Bonus points for physics test

My physics teacher told me that I get 10 bonus points on our next test if I do something impossible. That impossible thing is providing an example of a situation (and the necessary equations to prove it) in which the total entropy of the universe decreases. His example was freezing water: he showed that when you freeze water the entropy in the sample decreases, but increases in the surrounding environment more than it decreased in the sample, thus resulting in an overall increase in entropy.

There has to be some way to decrease entropy, even if it doesn't actually work in practice, it doesn't matter, as long as I can show it to work mathematically. People of FES who know what they are doing, UNITE! Help me get bonus points.

OK I found this:
"Unlike most other laws of physics, the Second Law of Thermodynamics is statistical in nature, and its reliability arises from the huge number of particles present in macroscopic systems. It is not impossible, in principle, for all 6 × 10²³ atoms in a mole of a gas to spontaneously migrate to one half of a container; it is only fantastically unlikely—so unlikely that no macroscopic violation of the Second Law has ever been observed."

"Consider the situation in which a large container is filled with two separated liquids, for example a dye on one side and water on the other. With no barrier between the two liquids, the random jostling of their molecules will result in them becoming more mixed as time passes. Now imagine that the experiment is repeated, this time with only a few molecules, perhaps ten, in a very small container. One can easily imagine that by watching the random jostling of the molecules it might occur — by chance alone — that the molecules became neatly segregated, with all dye molecules on one side and all water molecules on the other. That this can be expected to occur from time to time can be concluded from the fluctuation theorem; thus it is not impossible for the molecules to segregate themselves."

So that gives you one starting point, that part of how entropy works is based on probability, and any probability is always a possibility no matter how unlikely.

Secondly, it's possible to trick your way round the question using the notion that entropy is a time dependent rule. It assigns a direction to time - however, if one treats time as a dimension in the same way as the dimensions of space are, there's nothing to say you can't treat time as if it were flowing in the other direction. If your physics teacher didn't specify you weren't allowed to make time flow backwards, then I see no reason you shouldn't treat it as a changeable variable and use the exact same equations that govern normal entropy but with time flowing the other way. I think it likely that at least on paper this would be possible, even if not in the real world. Mind you, in order to prove it was not possible in the real world your physics teacher would need to present a proof that time cannot flow in the other direction, which I don't think anyone has ever been able to do.

Sorry if this is garbage, most of my physics knowledge deals with radiation.

The Big Crunch.

Secondly, it's possible to trick your way round the question using the notion that entropy is a time dependent rule.

Actually, it isn't.

The Big Crunch.

Actually, it increases during this phase, too.   

on the very small scale it is possible. http://www.newscientist.com/article/dn2572-second-law-of-thermodynamics-broken.html
so one part of the universe entropy decreases while the not increasing it any anywhere else.
not the best source but you can find other sources for this if you look. yes I am lazy.

Is he a sci-fi literature buff?

If so, give him a printout of Isaac Asimov's The Last Question for full bonus points.

Quote from: Thermal Detonator on February 18, 2010, 06:12:07 PM

Secondly, it's possible to trick your way round the question using the notion that entropy is a time dependent rule.

Actually, it isn't.

Irrelevant.

Entropy is time dependant? You mean it can determine the arrow of time? Wha? As to the original question, i'm not really sure my instinct is theres no process that can reduce entropy. It may be able to happen stochastically even then i'd have to think about it, i'm not totally convinced.

Entropy is time dependant? You mean it can determine the arrow of time? Wha?

Yes, it can.

Quote from: bowler on February 19, 2010, 02:03:23 PM

Entropy is time dependant? You mean it can determine the arrow of time? Wha?

Yes, it can.

No, it can't.

Yes it can.

Entropy [|||||||]

Entropy later [||||||||||||||||]

I guess time went forward.

Yes it can.

Entropy [|||||||]

Entropy later [||||||||||||||||]

I guess time went forward.

This is not generally true.

Quote from: SupahLovah on February 19, 2010, 07:23:36 PM

Yes it can.

Entropy [|||||||]

Entropy later [||||||||||||||||]

I guess time went forward.

This is not generally true.

Irrelevant.

"Entropy is the only quantity in the physical sciences that "picks" a particular direction for time, sometimes called an arrow of time. As one goes "forward" in time, the second law of thermodynamics says, the entropy of an isolated system will increase when no extra energy is consumed. Hence, from one perspective, entropy measurement is thought of as a kind of clock—although not really an accurate measure of time. There is also the complication that, locally, entropy can decrease with time: many systems, including living systems, reduce local entropy by expenditure of energy at the expense of a net increase in the entropy of their environment."

Parsifal! I need your help! I'm thinking of just printing that article, but if you can give me something better I'll go with it.

Parsifal! I need your help! I'm thinking of just printing that article, but if you can give me something better I'll go with it.

One of my own ideas is something called an anti-star, which is much like a star except that it fuses atoms backwards in time. From our perspective, it would look a bit like a black hole, absorbing light and thermal energy in order to convert helium into hydrogen. You could simply provide the equations which govern the function of a star and challenge the teacher to find a reason why time needs to flow in a particular direction for them to work.

Quote from: Taters343 on February 20, 2010, 04:39:48 AM

Parsifal! I need your help! I'm thinking of just printing that article, but if you can give me something better I'll go with it.

One of my own ideas is something called an anti-star, which is much like a star except that it fuses atoms backwards in time. From our perspective, it would look a bit like a black hole, absorbing light and thermal energy in order to convert helium into hydrogen. You could simply provide the equations which govern the function of a star and challenge the teacher to find a reason why time needs to flow in a particular direction for them to work.

Very few equations in physics do not have time-reversal symmetry. Its just the second law of thermodynamics stops it happening. In quantum field theory the whole issue becomes more ambiguous where an electron going backwards in time is the same as an anti-electron going forwards. You could reverse the hydrogen fusion equation. If you gave enough energy to a helium you could probably break it into 2 hydrogen atom. Unfortunately a situation where you have to pump in a load of energy doesnt make a good star.

just look up second law of thermodynamics on the very small scale as I said before. you just need to show that something can become more ordered without causing disorder somewhere else right? you can find many source with that.

just look up second law of thermodynamics on the very small scale as I said before. you just need to show that something can become more ordered without causing disorder somewhere else right? you can find many source with that.

Ok. I'll do that.

Very few equations in physics do not have time-reversal symmetry. Its just the second law of thermodynamics stops it happening. In quantum field theory the whole issue becomes more ambiguous where an electron going backwards in time is the same as an anti-electron going forwards. You could reverse the hydrogen fusion equation. If you gave enough energy to a helium you could probably break it into 2 hydrogen atom. Unfortunately a situation where you have to pump in a load of energy doesnt make a good star.

Not necessarily. The Universe could be temporally symmetric, with a big bang at each end. We live in a time where stars exist because we require their energy production to exist, whereas in a few hundred billion years, after all the stars have died, the debris from them could have collapsed into a plethora of anti-stars which in fact derive their fuel from the big bang at the other end (what we would describe as a "big crunch") and operate backwards in time.

Quote from: bowler on February 20, 2010, 10:50:22 AM

Very few equations in physics do not have time-reversal symmetry. Its just the second law of thermodynamics stops it happening. In quantum field theory the whole issue becomes more ambiguous where an electron going backwards in time is the same as an anti-electron going forwards. You could reverse the hydrogen fusion equation. If you gave enough energy to a helium you could probably break it into 2 hydrogen atom. Unfortunately a situation where you have to pump in a load of energy doesnt make a good star.

Not necessarily. The Universe could be temporally symmetric, with a big bang at each end. We live in a time where stars exist because we require their energy production to exist, whereas in a few hundred billion years, after all the stars have died, the debris from them could have collapsed into a plethora of anti-stars which in fact derive their fuel from the big bang at the other end (what we would describe as a "big crunch") and operate backwards in time.

This has been disproved. Stephen Hawking outlines why in A Brief History Of Time.
Also, irrelevant.

just look up second law of thermodynamics on the very small scale as I said before. you just need to show that something can become more ordered without causing disorder somewhere else right? you can find many source with that.

please be careful, generally entropy is not order or disorder in a literal sense (symmetry) .imo that's only partially true for crystals. a better explanation is (please correct me if that's wrong), that the entropy of a system depends on the amount of possible microscopic states for given macroscopic boundary conditions. so it can be seen as a measurement for the information that is missing to fully decribe the system when you only know the boundary conditions.

Quote from: optimisticcynic on February 20, 2010, 11:44:08 AM

just look up second law of thermodynamics on the very small scale as I said before. you just need to show that something can become more ordered without causing disorder somewhere else right? you can find many source with that.

please be careful, generally entropy is not order or disorder in a literal sense (symmetry) .imo that's only partially true for crystals. a better explanation is (please correct me if that's wrong), that the entropy of a system depends on the amount of possible microscopic states for given macroscopic boundary conditions. so it can be seen as a measurement for the information that is missing to fully decribe the system when you only know the boundary conditions.

ok bad definition I know. however my point stands. there are events such as Brownian motion that can cause a closed systems entropy to decrease for a limited time. it happens surprisingly often at the small scale.

Quote from: bowler on February 20, 2010, 10:50:22 AM

Very few equations in physics do not have time-reversal symmetry. Its just the second law of thermodynamics stops it happening. In quantum field theory the whole issue becomes more ambiguous where an electron going backwards in time is the same as an anti-electron going forwards. You could reverse the hydrogen fusion equation. If you gave enough energy to a helium you could probably break it into 2 hydrogen atom. Unfortunately a situation where you have to pump in a load of energy doesnt make a good star.

Not necessarily. The Universe could be temporally symmetric, with a big bang at each end. We live in a time where stars exist because we require their energy production to exist, whereas in a few hundred billion years, after all the stars have died, the debris from them could have collapsed into a plethora of anti-stars which in fact derive their fuel from the big bang at the other end (what we would describe as a "big crunch") and operate backwards in time.

I guess it depends on how much physics your going to re-write. I mean as I understand it you idea involves a start 'burning' a single helium atom to produce two hydrogen atoms. In this universe this is unconditionally forbidden. In our universe,
H + H -> He + some energy (few MeV I think, depending on the cycle).

You could do this process is reverse, puming in energy could split helium, probably, other factors come into play. Obviously conservation of energy stops this happening in a self sustaining reaction because it requires far more energy that it puts out. Conservation of energy is a pillar on which modern physics is based. I can think of a situation where most laws are shown to be incomplete, not conservation of energy, you can kinda of borrow it on a credit system but that about it. Its not clear to me how a star can work in reverse and still be an exothermic process. If i'm missing something you may be best replying with some more rigorous detail that way I won't get lost if we're at cross-purposes in the wording.

I guess it depends on how much physics your going to re-write. I mean as I understand it you idea involves a start 'burning' a single helium atom to produce two hydrogen atoms. In this universe this is unconditionally forbidden. In our universe,
H + H -> He + some energy (few MeV I think, depending on the cycle).

You could do this process is reverse, puming in energy could split helium, probably, other factors come into play. Obviously conservation of energy stops this happening in a self sustaining reaction because it requires far more energy that it puts out. Conservation of energy is a pillar on which modern physics is based. I can think of a situation where most laws are shown to be incomplete, not conservation of energy, you can kinda of borrow it on a credit system but that about it. Its not clear to me how a star can work in reverse and still be an exothermic process. If i'm missing something you may be best replying with some more rigorous detail that way I won't get lost if we're at cross-purposes in the wording.

In the far future, time essentially would flow backwards. From our point of view, the energy would be sourced from large amounts of light all directed at the star, which in fact is simply being emitted from it where time is flowing in the opposite direction.

Quote from: bowler on February 21, 2010, 10:07:46 AM

I guess it depends on how much physics your going to re-write. I mean as I understand it you idea involves a start 'burning' a single helium atom to produce two hydrogen atoms. In this universe this is unconditionally forbidden. In our universe,
H + H -> He + some energy (few MeV I think, depending on the cycle).

You could do this process is reverse, puming in energy could split helium, probably, other factors come into play. Obviously conservation of energy stops this happening in a self sustaining reaction because it requires far more energy that it puts out. Conservation of energy is a pillar on which modern physics is based. I can think of a situation where most laws are shown to be incomplete, not conservation of energy, you can kinda of borrow it on a credit system but that about it. Its not clear to me how a star can work in reverse and still be an exothermic process. If i'm missing something you may be best replying with some more rigorous detail that way I won't get lost if we're at cross-purposes in the wording.

In the far future, time essentially would flow backwards. From our point of view, the energy would be sourced from large amounts of light all directed at the star, which in fact is simply being emitted from it where time is flowing in the opposite direction.

Ah I think I see what your getting at. Reminds me of an episode of Red Dwarf. Unfortunately thats not really how the big crunch theory goes. At least the current Physics version.

Ah I think I see what your getting at. Reminds me of an episode of Red Dwarf. Unfortunately thats not really how the big crunch theory goes. At least the current Physics version.

I know it's not how conventional big crunch theory works, but I don't see any reason why it couldn't work that way. Of course, this is all hypothetical, since any environment in which time runs in reverse would be not only inhospitable, but antihospitable - we would literally have the life sucked out of us.

Is he a sci-fi literature buff?

If so, give him a printout of Isaac Asimov's The Last Question for full bonus points.

I was going to post this

Quote from: bowler on February 21, 2010, 10:33:53 AM

Ah I think I see what your getting at. Reminds me of an episode of Red Dwarf. Unfortunately thats not really how the big crunch theory goes. At least the current Physics version.

I know it's not how conventional big crunch theory works, but I don't see any reason why it couldn't work that way. Of course, this is all hypothetical, since any environment in which time runs in reverse would be not only inhospitable, but antihospitable - we would literally have the life sucked out of us.

Well obviously it breaks most laws of physics, conservation of energy, thermodynamics, quantum mecahnics. Probably most of the others. Though it is a fun philosophical  idea i'll grant you.