Wikipedia:Reference desk/Archives/Science/2016 April 17

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April 17

Torch beam question

Does the beam from a torch exert a force (obviously a minuscule one) backwards on the torch? If so, does that mean that there must be mass ejected in the light beam, and, if so, where does that mass come from? Does the torch get lighter as a result of expulsion of mass? 86.151.118.252 (talk) 01:09, 17 April 2016 (UTC)

Note for my fellow Americans: in British English they use "torch" for what we call a "flashlight". Anyway, answers: a) yes, it exerts radiation pressure. b) mass and energy are the same thing, so the answer is yes. It is true that photons have no rest mass. c) Yes, very slightly so, because of the loss of the stored energy contained in the batteries (or whatever the power source is). I recommend these videos for learning more about mass and energy in physics: [1] [2] (and really, every video on that channel is great). --71.110.8.102 (talk) 02:05, 17 April 2016 (UTC)

Also New Zealand English, and I believe Australian English, and perhaps other former British Empire countries. Akld guy (talk) 06:21, 17 April 2016 (UTC)

"former British Empire countries" isn't a useful distinction, since part of the US and all of Canada would qualify. StuRat (talk) 22:33, 18 April 2016 (UTC)

@StuRat: Really? I was thinking of India, Pakistan, Hong Kong, Rhodesia, Palestine, Malaya, Singapore, Sri Lanka, Falkland Islands, Mauritius, Malta, Jamaica, Trinidad, Barbados, British Virgin Islands, Guyana, Fiji, Vanuatu, Tuvalu, Solomon Islands, Papua New Guinea. This list has been thrown together quickly because I don't want to waste any more time on this, but you get the meaning. Akld guy (talk) 00:56, 20 April 2016 (UTC)

Right, but "former British Empire countries" is pretty much synonymous with "English speaking nations", including those which now have a rather distinct dialect from the UK. StuRat (talk) 02:57, 21 April 2016 (UTC)

Yes, chemical potential energy is expelled. The mass is reduced in the torch as the energy expelled is expelled. Charging the battery adds mass as well. It's much too small to be observed as the mass of a charged battery is not measurably different from a discharged battery even though we know there is a difference, --DHeyward (talk) 08:44, 17 April 2016 (UTC)

The amount of mass lost by the batteries when they discharge can be calculated by E=mc². For a typical 2AHr AA battery, it is 120 femtograms. LongHairedFop (talk) 11:35, 17 April 2016 (UTC)

Just to pique your brain a little bit harder than usual: the theory of relativity doesn't actually formally prove the conservation of energy; it just provides a bunch of new coordinate-change equations that help relate energy and position.

So, even if we admit that the emitted light does carry both energy and momentum, this is not in itself proof of a mass-decrease in the flashlight. That mass-decrease depends entirely on an unrelated and unproven physical concept - conservation of energy. Nearly every experiment validates this principle; nearly every intelligent and educated scientist believes this principle; but it is not actually a requirement of special- or general- relativity. For the record: I also believe that energy and mass are normally conserved, at least on average, and I believe there is no reason to doubt this premise in normal circumstances.

But the story does not end there: science does not rest on "strong beliefs." We need complete theories, and we need experimental evidence to validate those theories. This is called the scientific method.

In fact, it remains an open question whether we could ever construct some metaphorical flashlight that emits energy and momentum but loses no mass. This would be symmetry breaking; more specifically, it would be a break in "T-symmetry", or "time symmetry." To my knowledge, no reputable scientist has ever seen this happen; but there are lots of theorists who study it, because it would have incredible consequences that can help us describe the evolution of our entire universe.

Here is an article from none other than Symmetry Magazine, a joint publication of SLAC and Fermilab: Fermilab and symmetry breaking (2008). It helps explain why the 2008 Nobel Prize in Physics was awarded to a trio - (they worked in groups of three) - of very smart physicists, who worked through the very subtle theoretical consequences of CP-violation. This is different physics; but in terms of my intuition, it superficially appears equally un-physical as a flashlight that can emit energy and momentum without losing mass. These scientists propose a violation of a conservation law. Yet, this theory was Nobel-worthy: the math does not forbid it. The fact is, you're here as a carbonic bag full of salt water born on a cluster of used-up stardust, instead of existing only as a bunch of thermal-equilibrium-in-a-vacuum. This in itself proves that there must have been some condition in the past in which some conservation law was violated, and we still have an incomplete explanation about the how- and why-.

Nimur (talk) 14:56, 17 April 2016 (UTC)

To keep us on the right side of the physics/metaphysics line, weak anthropic principle might be a better candidate for Nimur's last link than the cogito. Thought without material existence is not a priori impossible. Tevildo (talk) 16:06, 17 April 2016 (UTC)

Sure, I can agree with your statement! I meant to suggest that thought implies thermal non-equilibrium, and non-equilibrium implies imperfect symmetry. I assert that these statements are distinct from the less logically-closed statement, thought implies material existence! You can read Descartes' own impressions - for example, he is republished in English in Great Books of the Western World - and in my reading, his position seems to be that of the more restrictive, almost solipsistic stance - material existence may occur, though it is largely superfluous; but thought must occur! The man had a lot to say on the scientific method, and his work predates the academic distinction between "science" and "philosophy," so we can only imagine what he might have thought about the way more modern thinkers have formulated anthropic principle! Nimur (talk) 20:15, 17 April 2016 (UTC)

The Symmetry article is interesting, but seems less approachable than our own Cabibbo–Kobayashi–Maskawa matrix article. It seems to differ in at least one regard - it says that the CKM matrix predicted three generations of quarks, while our article makes it sound like it was written to account for them. Wnt (talk) 21:46, 17 April 2016 (UTC)

Nimur's post is completely wrong. He seems to have confused time-reflection symmetry with time-translation symmetry. The former is called T symmetry and is dual to CP symmetry. The latter is the symmetry associated with energy conservation in Noether's theorem. They are not the same thing.

Any theory defined by a special-relativistic Lagrangian must conserve energy by Noether's theorem. Unitarity implies conservation of energy. A form of energy conservation follows from the Bianchi identities of general relativity. -- BenRG (talk) 01:21, 18 April 2016 (UTC)

Sir, as many regular readers of this reference desk may know, there is a fascinating history of individuals who would like to tell me that I am completely wrong. I will completely admit that I am sometimes wrong, and sometimes I am even completely wrong, but in this particular instance, I feel that you are weakening your position when you allege that everything I wrote was "completely wrong." Perhaps I wrote unclearly; or perhaps in the spirit of writing toward a general audience I may have elided some subtle detail, but I do not believe - in this instance - that my post was "completely wrong."

There is a part of me that wishes to call out "" and have you present your case using a reliable encyclopedic resource outside of a Wikipedia article. I'm fairly sure that I know how to find such resources for myself, but we have many readers who don't find particle physics particularly accessible.

So, instead of fruitlessly debating whether I am completely mistaken, I feel that it would be more productive if we and the other readers of this reference desk spent our time reading and referencing what some very smart scientists have written on these topics, distilled into a format that is a little bit more accessible to the generally-interested reader. In that spirit, here are a few items from the very same journal:

• Emmy Noether: a mathematician to know (this article was published last summer);
• Charge-parity violation, on CP symmetry and its physical implications;
• Symmetry, a user-friendly introduction to the topic.

If you should ever find yourself in my neighborhood, I'd be very happy to join you for a visit to the local particle accelerator, and we can ask a scientist to clarify any of the more uncertain points. If I am wrong, I would very much appreciate a correction of my error.

Nimur (talk) 17:09, 18 April 2016 (UTC)

I corrected your error in my last reply.

We clash frequently because you frequently post about things you don't understand. I would get equally frustrated with anyone else who did that (I am equally frustrated with StuRat). There's nothing wrong with reading about symmetries in Wikipedia and science magazines and getting confused about two different symmetries with the word "time" in them. If you'd opened a new thread asking whether energy conservation might be violated because CP is violated and CP is dual to T, I'd have been happy to explain that, no, they're unrelated because they involve different symmetries of time. That's a good use of the ref desk. But when you post your misunderstanding as a reply to someone else's question, that's different. It contributes negative value to the ref desk because it distracts from better answers, especially when it's as long as all the other answers put together, as your posts tend to be. I don't want to make you or anyone angry. But every time you do this, I have to decide whether to let it go or try to limit the damage.

When I say completely wrong, I don't mean 100% wrong on the facts. On a true-false test, it's just as hard to get everything wrong as to get everything right. If you don't know anything, you will score about 50%. The information content of the answers is minimal at 50%. That's closer to what I mean by wrong.

Again, there's nothing wrong with being a beginner in some subjects. The problem is that you don't seem to know which subjects those are. -- BenRG (talk) 19:18, 18 April 2016 (UTC)

Well you attempted to correct him, but you did so in a way that was also hard (for me) to follow. I understand what you mean when you say time reversal and time translation are two different types of symmetry, and I understand what you mean when you say Unitarity implies conservation of energy. I'm just not so sure how those statements apply to Nimur's original statement. If he struck out this bit --'more specifically, it would be a break in "T-symmetry", or "time symmetry."'-- would you still object to the rest? I mean the CP bit may then be off-topic, but not to my eye saying anything literally incorrect, just perhaps less useful... I mean is Noether's theorem really part of Relativity? Or is that an outside finding? Perhaps are you talking past eachother on the 'unproven' bit? E.g. some people think formal proofs of necessary implications from good models constitute scientific proof, while others may demand something more empirical. But then these ultra-empiricists should abandon the term "proof" to the mathematicians, and only speak of evidence. Anyway, I kinda like it when you two argue. You're both smart people and and some of our best respondents, so it's way more informative than most arguments here. And all of us would do well to try to stick to what we're good at ;) SemanticMantis (talk) 22:10, 18 April 2016 (UTC)

Note that light has no rest mass but it contains relativistic mass, i.e. energy. If you look up photon you can get a figure for the momentum. Also note that photons carry angular momentum, so in theory you can screw something in with a torch ... it just isn't very much (and neither is the mass). The use of light pressure for solar sails is well known, and is the basis for stabilizing the Kepler telescope; and is proposed in stuff like this. And for every action there is a reaction, as far as anyone has ever found. (Possible violation is being considered as one explanation for Emdrive, but few believe that works at all, let alone that it is truly a reactionless drive) Wnt (talk) 21:37, 17 April 2016 (UTC)

• (OP) Thanks for the replies. 86.151.119.176 (talk) 01:00, 18 April 2016 (UTC)

Limits of human cognition: smallest pixel size/broadest range of colors

Does it make sense to produce screens with smaller and smaller pixels, and more and more colors? Will we reach a point (sorry for the pun) when the human eye cannot see any difference anymore in size/color palette of a dot? --Scicurious (talk) 17:55, 17 April 2016 (UTC)

We has already reached this point at least a few years ago. Ruslik_Zero 18:36, 17 April 2016 (UTC)

And what is the threshold for size/number of colors (for non colorblind people)? --Scicurious (talk) 19:11, 17 April 2016 (UTC)

For colors it is 24-bit. For size it is probably >250 dpi. Ruslik_Zero 20:24, 17 April 2016 (UTC)

See gamut. 24-bit RGB colour does _not_ adequately cover the entire human visual range. Tevildo (talk) 20:58, 17 April 2016 (UTC)

So, why would someone use higher than 250dpi then? --Scicurious (talk) 21:23, 17 April 2016 (UTC)

If you're presenting a wide-screen motion picture, such as a Star Wars entry, wouldn't you need a lot more dots per inch than in a pocket-sized photo? ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:28, 17 April 2016 (UTC)

Actually you'd most likely need less. You'd need a higher resolution i.e. more total pixels ("dots") which is a different thing. Nil Einne (talk) 23:16, 17 April 2016 (UTC)

I'm talking about the source. A photo on your monitor might look pretty good, but if you project it to room-size, you might more easily notice the individual pixels. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:37, 17 April 2016 (UTC)

You might be mixing up dots per inch with Display_resolution in pixels. So: say I have an image at 500x500 px. If I display it at a size of 1x1 inch, then I have 500 DPI, and it looks sharp. If I display it at 100x100 inches, it has 5 DPI, and looks chunky, unless I stand really far away. So changing the projection size changes the DPI while holding the resolution constant. For a movie, they have far less DPI than you normally use on your home computer. This is because nobody needs to stand 2 feet in front of the movie screen and be able to read 11 pt font. You need to think about both DPI and resolution (and viewing distance, and lots of other things) to understand how an image will look. SemanticMantis (talk) 14:04, 18 April 2016 (UTC)

Also, I believe a movie shown on any device requires less resolution than a still, because the mind averages out moving pixels, while you can make them out more precisely if you look closely at a still for an extended period of time. StuRat (talk) 14:27, 18 April 2016 (UTC)

Also see retina display. --Stephan Schulz (talk) 21:46, 17 April 2016 (UTC)

• For resolution, the problem with talking in terms of pixels is that the apparent size of a pixel depends on how far away it is. The maximum resolution of human vision is generally considered to be on the order of one arc-second -- 1/3600 of a degree. To convert that to pixel size it is necessary to specify the distance from the viewer to the screen. Looie496 (talk) 21:44, 17 April 2016 (UTC)

• The smallest pixels are needed when the screen is closest to the eyes, such as in some VR headsets. StuRat (talk) 05:32, 18 April 2016 (UTC)

Mechanics of human eye

Why does the human eye have such a high yield stress? What gives it that property? 2A02:C7D:B907:6D00:D185:96E9:64E3:36EB (talk) 21:55, 17 April 2016 (UTC)

Be more specific. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:17, 17 April 2016 (UTC)

Because it's basically a sack filled with fluid? Vespine (talk) 22:24, 17 April 2016 (UTC)

As in its able to undergo elastic deformation at very high stresses. It's not fluid though, it's like some sort of hard jelly. I think it's called vitreous humour but I want to know what gives it this property at a molecular level. 2A02:C7D:B907:6D00:D185:96E9:64E3:36EB (talk) 23:30, 17 April 2016 (UTC)

The main part of the eyeball is filled with aqueous humour, a very runny sort of sticky liquid. Vitreous humour "is the clear gel that fills the space between the lens and the retina of the eyeball". Have a read of those blue links, but please get back to us if they haven't answered your question. Alansplodge (talk) 00:30, 18 April 2016 (UTC)

That should be: The front part of the eyeball between the cornea and lens is filled with aqueous humour, a very runny sort of sticky liquid. Vitreous humour "is the clear gel that fills the space between the lens and the retina of the eyeball". Bazza (talk) 16:01, 18 April 2016 (UTC)

In mechanics, a "high yield stress" means that the physical object or material remains elastic under high stress loads, and does not undergo irreversible (plastic) deformations. The eye indeed has to retain its shape and structural integrity to be able to function properly. Saccadic loads on an eye can be quite high, eyeball reaching angular velocity of a few hundreds of degrees per second in a few tens of milliseconds. The elastic properties of the eyeball, AFAIK, are mostly due to the collagen and other fibers in the sclera; these two wiki articles are probably the best place to start. For the mechanics of the eyeball, the best place to start is probably the classical paper by David Robinson, here. There are newer papers on eyeball mechanics, but this one is very well worth reading. If you are interested in neuroscience (and neuropathology) of eye movement, I suggest the Leigh and Zee book. Hope this helps. --Dr Dima (talk) 16:59, 18 April 2016 (UTC)