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Time’s Almost Reversible Arrow

quantamagazine.org

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Re: Time’s Almost Reversible Arrow

#21

Post by TTPrograms »

roberto wrote: post #19
maho wrote: post #7

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the…

> Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic. This one thing I don't understand about the 2nd law of thermodynamics: given enough time won't the system go back to a more organized state simply by chance? And in that case, wouldn't the total entropy be reduced?

Yes, that can happen, but with overwhelmingly small probability. Say if you consider the probability that all the gas atoms in a box spontaneously are in one half of the box, the chance is approximately (1/2)^N, where N is on the order of Avogadro's number.

Re: Time’s Almost Reversible Arrow

#22

Post by mrfusion »

mrfusion wrote: post #10

What s the current thinking on being able to run Schrödinger's cat backwards? How can time be reversed on something in an indeterminate state?

Also perhaps the uncertainty principle in general could present problems reversing time?

How would the reversing process know the direction and momentum to reverse for each particle?

Re: Time’s Almost Reversible Arrow

#24

Post by tbabb »

jsprogrammer wrote: post #17
mrfusion wrote: post #10

What s the current thinking on being able to run Schrödinger's cat backwards? How can time be reversed on something in an indeterminate state?

The indeterminacy may not be real. It's just that you don't know the state. I don't believe there is a rule that says you get to know everything.

Are you proposing a "hidden state" which is unobservable? Such a formulation is not consistent with quantum mechanics, as proven by Bell's theorem, unless you permit instantaneous influence from arbitrarily distant parts of the universe.

Re: Time’s Almost Reversible Arrow

#25

Post by nabla9 »

roberto wrote: post #19
maho wrote: post #7

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the…

> Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic. This one thing I don't understand about the 2nd law of thermodynamics: given enough time won't the system go back to a more organized state simply by chance? And in that case, wouldn't the total entropy be reduced?

You understand it just fine. What may be nonintuitive is the incredibly low change of that happening even in microscopic scale. Difference between macroscopic 'organized states' vs 'unorganized states' just mind mindbogglingly huge.

Re: Time’s Almost Reversible Arrow

#26

Post by bronson »

danielsju6 wrote: post #20
mrfusion wrote: post #8

Earlier quoted context omitted.

But if gravity isn't reversed how would planet formation be reversed? What would allow the matter in a planet to fly apart?

You'd see radiation hitting the planet warming the mantel, complex chemicals breaking down & releasing heat, all warming the planet significantly to the point where it was a molten ball, and finally lots of instability where large explosions were throwing pieces of it into space. Basically take the movie and play it backwards.

I find it very hard to picture a molten ball throwing a giant chunk of cold comet deep into space.

Re: Time’s Almost Reversible Arrow

#27

Post by tbabb »

bronson wrote: post #26
danielsju6 wrote: post #20

Earlier quoted context omitted.

You'd see radiation hitting the planet warming the mantel, complex chemicals breaking down & releasing heat, all warming the planet significantly to the point where it was a molten ball, and finally lots of instability where large explosions were throwing pieces of it into space. Basically take the movie and play it backwards.

I find it very hard to picture a molten ball throwing a giant chunk of cold comet deep into space.

That is what is so unsettling about T-symmetry. What you describe is merely improbable, not impossible. With appropriate starting conditions, it is certain. Basically we'd require all the particles in the comet to bounce into each other in such a way that their heat motion cancels and organizes into momentum in one direction, away from the planet. This is physically permitted in either time-direction, but we only see it in one.

Re: Time’s Almost Reversible Arrow

#28

Post by Sharlin »

bronson wrote: post #26
danielsju6 wrote: post #20

Earlier quoted context omitted.

You'd see radiation hitting the planet warming the mantel, complex chemicals breaking down & releasing heat, all warming the planet significantly to the point where it was a molten ball, and finally lots of instability where large explosions were throwing pieces of it into space. Basically take the movie and play it backwards.

I find it very hard to picture a molten ball throwing a giant chunk of cold comet deep into space.

Yes, but that has nothing to do with gravity and everything to do with thermodynamics.

Re: Time’s Almost Reversible Arrow

#29

Post by darkmighty »

maho wrote: post #7

This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time. Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the…

Those questions are very old. You may think the anthropic principle is sufficient here: complex systems seem to rely on an entropy gradient (no life exists in an uniformly chaotic or static state). Ludwig Boltzmann made this argument in the 19th century [1].

However, the argument can apparently be extended to the conclusion that, in a probabilistic sense, your consciousness is just a sliver of a random fluctuation of a near-maximal entropy state. Feynman argued[2] that this shows we do not arise from a random fluctuation, because (even though we're conscious), when we look far away the universe looks highly organized.

Note that those arguments make a bunch of assumptions that may not be valid in our universe, and may not make cosmological sense (entropy is not well defined cosmologically).

[1] https://en.wikipedia.org/wiki/Boltzmann_brain

[2] http://blogs.discovermagazine.com/cosmicvariance/2008/12/29/...

Re: Time’s Almost Reversible Arrow

#30

Post by jarin »

tbabb wrote: post #24
jsprogrammer wrote: post #17

Earlier quoted context omitted.

The indeterminacy may not be real. It's just that you don't know the state. I don't believe there is a rule that says you get to know everything.

Are you proposing a "hidden state" which is unobservable? Such a formulation is not consistent with quantum mechanics, as proven by Bell's theorem, unless you permit instantaneous influence from arbitrarily distant parts of the universe.

Put another way, aside from the possible "instantaneous influence" exception above, there are no "hidden variables" or mechanisms under the surface that we just haven't figured out yet. At that level it is truly probabilistic.
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