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Time’s Almost Reversible Arrow

quantamagazine.org

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Re: Time’s Almost Reversible Arrow

#61

Post by [deleted] »

tbabb wrote: post #34
jsprogrammer wrote: post #32

Earlier quoted context omitted.

I'm saying that the construction "indeterminate" is only a statement about the knowledge of the person making the statement. Are you proposing that the universe doesn't act unless you are considering it?

https://en.wikipedia.org/wiki/Bell's_theorem Certain observations are probabilistic, and the correlations in them predicted by quantum mechanics cannot be reproduced by ascribing them to the traits of particles. It means that at the lowest level, the universe is (barring faster-than-light communication, generally regarded to be false) "truly random". (No concept of an "observer" needs to be devised in order to arrive…

[deleted]

Re: Time’s Almost Reversible Arrow

#62

Post by trhway »

effie wrote: post #57
trhway wrote: post #55

Earlier quoted context omitted.

>If you mix oil and water, you will see the result does unmix itself again. Does this mean entropy decreased and time direction got reversed? Nah. Moving an object to higher gravitational potential decreases the entropy of the system (and thus requires work, unsurprisingly). Mixing oil into water means you displace some volume of water which were closer to Earth with oil bubbles (which are lighter than equivalent vol…

When the fluids are mixed by a spoon, surely work has been done by the stirring spoon on the system and energy of the system increased. But this does not by itself imply entropy of the system increased. The reason is the system got to a non-equilibrium state which has no macroscopic thermodynamic description and no unique way to assign entropy and detect its changes.

>But this does not by itself imply entropy of the system increased.

doesn't matter what way it got there, entropy of the gravitational system oil-water-Earth is lower and the gravitational potential is higher when oil bubbles are inside the water when compared with the same system in the state when all oil is on top of the water. Thus system naturally moves from the former state to the latter, from lower entropy to the higher, from higher gravitational potential to the lower.

Re: Time’s Almost Reversible Arrow

#63

Post by wildmusings »

effie wrote: post #57
trhway wrote: post #55

Earlier quoted context omitted.

>If you mix oil and water, you will see the result does unmix itself again. Does this mean entropy decreased and time direction got reversed? Nah. Moving an object to higher gravitational potential decreases the entropy of the system (and thus requires work, unsurprisingly). Mixing oil into water means you displace some volume of water which were closer to Earth with oil bubbles (which are lighter than equivalent vol…

When the fluids are mixed by a spoon, surely work has been done by the stirring spoon on the system and energy of the system increased. But this does not by itself imply entropy of the system increased. The reason is the system got to a non-equilibrium state which has no macroscopic thermodynamic description and no unique way to assign entropy and detect its changes.

Entropy always increases over time within a closed system, except in idealized interactions that leave entropy unchanged, which are not practically possible. From what we know, the only thing that might actually be a true closed system is the universe. If you've identified some system that has decreased in entropy, then you've failed to notice something outside of it that has increased in entropy by a greater amount.

Is it possible that our understanding of entropy is flawed? Yes. But unless you have some novel physics experiment or mathematical model to share with us, I'll stick with the second law of thermodynamics.

>But this does not by itself imply entropy of the system increased. The reason is the system got to a non-equilibrium state which has no macroscopic thermodynamic description and no unique way to assign entropy and detect its changes.

Entropy is the tendency to equilibrium. Your earlier statement, that entropy is only a measure of systems that are in equilibrium, is false. If you were right, entropy would be a totally meaningless notion because there are no closed systems and true equilibrium (as opposed to the near-equilibriums that we work with in practical terms) can only exist in one.

Re: Time’s Almost Reversible Arrow

#64

Post by d4rkph1b3r »

effie wrote: post #53
d4rkph1b3r wrote: post #50

Earlier quoted context omitted.

>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. Oh that is an interesting and compelling argument I have not heard! Thank you for sharing.

Both Boltzmann's idea of fluctuation and Feynman's argument against it are unfortunately devoid of meaning, because we do not know how highly organized and thermal death universe look like. We never saw and probably never will see another state of the universe and there is no way to experiment with it.

His argument is sound. If there is infinite #s of universes, or the universe keeps reforming infinitely (implication being it will have varying degrees of uniformity), then it's mathematically probable that we would be boltzman brain's in a less coherent universe. We don't observe that.

Re: Time’s Almost Reversible Arrow

#65

Post by Retric »

darkmighty wrote: post #29
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…

I am not sure what is meant by that. Evolution is not a purely random process nor is say star formation.

Re: Time’s Almost Reversible Arrow

#66

Post by dsmolovich »

prewett wrote: post #36
TeMPOraL wrote: post #31

Earlier quoted context omitted.

As others said, your understanding is fine - you just need to gain the appreciation to how much more unordered states are there for a system to be in than the ordered ones. My physics professor used to underscore it by drawing diagrams that show a line that suddenly goes from almost 0 to "the tip of this line is somewhere in the next galaxy". Or to give you a somewhat similar problem - consider a 32x32px 8bit image (…

The Digital Library of Babel ( https://libraryofbabel.info/ ) was posted here a while ago, which is a tangible way to search through this space. The library comes from a book by Jorge Luis Borges, which has all possible books, meaningful and not. The people in the book are searching for books with meaning, but the chance of finding one is really small. The digital library lets you do the search electronically. Kind o…

It's funny though as I had a very similar idea some years ago sticking much with virtual machines. I thought that if VM's disk image is just a file represented by sequence of zeroes and ones and what if someone would generate a final number of those images starting from 0...0 to 1...1 and then probe them all on a given emulated hardware what would be a chance to discover some viable/useful software? How would we detect it? How would we know or reverse engineer it to understand what it could be used for and how to use it, interact with it etc.?

So I ended up thinking that it would be a project similar to SETI but on a smaller scale (let's say if the image size would be limited to just 1Gb).

Re: Time’s Almost Reversible Arrow

#67

Post by bcbrown »

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…

> The real mystery, for me, is: Why was the initial state of our universe so completely non-chaotic?

Here's a possibility: if you "start" at a random state, after enough exploration of the state space, there would be a local minima that was arbitrarily well-ordered. Both directions of time would then view that as the "past", and the other direction as "future".

These perceptions as to the arrow of time would last until a state equivalent to heat death is reached, at which point the "arrow" of time would flip around again.

Re: Time’s Almost Reversible Arrow

#68

Post by [deleted] »

Retric wrote: post #65
darkmighty wrote: post #29

Earlier quoted context omitted.

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…

I am not sure what is meant by that. Evolution is not a purely random process nor is say star formation.

[deleted]

Re: Time’s Almost Reversible Arrow

#69

Post by darkmighty »

Retric wrote: post #65
darkmighty wrote: post #29

Earlier quoted context omitted.

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…

I am not sure what is meant by that. Evolution is not a purely random process nor is say star formation.

The context of those arguments is almost metaphysical. When you ask 'Is it more likely my consciousness is a local random fluctuation of some large system or the result of a one-off process?' you have to imagine you may be part of some abstract system that satisfies our notion of time and has rich enough dynamics to comport entropy and intelligence, but this universe could look nothing like ours, and you have to notice you are abusing probability, which is nothing more than a model of uncertainty and not an Oracle of reality.

But if you're willing to make some assumptions, Feynman's evidence for the latter seems valid.

Re: Time’s Almost Reversible Arrow

#70

Post by tbabb »

guscost wrote: post #45
tbabb wrote: post #39

Earlier quoted context omitted.

Pseudorandom means the choice is deterministically chosen by according to the state of hidden variables. Bell's theorem says there are no hidden variables that completely explain the predictions of quantum mechanics. A many-worlds interpretation (which is consistent with QM) is basically the same as saying the result is random, because you can't predict exactly which branch any particular interaction will follow.

No local hidden variables. Hugely important distinction.

Yes, see the bit in my original post and the one that follows about instantaneous communication.
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