Wikipedia:Reference desk/Archives/Science/2015 February 2

= February 2 =

Taking a picture of the past without a camera
Is it theoretically possible to use Quantum cloning to reconstruct photons that existed in the past, like say photons that reflected off of a tyrannosaurus so we can see what it actually looked like without dealing with the messy business of time travel. ScienceApe (talk) 00:29, 2 February 2015 (UTC)


 * No, entropy and non-linear dynamics, i.e., chaos theory, will quickly make this impossible. Piers Anthony's pulp sci-fi novel Macroscope addresses this supposition. μηδείς (talk) 01:41, 2 February 2015 (UTC)
 * That's a relief. InedibleHulk (talk) 01:59, 2 February 2015 (UTC)


 * I don't follow, how does any of that make it impossible? ScienceApe (talk) 04:10, 2 February 2015 (UTC)


 * The article you linked to says quantum cloning is forbidden by the no-cloning theorem. --Bowlhover (talk) 02:34, 2 February 2015 (UTC)


 * Now that's got to be the second worst written article I have ever read on WP. If specialists can't explain themselves without jargon (i.e., explain the basis of their thought in reality) they really aren't specialists, just theologists. See WP:ONELEVELDOWN μηδείς (talk) 03:12, 2 February 2015 (UTC)
 * It's all made perfectly clear in Hom functor. InedibleHulk (talk) 07:47, 2 February 2015 (UTC)


 * The article I linked to also says imperfect cloning is still possible. ScienceApe (talk) 04:08, 2 February 2015 (UTC)


 * Why do you think quantum cloning has something to do with taking pictures of the past? -- BenRG (talk) 09:45, 2 February 2015 (UTC)
 * You can clone the photons that bounced off of objects that existed in the past. If you clone those photons they should be able to produce a picture. ScienceApe (talk) 20:08, 2 February 2015 (UTC)
 * How would you know which ones to clone? ←Baseball Bugs What's up, Doc? carrots→ 20:32, 2 February 2015 (UTC)
 * How the hell would I know? ScienceApe (talk) 21:30, 2 February 2015 (UTC)
 * Are you confused by the article lede saying that you can clone an "arbitrary" quantum state? That just means anything you could prepare in the lab. It doesn't mean any event in the history of the universe. It's like a camera being able to photograph an "arbitrary" scene. Except that quantum cloning isn't possible, but even if it were, it would have nothing to do with seeing into the past... -- BenRG (talk) 00:22, 3 February 2015 (UTC)
 * Or is the idea that by cloning the system many times and measuring the clones you could determine the complete wave function, then evolve that back in time? I suppose that's an argument that cloning, if it were possible, would make seeing into the past just as feasible as it would be in a classical world. But it isn't feasible classically because of chaos, as people already said. And because some of the information you need is travelling outward at the speed of light from the original event, and there's no way to collect it back at a point. -- BenRG (talk) 00:29, 3 February 2015 (UTC)


 * Every photograph is a picture of the past... SemanticMantis (talk) 20:51, 2 February 2015 (UTC)
 * Taking a picture of the past without a camera ScienceApe (talk) 21:30, 2 February 2015 (UTC)


 * To ScienceApe above, we don't seem to have a good article for path independent, versus path independent qualities. There are Nonholonomic system and path-dependence.  But imagine you've got a cup of water at 32F sitting in front of you.  What temperature was it at the sae time yesterday?  You can't deduce that from the current state of the water alone.  Yt, if I give you a piece of magnetized metal, you can surmise that at some point in the past it was subject to a magnetic field strong enough in a given temperature to allow its structure to polarize magnetically.  The current temperature is path-independent.  Whether the nail is magnetized is path dependent.  Now, even if everything were path-dependent, which it isn't, and even if we could gather all the data needed to reconstruct the past history, we wouldn't have a computer large enough (i.e., smaller than the universe itself) powerful enough to predict where any given air molecule was last month, let alone "photograph" a Tyrannosaur.  Basically put, the butterfly effect works backwards, and even things like the orbit of Pluto moving in the "vacuum" and assumed to be subject only to gravity cannot be calculated past or before certain dates. μηδείς (talk) 21:14, 2 February 2015 (UTC)
 * (Cup of water) Actually, you could tell that the oxygen atoms used to be helium (and most of them were hydrogen before that), but everything changed when stellar nucleosynthesis happened. - ¡Ouch! (hurt me / more pain) 08:29, 3 February 2015 (UTC)


 * In some of the old Superman comics they hypothesized that Superman could fly faster than light, so that he could overtake light that had left the earth and thus observe past history. That sounds about as plausible as cloning photons to get a picture of a living T-Rex. ←Baseball Bugs What's up, Doc? carrots→ 21:38, 2 February 2015 (UTC)
 * Even if he could, he'd be running up against the inverse-square law. If he could see an object at one mile with an X resolution, by travelling a light second away (186,282 miles) he would see the same object with an X/34,700,983,524 resolution. Not very helpful. μηδείς (talk) 22:13, 2 February 2015 (UTC)
 * He would need a huge eye, thousands or millions of light years in diameter. More likely is that a natural camera existed that took a photo of your dinosaur. Perhaps a cave with a pinhole formed an image on a layer of algae on the back wall, and the T. Rex stood still for a few days to make an image. Or perhaps a gamma ray burst projected a shadow in altered isotopes on a rock surface.  Or perhaps a giant meteorite strike made a huge flash of light that burnt a shadow on the ground. Graeme Bartlett (talk) 22:20, 2 February 2015 (UTC)
 * You're forgetting about Superman's super-vision. ←Baseball Bugs What's up, Doc? carrots→ 22:29, 2 February 2015 (UTC)
 * Or any kind of adult supervision for that matter. - ¡Ouch! (hurt me / more pain) 07:28, 3 February 2015 (UTC)


 * This is out of my depth, but it seems to me the salacious bit involves the relationship of quantum entanglement and local hidden variable theory. In theory, every electron is the same electron, and every photon is the same photon, except for the momentum and spin they carry.  Yet... if one particle is entangled, and the other isn't, does that give you a way to distinguish them?  Can you measure whether a photon has an (unknown) entangled partner by experimenting on it?  Can you tap on a molecule and somehow tell the last time it absorbed or emitted a photon?  It seems like a slippery slope that would have basic particles as artifact-encrusted as our Earth.  But in my very limited knowledge, the experiments have resisted such interpretation - you can only measure whether two particle are entangled by viewing them both.  And by the same token, you can't tap on a molecule to resurrect the ghosts of dead photons that once fell upon it. Wnt (talk) 13:22, 3 February 2015 (UTC)
 * We have a fun little article at one electron universe - I don't think anybody takes it too seriously today, but it's a fun thought experiment. OP should also see Identical_particles - even if we take it as a given that there are many photons out there that have hit a T. rex, how are we to know which they are? SemanticMantis (talk) 14:28, 3 February 2015 (UTC)

Fastest speed
What's the fastest speed record for rotational and linear object? I read for a micro-object is a research in St. Andrew link1, but what's the fastest for object larger than 1 mm or 1 cm? What happened when 2 high speed iron object hit each other, will it bounce back or melted into one object? I supposed for stone object the collision will create massive explosion, correct? roscoe_x (talk) 04:10, 2 February 2015 (UTC)


 * For linear motion, the limits would be air resistance, or, in vacuum, the only limit would be the amount of energy that would be required and the length of the "barrel" needed to accelerate the object. Eventually you'd approach the speed of light and more and more energy would be required for each small increase in speed.


 * For rotational speed, keeping the object from flying apart would be problematic.


 * Assuming we aren't talking about speeds where nuclear reactions would occur, then the objects would vaporize each other if they hit exactly straight on, but any slight deviation would send larger chunks flying out. Beyond a certain speed, the material wouldn't much matter, as it would all vaporize. StuRat (talk) 06:17, 2 February 2015 (UTC)


 * May I suggest Orders of magnitude (angular velocity). It lists a bunch of high-speed rotations.   I'll have to leave it up to you to decide which ones count and which ones don't. SteveBaker (talk) 15:48, 2 February 2015 (UTC)


 * Researchers and practical people who spend a lot of time pushing the limits of very high velocity objects usually switch units - eventually, they get to the point of measuring some physical parameter other than velocity. It's not because they cannot measure velocity; but that velocity is less interesting than other properties, like momentum or energy.  For example, consider the people who work with:
 * terminal ballistics - the common way to describe and compare really fast bullets is to talk about muzzle energy and the momentum of the projectile.
 * fluid dynamics experts - the way we talk about air movement at high speed is as a ratio of the velocity with respect to a different velocity. This is called the Mach number, and it is more useful in many high-speed aerodynamics calculations than a measure of straight line movement per unit of time.
 * particle accelerators - in scientific experiments where physicists create very fast-moving, very tiny particles, the usual unit of measurement is average energy per particle - and that can be expressed as a temperature or an energy, depending on context
 * Physicists know how to convert these units back into a corresponding velocities. But we'd rather use the most appropriate unit for the physical context.  If we want to talk about the weather, though, we don't usually say "the nitrogen molecules are convecting at around mach 1.5 today, even though this may be factually correct.  That's a useless measurement of linear velocity!  It's much more informative to say that it's 25º C; and similar logic applies (but with a more difficult calculation) for the molecular angular momentum.  The same logic even applies if we talk about something macroscopic, like an artillery shell: we can describe its linear velocity; but it's more useful to talk about its momentum.
 * Here's a practical example: consider a few "really fast bullets." Let's look at the M-16 rifle, which fires a supersonic round.  Let's compare it to the USS New Jersey's main battery, which fired a much much much slower round - 200 meters per second slower!  But when the New Jersey lobbed shells into the mountains of Lebanon, the barrage of shells were famously described as "flying Volkswagens", not "bullets that fly 25% slower than an ordinary rifle round."  Intuitively, you already know why the velocity does not matter.  Those shells had the momentum of a VW bug that could fly for twenty or thirty miles.  Regular press-reporters and physicists alike all knew that the velocity of these shells simply did not effectively describe the situation.
 * So if you are looking for the "fastest thing ever made by humans," it's going to be a very fruitless search. Obviously, the fastest thing we ever make is a photon - and we make those every time we turn on a lightbulb.  In fact, we emit photons every time we stand around just existing - because blackbody radiation means that we are releasing particles that travel at the speed of light.  If you want a photon that travels at the speed of light and is also very large, then consider the radio photons emitted by NLK - each photon moves at the speed of light and is about the size of a very large mountain-valley.  It is more interesting to talk about the things we make that have very high energy, or very large momentum.
 * Nimur (talk) 19:09, 2 February 2015 (UTC)


 * My understanding of this question was, what would happen if we rotated a disk 29,647.8 miles in radius once a second (or a smaller one proportionally faster)? μηδείς (talk) 22:06, 2 February 2015 (UTC)
 * I don't understand where you got that understanding from - but the situation you describe is covered by the Ehrenfest paradox. However, our OP seems to be talking about the fastest rotational speed we've actually obtained from a macroscopic object...not the theoretical maxiumum. SteveBaker (talk) 04:53, 3 February 2015 (UTC)
 * I suspected perhaps that the OP was wondering if there is an upper limit on the rotational speed material objects. μηδείς (talk) 18:54, 4 February 2015 (UTC)

What is the liquid sprayed at the base of the rocket in this rocket launch animation?
The CNN website has a video with an animation of the launch a SpaceX Falcon Heavy rocket. About 23 seconds into the video, something is spraying a liquid at the base a firing rocket. What is the liquid supposed to be and what is it supposed to do? --173.49.17.60 (talk) 12:00, 2 February 2015 (UTC)


 * It's water, coming from the large water tower also shown in that video. Our launch pad article, describing launch pads in general, mentions that, "a sound suppression system spraying large quantities of water may be employed".  The Space Shuttle sound suppression system is described at this NASA page.  There are also some wonderful videos of the shuttle's sound suppression system being tested that you should be able to find searching on YouTube.  I'm not sure what specific information is available about SpaceX's sound suppression systems, but note that they are modifying Kennedy Space Center Launch Complex 39A (SLC-39A), one of the two old Space Shuttle launch pads, for the Falcon Heavy East Coast launches.  LC-39A gives a bit more information on their use with the Apollo program Saturn V.  For the West Coast FH launches, they will be using Vandenberg AFB Space Launch Complex 4 (SLC-4). -- ToE 12:25, 2 February 2015 (UTC)


 * LC-39A mentions the Rocketdyne F-1, implying that the sound suppression system was used during Apollo, but the sources I've looked at imply that it was first installed for the Space Shuttle, so I've raised the question at Talk:Kennedy Space Center Launch Complex 39. -- ToE 13:04, 2 February 2015 (UTC)


 * There's a good thread on water sound suppression systems at StackExchange: -- The Anome (talk) 12:32, 2 February 2015 (UTC)


 * Thanks for all the answers and links. Much appreciated. --173.49.17.60 (talk) 05:23, 3 February 2015 (UTC)

And here I thought the water was to keep the rocket blast from eroding the concrete launch platform. Huh. Learn something new everyday.50.43.56.168 (talk) 03:26, 4 February 2015 (UTC)

Do all batteries decay with cycles of charging and re-charging?
Known and possible batteries are meant. Why can't batteries be charged and re-charged without (too much) wear? Noopolo (talk) 22:53, 2 February 2015 (UTC)


 * Entropy would be a first principles start but isn't particularly satisfying numerically. In short, nothing is for free in Thermodynamics even  reverse.  --DHeyward (talk) 01:44, 3 February 2015 (UTC)


 * I don't think we can invoke entropy here. The battery isn't a closed system - electrical energy goes in, heat and electricity comes out.  The battery doesn't have to suffer from entropy (at least in principle). SteveBaker (talk) 04:47, 3 February 2015 (UTC)


 * For some reason chemistry just doesn't work well to provide a reusable way to store energy. Some mechanical and hydraulic systems that seem like they would be a joke actually work better in some circumstances, like a flywheel or pumping water into a high tank. StuRat (talk) 03:18, 3 February 2015 (UTC)


 * Side reactions are inevitable. shoy (reactions) 14:00, 3 February 2015 (UTC)


 * How about a large capacitor? Won't that do? SteveBaker (talk) 04:47, 3 February 2015 (UTC)
 * Flywheels and capacitors and things like Pumped-storage_hydroelectricity seem to get many more "Charge_cycle" than chemical batteries. Some other alternatives at Energy_storage and Grid_energy_storage. But outside of idealized models, these will still wear out eventually. There are so many different types of chemical battery that I'm not sure if there's one general explanation of why they can't be used for more cycles. But hopefully someone here can clarify the issue. :) SemanticMantis (talk) 14:25, 3 February 2015 (UTC)


 * Entropy is of course, relevant. With each recharging the battery will become less and less useful.  That's not gremlins.  The packaging will indicate that the batteries have a limited life--it simply won't say "due to entropy". μηδείς (talk) 16:25, 3 February 2015 (UTC)


 * Entropy is relevant - but you can't use the laws of thermodynamics to say that you can't build an infinitely rechargeable battery because those laws only apply to closed systems and unless you're talking about a thought-experiment battery that is 100% efficient (which is impossible), there will be energy entering into the battery as you charge it which is leaving as heat. That allows the battery (in principle) to fight entropy by increasing the degree of chaos in the rest of the universe.   This situation is no different than (for example) a computer with memory full of random numbers that is computing and storing the digits of pi using incoming electricity.  Entropy is reversed inside the computer at the expense of emitting heat as it does its calculations.


 * I'm not saying that a battery could definitely be made to recharge an indefinite number of times - only that you cannot use the laws of thermodynamics to rule it impossible on grounds of entropy. SteveBaker (talk) 18:39, 3 February 2015 (UTC)


 * Yes, well, if we're talking about a locally open system, then the ability to go to the store and get a new battery (or to build your own lab to recycle and reconstitute batteries) rules out the problem of entropy. μηδείς (talk) 22:45, 3 February 2015 (UTC)


 * Most common batteries consist of two solid electrodes separated by a fluid electrolyte (liquid or gel). One or both of the electrodes will be chemically altered during charging and discharging, and it is often the case that the electrodes aren't the same after each cycle.  Typical defects include the growth of filaments or the entrapment of other substances in the electrode lattice.  Such degradation of the electrodes is a common reason that rechargeable batteries wear out over time.  For industrial applications, liquid metal batteries can provide a solution.  In the typical liquid metal case, both electrodes are liquids and the barrier playing the role of the electrolyte is solid.  That scenario makes the battery much more resistant to degradation since the liquid electrodes can't be physically altered the ways solids do.  This allows good efficiency to be maintained for thousands of charge cycles.  Unfortunately liquid metal batteries only really make sense for large-scale applications because they require high temperatures in order to keep their molten metal components liquid.  Dragons flight (talk) 23:32, 3 February 2015 (UTC)


 * Why not use mercury for the electrodes (or at least one) ? StuRat (talk) 06:04, 4 February 2015 (UTC)


 * The electrodes have to participate in the chemical reactions of the battery. Unfortunately I don't know enough about battery electrochemistry to say whether mercury could be a viable choice or not.  I can imagine though that health and safety concerns might also weigh against the widespread use of liquid mercury in consumer batteries.  Dragons flight (talk) 17:44, 4 February 2015 (UTC)


 * Touching liquid mercury at room temperature has got to be a lot healthier for you than touching another metal at molten temperatures. StuRat (talk) 06:36, 6 February 2015 (UTC)