be a good thing to do; we don't want to approve a drug that doesn't actually do anything!  All drugs have side effects, and all drugs cost money, so drugs that don't do anything are bad. Now, if you've been paying attention, you might note that the p-value just talks about the likelihood of the null hypothesis.  And we don't actually care about the null hypothesis at all! We want to know if our hypothesis is correct, not if our null hypothesis is wrong.   This is a fundamental limitation of p-values and the philosophy of statistics that spawned them, because that philosophy does not really believe in probabilities, only proportions.  So it isn't about the probability of a study being wrong, it's about the proportion of studies that we're willing to accept being wrong in a certain way.  Asking about the probability of a hypothesis is a nonsense question; hypotheses must be either true or false. The other philosophy of statistics is perfectly happy to assign probabilities to hypotheses, with the probability representing our own uncertainty about whether the hypothesis is true or false.  In reality, it's one or the other, but since we don't know, probabilities are a great way to measure our uncertainty. Unfortunately the other philosophy of statistics often involves math that was untractable before computers, and the p-value philosophy had a very aggressive advocate, and so p-values became extremely popular, even though 99% of the people using them are using them incorrectly because they forget that the p-value is the probability of the data GIVEN THAT THE NULL HYPOTHESIS IS CORRECT.  They leave off that part and just treat the p-value as the probability that the null hypothesis is correct.   This ignores the probability of the null hypothesis given everything else you know, which is what you need to transform from what the p-value actually is to what people think it is.  And if the hypothesis your testing is deeply unlikely to be true, which is extremely common since there are many, many possible hypotheses, then you need WAY more evidence before you decide to assign high probability to it, and just taking the p-value will mess you up rather badly.   This is all also ignoring the fact that if you really want to reject the null hypothesis, you can just run 20 studies and one of them will come up rejecting just by chance (at a p-value of 0.05).  Going to 0.005 would require 200 studies to get one to come up by chance.   And there are many, many, many ways to fiddle with your data to turn one dataset into 20 datasets, find one that rejects, and then not really talk about the other 19.  Or to otherwise get a low p-value when you shouldn't.  I've seen analyses where the math in a paper would reject the null hypothesis something like 60% of the time for totally random data.   So, if we lower to 0.005, it will be harder to publish papers, because papers only get accepted if they say ""we rejected the null hypothesis (which is taken to mean 'my hypothesis was right')""; saying 'we failed to reject the null' is taken as 'my study didn't show anything and was a giant waste of time'.   Whether this would be catastrophic, I couldn't tell you; it will certainly be bad for labs publishing papers about results that aren't real. ",329,askscience,87% ,Fri Sep 8 00:19:38 2017 UTC,41,comtedeRochambeau,2168,2297,
+Why Dimensional Analysis / Buckingham Pi Theorem Works?,"I know that we can use dimensional analysis to experimentally correlate several variables and to reduce the amount of experiment that you need to do to determine those correlation. I also know that it can be used to scale things up. Example: http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/dimension/node6.html In the example above, we want to study how drag (F) is effected by fluid velocity (v), viscosity (mu), density (rho) and diameter (D). Using dimensional analysis / Buckingham Pi Theorem, we can reduce the variables into Drag Coefficient and Reynold numbers. But why is this valid? Why does Buckingham Pi Theorem led to a conclusion that increasing fluid velocity and increasing fluid density will have the same effect on the drag coefficient because both conditions led to the same Reynolds Number? Buckingham Pi Theorem did not take into account any fundamental principles. Every readings I encountered only explained why dimensional analysis is necessary and how to do it. I never found any resources that tried to explain why Buckingham Pi Theorem is justified. ","Not to put words in your mouth, but I think your main question is ""How do we make the jump from Buckingham Pi to Reynold's number?"" At least, that's what I've always struggled with. It's really just an oversimplification textbooks and professors use when teaching aerodynamics. It is entirely possible to create non-dimensional numbers that aren't helpful in a given problem. Reynold's number is actually the result of a lot of experimentation and doesn't flow directly from Buckingham Pi. Buckingham Pi is the proof that the set of non-dimensional numbers including Reynold's number is sufficient to scale the system. It may have also been helpful in selecting a few different non-dimensional numbers to experiment with. In short, the experimental results of Reynold's number and mathematical proof of Buckingham Pi combine to give you the relationship you're looking for, but it is often taught that one led directly to the other. ",3,askscience,64% ,Fri Sep 8 23:58:19 2017 UTC,6,dkurniawan,275,4222,
+Can solar flares affect the earths tectonic plates?,I noticed there was a solar flare last night that hit earth and then a couple hours later a massive earthquake occured in mexico. could these events be related in any way and if so how? ,"Short answer: No. Nor has Irma triggered it.  Every time there is an earthquake there is going to have happened something unusual somewhere on Earth a short time before. We tend to connect the two, but it is almost always just a coincidence.  ",0,askscience,46% ,Fri Sep 8 12:56:08 2017 UTC,4,Structuresaurus,1,508,
+Why are fires a major problem in the western United States but not so much in the forests of the east or midwest?,"At first I was thinking it may be related to moisture levels, but the PNW is obviously having serious fires right now, where it rains frequently. Is it the sheer expanse of forest present out west that raises the chances of serious fires?  ","Western forests are rather different from Eastern forests.  One of the key differences is in the way that they recycle nutrients; in Eastern forests, materials decay, where in Western forests, they recycle through fire.  Now, when forests are allowed to burn on a regular cycle, these fires are low heat surface fires that are not destructive.  Based on fire scars, the natural burn cycle in the Sierra Nevadas was about every 20 years.  This is frequent enough to clear out the fuel lying on the forest floor and any underbrush.   Tree species here are adapted to survive fire; compare the thick bark of a Ponderosa Pine to something like a Birch.  Some species even specially adapted to propagate during forest fires and have seritonous cones that only open in that much heat. However, this all stopped in the 1910s with the advent of humans and fire suppression.  Now instead of forests burning out the fuels, they're allowed to sit and accumulate.  Fires that do happen have significantly more fuel and underbrush has been allowed to grow creating ladder fuel that turns surface fires into crown fires.  Fires are infinitely more destructive, killing everything in the burn path, and destroying things all the way down to mineral soil. EDIT: I completely ignored Native Americans and their use of fire in my post above. ",63,askscience,77% ,Thu Sep 7 16:56:17 2017 UTC,37,FoggyTitans,2798,642,
+"[Seismology] Someone on twitter was claiming the earthquake in Mexico might be related to Houston, Tx sinking 2 cm, is there any truth in this?","Obviously, I have no idea about seismology, so I need to ask the question in a number of phases  Would City sized areas of the earth sink noticeably (a couple cm) under flood water? Does it happen? If that is possible, did Houston really sink 2 cm (or any other measurable value that we figured out) Would such a sinking possibly cause an earthquake? Bonus: If Houston did indeed sink, or if any other city sinks after flooding, does it ever rise back up?  Thanks "," 1 Would City sized areas of the earth sink noticeably (a couple cm) under flood water? Does it happen? 2 If that is possible, did Houston really sink 2 cm (or any other measurable value that we figured out)  Yes, your twitter friend is likely referring to this map (top image in this story) produced by the Nevada Geodetic Lab. What you're seeing is vertical motion (in particular downward vertical motion) measured by GPS stations post Harvey caused by the weight of the flood waters. What remains unclear is how exactly this surface lowering was accommodated, namely was it caused by 'flexure' of the earths crust (i.e. elastic deformation of the crust that will at least partially recover after the load is removed) or compaction (i.e. closing of pore spaces within the underlying sediment). As that article nicely points out, we see small changes (mm to cm) of surface elevation change in relation to water content changes all the time in similar data, but these are usually on a seasonal cycle as groundwater or storage of water in lakes/glaciers/etc change over the course of a wet-to-dry seasonal cycle.  3 Would such a sinking possibly cause an earthquake?  Not likely in this region and definitely not in the sense that the flooding in Houston had anything to do with the recent M8 Mexican earthquake. There has been evidence that in tectonically active regions, seasonal changes in water balance can influence the timing of small magnitude earthquakes (e.g. this study showing this in California). The important bits here are that 1) this is very local in that the stress changes caused by seasonal variation in water storage are influencing seismicity rates on essentially adjacent fault systems and 2) these are influencing the timing of earthquakes and statistics of when particular types of events happen via small stress changes, but these are earthquakes that would already happen because of build up of tectonic stresses (i.e. the stress changes from the seasonal water variations are the last little nudge, but virtually all of the stress released in these events is built up from plate motions). In the case of the M8 Mexican earthquake, the flooding in Houston is WAY too far away to in anyway influence that and the Mexico earthquake is in no way surprising (it happened on a subduction zone, where virtually all large magnitude M8 or greater earthquakes happen, though this one is interesting as it appears to be caused by deformation/bending of the subducting plate as opposed to on the interface between the two plates).  4 Bonus: If Houston did indeed sink, or if any other city sinks after flooding, does it ever rise back up?  This goes back to whether the surface lowering was caused by compaction or deformation of the earth's crust, and at this point we don't know. If it's compaction, there is no force to reopen pore space so that's permanent. If it's deformation, some of it may be recovered. In an idealized scenario where flexure of earth's crust due to a load is accommodated by elastic deformation of a homogeneous material (imagine putting a weight on a stretched rubber sheet) all of the deformation would be recovered when the load is removed. In the real world, the crust is not totally elastic so some of this deformation (if it is deformation as opposed to compaction) could be permanent. We will be able to tell with continued measurements with GPS as the floodwaters recede as we measure whether the surface rebounds or does not. ",4,askscience,64% ,Fri Sep 8 08:04:52 2017 UTC,4,c0d3M0nk3y,3651,2331,
+After recently watching a Kurzgesagt video about bringing a piece of the sun to Earth: Are fusion reactors truly safe when they fail?,"Reference video: https://www.youtube.com/watch?v=J0ldO87Pprc In the video, Kurzgesagt discusses the possible results of bringing a piece of the sun, gathering from different areas, to Earth and the effects of bringing the pieces. Most of the effects basically involved large, planet affecting explosions. Fusion reactors are described as safe and don't 'melt down'. If we are recreating a process much much hotter than the sun in a fusion reactor, why would a containment fail still be considered safer than a fission reactor, other than the obvious radiation issue? EDIT: Thank you for all the answers. I didn't consider the density of the material as a large factor determining the results of a failure. "," a piece of the sun, each the size of a house  Note that the core of a star is very dense, so dense it can fuse elements at much lower temperatures because of the extreme pressure. In fusion reactors they have to compensate for the lack in pressure with higher temperatures, however, the amount of stuff being heated is much, much smaller and less dense. Take ITER's chamber, it will be a vacuum housing only a couple of grams of material in gas form and then heated to a plasma. If confinement is lost, there is so little mass that it will cool down immediately and stop the fusion process.  Edit: if my calculations are right, the 500 MW ITER reactor would release about this much energy in case of a breach (around 120 kg of TNT equivalent). https://www.youtube.com/watch?v=vai5S0mI9u0 I wouldn't want to be near the chamber if it implodes and the heat is released, but it's hardly a world ending event. ",169,askscience,85% ,Thu Sep 7 13:16:08 2017 UTC,34,Cryptonat,1027,9537,
+At what point does the speed of air moving over a surface start heating from friction rather than cooling by carrying away heat?,,"Aerodynamic heating is, in most if not all circumstances, caused by compression and not by friction. This principle is very pronounced in things like diesel engines and fire pistons, which use compression heating to ignite a fuel. Any object moving through the atmosphere compresses the air in front of it, causing it to heat up. At low speeds this is negligible,see Bernoulli's Law, dynamic pressure is 1/2 density multiplied by velocity squared. Similar to Bernoulli's law for pressure we can calculate a parameter called Total Temperature, which is the highest possible temperature that an airflow can cause. The equation for total temperature is: T_t = T_local * (1 + a M2 ) Where M is the Mach number of the flow, and a is a constant of approximately 0.2. The local and total temperatures are measured in Kelvin. Looking at this formula we see that, flying at the speed of sound, M=1, the air will be heated by, at the most, 20%. At standard sea level conditions this is a 60 Kelvin increase. At altitude the local temperature tends to be 50 or 60 Kelvin colder than at sea level, so your plane will hardly notice anything. Since the relation is quadratic the total temperature rapidly increases at higher speeds. At Mach 2 the total temperature is almost twice the air temperature, which is generally considered as the point at which designers have to start accounting for it in the design and materials used in the aircraft.  Do note that these extreme temperatures only occur at the front of the aircraft and wings, where the compression occurs. The flow past the aircraft fuselage and wing surfaces will only be partially compressed, and therefore only partially heated. ",15,askscience,87% ,Wed Sep 6 21:24:04 2017 UTC,5,LBoss9001,385,2242,
+Any studies/data on global temperature change that's separated by latitude?,"I am frequently dismissed whenever I try explaining the catastrophic possibilities of a global 1-degree temperature change. Is this temperature change evenly distributed, or do some latitudes experience a greater change than others? ","There's a large latitudinal difference in global warming, with the most obvious feature being Polar amplification. This figure is the temperature anomaly from Jan-Mar (2016) with respect to a 1961-1990 baseline, and you can see the exaggerated warming in the Arctic. The precise cause is still disputed, but certainly an important factor is the ice-albedo feedback. Arctic amplification has consequence for the weather beyond the Arctic. You may have heard of the ""polar vortex"" and it's effect on weather in Nth America. The actual link between the Arctic and the mid-latitudes is an area of very active study - which means we don't yet understand all the consequences of the Arctic temperature amplification! There are a number of papers on it, like Overland, 2016 and Francis, 2017. On a side note, another reason why 1C of mean warming is important is because that implies a shift the ""normal distribution"" of temperatures. What this means is that at the extreme ends of the distribution, the ""tails"", a small 1C change in the mean can lead to a large increase in the number of extreme events, and it's the extreme events that are most dangerous for humans. More info here. ",1,askscience,60% ,Fri Sep 8 17:47:40 2017 UTC,3,King_Blotto,326,2020,
+What is that clicking sound you hear when you swallow?,,"Could be a couple things, but what you are probably hearing is the opening of the Eustachian tubes. The basic structure of the ear is as follows.  The outer ear consists of the ear canal to the ear drum. It has nothing but air in it (well, and some wax) The middle ear runs from the ear drum to the tympanic membrane. There are 3 little bones in there that transfer the vibration of the ear drum to the tympanic membrane. The middle ear is also filled with air. The inner ear is the fluid filled part. There are little hairs in there that detect the vibrations in the fluid and send the info to the brain via the auditory nerve.  Now, the air pressure around you isn't constant. It can be higher or lower. For the outer ear, this is no problem as the direct connection to the atmosphere keeps everything at the same pressure. But that middle ear is between 2 membranes, so can't directly equalize pressure with the air. That's where the Eustachian tubes come into play. They run from the middle ear to the throat and can open to equalize the pressure in the middle ear. When you get on an airplane and it takes off, the pressure drops and your ears pop. That's the Eustachian tubes opening. Chewing and swallowing can also open the tubes, so that's probably the clicking you're hearing.  ",7,askscience,100% ,Fri Sep 8 05:30:33 2017 UTC,3,nekminnit4,4009,6597,
+How do we know the how old the universe is?,Heard my prof mention it today in our lecture and it got me thinking as to how the hell we were able to measure that.  ,[removed] ,8,askscience,63% ,Fri Sep 8 03:28:58 2017 UTC,9,youguesseditt,95,23,
+Are there any known human diseases that come from archaea?,My bio teacher slid this fun fact in class today. Is this true? Why?  ,"I'd check this link out for more info and sources: https://www.reddit.com/r/askscience/comments/h4ws4/why_arent_archaea_human_pathogens/ I think it basically boils down to the environments that archaea typically populate and their comparison to humans, as well as genetics of archaea vs. bacteria, specifically the role of phages in pathogenicity. Archaea live in crazy environments - low pH, high temp, anaerobic, etc. They evolved very different mechanisms for dealing with these stressful conditions, including a completely different cell membrane. Humans don't really have a similar environment, the exception being our gut is anaerobic. Some archaean methanogens have been found in gum disease, but we don't know how they got there and if they caused the disease or happen to be eating the leftovers from the bacteria causing the disease (deemed most likely by a few scientists). Additionally, archaean phages don't play nice with bacterial phages and vice versa. A lot of the molecular machinery for infection is missing in archaea because of this.  ",8,askscience,84% ,Thu Sep 7 04:05:46 2017 UTC,5,kaptainbrunch,4535,3301,
+Why does the Inverse Compton Effect prevent the atmosphere from lighting up from an atomic explosion?,"So Inverse Compton Effect is that a charged particle and give some of its energy to a photon, and somehow that prevents the atmosphere from lighting up in an atomic explosion. https://en.wikipedia.org/wiki/Effects_of_nuclear_explosions ","The practical result of the Inverse Compton Effect is that high-energy electrons lose energy (by scattering low-energy photons). The electrons in question are attached to the atoms you are trying to fuse (e.g., nitrogen, if you are imagining the atmosphere burning). It's another way to say, ""the nitrogen atoms are going to be cooled because they are getting hit with all of these low-energy photons that are going to remove some of that energy, so they won't fuse."" This is a general problem with thermonuclear reactions in situations where the number of photons created is just astounding. It doesn't mean they're impossible, but it a form of energy loss that has to be accounted for (in a thermonuclear weapon, high compression, for example, helps counteract the effect; in the atmosphere, the fuel density is too diffuse for it to spread). As the cited report puts it:  Propagation of the reaction demands that the energy production in each newly entered region exceed the losses from that region. The energy appears in the form of kinetic energy of particles which are products of energetic nuclear reactions. The product particles, through collisions, share their energy with the particles of the air and help maintain the temperature. On the other hand, the share of the energy given the electrons is rapidly radiated way. This constitutes the chief energy loss. The radiation cannot help to maintain temperature because of the great transparency of air, and because, even if the heated volume is great enough to ""contain"" the radiation, the heat capacity of space for radiation is so great that the energy produced in the reaction is many orders of magnitude too small to maintain the needed radiation temperature.  The bolded part refers to inverse Compton reactions. It's an important part of the energy loss, but it's worth noting that it is part of an overall ""system"" that explains why it can't happen in that situation. ",3,askscience,64% ,Thu Sep 7 08:17:52 2017 UTC,2,yosimba2000,1005,2085,
+What type of explosive is being used for the H-Bomb's explosive lenses (Fuse for the fission reaction)?,Is RDX and TNT still used? ,"The specifics are very seriously guarded secrets. I would advise against seeking that information.  In general, explosive lenses use geometry and multiple types of explosives to shape a detonation shock front. Whatever explosive they use, it will be chosen to most closely match the designed behavior (maximum reliability) and to be stable against anything except the trigger (safety.)  ",5,askscience,65% ,Thu Sep 7 10:37:00 2017 UTC,7,monkeymonkeym,85,44,
+"Do we have any way of tracking the average lifespan of animals (domesticated or not)? If so, have we had a noticeable impact on animal life spans?",,"Animals is almost certainly too broad a category to get any meaningful data, the average would be completely dominated by things like insects, spiders and a whole bunch of microscopic animals (like plankton). Limiting it to mammals would help, but even then I suspect the average would be heavily skewed towards rodents and other small animals.  I either case, I'm just nitpicking your question, I have no idea how to find the information you are looking for :) . ",0,askscience,36% ,Thu Sep 7 22:46:17 2017 UTC,2,Erebus136,870,889,
+Is there a map of the human genome?,"Is there an accurate map of the human genome? That is maybe an interactive map and says what dna does what in theory. Like showing a giant full length dna sequence of examples and when you click on a certain section it shows eye color, circadian rhythm, and other fascinating theoretical sequences. ","There are probably several, but you might start here: https://www.ncbi.nlm.nih.gov/projects/genome/guide/human/ When you click on the chromosome it takes you to a map with the genes shown. You can view the sequence in the little tool that shows up after. It takes a few minutes to get used to, but it's free and works in your web browser. There's a search tool on the left that lets you search for 'phenotype' - phenotype is (more or less) a fancy word for trait. Typing in eye color pulls up numerous genes that seem to be involved in eye color determination. Most of your traits are influenced by many gene sequences. I am not sure how comprehensive this resource is and I haven't spent a lot of time using it - I'm a microbial geneticist. If someone with a technical knowledge of human genetics wants to chime in, I would appreciate it. ",6,askscience,88% ,Thu Sep 7 09:26:27 2017 UTC,2,ArmoredBattalion,900,1152,
+"If you were to randomly find a playing card on the floor every day, how many days would it take to find a full deck?","The post from front page had me wondering. If you were to actually find a playing card on the floor every day, how long would it take to find all 52? Yes, day 1, you are sure not to find any duplicates, but as days pass, the likelihood of you finding a random card are decreased. By the time you reach the 30th card, there is a 22/52 chance of finding a new card. By the time you are looking for the last card, it is 1/52. I can't imagine this would be an easy task! ","This is a rephrased version of the coupon collector's problem, where an item is chosen at random, with replacement, from a collection of n distinct items, and we want to know how many tries you would expect to take before you drew every item at least once. The answer turns out to be, for n items, n*Hn, where Hn is the nth harmonic number (i.e. Hn = 1 + 1/2 + 1/3 + 1/4 + ... + 1/n). For n = 52, this gives an average result of almost 236 days. ",12064,askscience,84% ,Tue Sep 5 16:25:25 2017 UTC,1025,Eddie_shoes,5489,53627,
+Why is the impact from being hit while wearing a bulletproof vest so much worse than the recoil from a rifle?,"From my understanding, recoil is the force the bullet exerts on the mass of the gun, and the impact felt by the person being hit is the energy of the traveling projectile, spread through the mass of the vest. The force of the bullet is the same in both cases, so why is the reported damage so different? ","The butt of rifle is designed to spread the foot pounds evenly across the shooter's shoulder. Conversely, the bullet is intended to concentrate all of its force in a very concentrated area. Even with a bulletproof vest, the force is distributed unevenly, strongest at the point of impact. In short, the bulletproof vest does not distribute the foot pounds as effectively as the butt of a rifle.  ",35,askscience,92% ,Thu Sep 7 01:14:38 2017 UTC,42,HelviusCinna,23,6,
+Is it possible to achieve a fusion reaction in a laboratory setting using only protium?,"In most man-made fusion reactions, deuterium/tritium is the most common substance used, but is it possible to achieve fusion in a laboratory setting with normal hydrogen? If so, would this require more energy to do? ","The Coulomb barrier is a much stronger function of Z than A, so the Coulomb barrier for protons, deuterons, and tritons are all essentially the same. The only nontrivial reaction you can have between two protons (at least below the threshold for production of new particles) is p + p -> d + e+ + νe. This is the very first reaction in the stellar pp chain. Since a neutrino and a charged lepton are produced, this reaction involves the weak force. Since the weak force is very weak on nuclear physics scales, it has a very low probability of occurring. It's the rate-limiting step in the pp chain, because it's slow and it's necessary to produce neutrons. This reaction has a small positive Q-value, which means that it produces a little bit of energy. So overall this reaction is very unfavorable. DD and DT fusion reactions happen with much greater probability, have the same Coulomb barrier, and release a larger amount of energy. ",4,askscience,61% ,Wed Sep 6 15:09:06 2017 UTC,3,Siphonophore-,168,116,
+"With the current U.S. opioid epidemic, do we have a way to estimate the amount of people who abuse these drugs but don't overdose?","Listening to NPR today, there was a teaser for a story about Muncie, IN, where apparently the opioid epidemic has gotten so bad that it's affected some companies' ability to hire drug-free employees. It got me thinking - ""Do we have a way to know how many people actually abuse these drugs when not all of them OD?"" Obviously overdose statistics are kept, but not everyone who abuses narcotics necessarily overdoses. Thanks in advance for any info! ",[removed] ,1,askscience,67% ,Wed Sep 6 22:52:50 2017 UTC,2,wut_r_u_doin_friend,425,4201,
+"Why do Ammonites seem to be the most common fossil, and why are they referenced so often to the Cretaceous–Paleogene extinction event, compared to other species who went extinct?",,"While I wouldn't say ammonites are ""the most common fossil” (although they might be in the rocks in your area), they certainly are a common fossil for the marine sedimentary rocks ranging from at least the Permian to the K-T extinction.  What makes ammonites important? Well, they have all the characteristics of an important group of index fossils. Index fossils are special, and sought after because they provide reliable time markers within a rock sequence. A good group of index fossils will:  Have a wide spatial range – ammonites were pretty much global within the world’s oceans, and marine rocks make up most of the worlds sedimentary record.  Have easily fossilisable and uniquely recogniseable hard parts - Ammonites have hard shells which are distinct for each species, unlike squids for instance, so check that! Have a wide temporal range (as a group) within which occur a wide variety of distinct & recognisable and rapidly changing species with a definite time range may be recognised. Ammonites have all of this, allowing the recognition of most of their numerous species to translate into a time stamp (example). Be physically numerous when present, at least some of the time. As you have stated, this is the case in several rock assemblages, including the ones where you live (myself, I live in an area too old to contain ammonites).  So …. Because of all that, geologists will pay strong attention to the ammonite assemblages in their rocks (when they are lucky enough to have them) in order to know what is their age and figure out where in the stratigraphy they are. This entails that they will be referenced a lot, almost in a mandatory fashion, as they are in a way the reference calendar we are using when working in those rocks. And their disappearance at the K-T extinction deprives us of a reliable and useful tool. They are missed. There are other groups of fossils which I would say are at least as important as ammonites for other time periods or environments, such as: graptolites, brachiopods, and foraminifers. ",7,askscience,68% ,Wed Sep 6 09:02:22 2017 UTC,1,sconestm,977,2746,
+What is the speed of breaking glass?,"To our eyes, glass shatters instantaneously, but there has to be a propagation speed...... ","While you might think that the fracture would spread at the speed of sound in glass (because that's the speed at which pressure changes propagate), a fracture in something like glass typically grows more slowly than that (although in certain materials, the fracture has been shown to actually move faster than the speed of sound - or at least, a certain definition of the speed of sound, because different waves travel at different speeds).  Fracture is something that isn't fully and accurately modeled yet in mechanics of materials so the most accurate data on this comes from measurement. And fracture speeds are not independent of material shape and loading.  Nevertheless, the speed of fracture in glass is about 1500 m/s or about 5000 ft/s (or about 5400 kph or 3300 mph).  ",2,askscience,67% ,Wed Sep 6 19:38:03 2017 UTC,6,stupre1972,3182,756,
+How long would you need to survive to out live a zombie apocalypse?,"Zombies are just decomposing bodies out in the elements, so how long would you need to survive before they reach a level of decomp that they no longer pose a threat of being able to chase you down and attack you? ","I'll just step in while we wait for a researcher with expertise in zombyism to show up.  Zombies are just decomposing bodies out in the elements  Well ... for one thing, they are fictive ""decomposing bodies out in the elements"" ... for the purpose of your question, it matters. It matters because there are so many different biological models out there for zombyism that it may not necessarily be so. These include viral infection with varying degrees of physical mortification (28 days), brain controlling parasites (Night of the Creeps), partial brain destruction with preservation of living function (the Walking Dead), actual death and colonisation by hive organisms (Demon, by John Varley) and dead and resurrection through unknown means (the Living dead).  On one hand, this is a topic with a surprisingly large amount of research behind it. On the other hand, that research exposes the fatal flaw within the question: the answer depends on what biological & epidemiological models you use for the zombie epidemic. The answers range from a few months to impossibility, depending on the parameters selected for the biological and epidemiological model. You'll find more discussions of the topic here: Alemi, Alexander A., et al. ""You can run, you can hide: the epidemiology and statistical mechanics of zombies."" Physical Review E 92.5 (2015): 052801.  Kruvand, Marjorie, and Fred B. Bryant. ""Zombie apocalypse: can the undead teach the living how to survive an emergency?."" Public Health Reports 130.6 (2015): 655-663.  Verran, J., Crossley, M., Carolan, K., Jacobs, N., & Amos, M. (2014). Monsters, microbiology and mathematics: the epidemiology of a zombie apocalypse. Journal of Biological Education, 48(2), 98-104.  Witkowski, Caitlyn, and Brian Blais. ""Bayesian Analysis of Epidemics-Zombies, Influenza, and other Diseases."" arXiv preprint arXiv:1311.6376 (2013).  ",0,askscience,43% ,Thu Sep 7 09:23:39 2017 UTC,12,the_dying_punk,194,6253,
+Why can't I see around a corner?,"Think about a mirror, if you tilt the mirror, you can see light that was directly repelled from the mirror and into your eyes effectively allowing you to see light that bounced off objects not in your field of view. But why can we only see from that one ""Bounce"" and not the previous bounce, maybe light hit a TV then bounced off a dresser then hit the mirror, but only the dresser is shown in the mirror. Another example might be a square room with a wall in the middle that is too tall to look over, one side has a light source, the other side doesn't, it's dark. The dark side gets indirect light from a slit at the top of the wall connecting the other room. Why can I not see the room where the light came from if I'm in the dark room. ","There are two types of reflection, specular and diffuse.  A mirror has specular reflection, it reflects all light at the same angle that hits it. That is why you can see an reflected image, the light rays continue taking the same path relative to each other after bouncing off the mirror. A diffuse reflection, however, reflects light that hits it in a random direction (or rather many random directions at once). That means every point on the object is reflecting light rays from every light source (light emitter or light reflector) in the room at once, so the individual light rays take random paths to reach you. Most objects have diffuse reflections rather than specular reflections. That means that the light coming from different directions gets all mixed together, making it impossible to form an image. ",1,askscience,67% ,Wed Sep 6 08:27:22 2017 UTC,4,Koyhaku,8,13,
+"I just looked at the sun with my eclipse glasses, and there are two black dots on the sun. What are those?","If you have your eclipse glasses, go look. Are they solar flares visible to the naked eye? Or are they planets? ","Those are called sunspots. They are areas of the sun's surface that are cooler than the rest of it. They're still very hot, just not hot enough for the light they're emitting to be enough to see through eclipse glasses. Basically, there are strong magnetic fields in those locations that are inhibiting the normal surface convection of those areas, thus keeping them somewhat cooler than the rest of the sun. ",15272,askscience,87% ,Mon Sep 4 20:52:28 2017 UTC,1111,supaiderman,6256,591,
+Why do larger elements have more neutrons in comparison to their protons?,,"If you apply the liquid drop model of the nucleus, you see that the repulsive Coulomb energy term increases like Z2. As you go higher in Z, it becomes favorable to have more neutrons than protons, because neutrons don't contribute to the repulsive Coulomb force. ",8,askscience,66% ,Wed Sep 6 14:18:13 2017 UTC,3,ryzikx,5470,4671,
+Why do we use gasoline in cars instead of methanol fuel?,"I was watching a video and it was said that methanol fuel emits less than four pounds of carbon dioxide per gallon while gasoline emits nearly twenty. Why do we use gasoline when methanol fuel, which is what F1 cars use, is more environmentally friendly? ","Methanol is more costly than gasoline and is easily contaminated with water. It's less energy dense, because there are 4 hydrogen molecules and one oxygen for every carbon, while gasoline has about 2 hydrogens for every carbon, so a gallon of gasoline has twice the energy of a gallon of methanol. Methanol has to be chemically produced, while gasoline is refined from oil. ",7,askscience,64% ,Wed Sep 6 14:17:19 2017 UTC,16,JohnMarston208,1604,5210,
+How inaccurate are typically earthquake magnitude estimates?,"Estimates of the magnitude of the earthquake created by the most recent North Korean nuclear test range from 5.6 to 6.4. That's a range of 0.8, corresponding to a factor 15 uncertainty in energy release. Is an uncertainty this big typical for an earthquake of this size? ","There are two things at play here: 1) Initial Estimates != Final Reviewed Magnitudes: There is a difference between initial estimates of earthquake magnitudes (which often appear within minutes of an event) and final, reviewed determinations of earthquake magnitudes (which usually take a few days to determine). Most agencies, like the USGS, which report earthquake magnitudes do give an estimate of the uncertainty. If you look at the detailed description of the North Korea nuclear test, you'll see that the uncertainty is listed as 0.0 (though this is rounded, as you'll see shortly) and the magnitude is reported as 6.3. Initial estimates of earthquake magnitudes are often determined automatically from a limited number of observations (i.e. data from a limited number of seismometers) seconds-minutes after seismic waves are first recorded. Over a few days, more observations are put together, often requiring the sharing of data between different institutions and organizations across multiple countries, and the measurement of magnitudes of events get more precise and more accurate. This is true for earthquakes whether they are natural or man-made as in the case of the North Korea nuclear test. 2) There are different types of magnitude measures: Depending on how you measure earthquake magnitude (i.e. what part of the seismic wave do you use, how do you scale it, etc) there are a few standard ways of reporting magnitudes. If you look at the list of magnitudes as determined by the USGS (same link as above, but click on the 'mangitudes' tab), you'll see that the 6.3 magnitude is for body waves (abbreviated mb) with an uncertainty of 0.02 (there's that rounding), but that the moment magnitude (in this case abbreviated as Mww, which is one subtype of moment magnitudes) is 5.5 with an error of 0.066. For the most part, for natural earthquakes, it's more common to report moment magnitudes (these magnitudes are roughly equal to the old 'Richter Scale' magnitudes). I don't why people are using the body wave (mb) magnitude in this case, someone with a stronger background in seismology than myself might be able to answer that, but the takeway is that the range of magnitudes does not reflect uncertainty, just different ways of reporting an earthquakes magnitude. ",15,askscience,68% ,Tue Sep 5 17:16:32 2017 UTC,5,amaurea,440,15513,
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+Why do swallowed hormones (e.g. The pill) work? Shouldn't the acid & proteases render them non-functional?,,"The gastrointestinal tract does indeed present barriers to reliable absorption of medicine taken by mouth. That said, all of the medicines which are totally incapable of being absorbed through the GI tract have one thing in common: You don't take them as pills. (There's so many other non-oral routes: Intravenous, intraosseous, intramuscular, subcutaneous, sublingual, buccal, intranasal, transdermal, intrathecal, rectal...) Anyway, let's say you have a medication that's sensitive to stomach acid. You'd want an enteric coating to get it safely to the gut. This can also spare the stomach from drugs that irritate it, like aspirin. (Though there's times where the faster release of a non-enteric aspirin is preferable, like a heart attack.) Proteases chop up proteins and peptides, which only applies to a small fraction of medications. (Oral contraceptives contain various versions of steroid hormones, which are closer to cholesterol than any protein.) Insulin is the big one, which is why it always comes by some kind of needle or tube. (Actually, there's inhaled insulin now, but that's a whole other minor footnote.) Monoclonal antibodies are a newer group of drugs which are protein, and also given by needle. ",7,askscience,74% ,Tue Sep 5 22:32:03 2017 UTC,7,timepiggy,147,1214
+There have been a dozen or so species in genus Homo; why did all but one sub-species disappear?,There may still have been H. erectus in Java when H. sapiens sapiens were building pyramids in Egypt.  Where is everybody? ,"You've asked a good question, and the only real answer is that archaeologists and paleoanthropologists simply do not have enough data to fully support any hypotheses on the extinctions of pre-modern Homo species.  Among the hypotheses that have been promoted are, of course, intraspecies violence, miscegenation, being out-competed by modern humans, and simply dying off due to failure to adapt to changing environmental conditions. To my knowledge, no evidence exists of any intraspecies Homo violence, though this does not mean it did not occur. Miscegenation, or interbreeding between different species and groups, certainly did happen between modern humans, Neanderthals, and Denisovans.  We don't know exactly to what extent, but likely not on the level of integrated cultures on a large scale.  Enough, though, that most modern humans contain detectable quantities of Neanderthal and Denisovan genetic material.  I believe Papua New Guineans have the most ""non-modern"" DNA at something like 7% Denisovan. Competition between prehistoric humans and modern humans would need evidence of coexistence, which is rare.  The times at which various ancient populations existed can be hard to define, and being out-competed would require coexistence for some length of time.  As dates of late Neanderthal remains are refined, and new sites are discovered, more evidence should come to light. It is, in my opinion, most likely that changing environmental conditions put pressure on prehistoric human species, and gradually reduced their genetic integrity and geographic diversity.  It seems likely to me, though this is conjecture, that the arrival of modern humans did play a role in preventing a recovery of Neanderthal populations in Europe, though this assumes a lot in terms of time and relative population size. All in all, we can't answer that question.  We can come closest by looking at potential interactions between modern humans and prehistoric humans like Neanderthals, which included interbreeding, tool technology sharing, and possible cultural behavior transmission.  As for more ancient interactions, say between H. ergaster and H. habilis, there's practically no data available. As a last note, I don't know of any evidence suggesting H. erectus was alive in historic times.  The remnant species H. floresiensis, the Indonesian ""hobbit"", was at one point thought to have survived to the end of the Pleistocene, or even later, but last I heard that date had been pushed back to something like 50 kya. ",4,askscience,75% ,Wed Sep 6 00:54:44 2017 UTC,14,malvoliosf,39642,185119
+"Ask Anything Wednesday - Biology, Chemistry, Neuroscience, Medicine, Psychology","Welcome to our weekly feature, Ask Anything Wednesday - this week we are focusing on Biology, Chemistry, Neuroscience, Medicine, Psychology Do you have a question within these topics you weren't sure was worth submitting? Is something a bit too speculative for a typical /r/AskScience post? No question is too big or small for AAW. In this thread you can ask any science-related question! Things like: ""What would happen if..."", ""How will the future..."", ""If all the rules for 'X' were different..."", ""Why does my..."". Asking Questions: Please post your question as a top-level response to this, and our team of panellists will be here to answer and discuss your questions. The other topic areas will appear in future Ask Anything Wednesdays, so if you have other questions not covered by this weeks theme please either hold on to it until those topics come around, or go and post over in our sister subreddit /r/AskScienceDiscussion , where every day is Ask Anything Wednesday! Off-theme questions in this post will be removed to try and keep the thread a manageable size for both our readers and panellists. Answering Questions: Please only answer a posted question if you are an expert in the field. The full guidelines for posting responses in AskScience can be found here. In short, this is a moderated subreddit, and responses which do not meet our quality guidelines will be removed. Remember, peer reviewed sources are always appreciated, and anecdotes are absolutely not appropriate. In general if your answer begins with 'I think', or 'I've heard', then it's not suitable for /r/AskScience. If you would like to become a member of the AskScience panel, please refer to the information provided here. Past AskAnythingWednesday posts can be found here. Ask away! ","Why does old urine smell like a truck stop, but new urine doesn't smell at all? ",2,askscience,55% ,Wed Sep 6 15:06:56 2017 UTC,6,AutoModerator,1778,445850
+Can fingers regrow their unique prints after having a chunk cut off?,How similar would it be? What if there were previous prints taken? ,"The structural information that determines the shape of your fingerprints is present in the dermis, which is the layer of skin just beneath your epidermis, which is the outer layer. Ridges/folds called dermal papillae exist at the boundary of these two layers and from these arise the prints. If you damage the dermis, you will have a scar that will change your fingerprint. If you cut a whole chunk of the dermis off, you're definitely going to have a scar. ",3,askscience,72% ,Wed Sep 6 04:05:28 2017 UTC,6,Astroki,1379,19
+What exactly is a laser composed of and what are the differences between different types of lasers?,"I understand that lasers are light (focused?) and that's about it. Lasers fascinate me. You can buy a pointer at pretty much any corner shop and those are very fun to play with. What interests me more are those lasers that cause damage. The ones that cut through metal and can disintegrate fabric/paper. What is the difference between a basic laser pointer and a laser capable of cutting through six inches of metal like a knife through butter? Is it simply the intensity of the light? And if that's the case, through what means do we control the intensity? I apologise if this question seems overly basic but, generally speaking, I am not a man of science. Thank you in advance for your answers and I look forward to being educated further on the topic. ","A laser is a source of highly monochromatic (one wavelength) and coherent light, which means that all the peaks and troughs in the emitted wave match up perfectly. They’re basically tools for creating really well controlled streams of light. Lasers broadly can be categorised into two types - pulsed and continuous beam. It’s the power output (amount of energy per second) of the laser that determines whether or not it can cut through things, typical laser pointers that you can pick up from a shop will be around 1 mW.  The colour of the laser is determined by a thing called the gain medium; if the medium is a mixture of helium and neon then the laser will be red, and if the medium is neodymium yttrium-aluminium-garnet, then the laser will be green (after a process called frequency doubling). You can effectively combine lasers together using a process known as laser pumping, which combines properties of laser outputs to generate the sort of light you require.  The most powerful lasers are typically pulsed, and they emit a very high amount of energy in a very short amount of time.  ",15,askscience,72% ,Tue Sep 5 09:29:35 2017 UTC,12,ChosenNeravarrine,14,0
+What was weather like during the ice age?,"Nowadays, we have lots of interesting weather, like thunderstorms, hurricanes, tornadoes, snowfall, droughts, etc. How does ice age weather compare? ","Just a brief word or two about the term ice age. The Earth is currently in an Ice Age, known as the Quaternary or Pleistocene glaciation, which began 2.6 million years ago An ice age is a period of long-term reduction temperature of Earth's average global surface and atmospheric temperatures, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers.    Within a long-term ice age, the long stretches of time with cold climate are termed ""glacial periods"".  However, the long term cold is intermittently interrupted by relatively brief warmer periods called ""interglacials"".    What is commonly referred to as ""The Ice Age"" was merely the glacial period that ended when the current interglacial period began about 10000 years ago.  At some point this interglacial will give way to the next glacial period.  (Prior interglacials have lasted 10000-40000 years).  So I'll change your question to ""What is the weather like during the glacial periods of this ice age?"" I don't know how each individual weather phenomena will fare when the cold takes hold.  But in general, we can expect it to be drier, dusty (less precipitation & dropping sea level exposes ocean floor sediments), windy near the advancing ice shelves, and COLD, like our hunter/gatherer ancestors survived, but like modern human civilization has never experienced on a large scale (temperatures lower by up to 10°C (18°F) from their interglacial (i.e. current) norm in some regions). ",2,askscience,67% ,Tue Sep 5 19:00:40 2017 UTC,3,Sidnoea,14095,4587
+Are we aware of correlations between photons/light/massless particles and causality that would hint at why light travels at c rather than some slower speed (or a converse; see question text)?,"Sorry if the title is misleading or confusing. I guess the tl;dr here is ""what's so special about light that it happens to travel at the speed of causality?"" Is there some intrinsic relationship between photons/particles lacking mass and causality/information? Or is this just one of those ""That's how the Universe works; we don't [yet] know why, or even if there is a particular reason"" ? Could also be a sort of chicken vs. egg question; please read on with my apologies for failing to express these thoughts concisely. From what I understand, c (~3 x 108 m/s) is the speed of causality, or of information. So, if parties A and B are spacelike-separated by some distance D meters, and an event X occurs in the immediate vicinity of party A, party B cannot be aware of that event until (at the earliest) D / c seconds later. So if party B is ~9 x 108 meters from party A, party B cannot possibly know that the event X has occurred until at least 3 seconds has passed (let's say parties A and B are in the same inertial frame to avoid relativistic effects for now). Fine; I can accept that as a physical rule of our Universe. There is some finite maximum speed at which information can propagate. What I don't understand is why light / massless particles are so fundamentally related to causality so as to have an identical propagation speed. Is it because information can be conveyed by almost any conceivable means, so if information about an event happens to be carried by massless particles, and massless particles travel at a particular maximum speed, which happens to be the highest speed we've ever observed, we conclude that causality travels at the same speed? If THAT preceding paragraph is true, then how do we prove that causality cannot propagate faster than c and we simply remain unaware of any means by which to test/falsify that hypothesis, since the fastest way we know to verify the propagation of causality is via (generally) EM radiation/photons? Is this just a really extended question about the proof of the relativity of simultaneity and I'm just going about thinking about it in an extremely convoluted, flawed fashion? I am aware of the scenarios postulated showing that traveling faster than light would effectively result in traveling backwards in time, causing a paradox, but don't they all hinge upon the assumption that the speed of light IS the speed of causality, rather than the possibility that the speed of light is slower than the speed of causality? I feel like I'm missing something relatively simple making this a sort of dumb question. Maybe a better tl;dr: What is the proof that the speed of light in a vacuum is exactly equal to the speed of causality, instead of the speed of light in a vacuum potentially being slower than that of causality, even if we are unaware of any phenomena traveling faster than light (or even if no such phenomena other than causality itself exist)? ","The answer is that the mathematics of relativity force particles with mass to move at less than c, and also force particles without mass to always move at c. The mathematical framework is just such that those are both true statements. I'm not sure if there's a better explanation than that. ",3,askscience,71% ,Tue Sep 5 18:26:21 2017 UTC,8,FrontColonelShirt,58,343
+Did the Big Bang originate from a Vacuum Decay?,"From what i understand, the Higgs Boson stays in a false vacuum, that if disturbed in any way, can result into a Vacuum Decay, growing at the speed of light and obliterating anything on it's way. Inside this bubble of destruction, lies entirely new laws of physics, changing how everything is.  Could the Big Bang be a Vacuum Decay that originated in another universe before it, meaning that beyond the expansion of the universe, is an entirely different universe made up by weird laws of physics? ","The current standard model predicts that, given the measured mass of the Higgs, it is in a metastable state for our universe. Metastable means that although there might be some lower vacuum, it is separated from our by a huge barrier and so the probability for tunneling is tiny. The time it would take before we expected one tunneling event is several ages of the universe. Also many people think this metastability is a sickness of the theory because it is solved in extensions of the standard model. https://en.m.wikipedia.org/wiki/False_vacuum You're referring to the idea of chaotic or eternal inflation (see also the great Wikipedia pages).  Stephen Hawking just had a paper out on this topic, although it's quite technical. Essentially they're trying to work out the details of what sort of eternally inflating universe could end up ""nucleating"" one like ours but it turns out to be hard to do, i.e. eternal inflation initially was more of a conjecture. It turns out to be hard to quantize to gravity as you would need to do to do this properly.  ",5,askscience,86% ,Tue Sep 5 17:40:54 2017 UTC,3,Wampxz,13488,16469
+"Would it be theoretically possible to see an ""Interstellar like"" accretion disk around a black hole from earth?",Just curious if we found an accreting black hole close enough; would we be able to take pictures of the extreme gravitational light bending like that seen around Gargantua? ,no comments,3,askscience,81% ,Tue Sep 5 18:02:23 2017 UTC,2,nyxo1,5319,2467
+Can we measure density of a black hole ? Would it be a cosmic constant ?,"Is the mass of a black hole directly proportional to the radius of it's even horizon ? If so, is there a density constant for every black hole in the universe ? Sorry for multiple questions. ","Black holes don't really have a density. You can see this old answer of mine to the same question. The volume bounded by the event horizon of a black hole is observer-dependent, for one. To the faraway observer hovering at infinity, the volume is not even defined. The mass is also not really located anywhere, so it's not correct to say the black hole is some spherical object with a mass and a volume. The best you can really say is that the mass is somehow located on the horizon, which has an observer-independent area of 4piR2 with R = 2GM/c2. So the mass per unit area, if interpreted as a useful quantity, is observer-independent and scales like 1/M, where M is the mass of the black hole. ",8,askscience,77% ,Tue Sep 5 15:01:18 2017 UTC,7,Epi_gene,6,0
+I know that combustion and stellar fusion are very different processes. But what would happen if a star made of oxygen collided with a star made of hydrogen?,"A recently discovered star is composed almost entirely of oxygen. If this star collided with a star made almost entirely of hydrogen, would combustion actually happen? While combustion was taking place, what percentage of the star's total energy output could it be? ","Stellar collisions are very rare, but they do happen on occasion. Perhaps contrary to your intuition, stars are generally significantly too hot to support any sort of chemical combustion, even in their outer atmospheres. Most of the gas, in either this oxygen-rich white dwarf or in any ordinary hydrogen-rich star, will probably be either atomic gas (not in any molecular arrangement) or ionized gas (stripped of some or all of their electrons.) A molecular reaction, such as a hydrogen-oxygen burning, would be very rare, and the product would be short-lived. Whenever it would happen, the product (water) would be quickly destroyed (either from strong collisions from other gas particles or from the absorption of intense radiation), which would in the process remove any energy added to the gas in creating it. In other words, no combustion would occur. However, because this oxygen-rich star is a white dwarf, a stellar collision would probably trigger a Type Ia supernova. This would dwarf any other processes in the system. Stellar collisions of any kind tend to be pretty violent events, even if they don't result in a supernova. A small, dense object like a white dwarf is liable to punch straight through a larger star, especially giants, whose envelopes are pretty low density in comparison. This would be pretty spectacular.  ",5,askscience,60% ,Tue Sep 5 16:12:39 2017 UTC,5,Chamale,61228,50947
+"Are gas giants (up to 12 Jupiter masses) more likely around hot stars (O-A class) or cold stars (K, M, Brown Dwarf)?",I can't find reliable data on planetary likelihood per stellar class. ,"There is evidence that gas giants are more likely around high-mass stars, and low-mass stars are more likely to have smaller planets. https://www.newscientist.com/article/dn11944-heftier-stars-are-more-likely-to-host-planets/ Also, stars with high metallicity are more likely to have gas giants. ",7,askscience,89% ,Mon Sep 4 17:03:16 2017 UTC,4,MrJadexxxxxxx,13,168
+What types of magnets are used in motors and generators?,"For example, in wind turbines/electric cars. What are the advantages/disadvantages of different materials? ","All motors require some form of field magnet as you've suggested. The only reason to not use an electro-magnet as the field magnet is if you are trying to conserve power or have size constraints. In alot of the small motors I've messed around with there are curved permanent magnets, however most of the larger motors I've worked on in the military have used electro-magnets. So to answer your question depending on size and power limitations/availability different types of magnets are used for the field magnet. ",5,askscience,86% ,Tue Sep 5 03:54:36 2017 UTC,2,JackA7X,138,775
+"When the immune system damages the gut of a coeliac person who eats gluten, does the mechanism of action differ significantly from that of an allergy (igE+histamine+basophils)?","Coeliac's is often described as an ""allergy"", but people who've been educated in the field say it's ""technically not an allergy and technically not autoimmune, though very similar to both"". I'd like to better understand what this means. Bonus question: Do allergies have something similar to ""levels"" like coeliac's? Or is it more like you're either allergic or non-allergic? ","Your gut lining and mucus membranes secrete IgA to protect from infection. With celiac disease, the interactions of gluten and IgA lead to signficant immune response which causes the chronic inflammation and destruction of gut tissue. It does this by activating T cells to an abnormal state where they mount an immense immune reaction against the body(gut specifically) but not via IgE (it is very much like an autoimmune disease but it also has a clear defined environmental trigger which is more of an allergy trait). However it is really isnt fully understood still in terms of pathophysiology but many suggestions are in the literature. ",15,askscience,73% ,Mon Sep 4 12:18:14 2017 UTC,5,katinla,3006,9666
+How much effect does the gravitational spin of the Milky Way galaxy have on our solar system?,,no comments,1,askscience,60% ,Tue Sep 5 02:18:56 2017 UTC,1,HappySadChap,3554,291
+How can we tell what path a hurricane will take days in advanced? How come we can't determine high probability paths for the hurricane further in advance?,I'm a dumb kid. ,"Short answer is chaos theory. Weather is the most commonly cited example of a chaotic system where even small changes in initial conditions have a drastic effect on future weather even a week out. Even if we knew the position and temperature of every molecule of air on the whole planet, and a computer that could model it much faster than real-time, we still wouldn't be able to have a perfect prediction of the weather much in advance. Since our current system is based on extremely sparse and inexact data, we're lucky to have a general idea of what's coming a week out. ",4,askscience,60% ,Mon Sep 4 21:05:16 2017 UTC,6,The_Thrill17,15,1966
+"In a flood situation, what walls are holding up the water?",,"Levees. We tend to associate the word 'levee' with a man-made ridge built along the banks of a river to attempt to prevent floods, but these are modeled after (and in some cases enhancements of) natural features. Natural levees form through multiple flood events on a river. Water within a river channel is transporting sediment and the ability to transport sediment is dictated by the flow speed of the water. When a river overflows it banks, it is 1) entering an area that is typically extremely flat (i.e. a floodplain) so there is virtually no gravitational force to move it and 2) is often disconnected from the rest of the flow so there is no other water to 'push' it. This results in a rapid reduction in flow speed, meaning that the ability for the water to carry sediment is virtually nil once it overtop its banks, so the sediment that is suspended in that flow drops out and begins to form a deposit near the banks of the rivers. If this happens multiple times (over multiple flood events) this deposit will increase in size and begin to form a levee. ",2,askscience,100% ,Tue Sep 5 01:56:40 2017 UTC,2,j_B00G,501,7620
+Do we have any idea what caused the Oh My God Particle to be moving so fast?,,"No, but simulations give us a tentative idea of where it came from. Energy measurements of individual cosmic rays aren't enough to provide us with a good idea of the mechanism of production. ",5,askscience,65% ,Tue Sep 5 00:49:20 2017 UTC,4,The_Telescreen,7,0
+How soon after an organism 'dies' are all of its cells dead? its bacteria?,"Let's say an organism dies instantaneously (no previous oxygen deprivation). How long would the cells continue to be alive despite the macro organism being considered dead? How long would the bacteria within and on the body last? How much of a time difference would it be, if any, between say an ant and a blue whale? ","There is a problem with your question - ""die's instantaneously"" isn't the right way to word things, since dying is a process and a spectrum. What is death, and when is the ""macro-organism"" considered dead? Both very good questions, without simple answers. A simpler definition would define death as the loss body's ability to maintain the biologic functions necessary for life - a poor definition IMO, since it argues that death is the absence of life. Before modern medicine, death was the lack of breathing and a heartbeat/pulse. However, with CPR, electrical defibrillation, and medications, we are able to continue oxygenation of the organs, and potentially return a stable heart rhythm. This definition is no longer valid in the modern era. The modern medical definition separates death into brain death and cardiopulmonary death (the heart and lungs). Death of an organ is defined as ""irreversible damage causing the cessation of function of that organ"". Death of other organs is possible, but can be (at least partly) medically managed, or does not lead to the immediate cessation of functioning of the organism - e.g. one can live without renal function on dialysis, and complete loss of function of the liver would be a slow ""death"". So instead, let's frame the question differently - how long after the complete interruption of oxygenation do different organs and cells survive?  The most sensitive organ in the body is the brain. A general estimate is that the brain can survive for up to 10 minutes without oxygen, before there is widespread irreversible damage. This timing can be affected by many different factors - e.g. a low body temperature (hypothermia) extends survival, because it decreases the oxygen demands of the tissues. When dealing with hypothermic patients, ""you're not dead 'til you're warm and dead"". Back to the brain, giving a range for hypoxic time until brain death would be safer - I've seen sources give as little as 3 minutes; 10 minutes, as above, seems to be average; and longer is possible in certain scenarios. But the cells of the brain do not die evenly. Neurons (the ""most important"" brain cell) are the most sensitive to hypoxia, while other cells of the brain can survive longer. Neurons of one specific area (CA1 of the hippocampus, Sommer's area) is most sensitive, and irreversible damage to these cells usually occurs after 3 minutes of hypoxia. During autopsy, looking at these cells can be a helpful clue that significant hypoxia occurred prior to death. For other organs and tissues, the time from hypoxia to organ death varies; kidney and liver will survive for about 20 minutes, skeletal muscle for an hour and a half, and skin may survive for a few days. The cornea (part of the eye) remains healthy enough for transplantation up to 18 hours after death. As for bacteria - it probably depends on the type of bacteria. It also depends greatly on the conditions where the body is left. In a warm, moist, oxygenated environment, some species of bacteria will not only survive, but will thrive and multiple - causing putrification of the corpse. These bacteria (or at least their offspring) will survive until the corpse is rotted away to a point were there is no food left for the bacteria. If it is too dry, the bacteria will die, and the body will become mummified. If the body is embalmed, the embalming solution will kill the bacteria. TL:DR Death is a complicated concept, and is better thought of as a process, rather than a single event. There are different definitions of death, but modern medicolegal definitions define cardiopulmonary death and brain death separately. Different organs can survive for different amounts of time without oxygen - the brain is the most sensitive organ to hypoxia. ",140,askscience,85% ,Mon Sep 4 05:12:43 2017 UTC,22,zincinzincout,1884,3870
+Why does clipping a sine wave create a square wave?,"I understand why an infinite series of odd harmonics at proportional amplitudes and frequencies creates a square wave, and I understand the additive properties of waves. I also understand why overdriving a sine wave through an amplifier/circuit cuts off the top and bottom of the wave, giving it a ""square"" appearance. I guess I don't understand the reasoning the other way around. How and why does the fairly simple operation of clipping turn one sine wave into an infinite series of sine waves? It may seem like I'm asking a question I've already answered, but that's not my intention, or maybe I'm not getting something. Let's say you didn't offhand know the harmonics of a square wave. How could you derive them from the shape of a square wave? Odd harmonics generate square/triangle waves. Why do square/triangle waves generate odd harmonics? Does that make sense, or is the answer just, ""dumb dumb, what about the transitive property don't you understand?"" ","It ends up being like you said a sort of transitive property because you're dealing with Fourier analysis. The Fourier analysis is basically working off the theorem saying you're working on a continuous, periodic function and it can be expressed in terms of the sum of its component sin/cos terms with phase and amplitude coefficients. When you go through an analysis on a square wave you'll come back with those odd component frequency components. And part of it being a Fourier series is that you can go back and forth between frequency space and time space as the two are fully inclusive. If you know the component sinusoids of a periodic continuous function you know the function. If you know the function you can run it through a spectrum analyzer (like your music visualizer) and analyze the frequency component's amplitudes and phases. The math of why it is only the odd components comes down to how you integrate the square wave function when doing the Fourier series component calculations and I believe you end up with something like a (cos(k*pi) -1) where k is the harmonic and so all the even harmonic wind up with ((1)-1 = 0). A cool way to see it is finding a music program and getting a sine vs a square wave going. You can try using the amplitude ratios and set the frequency. I did it before as a demo in a music theory class with I think 12 sine waves. Also can model in pspice or matlab if you're engineering type and see both way what's happening. ",5,askscience,69% ,Mon Sep 4 19:57:28 2017 UTC,4,diamondbackmanhunter,368,963
+Why Tsar Bomba - the most powerful bomb ever detonated - could have destroyed the Earth according to its creators?,"According to the Russian physicists who created the bomb, the explosion could have started a self-sustaining nuclear reaction in the ocean, leading to the planet's destruction. How is that possible? ","Do you mean the Trinity test rather than Tsar Bomba? The thought was that the temperature would be high enough to ignite a fusion chain reaction in the atmosphere. Of course it didn't happen, because we're still alive. ",15,askscience,64% ,Mon Sep 4 18:27:26 2017 UTC,12,EchoOne11,17,0
+What are some other mathematical axioms/systems?,"To my understanding, in every system, axioms of what ""is"" and ""isn't"" must be defined and accepted.  In our current system of mathematics, for example, we know that 1 has the same identity as that of 1. We accept the value of 1, and of how it relates to other such values. From there on we branch out into complex number theory and the whatnot. For instance, very rudimentarily explained, Euclidean geometry differs from non-Euclidean geometry in the nature of parallel lines. Obviously the system of math that we use today won out over all others -- passed down century upon century from the first Egyptian etch or the prehistoric Chinese value system -- but what other mathematical axiomatic systems were/are made before the ""math"" that we know today? ","What you have defined are not really what the axioms of math are. There is no universal standard for mathematical axioms. Depending on what you do, you could decide to go with one or another, and there are different consequences for both. This goes deeper than just whether or not you have Euclidean or non-Euclidean geometry, though this is a very light example of some of how things can be different. There are also many different ""levels"" at which mathematicians work. At the high level, we just assume that we are working in some framework where everything makes sense. At this level, we might have things like ""Euclidean Geometry Axioms"" or ""Group Theory Axioms"" or ""Topological Space Axioms"", but these generally refer to different kinds of objects that we can create within our chosen mathematical framework. They aren't fundamental axioms, and function more like definitions/labels for things that implicitly depend on our higher axioms to make. Then there is the low-level stuff. The ""Assembly"" of math. These are axioms based on first principles (assuming some kind of formal logic) that create the framework that most working mathematicians assume exist. These are the axioms that we choose to define things like Set Theory, Category Theory or even something more exotic. These are very sensitive to what axioms you choose and how you decide to write them. Even within set theory, there are many different choices.  It is definitely not the framework handed down from the Egyptians or Greeks, and it took a lot of work in the last 150 years to figure out what the hell axioms are and how they fit together to form a mathematical framework. Egyptians, Greeks, Persians, Indians, etc all found the kinds of objects that we would like to have axioms be able to describe, but they gave us next to nothing for anything foundational about math (vague philosophical murmurs at best, and one example of a high-level axiomatic system). If you want to look for ""exotic"" types of math, then the past is not where you should look. It's the present. The math of the past was limited to basic counting and geometry, and while deep for the time, there was nothing anywhere near as exotic as what is around today. The many types of Set Theory: ZF, ZFC, ZF+CH, NBG, TG. Topos Theory. Even Formal Logic, nonstandard analysis and Universal Algebra are, in their own way, tangentally related to these ideas.  ",2,askscience,58% ,Sun Sep 3 21:32:19 2017 UTC,8,Kalidic,3,0
+Do black holes show any promise in the hunt for dark matter/energy?,"Wouldn't we have a greater chance of finding an example of dark matter/energy near a black hole? If they suck in everything including light, wouldn't they suck in dark matter/energy, therefor resulting in more dark matter near the black hole? ","Black holes don't really ""suck"", they have the same force of gravity that everything else does. The main difference is that black holes are denser so you can get closer to the center without diluting the mass. If we replaced the sun with a black hole of the same mass, the Earth's orbit wouldn't change a bit. As you move towards the black hole sun, it remains the same as the old sun until you get to where the surface of the sun used to be. The Sun has a radius of 700,000 km, so if you are 400,000 km from the center, you've now got quite a bit of matter above you, so the gravitational pull is less than it would be if all the matter were still below you at the same radius. With a denser object of the same mass, all the mass is still below you, pulling you down. At the center of an object like the Sun or the Earth, the gravitational force pulling on you is zero (though the pressure from all of the stuff above you being pulled on is incredible). A black hole, though lets the parabolic increase in acceleration due to gravity with diminishing radius continue to much smaller radii. For a little while, people thought maybe dark matter was black holes. They called them ""Massive Compact Halo Objects"", or ""MaCHOs"" for short. In the late 80s, people set up projects to look for gravitational lensing around these black holes, which, if they existed, should be numerous. They pretty quickly proved that there was no secret population of black holes in the galactic halo responsible for dark matter, though. Now, the favorite candidate for dark matter is Weakly Interacting Massive Particles, or ""WIMPS"" - you know, instead of ""MaCHOs"". Particle physicists at Fermi Lab and CERN are looking for these in their colliders, but haven't found any dark matter candidate particles yet.  ",6,askscience,80% ,Sun Sep 3 17:50:17 2017 UTC,7,itstatum,11,112
+"What was the weather like in northern parts 13,000 years ago that humans would consider and make the trek from Asia to NA?",,"I recall learning years ago the prevailing idea was that nomadic groups of people followed the migration of the megafauna they hunted. Looking at today's Inuit (eskimo), and other Arctic indigenous peoples, it's unlikely that winter conditions especially when such a large portion of the Earth's surface was covered by glaciers, would deter the movement of determined (read: hungry) humans. ",1141,askscience,89% ,Sat Sep 2 13:31:35 2017 UTC,91,JTsyo,3149,65460
+Do white (gray) hairs grow faster than non white hairs?,,"Yes, gray hair also grows faster and for a longer time than black hair. A study of scalp and eyebrow hair revealed that the genes responsible for producing two of the main structural proteins in hair are twice as active in white hair as in black hair. Hair, like skin, gets its colour from a compound called melanin. Info from - http://news.health.ufl.edu/2012/19231/multimedia/health-in-a-heartbeat/gray-hair-is-a-sign-of-life/ ",13,askscience,88% ,Sat Sep 2 21:26:32 2017 UTC,5,PimpangryMX,436,2916
+Can a star have more than one fusion core?,"Other than supernova, could for example a blue supergiant/hypergiant have hotspots not in the center that cause fusion? ","The Evolution of some very massive stars (> 8M⊙) leads them to a state in which fusion is occuring at numerous locations within the star.  For example: Initiallly H fuses into He in the core After 10 Myr,  core H is exhausted,  He core contracts, heats. H fuses to He in shell around the contracting core  When core temp reaches 170M K,  core He begins fusing into C and O H fusion continues in a shell surrounding the He burning core.  After ~1 Myr,   core He is exhausted. C/O core contracts and heats He fusion continues in a shell surrounding the contracting core H fusion continues in a shell surrounding He shell.  At core temperature of 600M K,   core C begins fusing into O, Ne, and Mg. He fusion continues in a shell around the C burning core H fusion continues in a shell around the He shell.  After < 1000 years, core C is exhausted,   the O/Ne/Mg core contracts and heats C fusion continues in a shell around the contracting core, He fusion continues in a shell around the C shell, H fusion continues in a shell around the He shell  At core temperature of 1.5B K,   core Ne begins fusing into O, Mg, ... C fusion continues in a shell around the Ne burning core, He fusion continues in a shell around the C shell, H fusion continues in a shell around the He shell  After a few years,   core Ne is exhausted, O / Mg core contracts, heats Ne fusion continues in a shell around the contracting core, C fusion continues in a shell around the Ne shell, He fusion continues in a shell around the C shell, H fusion continues in a shell around the He shell  At core temperature of 2.1B K,   core O begins fusing into Si and others Ne fusion continues in a shell around O burning core, C fusion continues in a shell around the Ne shell, He fusion continues in a shell around the C shell, H fusion continues in a shell around the He shell  After a few months,   core O is exhausted, Si core contracts, heats O fusion continues in a shell around the contracting core, Ne fusion continues in a shell around the O shell C fusion continues in a shell around the Ne shell, He fusion continues in a shell around the C shell, H fusion continues in a shell around the He shell  At core temperature of 3.5B K,   core Si begins fusing into Ni and Fe O fusion continues in a shell around the Si burning core, Ne fusion continues in a shell around the O shell, C fusion continues in a shell around the Ne shell, He fusion continues in a shell around the C shell, H fusion continues in a shell around the He shell  This lasts perhaps a day.  Tomorrow the star explodes. ",34,askscience,74% ,Sat Sep 2 23:40:46 2017 UTC,14,UntamedOne,3197,4631
+Why is a kettle so much more efficient than a saucepan at boiling water?,,"It traps the steam inside. If you have a hot cup of coffee or bowl of soup, you can cool it off by blowing on it.  This removes the layer of steam from over the surface of the liquid.  When the water vaporizes off the top of the hot liquid, it takes a lot of heat with it.  If it just sits there, it can collide with the liquid and return a bunch of that heat.  If it is blown away, it can't. A saucepan, without a lid, lets that hot steam just dissipate into the air, taking a bunch of heat with it.  A closed kettle keeps that steam trapped inside, and holds onto a bunch of that heat. ",9,askscience,77% ,Sat Sep 2 17:59:12 2017 UTC,6,Zekhaze,209,74
+"With regards to the Diving Reflex, can we hold our breath longer while underwater than we can on land?",,"So the Diving Reflex is the idea that when our bodies are submerged in cold water, the body stops some not as important physiological processes, for example digestion. WHat actually happens is the blood vessels supplying oxygenated blood to the stomach are constricted, thus not allowing as much (or in some cases any) blood to reach the stomach. Cells need oxygen to function (cellular respiration), and without it, the cells won't do their jobs.  So, certain parts of the body are no longer receiving oxygenated blood. There was no blood loss, so the unused oxygenated blood must be going somewhere. It stays in the bloodstream and delivers it's oxygen to the important sites, for example the brain. Now there's less oxygenated blood being ""wasted"" on non-essential processes, therefore there is less oxygen being wasted. Deoxygenated blood becomes oxygenated again in the alveoli of the respiratory system. The oxygen it uses comes from the air we breath in, so the reserved air you have (your breath) can stretch longer.  Assuming you're on land and not in extreme cold, your body doesn't do this. So, on land you continue to supply oxygen to non-essential functions (Assuming you're not in some other dire situation). On land, your stored reserve of oxygen (your breath) is used up faster.  Source.} ",17,askscience,89% ,Sun Sep 3 14:48:08 2017 UTC,4,cmuir37,64,16
+"Why do nuclear explosions in the atmosphere cause little radioactive fallout, while explosions close to ground level cause massive amounts of radioactive fallout?",,"They both cause the exact same amount of fallout, but it gets distributed differently and at different time scales.  A surface or near-surface burst, where the fireball either is in contact with the ground or sucks up a lot of debris into it, has lots of relatively large dirt particles mixed into it. ""Relatively large"" here means the size of a snowflake or so. The radioactive fission products (the remaining ""halves"" of those atoms that are split) attach themselves to the dirt. This makes them much heavier, and a lot of them ""fall out"" of the cloud within a few hours. That means that you get a relatively local (hundreds of miles) plume of radioactive contamination within a day or so of the blast. This means the particles are still very ""hot"" from a radioactive standpoint. So that creates a real health hazard: high concentrations of highly radioactive particles in a limited (but large) area. This is known as ""local fallout"" because of its relatively local area (i.e. ""just"" downwind of the mushroom cloud, but that can still be many thousands of square miles affected for large weapons).  In an airburst, the dirt isn't mixed in, so those particles remain ""light"" and hot. They stay in the upper atmosphere longer. They still eventually ""fall out"" of the cloud, but it's over a much large physical area — like the size of a continent or even hemisphere — and usually weeks later. That means two things: 1. the concentration on the ground at any given place is not very high, so it only adds a bit to the background rate of radioactivity, and 2. by staying up in the clouds for so long, only relatively longer-lived fission products still remain when it falls down (e.g. things with longer half-lives) so the overall radioactivity is less. This is called ""global fallout,"" and it more or less raises the background level of radioactivity globally (though some areas can get a bit more than others depending on how the winds circulate) by a small amount. Where the ""local fallout"" of the ground burst is a ""kill you or hurt you in a short amount of time"" sort of level of radioactivity, ""global fallout"" is a ""adds to your overall radioactivity exposure in a small amount"" sort of hazard. Over very large populations (e.g. the entire world) this adds up to an up-tick in fatal cancers contracted over decades, but the up-tick is small enough that it is very hard to isolate from other carcinogens (even though it still might be what you might consider a large ""raw number"" of cancers — e.g. some tens or hundreds of thousands of excess cancers from atmospheric nuclear testing in the Cold War, which is a lot of people but a very small number compared to the number of people who got cancer from other sources over that time period).  So, to recap, both release the same amount of fallout. But in surface bursts, it comes down fast, concentrated, and ""hot."" For higher altitude bursts, it generally (with some exceptions, e.g. if it rains immediately afterwards) comes down slowly, diffusely, and less radioactive (because of the ""slowly"" part). ",12,askscience,79% ,Sun Sep 3 15:26:09 2017 UTC,6,dev_json,183,33
+If it's impossible to know both the speed and the location of an atom at any given time is it still possible to predict where the atom will be after measuring the speed?,And if so is there an efficient enough way to predict that location? This thought came to me after watching a Vsauce video in which a little diorama had electrons moving around it in a predictable motion so thought what if we could predict the the distance of an atom after measuring the speed? ,"The mathematical form of the Heisenberg Uncertainty Principle (at the top of the page) states that the uncertainty in the position times the uncertainty in the momentum must always be greater than or equal to a certain constant. Whatever experimental setup you use, the more precisely you try to measure the momentum, the less precisely you can measure the position. However, you can know both of them to within a certain margin of error at the same time. Therefore, you can predict where the particle will be a short time later, but not precisely. There will always be a margin of error in the predicted position, and no matter how you try to improve your experiment, you'll never be able to get that below a certain value. ",7,askscience,64% ,Sun Sep 3 01:39:49 2017 UTC,6,_EliasJR_,829,43
+Are we capable of fission reactors for electrical space propulsion?,"Considering future interplanetary exploration using electrical engines we will need powerful and efficient electrical power sources. As far as I know solar panels are not capable of supplying a mass per kilowatt efficiency required by power heavy engine types like MPD arcjet or Vasimr, especially for the exploration of the gas giants.  Would proper fission reactors (other than the weak RTGs) be feasible in our current technological and industrial capabilities? Or are they currently science fiction? ","We sent a fission reactor into space in 1965 - it's still up there, though no longer functional - https://en.wikipedia.org/wiki/SNAP-10A Right now NASA is actively developing a small fission reactor for possible missions to Mars. The goal is to produce a 40kW reactor to provide power (but probably not propulsion) for the mission - https://www.scientificamerican.com/article/nasa-seeks-nuclear-power-for-mars/ Likewise with nuclear propulsion the NERVA engine was certified ready for a manned mission to Mars in the late 1960's - https://en.wikipedia.org/wiki/NERVA NASA has also recently contracted with BWXT Nuclear Energy to develop an updated NERVA-type engine using low-enriched uranium as fuel - https://www.nasa.gov/centers/marshall/news/news/releases/2017/nasa-contracts-with-bwxt-nuclear-energy-to-advance-nuclear-thermal-propulsion-technology.html So no, these are very much real things which do currently exist and are actively being further developed with an eye towards deploying them on future missions. ",45,askscience,74% ,Sat Sep 2 12:21:14 2017 UTC,20,88880,58,17
+Can someone explain the comparative strength of household magnets?,"I'm interested in making household (i.e., refrigerator) magnets as a craft project by gluing some gewgaw to a plain, ordinary magnet. The plasticky ""magnet tape"" is notoriously weak, and the hard black ones are too big, so I ended up with some neodymium magnets from the hardware store. After some reading, I am worried that these are too strong and may be difficult to remove from a refrigerator without tearing off the gewgaw. Internet research is just getting me a lot of industrial supply company FAQs and Wikipedia articles with tesla measurements. I don't even know what a tesla is a function of: mass of the magnet? area of the pole? something more esoteric? I would appreciate a quick-n-dirty guide comparing household magnet types in some sort of strength-for-size way. Thank you! ","Magnetism is a function of a whole bunch of stuff to do with the material and the alignment of the atoms within, and I don't feel fully confident explaining it. But a Tesla is a unit of magnetic flux density, which (basically) is how strong the field (and therefore the force) is. My actual practical advice: do not use rare-earth magnets on your fridge, they're massive overkill and might scratch it up and you'll struggle to remove them. Buy some whiteboard magnets and superglue your craft stuff onto that.  ",4,askscience,67% ,Sun Sep 3 01:18:36 2017 UTC,2,SciviasKnows,19545,18642
+"Need help Understanding JFET's, Specifically how the depletion region is controlled and its relationship to drain current?","So we just started studying FET's in my Electronics circuits and device theory class, I have a basic understanding of how N-channel JFET's work. I am having trouble wrapping my head around how the depletion region is affected by VDS/Drain current when VGS is set to 0V. I think the larger the drain current(ID) becomes the more free electrons are ""pulled""(not sure if correct term) from the N-type making the depletion region have more positive ions. I am trying to understand what is physically happening in the depletion region when VGS=0 and ID increases. I also want to understand the other side, Where ID is a function of VGS, How does applying a larger negative voltage make the depletion region larger and how does that effect the resistance of the N-channel. What is the difference between the depletion region when VGS=0 and ID is at saturation compared with VGS= Some negative value and ID = some lower saturation current. What physically happens to the charges in the depletion region? I can clarify if what I'm asking is unclear. ","Let's start with fundamental materials first. N-type material is doped with pentavalent elements and P-type is doped with trivalent elements.  An n-type jfet has an excess of electrons in it's source to drain conducting channel due to N-type doping, and an excess of holes on the gate due to P-type doping. Therefore, when N-type and P-type are placed together to form a junction, electrons migrate into the P-side and holes migrate into the N-side. Diffusion of an electron from the N-side to the P-side leaves a positive ion behind on the N-side, and diffusion of a hole leaves a negative ion on the P-side. This creates an electric field that provides a force opposing the continued exchange of charge carriers.  The formation and change in width of the depletion region depends on the voltage applied to the gate and voltage applied to the source(see construction of a n-type JFET). If you have a lower voltage applied to the gate relative to the voltage applied to the source ie a negative gate to source voltage, the PN junction is reverse biased, causing current flow from source to gate, creating more negative ions and more positive ions in their respective channels and an increase in width of the depletion region. As Vgs becomes more negative, the n-type material is depleted of electrons(more positive ions) resulting in greater resistance to current flow(from source to drain) and a smaller drain current. ",5,askscience,59% ,Sat Sep 2 20:58:41 2017 UTC,1,eoin34,114,7
+What determines the boiling and freezing points of a substance?,,"Long story short (in case someone else can't give a more technical response) is that every single molecule is attracted to other molecules of its kind in a pure solution of it. That being said, some molecules are really good at attracting each other (things that are solid at room temperature) and other things aren't as good at it (gases at room temperature). Normally, this ability to attract other molecules depends on uneven charges molecules. Think of them as really tiny magnets almost because there are more electrons in one part of the molecule than in other parts. Elements like Oxygen, Fluorine, and other halogens are really good at pulling electrons in, but other elements like Hydrogen and Carbon aren't so good. This is called the electronegativity of an atom. In any event, the slightly positive part of a molecule will line up with the slightly negative part of a different molecule. It's not a full bond because electrons aren't being shared, but it's evough of an attraction to make a noticeable enough difference, especially when there's just SO MANY molecules in a test tube normally (on the order of 1023 for 18 g of water). If there are no slightly charged areas, like in methane which is a carbon surrounded by 4 hydrogens, then molecules have a hard time attracting each other. This is why methane is a gas at a LOT of temperatures. The physical temperatures required the change states are pretty unimportant, they are really just a result of a molecule having greater or weaker attractions to other molecules.  Intermolecular forces is the subset of thermodynamics you are interested in by the way if you wanted to do a quick Google search for it! Source: Chemical Engineering student ",7,askscience,78% ,Sat Sep 2 22:51:12 2017 UTC,6,ClockPuns,397,1377
+How does speakers make multiple frequency at once?,So I understand that a speaker oscillates to a specific frequency but how is possible for it to make a mixtures of highs and lows at once ,"Essentially, think of it as different frequencies are superimposed on top of each other to form complicated waveforms that result in complicated movement. With a simple sine wave of one frequency, the movement is simple but with multiple frequencies and different sounds, the wavelength is complex and so is the movement of the cone/driver. Furthermore, any decent speakers at least split the hi from the mid/bass so different specific drivers handle a narrower band that they are best suited to reproducing and only that frequency band is sent to that driver via a crossover. Usually, The better speakers split that further so that dedicated driver whether it be the Tweeter/Horn (hi) mid driver (mid)  and bass driver only has to handle that band. It's becuase the waveform is complex that usually single driver speakers that have to do the entire frequency range struggle compared to multiple driver speaker systems. ",5,askscience,56% ,Sun Sep 3 00:01:06 2017 UTC,6,princehermit,190,486
+Is there radiation associated with the creation of a charged particle?,"I was studying for E&M and I noticed that radiation fields seem to be accompanied by a changing current somewhere. I then tried to figure out how to get radiation without a current this is the best I got. I know the question is kind of nonphysical since you can't just create a lone electron for example but supposing you could, does that release radiation? I'm assuming there's some delta function like object but I'm not sure.  I recognize that this is kind of a silly question but I wanted to procrastinate and nobody else is in the office with me.  Edit: Turns out this is an end-of-chapter question in Zangwill. 20.8 ","These are the electric and magnetic fields produced by a point charge moving arbitrarily. Whenever the particle is accelerating, it produces electromagnetic radiation. ",8,askscience,71% ,Sun Sep 3 01:20:22 2017 UTC,19,MarsOfDickstruction,237,1997
+Why do some things burn and some things melt?,,"Burning and melting are two different reactions chemically. Burning happens when the substance reacts with a gas in the air, rapidly oxidizing (oxidation is where a substance loses electrons). Usually, it is being oxidized by oxygen (hence the name). The oxidation reaction produces a lot of light, which is what you see as fire, along with heated soot from the substance flying up into the air. Melting on the other hand is where the molecules in a substance start shaking so violently that they lose structure, like a building collapsing due to a hurricane. We can predict whether a substance will burn or melt at a high temperature based on whether it has a low electronegativity/ionization energy, and loses electrons to oxygen under heat. Without oxygen/some other oxidizing agent, the substance would continue to heat until it melts. ",11,askscience,80% ,Sat Sep 2 18:05:38 2017 UTC,6,elliotjmbird,767,383
+Where do heat is transmitted in space?,"If there is nothing in the space, why do objects cool/freeze and where does that energy goes?  ","They're are several ways energy can be transmitted:  conduction - you touch a hot object,  you get burned.  convection - think a hot wind.  radiation - you step out of the shade and into the sun, and instantly feel it's warmth.   Indeed,  a vacuum stops conduction and convection.  That's why Thermos bottles work so well.   But there's still radiation.  Sunlight can warm a spaceship (that's why the lunar modules had gold-colored ""foil"" shields), or if the object radiates energy off to the darkness, the object will get cold.  There are other modes,  such as evaporation.  ",0,askscience,36% ,Sat Sep 2 18:54:35 2017 UTC,6,vatojavier,3,1
+"Why do bridges need these ""sail"" like objects on top of them?",Relevant: https://ichef-1.bbci.co.uk/news/624/cpsprodpb/69CB/production/_97638072_new3.jpg ,"Not all bridges do. However the new forth crossing bridge and the forth road bridge (both pictured) are suspension bridges, and the cables (which you describe as the 'sails') are effectively what holds the bridge up as there is not enough weight underneath to provide structural support. ",0,askscience,50% ,Sat Sep 2 16:34:26 2017 UTC,9,WuSin,159,8417
+Will the electrons in the wire of the generator run out?,"This sounds like a dumb question. I'm a teen and from my understanding in class. Electricity is produced by cutting a conductor through a magnetic field to ""push"" the electrons. So when we us electricity, will the electrons be used up and you have to change the metal in the generator or the electrons are still there but just the energy they carry is transferred. I asked my teacher and all he said was ""It isn't in the syllabus, don't bother"" ","Not a physicist or engineer, but nobody seems to have stepped in yet.  In order for electricity to flow, you need a circuit (please tell me they covered that).  The circuit is in fact to permit electrons to move in a complete loop around your current path, which means no, you can't run out.  As soon as they can no longer cycle, current stops.  Although it is horribly misleading as to the more precise understanding of electricity, think of the water analogy.  If you had a hose sealed on both ends, you wouldn't be able to pump water through it - but make a loop and the water will cycle and you can attach turbines or other devices and make it do 'work' at a location in the loop somewhere other than where the pump is. ",7,askscience,82% ,Sat Sep 2 07:00:34 2017 UTC,10,poi_slayer,4,561
+Have we contaminated space with life from earth?,"For example, do we know if the Voyager probes or the Curiosity rover did not carry (living or dead) bacteria, viruses, or other microorganisms (e.g. tardigrades) into space or other planets? Are there normally steps taken before launch to prevent this? ","We almost certainly did include those microbes on Voyager, since cleaning protocols are only used on probes going to places we expect to find life.  Since Voyager did not land/crash anywhere, no cleaning. NASA has a protocol for cleaning these probes and rovers and the program is overseen by the NASA Planetary Protection Officer. ",21,askscience,72% ,Sat Sep 2 00:48:33 2017 UTC,7,TrainFan,4732,25110
+Do members of the same species share the same number of nerves or the same concentration of nerves?,"For example, do humans all have X nerves in their finger tips or Y nerves per cm? Would a larger adult human have more nerves than a similar aged human of smaller stature? ","Yes and no. Talking about the major nerves, like for example the nervus medianus which innervates parts of the arm, shoulder and hand, they look basically the same in every human, though they often show multiple patterns of branching into smaller nerves. Often there is a hand full of distinct branching patterns as shown here: tibial nerve  Now when talking about receptors and receptor cells that's another matter. A bigger hand will have more nerve cells expressing for example pain receptors. They, however, will likely produce the same receptor density as a smaller hand with fewer cells. So while a larger hand might have more receptors, their sensitivity will be about the same, as they display the same density. This of course varying in different individuals in a small range. Each of these cells expressing sensory receptors connects to a neuron, so a bigger hand has more nerve endings but you would not say it had more nerves ",1,askscience,57% ,Sat Sep 2 17:32:27 2017 UTC,2,thatguyChristophu,4383,42
+"Is there such a thing as ""peak physical form""? If so, how is it defined?",,"This is a tricky question but as someone who has load of experience with this ill help. There are 5 divisions of fitness. Flexibility, Strength, Endurance, Agility and Speed. People who work out often only train three (strength, endurance and speed) but lets assume your training all 5 over 5 different training days. You would be considered ""peak physical"" form when you hit your genetic limit (genetic limits are tricky asf, you cannot measure the absolutes but a 5""7' white guy will not be hitting the same ""peak"" as a 6""5"" samoan).  Hitting your limit takes a LONG time, and even when you think youve hit it, youll often find youve just plateued and can work around it.  So the correct answer is in a real life situation, never. Until youve been training for years and have hit an equilibrium with weight (weight gained = weight lost no matter how hard you train) and all 5 categories (cannot get faster, if you do get faster, endurance drops slightly etc) then you will never be at an absolute "" peak"". Why would you want to peak anyway? Theres only one way to go from there, down. ",0,askscience,42% ,Sat Sep 2 22:26:00 2017 UTC,3,Big_Dick_Jones,327,436
+How low to the surface can a lunar orbit be?,"With no atmosphere, could a lunar orbit theoretically be just high enough to clear the highest peak on the surface?  ","In theory, yes, but in practice density differences make the lunar gravitational field pretty ""lumpy"". In 2011-2012, the GRAIL pair of spacecraft orbited the Moon just tens of kilometers above the surface, but maintaining such a low orbit required frequent correction burns. Without thrusters, the probes would have crashed within weeks or months. ",2,askscience,67% ,Fri Sep 1 18:58:55 2017 UTC,4,johnnyseal27,157,551
+How and why do antenna extension cords work?,As far as I know an antenna's length has to be tuned to the exact wavelength it is supposed to receive. But what happens if you attach an extension cord to an antenna? For example if you attach a 5m extension cord to a WiFi-Antenna which is only 6.25cm long wouldn't that change the frequency the antenna receives? ,"Cables that hook up antennas are coaxial cables. They consist of a central conductor surrounded by a dielectric and then a cylindrical, grounded conductor surrounding that. All signals travel down the central conductor as current or in the dielectric as an electromagnetic wave in a waveguide like fashion. As you go to higher frequencies, it begins to act more and more like a wave guide. The outside grounded conductor (ideally) confines all electric and magnetic fields inside, preventing it from radiating. The outside conductor also (ideally) shields it from all external electric and magnetic fields, preventing it from recieving any signals. As such, it does not act as an extension of the antenna.  ",3,askscience,64% ,Fri Sep 1 16:56:18 2017 UTC,5,singinWhale,6,0
+What is mAh(milli ampere hour) or Ah(Ampere hour) and how it is connected with the voltage of a batteries?,Edit :- voltages of batteries* ,"TL;DR: Amperes (Amps) are a measure of current, Ampere hours are a measure of the capacity of a battery, and it's actually nothing to do with voltage. When you divide two things that have units, the result is a number followed by the phrase '[unit type 1] per [unit type 2]'. For example, speed is measured by taking the distance you travel and dividing it by the time it takes to go that far. If you go 120 metres in 10 seconds, you calculate: 120 metres ---------- 10 seconds  in order to get: 12 metres    ------  1 second  though convention means we can just omit the '1' and assume everybody knows that's how many seconds we're talking about. The shorthand is therefore 12 metres per second or 12 m/s. 'Per' essentially means 'divided by' or 'divided between'. Take-home message: the way the units are written tells you something about what was multiplied or divided to arrive at that result.  With mAh (or Ah), you're seeing the result of exactly the same logic except it's the result of a multiplication instead of division. When you multiply, the numbers and their units all stay on the same (top) line of the calculation: 2 Amperes x 4 hours = 8 Ampere hours (or Ah) (Note: since 1 Ampere = 1000 milliamperes, this is the same as 8000 mAh.) This means you could draw 8 Amps for a period of 1 hour before the battery was drained. Alternatively, you could draw 1 Amp for 8 hours, or 2 Amps for 4 hours, or 0.5 Amps for 16 hours, etc. Voltage = current x resistance (V=IR), so voltage depends on the current drawn by the device attached to the battery as well as the resistance of its circuits. Maybe a physicist can tell us more about whether / how that curve drops off as a battery approaches depletion. ",2,askscience,62% ,Fri Sep 1 20:00:15 2017 UTC,8,Avijit97,152,-5
+"If a human being were crushed down past their Schwarzschild radius, would the resulting black hole pull in enough surrounding matter to sustain itself or would it shortly evaporate in a devastating explosion?",,"a crude estimate for the remaining lifetime of a black hole is given by t = 10-26 (mass in grams)3 seconds A 70kg BH would be gone in ~ 10-12 s. The total energy released is Mc2 ~ 6 * 1018 Joules, about 30 Tsar Bombas if you need a reference. ",28,askscience,77% ,Fri Sep 1 06:41:38 2017 UTC,17,mohawk_ADE,19771,859
+Do neutrons penetrate significantly further into matter than protons given the same energy?,"I would expect that neutrons would penetrate further due to their lack of an electric charge, so they would mainly interact with other matter through the strong force, while protons would would interact through both the electromagnetic and strong force. However, I'm unsure of how significant this would be. Is it something on the order of a few percent further? Or would it penetrate several times further? "," I would expect that neutrons would penetrate further due to their lack of an electric charge, so they would mainly interact with other matter through the strong force, while protons would would interact through both the electromagnetic and strong force.  Yes, this is exactly correct, at least at sufficiently high kinetic energy. However at low energies, neutrons are very easily captured by nearby nuclei, while positively-charged particles are not.  However, I'm unsure of how significant this would be. Is it something on the order of a few percent further? Or would it penetrate several times further?  A proton with ~ 1 MeV of kinetic energy will be stopped by a thin piece of aluminum. A neutron with a kinetic energy of ~ 1 MeV will pass right through the aluminum as if nothing happened to it. To stop fast neutrons, you want thick barriers of water or plastic (molecules with hydrogen atoms on them, since collisions with light nuclei are the best way to slow down neutrons). ",6,askscience,72% ,Fri Sep 1 18:48:03 2017 UTC,5,myosotis00,6,8
+"If a plane is flying at a higher altitude, does the wake region decrease due to the lower air pressure?",Edit: not sure if I phrased it correctly. Would the wave drag decrease? ,no comments,4,askscience,65% ,Fri Sep 1 20:33:17 2017 UTC,1,nhjb1034,16,887
+"If you could detect and measure Hawking radiation, could you tell what was inside of the black hole?",,"If information is to be conserved (a principle known as unitarity) then the information brought in that everything that fell in, and then stored in the black hole, must then be all contained in the total Hawking radiation emitted by the BH in its whole lifetime, after it's evaporated completely. If you had perfect, microscopic knowledge of the quantum state of the total Hawking radiation (and of the fundamental laws of the Universe, including quantum gravity) you'd be able to reconstruct the original state of the matter that fell into the black hole. However, this information is stored in the Hawking radiation not in some straightforward way as a certain number of bits per amount of radiated particles; the radiation is instead strongly entangled with itself and the information lies there. In practice, this means that even omitting a small part of the radiation makes most of the information unreadable. There's a more famous example of how this works: I'm Alice and I have a message to you, Bob. I write my arbitrarily long message in a piece of paper and throw it in a black hole. You try to decode the message from the Hawking radiation that comes out. It turns out you need to wait for half the whole BH (in horizon area) to evaporate before you can even decode one single bit from my message. This is because when the BH is partially evaporated, the information of the message is in great part encoded in entanglement between the Hawking radiation particles and the remaining portion of the black hole itself. (This cryptographic property of BH is very important in that it solves a very tricky problems with the holographic principle, known as the xeroxing paradox). In any case, all of this assumes you can acquire perfect microscopic knowledge of the state of Hawking radiation. In reality, you are a macroscopic observer unable to perform anything else but a few macroscopic measurements. In this sense the information in Hawking radiation is completely unreadable and it looks like a zero-information state from a macro perspective - equivalently, it has maximum entropy. That's why it's in thermal equilibrium. From this macro POV then BHs are ideal shredders, completely erasing useful information. Of course what really happens is that the info is not getting destroyed, but just getting very scrambled to the point of being macroscopically unretrievable.  ",13,askscience,72% ,Sat Sep 2 05:06:18 2017 UTC,8,Sebbano,1951,139
+Blindness vs Solar Eclipse?,"With the recent solar eclipse and the many people who decided it would be fun to view without protection, many of these people took the risk of damaging their eyes to a certain extent. Question:  If someone who is already blind decided to stare at the eclipse, would they suffer any permanent eye damage like someone who has the ability to see or would it be a meager injury such as irritation of the eyes? ","Blindness can happen when any of several different stages of signal capture in the eye and transmission to the brain are interrupted. If the blindness comes from a problem after light hits retina, the sun's light will have the same harmful effect on the cells it hits. If someone is at some stage of legal blindness (i.e. not total, absolute nothingness) as a result of some occlusion in the front part of the eye (like heavy, untreated cataracts) then there's going to be less sunlight entering. I don't know whether anyone has quantified how much less*, but I'd imagine it's still going to be over the threshold of damage.  Very difficult to measure. You can't ask someone to tell you when something is 50% as bright, as we don't have an objective internal reference scale. Doubly difficult to estimate because your perception isn't linear; there are the equivalent of bicycle gears for converting low, medium and high level light input into signals we perceive as equivalent; this is why you can be comfortable that you've got sufficient illumination outdoors and indoors even though one is lit by the sun and one by some lightbulbs. The 'gears' include your iris/pupil size, retinal cell types for different intensities, and nerve signals with different encoding. Ultimately, you'd need some sort of internal light probe in the eye, which nobody is going to do to a patient without a very good reason.  ",1,askscience,60% ,Sat Sep 2 03:34:42 2017 UTC,2,nymphfer,17,47
+Could a planet be size of a galaxy? Whats the biggest one could be?,,"At a certain size, the planet would have so much mass that it would collapse and form a star, and if you added even more mass it would easily become a black hole. The cutoff point between a planet and a star (specifically a brown dwarf star) is around 11-16 jupiter masses. ",6,askscience,71% ,Sat Sep 2 08:47:46 2017 UTC,10,mudb3d,1711,16132
+"If we can get 400Gb on a microSD card, why can't we use that level of miniaturization in full-size HDDs?","SanDisk just underlined a new microSD card. Is cost the only reason that we don't have petabyte side HDDs now? If not, what could be the reason and what is the actually possible largest Carriscoa HDD we can make it there foreseeable future? http://bgr.com/2017/08/31/sandisk-ultra-400gb-microsd-price-release-date-announced/ Edit: how much more data can we conceivably pack into microSD card? They can't get larger, can they? ","Quoting the linked article: ""The new 400GB SanDisk Ultra microSDXC UHS-I card has a full retail price of $249.99"". At that rate, a 4TB drive would cost $2500. In reality, a 4TB drive costs a lot less (a 4TB USB drive may even cost less than $250 or close). . In other words, unless you need the small size (like you do in certain devices), the price isn't worth it. For desktop and server computing, network storage systems (SANs) and large RAID or other multi-drive systems are much more cost efficient. ",3,askscience,64% ,Thu Aug 31 22:45:54 2017 UTC,5,Yellow_Odd_Fellow,194,17278
+How does low mass close binary star systems change over time and what is their ultimate fate?,"For example, what will happen in the future in a system like Kepler-47? ","Kepler-47 is a G6V/M4V binary with an orbital period of 7.45 days. That's close enough together (~18 solar radii) that when the G star evolves onto the giant branch and expands, it will overfill the gravitational equipotential surface in the co-rotating reference frame called the ""Roche-Lobe"", which is just a mathematical way of expressing when material is more attracted to the other star than it's original host star. The result is that material starts to fall off of the G6 star (now a giant) and onto the red dwarf.  Now when this mass transfer happens, it changes the distribution of mass in the system, which changes the Roche geometry and moves the center of mass of the system. The angular momentum of the system is ALSO redistributed, so the stars will move closer together since the transferring star is more massive than its companion. The stars moving closer together moves MORE material past the equilibrium point (L1), which moves the stars closer together, which moves MORE material past L1, etc, so the mass transfer is unstable.  At this point, a number of different things can happen, and predicting what exactly it will be for a given system is pretty hard.   In some systems, like Algol, the mass transfer shuts off once the mass ratio flips and the previous red dwarf now has the mass (and temperature and luminosity!) of a more massive star and the old more massive star is the lighter of the two.  This is the most interesting one (to me). Some systems will enter a common envelope with a shared atmosphere and remain in that state until they either merge or the cores spiral towards each other but their energy and angular momentum ejects the common envelope before the merge, leaving what looks like a main sequence red dwarf again orbiting a hydrogen-deficient core which can continue to evolve in a weird way, but at this mass ends up ultimately as a white dwarf. Eventually, the orbit will continue to decay (via magnetic braking or gravitational radiation, depending on the mass of the donor now - but a M4V star would be by magnetic braking) and the system gets into Roche Lobe contact again, this time with the red dwarf filling its Roche Lobe. Now, it's the less-massive object donating mass and the transfer can be stable (as long as M2/M1 < ~5/6, otherwise the star doesn't shrink as fast as the Roche geometry does when a bit of matter moves across). The falling material forms an accretion disk which serves to transfer angular momentum away from the infalling material and allows some of it to fall on the surface of the white dwarf.  This type of system is called a Cataclysmic Variable (CV) because sometimes enough material builds up on the white dwarf to have a classical nova eruption, but they can do lots of other stuff too and it's a very complex zoo. They can have Dwarf Novae (which are instabilities in the accretion disk, and there are several kinds of those systems), or can be in a persistently ""high"" accretion state called a nova-like, or they can be in a middle state and switch between having dwarf nova and being a nova-like (like Z Cam). If it's a white dwarf with a particularly strong magnetic field, it can channel material straight to the poles of the white dwarf from L1 or the accretion disk (Polars and Intermediate Polars, depending on if the white dwarf has tidally locked to the orbital period or not). Some CVs have been seen to go from one of these states to the other, which has led to some proposing that Classical Novae can knock the system out of equilibrium and drive accretion rates up until they settle down again, but that's still pretty speculative and not widely accepted. Anyway, where was I? Oh right, also... They can merge together. This is a rapid event and so is rare, but V1309 Sco was seen to do this in 2011. V1309 Sco was a contact binary before this happened, but instead of ejecting the common envelope the cores just merged together. They noticed a transient in the sky and went back and looked at it in archival survey data, and noticed that it was a binary star. They could even track the orbital period shrinking as it wound down. It's a pretty spectacular result.   I can't tell you exactly which of these Kepler-47 will do in 5 billion years, but it should be fun for whoever is around and not on a planet orbiting them.  EDIT: If you start with massive stars instead of a G star, the massive core can go core-collapse supernova after the common envelope phase and form a black hole or neutron star, which could unbind the binary or form a Low Mass X-ray Binary (LMXB). LMXBs can act a lot like CVs, having disk instabilities or being persistently bright and, for neutron star primaries, even equivalents of classical novae called X-ray bursts where hydrogen starts fusing on the surface and it all goes boom in a few seconds. LMXBs have lots of radiation coming out in X-rays, whereas CVs have it mostly come out in the UV (though they have X-ray emission too, just not as much as LMXBs do). LMXBs can be ~1 million times more luminous than the sun, but in X-rays instead of optical light, because the gravitational potential well for neutron stars and black holes is so deep.  ",3,askscience,79% ,Fri Sep 1 19:11:52 2017 UTC,4,YottaEngineer,114,21
+Why do our bodies physically fell something when we see others get injured?,,"We have neurological circuitry made in part from what we're now calling mirror neurons, which fire when we see something happen to someone and make us feel as if it was happening to us. This can manifest as a physical sensation (which is always ultimately some neurons in the brain firing, whether you're talking about someone else being hurt or you being hurt) or an emotional response.  The evolutionary argument for their development is that by being able to better understand the emotions of others, we function better as a society. Humans are a social species. (One microcosmic example of what this means for us is that since our young take many years to develop, it helps if others in the tribe help with upbringing. Neural pathways to help with empathy therefore contribute to the survival of the genes that encode their instructions.) ",0,askscience,50% ,Fri Sep 1 08:28:38 2017 UTC,4,Ragnoik,4977,2220
+Why do humans live so long compared to most animals?,Biology  ,"I think your question is already biased in thinking humans live longer than most animals. Hundreds of species of fish, whales, turtles, tortoises, live to over a 100 years old or even upwards of 200 years. Many species of birds also can live as long as humans if not longer. Some Corals, sponges and mollusks (all animals) can live 500+ years. Not to mention some of our closest relatives chimps and gorillas live 40-50+ years. Now if you're asking why humans live past their ability to have kids. It's been found that parents are obviously necessary in our social world, but having grandparents also helps children grow up to be more successful. We are social animals, so we have the ability to take care of the weak, older generation. It makes sense that natural selection could allow for people to live longer. Beyond that! Modern medicine..... But that's just common sense why we're living longer than ever. ",1,askscience,57% ,Fri Sep 1 05:18:05 2017 UTC,6,banditoderojo,22,1406
+Is it possible to make a 100kW nuclear reactor that's not an RTG?,"I'm looking up ""small nuclear reactor"" and mostly finding that ""small"" means ""enough to power 50,000 homes"". Outside of an RTG, is it possible to go small enough to make a nuclear powered range extender for an electric vehicle? Or are there constraints on the minimum size of nuclear power? ","I don't work in a nuclear power plant but I have some background in nuclear energy. RTGs are really the only kind of ""portable"" nuclear energy that I think would be feasible. If you look at a nuclear power plant, it's a big compound, but the actual nuclear fuel and reaction chambers don't make up most of it. There's a lot of supporting infrastructure required to actually get power from the fuel like turbines, pumps, cooling systems, and of course safety and waste management. Not only are those kinds of things hard to scale down, but nuclear power plant operations require constant attention from people who are physically on site, and it would be incredibly dangerous to move a reactor around. There are smaller nuclear reactors that exist, for research purposes. Funnily enough I looked up the one my campus used to have and it's rated for 100kW so here's what a 100kW nuclear reactor looks like. Not exactly portable.  ",20,askscience,78% ,Thu Aug 31 18:51:06 2017 UTC,16,mutatron,59332,358699
+"If Temperature is a measure of the average kinetic energy of particles in a medium, is there something that measures standard deviation?","And if there is a way of measuring the standard deviation, what are the implications of it?  My understanding of evaporation is that the higher energy molecules escape the liquid even though the overall temperature is below the boiling point.  If the standard deviation of the temperature is lower, would that imply that evaporation would occur at a slower pace? ","For a classical ideal gas at equilibrium, which follows the Maxwell-Boltzmann distribution, you can write down a linear relationship between the average kinetic energy and the temperature.  < E > = 3kT/2, per particle. If you calculate the variance of the distribution, you get < E2 > - < E >2 = 3(kT)2/2. This represents the typical fluctuations of the particle energies away from the mean, and it increases quadratically with temperature. The standard deviation is the square root of the variance. ",16,askscience,84% ,Thu Aug 31 17:39:20 2017 UTC,11,sacrelicious2,914,10359
+"Are there any pulsars that 'wobble' on their axis, so that sometimes their beam reaches us and sometimes it misses us?",,"Absolutely. If a pulsar has a close companion (another star, not necessarily a pulsar), then relativity causes it to undergo a process called 'geodetic precession', which is the wobble of the rotation axis. I saw a talk a while ago about a pulsar that we observed in the past, can't see now, but expect to come back in a few years. I can't seem to find anything on it through a quick web search, though... An interesting place to start learning more about this is this link, which is a video by a Dutch pulsar researcher. There's a paper linked in the description which discusses one such pulsar, which has vanished and isn't expected to point our way again until ~2170. ",6,askscience,100% ,Thu Aug 31 20:39:12 2017 UTC,2,malariadandelion,6462,1397
+I've seen ants huddle together on water to be able to float. How do the ants on the bottom level not drown and die? Or do they?,,This video explains it pretty well. ,21,askscience,79% ,Thu Aug 31 19:12:13 2017 UTC,8,MACKSBEE,26961,39000
+What is the consensus on creatine and its impact on baldness?,,[deleted] ,8,askscience,69% ,Fri Sep 1 00:45:56 2017 UTC,1,CaptainAdaw,471,5795
+"In a given season, do hurricanes (typhoons, etc) tend to follow a similar trajectory?",,"For Atlantic storms, there are several well established hurricane paths ( e.g. Into the Caribbean, up the US East coast). The one characteristic most share is a track recurve to the northeast, when they typically accelerate and then weaken. ",9,askscience,80% ,Thu Aug 31 01:12:05 2017 UTC,8,WhatsAMisanthrope,10132,57270
+Would encasing myself (or something capable of measuring time) inside a diamond induce a relativistic effect?,"Pretty straight forward, I think.  We know that the speed of light is effected by the medium through which it travels, but that it also must remain constant for the observers.  Light will travel slower through a diamond, so will the time experienced by something within the diamond adjust itself accordingly?  ","No, relativity only depends on the speed of light in a vacuum. The index of refraction of a material is very wavelength dependent; while visible light is slowed in a diamond, gamma radiation (for example) is hardly slowed at all. ",0,askscience,43% ,Thu Aug 31 16:17:25 2017 UTC,3,srooms,112,1048
+How do scientists fire subatomic particles at materials in their experiments?,"The two that I'm thinking about right now are the Davisson-Germer experiment, where electrons were fired as I've been told, and the Rutherford Gold foil experiment, where helium nuclei were fired at gold foil. How do scientists isolate these particles and how do they store them, and how do they manipulate these particles to ""fire"" them at a substance? Is the way these are fired the same as just atoms of an element? ","The way we do it now is a little different than how it was done in those old-fashioned experiments. For the Rutherford experiment, you just place an alpha source near a target (in some kind of vacuum chamber). But then you're limited to alpha particles, they only come out at the specific energies provided by your source, and they're emitted isotropically, so most of them probably don't even hit your target. We need to do better than this. In modern experiments we use particle accelerators. We can produce beams of many different species. We can accelerate them to a bunch of different energies. We can also optimize the properties of the beam (how large it is in space, how divergent it is in angles, how tight the momentum distribution is, etc.). We can steer and focus the charged particle beam using magnets and guide it onto our target much more precisely than simply placing an isotropic source near it. The particles in the (stable*) beam originate in an ion source. In a simplified picture, you just take chemically stable atoms of the desired species and heat them up to ""boil off"" some or all of their electrons (there are also sputtering sources which add additional electrons to the atom, or ECR sources, and others). Once the electrons are gone, what you've got left is a charged nucleus, which can be accelerating using electric fields. You can then guide your ions (could be hundreds, or thousands, or millions per second, maybe more depending on the machines) into an accelerator to speed them up to the desired energy. Once the beam is at the energy you want, you steer it around and keep it focused using magnets. Magnets don't change the energy of the particles. (* Producing a beam of radioactive species is more challenging, but I can go into the details on that too.) Finally your beam interacts with a target, or another beam in the case of colliders. This is where the experimentally interesting scattering and reactions occur, and some of the outgoing particles end up in your detectors to be measured. ",16,askscience,89% ,Thu Aug 31 01:13:26 2017 UTC,6,ShammaLamaMu,64,1998
+Why shouldn't I control a parallel load register using an AND gate?,I have been taught to use a mux to control whether you load a new data value into the D-Flop or load the Flop's current output back in to the flop.  Why is it not better to remove the multiplexers and just have a control input ANDed with the clock and use the output signal as the clock for the flops? Wouldn't doing this remove the constant reassigning of the flop's output back through the design?  ," Why is it not better to remove the multiplexers and just have a control input ANDed with the clock and use the output signal as the clock for the flops?  A multiplexer selects one out of two or more input signals to output based on an other (control) signal. Any way you choose to implement it, it will be a multiplexer nonetheless. The reason you don't want to use the output of a multiplexer as the clock of the subsequent flip-flop is it's introducing unnecessary propagation delay. Also, since the output of a multiplexer can be something other than your clock signal, you have to introduce additional circuitry to cover what happens when the clock is not selected - because if all inputs of the multiplexer were the clock, then it is useless. What you are describing in your question is conflating the usage of a multiplexer and a clock. Their purpose and functions are orthogonal.  Wouldn't doing this remove the constant reassigning of the flop's output back through the design?  You have to be more specific - what exactly is the circuit you have in mind? And what exactly do you mean by ""constant reassigning of the flop's output""? ",3,askscience,64% ,Thu Aug 31 06:31:16 2017 UTC,5,AsaJack,33,16
+"Why does being infected with vaccinia virus vaccine protect you from Smallpox but doesn't protect you from other ""pox"" such as chicken pox?","Due to my career field I was recently given the vaccinia virus because I was told it's a more ""mild"" form of small pox, this peaked my interest and I learned the difference between vaccinia and variola virus(small pox) but could not find an answer to my question in regards to the varicella virus (chicken pox). I probably have a misinterpretation on the correlation of the word ""pox"" but I'd like to know, thank you! ","Chickenpox is caused by the varicella zoster virus and smallpox by the  variola viruses.  Vaccina and variola are poxviruses, varicella zoster virus is not - so it is not surprising that vaccina immunization will not confer any immunity to chickenpox. ""Pox"" is an old term dating from the time before we had knowledge of infectious agents and was used to describe a number of diseases that seemed to spread easily and cause skin lesions. For example syphilis was once also known as the ""great pox"".  ",487,askscience,81% ,Wed Aug 30 03:18:00 2017 UTC,35,KannonTheKid,725,41
+"When we pop a joint like a knuckle or our back, its a realeases air. Where does that air go?",,"It dissolves back into the fluid. When you pop your knuckles, you're decreasing the pressure enough to allow the gas to escape the solution, which creates the popping noise. When you decrease the pressure again, it dissolves again. Think of a soda can; when you open the can you decrease the pressure and the gas escapes forcefully, creating a noise.  ",18,askscience,73% ,Wed Aug 30 22:21:55 2017 UTC,10,Wardragon117,8632,2494
+What is the average chemical composition of fossil fuels?,"'Chemistry' I like to do back-of-an-envelope calculations of things related to climate science. I'm trying to estimate the amount of water produced by fossil fuel consumption, and so I'd like to see some estimate on the chemical/elemental composition of fossil fuels. For petroleum, I've found the figure 10-14% (weight) hydrogen, and for coal some 1-5%. What I am after is more of a total average over all fossil fuels, for some recent year, say after 2005.  ","Fuels is a very broad topic. You're not going to find an average here. Some fuels are saturated and some are completely void of hydrogen e.g. Coal or Charcoal from wood. We also have to think about where these compounds are going and what they're being used for. Not all fossil fuels are being burned, but are used in more chemistry. Then, if you try to narrow that to say a field of how much water is produced by the combustion of gasoline, we first see that gasoline too is variable.  This is something that will be entirely speculation and I fear that without lots of clarification, any calculation will be overshadowed by its too statistically significant room for error. ",2,askscience,57% ,Thu Aug 31 10:27:38 2017 UTC,6,Gelnef,366,613
+Is there any concrete use of Geometric Mean in Physics? Does it relate to any physical quantity like Arithmetic Mean does to the Center of Mass of a body?,,"Geometric Mean Distance (GMD) is used when calculating the impedance of three-phase power transmission lines. Some line structures are imbalanced thus causing different inductance values on each phase. This can be mitigated by transposing the phase conductors in the middle of a line to give each the same average inductance. GMD is used to calculate the average (effective) inductance over the entire length. GMD is simply the geometric mean of the distance between each phase conductor.  Edit: I removed the word ""intrinsic"" from ""intrinsic impedance.""  Intrinsic impedance is a wave impedance of an electromagnetic medium.  ",7,askscience,72% ,Wed Aug 30 10:26:21 2017 UTC,6,sarthakRddt,7,0
+Why does the standard deviation formula have an (n-1) instead of n?,"The formula to calculate standard deviation involves adding the squares of differences between the point and mean for each data point, then dividing it by (n-1). Since standard deviation is a way to calculate spread that corresponds with mean, why isn't the sum divided by n? ","Essentially, you want to try and find the population standard deviation from a smaller sample. If the population is finite and you know all the relevant information (eg, you know everyone's age in the US), then you can compute the exact population standard deviation, and this has an 'N' in the bottom rather than 'N-1'. But this works because if we know absolutely everything about a population, then we can compute the actual mean of the population. If we only have a sample of the population, then the mean itself is an estimation. So every time you use the mean of the sample, you're introducing error. In fact, using the sample mean and dividing by 'N', you will always underestimate the actual standard deviation you would get if you knew and computed with the actual mean. Wikipedia has a pretty good explanation of why this happens. So computing the standard deviation using the sample mean (and dividing by N) will always give you a smaller standard deviation than you should get. Luckily, we can compare this computed standard deviation to the standard deviation you would get using the population mean instead, and the way to correct for the error that you get for using the sample mean is to multiply your result by N/(N-1), which is the same as just dividing by N-1 at the get-go. ",27,askscience,81% ,Wed Aug 30 23:20:57 2017 UTC,12,existentialepicure,18262,1502
+How many generations does it takes before medical history becomes irrelevant?,,"In theory, infinitely many. Consider for example a man that lived, say 30,000 years ago that had some recessive genetic disorder. He could pass this gene on for 1000 generations, and assuming no one else in the tree carried that gene, none of his descendents would developed this disease. Until after 1000 generations, a distant descendent of his has a child with a woman that also carries this gene and their child has a genetic disease.  There is no length of time where genetics or medical history becomes irrelevant. That said, the further back it goes the less influence it has. First of course, because with more generations you have more ancestors, and it is less likely you would have inherited a gene from any one ancestor. And second, because it becomes increasingly unlikely that a disease that was present in your family 30,000 years ago would not have been present in any of the 1000 generations before you.  ",2398,askscience,92% ,Tue Aug 29 23:02:19 2017 UTC,91,skuridat,2112,225
+"Do our skin cells, or liver cells, contain the genetic data for EVERYTHING in our bodies?","Do the DNA residing in the cells that live in our eyes, have genetic data that codes for how our feet should develop in humans? ","While there are some exceptions, almost every kind of cell in your body have a complete set of 46 chromosomes (all of your nuclear DNA). Red blood cells don't contain nuclear DNA (but they have mitochondria, and mitochondria have their own DNA) or mitochondria (thanks for catching that, u/JeremyFredericWilson), and sperm cell and egg cells have only half a set of chromosomes (though egg cells only do this at the absolute last minute, thanks u/Rather_Dashing). Edit: Biology is complicated. Never forget that. ",13,askscience,89% ,Thu Aug 31 03:07:55 2017 UTC,17,habibitee,1671,3572
+Shouldn't we be able to see black hole mergers via regular EM astronomy?,"When reading about the first detection of graviational waves, I saw this:  During the final 20 milliseconds of the merger, the power of the radiated gravitational waves peaked at about 3.6×1049 watts – 50 times greater than the combined power of all light radiated by all the stars in the observable universe.  Shouldn't this show up in the sky as as a star blinking into existence (or a slow ramp, not sure about the steepness of the curve)?  If so, is there a way to match the EM observation with the GW observation?  I imagine you can draw an arc using the two detectors and retroactively cross-reference with high energy EM events. Why haven't I heard of any detections of mergers prior to GW astronomy? ",http://adsabs.harvard.edu/abs/2012AAS...21914617D Yes we can. The issue is knowing where to point the telescopes. There is a small probability of finding a merger randomly in observations. This abstract describes using LIGO hits to direct observers to a merger. ,7,askscience,61% ,Wed Aug 30 14:57:20 2017 UTC,6,jammerjoint,32556,91434
diff --git a/tensorflow/examples/tutorials/mnist/board.py b/tensorflow/examples/tutorials/mnist/board.py
new file mode 100644
index 00000000000000..f0d0b0dc135353
--- /dev/null
+++ b/tensorflow/examples/tutorials/mnist/board.py
@@ -0,0 +1,211 @@
+# Copyright 2015 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the 'License');
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an 'AS IS' BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""A simple MNIST classifier which displays summaries in TensorBoard.
+This is an unimpressive MNIST model, but it is a good example of using
+tf.name_scope to make a graph legible in the TensorBoard graph explorer, and of
+naming summary tags so that they are grouped meaningfully in TensorBoard.
+It demonstrates the functionality of every TensorBoard dashboard.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import argparse
+import os
+import sys
+
+import tensorflow as tf
+
+from tensorflow.examples.tutorials.mnist import input_data
+
+FLAGS = None
+
+
+def train():
+  # Import data
+  mnist = input_data.read_data_sets(FLAGS.data_dir,
+                                    one_hot=True,
+                                    fake_data=FLAGS.fake_data)
+
+  sess = tf.InteractiveSession()
+  # Create a multilayer model.
+
+  # Input placeholders
+  with tf.name_scope('input'):
+    x = tf.placeholder(tf.float32, [None, 784], name='x-input')
+    y_ = tf.placeholder(tf.float32, [None, 10], name='y-input')
+
+  with tf.name_scope('input_reshape'):
+    image_shaped_input = tf.reshape(x, [-1, 28, 28, 1])
+    tf.summary.image('input', image_shaped_input, 10)
+
+  # We can't initialize these variables to 0 - the network will get stuck.
+  def weight_variable(shape):
+    """Create a weight variable with appropriate initialization."""
+    initial = tf.truncated_normal(shape, stddev=0.1)
+    return tf.Variable(initial)
+
+  def bias_variable(shape):
+    """Create a bias variable with appropriate initialization."""
+    initial = tf.constant(0.1, shape=shape)
+    return tf.Variable(initial)
+
+  def variable_summaries(var):
+    """Attach a lot of summaries to a Tensor (for TensorBoard visualization)."""
+    with tf.name_scope('summaries'):
+      mean = tf.reduce_mean(var)
+      tf.summary.scalar('mean', mean)
+      with tf.name_scope('stddev'):
+        stddev = tf.sqrt(tf.reduce_mean(tf.square(var - mean)))
+      tf.summary.scalar('stddev', stddev)
+      tf.summary.scalar('max', tf.reduce_max(var))
+      tf.summary.scalar('min', tf.reduce_min(var))
+      tf.summary.histogram('histogram', var)
+
+  def nn_layer(input_tensor, input_dim, output_dim, layer_name, act=tf.nn.relu):
+    """Reusable code for making a simple neural net layer.
+    It does a matrix multiply, bias add, and then uses ReLU to nonlinearize.
+    It also sets up name scoping so that the resultant graph is easy to read,
+    and adds a number of summary ops.
+    """
+    # Adding a name scope ensures logical grouping of the layers in the graph.
+    with tf.name_scope(layer_name):
+      # This Variable will hold the state of the weights for the layer
+      with tf.name_scope('weights'):
+        weights = weight_variable([input_dim, output_dim])
+        variable_summaries(weights)
+      with tf.name_scope('biases'):
+        biases = bias_variable([output_dim])
+        variable_summaries(biases)
+      with tf.name_scope('Wx_plus_b'):
+        preactivate = tf.matmul(input_tensor, weights) + biases
+        tf.summary.histogram('pre_activations', preactivate)
+      activations = act(preactivate, name='activation')
+      tf.summary.histogram('activations', activations)
+      return activations
+
+  hidden1 = nn_layer(x, 784, 500, 'layer1')
+
+  with tf.name_scope('dropout'):
+    keep_prob = tf.placeholder(tf.float32)
+    tf.summary.scalar('dropout_keep_probability', keep_prob)
+    dropped = tf.nn.dropout(hidden1, keep_prob)
+
+  # Do not apply softmax activation yet, see below.
+  y = nn_layer(dropped, 500, 10, 'layer2', act=tf.identity)
+
+  with tf.name_scope('cross_entropy'):
+    # The raw formulation of cross-entropy,
+    #
+    # tf.reduce_mean(-tf.reduce_sum(y_ * tf.log(tf.softmax(y)),
+    #                               reduction_indices=[1]))
+    #
+    # can be numerically unstable.
+    #
+    # So here we use tf.nn.softmax_cross_entropy_with_logits on the
+    # raw outputs of the nn_layer above, and then average across
+    # the batch.
+    diff = tf.nn.softmax_cross_entropy_with_logits(labels=y_, logits=y)
+    with tf.name_scope('total'):
+      cross_entropy = tf.reduce_mean(diff)
+  tf.summary.scalar('cross_entropy', cross_entropy)
+
+  with tf.name_scope('train'):
+    train_step = tf.train.AdamOptimizer(FLAGS.learning_rate).minimize(
+        cross_entropy)
+
+  with tf.name_scope('accuracy'):
+    with tf.name_scope('correct_prediction'):
+      correct_prediction = tf.equal(tf.argmax(y, 1), tf.argmax(y_, 1))
+    with tf.name_scope('accuracy'):
+      accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))
+  tf.summary.scalar('accuracy', accuracy)
+
+  # Merge all the summaries and write them out to
+  # /tmp/tensorflow/mnist/logs/mnist_with_summaries (by default)
+  merged = tf.summary.merge_all()
+  train_writer = tf.summary.FileWriter(FLAGS.log_dir + '/train', sess.graph)
+  test_writer = tf.summary.FileWriter(FLAGS.log_dir + '/test')
+  tf.global_variables_initializer().run()
+
+  # Train the model, and also write summaries.
+  # Every 10th step, measure test-set accuracy, and write test summaries
+  # All other steps, run train_step on training data, & add training summaries
+
+  def feed_dict(train):
+    """Make a TensorFlow feed_dict: maps data onto Tensor placeholders."""
+    if train or FLAGS.fake_data:
+      xs, ys = mnist.train.next_batch(100, fake_data=FLAGS.fake_data)
+      k = FLAGS.dropout
+    else:
+      xs, ys = mnist.test.images, mnist.test.labels
+      k = 1.0
+    return {x: xs, y_: ys, keep_prob: k}
+
+  for i in range(FLAGS.max_steps):
+    if i % 10 == 0:  # Record summaries and test-set accuracy
+      summary, acc = sess.run([merged, accuracy], feed_dict=feed_dict(False))
+      test_writer.add_summary(summary, i)
+      print('Accuracy at step %s: %s' % (i, acc))
+    else:  # Record train set summaries, and train
+      if i % 100 == 99:  # Record execution stats
+        run_options = tf.RunOptions(trace_level=tf.RunOptions.FULL_TRACE)
+        run_metadata = tf.RunMetadata()
+        summary, _ = sess.run([merged, train_step],
+                              feed_dict=feed_dict(True),
+                              options=run_options,
+                              run_metadata=run_metadata)
+        train_writer.add_run_metadata(run_metadata, 'step%03d' % i)
+        train_writer.add_summary(summary, i)
+        print('Adding run metadata for', i)
+      else:  # Record a summary
+        summary, _ = sess.run([merged, train_step], feed_dict=feed_dict(True))
+        train_writer.add_summary(summary, i)
+  train_writer.close()
+  test_writer.close()
+
+
+def main(_):
+  if tf.gfile.Exists(FLAGS.log_dir):
+    tf.gfile.DeleteRecursively(FLAGS.log_dir)
+  tf.gfile.MakeDirs(FLAGS.log_dir)
+  train()
+
+
+if __name__ == '__main__':
+  parser = argparse.ArgumentParser()
+  parser.add_argument('--fake_data', nargs='?', const=True, type=bool,
+                      default=False,
+                      help='If true, uses fake data for unit testing.')
+  parser.add_argument('--max_steps', type=int, default=1000,
+                      help='Number of steps to run trainer.')
+  parser.add_argument('--learning_rate', type=float, default=0.001,
+                      help='Initial learning rate')
+  parser.add_argument('--dropout', type=float, default=0.9,
+                      help='Keep probability for training dropout.')
+  parser.add_argument(
+      '--data_dir',
+      type=str,
+      default=os.path.join(os.getenv('TEST_TMPDIR', '/tmp'),
+                           'tensorflow/mnist/input_data'),
+      help='Directory for storing input data')
+  parser.add_argument(
+      '--log_dir',
+      type=str,
+      default=os.path.join(os.getenv('TEST_TMPDIR', '/tmp'),
+                           'tensorflow/mnist/logs/mnist_with_summaries'),
+      help='Summaries log directory')
+  FLAGS, unparsed = parser.parse_known_args()
+tf.app.run(main=main, argv=[sys.argv[0]] + unparsed)
\ No newline at end of file
diff --git a/tensorflow/examples/tutorials/mnist/boardtest/events.out.tfevents.1506353657.fluence-michael b/tensorflow/examples/tutorials/mnist/boardtest/events.out.tfevents.1506353657.fluence-michael
new file mode 100644
index 00000000000000..d4a83a854d2607
Binary files /dev/null and b/tensorflow/examples/tutorials/mnist/boardtest/events.out.tfevents.1506353657.fluence-michael differ
diff --git a/tensorflow/examples/tutorials/mnist/iris_test.csv b/tensorflow/examples/tutorials/mnist/iris_test.csv
new file mode 100644
index 00000000000000..5929d91f52e6b7
--- /dev/null
+++ b/tensorflow/examples/tutorials/mnist/iris_test.csv
@@ -0,0 +1,31 @@
+30,4,setosa,versicolor,virginica
+5.9,3.0,4.2,1.5,1
+6.9,3.1,5.4,2.1,2
+5.1,3.3,1.7,0.5,0
+6.0,3.4,4.5,1.6,1
+5.5,2.5,4.0,1.3,1
+6.2,2.9,4.3,1.3,1
+5.5,4.2,1.4,0.2,0
+6.3,2.8,5.1,1.5,2
+5.6,3.0,4.1,1.3,1
+6.7,2.5,5.8,1.8,2
+7.1,3.0,5.9,2.1,2
+4.3,3.0,1.1,0.1,0
+5.6,2.8,4.9,2.0,2
+5.5,2.3,4.0,1.3,1
+6.0,2.2,4.0,1.0,1
+5.1,3.5,1.4,0.2,0
+5.7,2.6,3.5,1.0,1
+4.8,3.4,1.9,0.2,0
+5.1,3.4,1.5,0.2,0
+5.7,2.5,5.0,2.0,2
+5.4,3.4,1.7,0.2,0
+5.6,3.0,4.5,1.5,1
+6.3,2.9,5.6,1.8,2
+6.3,2.5,4.9,1.5,1
+5.8,2.7,3.9,1.2,1
+6.1,3.0,4.6,1.4,1
+5.2,4.1,1.5,0.1,0
+6.7,3.1,4.7,1.5,1
+6.7,3.3,5.7,2.5,2
+6.4,2.9,4.3,1.3,1
diff --git a/tensorflow/examples/tutorials/mnist/iris_training.csv b/tensorflow/examples/tutorials/mnist/iris_training.csv
new file mode 100644
index 00000000000000..f5ae1054a16eeb
--- /dev/null
+++ b/tensorflow/examples/tutorials/mnist/iris_training.csv
@@ -0,0 +1,121 @@
+120,4,setosa,versicolor,virginica
+6.4,2.8,5.6,2.2,2
+5.0,2.3,3.3,1.0,1
+4.9,2.5,4.5,1.7,2
+4.9,3.1,1.5,0.1,0
+5.7,3.8,1.7,0.3,0
+4.4,3.2,1.3,0.2,0
+5.4,3.4,1.5,0.4,0
+6.9,3.1,5.1,2.3,2
+6.7,3.1,4.4,1.4,1
+5.1,3.7,1.5,0.4,0
+5.2,2.7,3.9,1.4,1
+6.9,3.1,4.9,1.5,1
+5.8,4.0,1.2,0.2,0
+5.4,3.9,1.7,0.4,0
+7.7,3.8,6.7,2.2,2
+6.3,3.3,4.7,1.6,1
+6.8,3.2,5.9,2.3,2
+7.6,3.0,6.6,2.1,2
+6.4,3.2,5.3,2.3,2
+5.7,4.4,1.5,0.4,0
+6.7,3.3,5.7,2.1,2
+6.4,2.8,5.6,2.1,2
+5.4,3.9,1.3,0.4,0
+6.1,2.6,5.6,1.4,2
+7.2,3.0,5.8,1.6,2
+5.2,3.5,1.5,0.2,0
+5.8,2.6,4.0,1.2,1
+5.9,3.0,5.1,1.8,2
+5.4,3.0,4.5,1.5,1
+6.7,3.0,5.0,1.7,1
+6.3,2.3,4.4,1.3,1
+5.1,2.5,3.0,1.1,1
+6.4,3.2,4.5,1.5,1
+6.8,3.0,5.5,2.1,2
+6.2,2.8,4.8,1.8,2
+6.9,3.2,5.7,2.3,2
+6.5,3.2,5.1,2.0,2
+5.8,2.8,5.1,2.4,2
+5.1,3.8,1.5,0.3,0
+4.8,3.0,1.4,0.3,0
+7.9,3.8,6.4,2.0,2
+5.8,2.7,5.1,1.9,2
+6.7,3.0,5.2,2.3,2
+5.1,3.8,1.9,0.4,0
+4.7,3.2,1.6,0.2,0
+6.0,2.2,5.0,1.5,2
+4.8,3.4,1.6,0.2,0
+7.7,2.6,6.9,2.3,2
+4.6,3.6,1.0,0.2,0
+7.2,3.2,6.0,1.8,2
+5.0,3.3,1.4,0.2,0
+6.6,3.0,4.4,1.4,1
+6.1,2.8,4.0,1.3,1
+5.0,3.2,1.2,0.2,0
+7.0,3.2,4.7,1.4,1
+6.0,3.0,4.8,1.8,2
+7.4,2.8,6.1,1.9,2
+5.8,2.7,5.1,1.9,2
+6.2,3.4,5.4,2.3,2
+5.0,2.0,3.5,1.0,1
+5.6,2.5,3.9,1.1,1
+6.7,3.1,5.6,2.4,2
+6.3,2.5,5.0,1.9,2
+6.4,3.1,5.5,1.8,2
+6.2,2.2,4.5,1.5,1
+7.3,2.9,6.3,1.8,2
+4.4,3.0,1.3,0.2,0
+7.2,3.6,6.1,2.5,2
+6.5,3.0,5.5,1.8,2
+5.0,3.4,1.5,0.2,0
+4.7,3.2,1.3,0.2,0
+6.6,2.9,4.6,1.3,1
+5.5,3.5,1.3,0.2,0
+7.7,3.0,6.1,2.3,2
+6.1,3.0,4.9,1.8,2
+4.9,3.1,1.5,0.1,0
+5.5,2.4,3.8,1.1,1
+5.7,2.9,4.2,1.3,1
+6.0,2.9,4.5,1.5,1
+6.4,2.7,5.3,1.9,2
+5.4,3.7,1.5,0.2,0
+6.1,2.9,4.7,1.4,1
+6.5,2.8,4.6,1.5,1
+5.6,2.7,4.2,1.3,1
+6.3,3.4,5.6,2.4,2
+4.9,3.1,1.5,0.1,0
+6.8,2.8,4.8,1.4,1
+5.7,2.8,4.5,1.3,1
+6.0,2.7,5.1,1.6,1
+5.0,3.5,1.3,0.3,0
+6.5,3.0,5.2,2.0,2
+6.1,2.8,4.7,1.2,1
+5.1,3.5,1.4,0.3,0
+4.6,3.1,1.5,0.2,0
+6.5,3.0,5.8,2.2,2
+4.6,3.4,1.4,0.3,0
+4.6,3.2,1.4,0.2,0
+7.7,2.8,6.7,2.0,2
+5.9,3.2,4.8,1.8,1
+5.1,3.8,1.6,0.2,0
+4.9,3.0,1.4,0.2,0
+4.9,2.4,3.3,1.0,1
+4.5,2.3,1.3,0.3,0
+5.8,2.7,4.1,1.0,1
+5.0,3.4,1.6,0.4,0
+5.2,3.4,1.4,0.2,0
+5.3,3.7,1.5,0.2,0
+5.0,3.6,1.4,0.2,0
+5.6,2.9,3.6,1.3,1
+4.8,3.1,1.6,0.2,0
+6.3,2.7,4.9,1.8,2
+5.7,2.8,4.1,1.3,1
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+6.3,3.3,6.0,2.5,2
+5.0,3.5,1.6,0.6,0
+5.5,2.6,4.4,1.2,1
+5.7,3.0,4.2,1.2,1
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+4.8,3.0,1.4,0.1,0
+5.5,2.4,3.7,1.0,1
diff --git a/tensorflow/examples/tutorials/mnist/nnetclass.py b/tensorflow/examples/tutorials/mnist/nnetclass.py
new file mode 100644
index 00000000000000..0477f4899974ef
--- /dev/null
+++ b/tensorflow/examples/tutorials/mnist/nnetclass.py
@@ -0,0 +1,87 @@
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import os
+import urllib
+
+import numpy as np
+import tensorflow as tf
+
+# Data sets
+IRIS_TRAINING = "iris_training.csv"
+IRIS_TRAINING_URL = "http://download.tensorflow.org/data/iris_training.csv"
+
+IRIS_TEST = "iris_test.csv"
+IRIS_TEST_URL = "http://download.tensorflow.org/data/iris_test.csv"
+
+def main():
+  # If the training and test sets aren't stored locally, download them.
+  if not os.path.exists(IRIS_TRAINING):
+    raw = urllib.urlopen(IRIS_TRAINING_URL).read()
+    with open(IRIS_TRAINING, "w") as f:
+      f.write(raw)
+
+  if not os.path.exists(IRIS_TEST):
+    raw = urllib.urlopen(IRIS_TEST_URL).read()
+    with open(IRIS_TEST, "w") as f:
+      f.write(raw)
+
+  # Load datasets.
+  training_set = tf.contrib.learn.datasets.base.load_csv_with_header(
+      filename=IRIS_TRAINING,
+      target_dtype=np.int,
+      features_dtype=np.float32)
+  test_set = tf.contrib.learn.datasets.base.load_csv_with_header(
+      filename=IRIS_TEST,
+      target_dtype=np.int,
+      features_dtype=np.float32)
+
+  # Specify that all features have real-value data
+  feature_columns = [tf.feature_column.numeric_column("x", shape=[4])]
+
+  # Build 3 layer DNN with 10, 20, 10 units respectively.
+  classifier = tf.estimator.DNNClassifier(feature_columns=feature_columns,
+                                          hidden_units=[10, 20, 10],
+                                          n_classes=3,
+                                          model_dir="/tmp/iris_model")
+  # Define the training inputs
+  train_input_fn = tf.estimator.inputs.numpy_input_fn(
+      x={"x": np.array(training_set.data)},
+      y=np.array(training_set.target),
+      num_epochs=None,
+      shuffle=True)
+
+  # Train model.
+  classifier.train(input_fn=train_input_fn, steps=2000)
+
+  # Define the test inputs
+  test_input_fn = tf.estimator.inputs.numpy_input_fn(
+      x={"x": np.array(test_set.data)},
+      y=np.array(test_set.target),
+      num_epochs=1,
+      shuffle=False)
+
+  # Evaluate accuracy.
+  accuracy_score = classifier.evaluate(input_fn=test_input_fn)["accuracy"]
+
+  print("\nTest Accuracy: {0:f}\n".format(accuracy_score))
+
+  # Classify two new flower samples.
+  new_samples = np.array(
+      [[6.4, 3.2, 4.5, 1.5],
+       [5.8, 3.1, 5.0, 1.7]], dtype=np.float32)
+  predict_input_fn = tf.estimator.inputs.numpy_input_fn(
+      x={"x": new_samples},
+      num_epochs=1,
+      shuffle=False)
+
+  predictions = list(classifier.predict(input_fn=predict_input_fn))
+  predicted_classes = [p["classes"] for p in predictions]
+
+  print(
+      "New Samples, Class Predictions:    {}\n"
+      .format(predicted_classes))
+
+if __name__ == "__main__":
+    main()
diff --git a/tensorflow/examples/tutorials/mnist/tenstest.py b/tensorflow/examples/tutorials/mnist/tenstest.py
new file mode 100644
index 00000000000000..56a7e4f6ba9f17
--- /dev/null
+++ b/tensorflow/examples/tutorials/mnist/tenstest.py
@@ -0,0 +1,21 @@
+import tensorflow as tf
+
+k = tf.placeholder(tf.float32)
+
+# Make a normal distribution, with a shifting mean
+mean_moving_normal = tf.random_normal(shape=[1000], mean=(5*k), stddev=1)
+# Record that distribution into a histogram summary
+tf.summary.histogram("normal/moving_mean", mean_moving_normal)
+
+# Setup a session and summary writer
+sess = tf.Session()
+writer = tf.summary.FileWriter("/tmp/histogram_example")
+
+summaries = tf.summary.merge_all()
+
+# Setup a loop and write the summaries to disk
+N = 400
+for step in range(N):
+  k_val = step/float(N)
+  summ = sess.run(summaries, feed_dict={k: k_val})
+  writer.add_summary(summ, global_step=step)