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Time’s Almost Reversible Arrow

quantamagazine.org

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Re: Time’s Almost Reversible Arrow

#51

Post by effie »

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…

> 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 result unmix itself again, even though the interactions between the individual molecules are perfectly reversible.

Entropy does not always increase in time, because entropy is rarely a function of time. The most common meaning of the word is a thermodynamic quantity that is not, in general, defined in non-equilibrium states. It is a function of state variables rather than time.

What people are talking about when they say "entropy increases in time" is actually some variant of minus Boltzmann H-function. This kind of resembles thermodynamic entropy, but is a distinct and less general concept. There are processes where entropy gets higher but "entropy" (minus Boltzmann H-function) get lower.

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. It is not even clear how to define entropy of the system in between, since it is in highly non-equilibrium state.

Re: Time’s Almost Reversible Arrow

#52

Post by e12e »

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…

>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 result unmix itself again, even though the interactions between the individual molecules are perfectly reversible.

Perhaps in zero-g, under the effect of no acceleration and/or gravity. But I've seen all (most of) the non-water bits settle in the bottom of a bottle left alone for a long time.

That doesn't detract from your overall point about entropy, but it's probably to use an example based on the quality of energy (try to generate electricity from the heat that leeches from the walls of your cabin in winter).

Re: Time’s Almost Reversible Arrow

#53

Post by effie »

d4rkph1b3r wrote: post #50
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…

>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.

Re: Time’s Almost Reversible Arrow

#54

Post by jberryman »

nickpeterson wrote: post #43

This is an honest question, how are we so certain time exists? Does anyone have a few quick references to books or articles that would explain it to the layman? I always found the concept of time completely artificial, created by observation of cyclical systems.

"created by observation of cyclical systems". Can you elaborate on that? I'm not sure what you mean, but it sounds interesting. It seems to me we believe time "exists" because of causality; things that happen at "this moment" affect things that happen "in the future" but not things that happen "in the past". We've also predicted and measured certain weird things about time, such as how it can go faster or slower.

Re: Time’s Almost Reversible Arrow

#55

Post by trhway »

effie wrote: post #51
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…

> 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 result unmix itself again, even though the interactions between the individual molecules are perfectly reversible. Entropy does not always increase in time, because entropy is rarely a function of time. The most common meaning of the word is a therm…

>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 volume of water) while you force the displaced water volume higher/further from Earth. Thus by mixing lighter oil with heavier water you decrease the entropy of the gravitational system consisting of the oil, water and Earth. Left to its own devices, the system evolves along the entropy increasing gradient - heavy water goes down displacing lighter oil up, thus decreasing the gravitational potential of the system (the friction during that "unmixing" is the work of the gravitational force that ultimately results in the heat that is lost away into the environment).

Re: Time’s Almost Reversible Arrow

#56

Post by gizmo686 »

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 equations that govern quantum mechanics are time reversible, and describes a system of consisting of superimposed states.

The Copenhagen (traditional) interpenetration of quantum mechanics posits that when a system gets large enough [0] these superimposed states collapse down to a single state. This interpenetration does pose a problem with time reversibility, as if you run time backwards, then forwards again, it is not guaranteed that you will arrive in the same state after collapse.

A more modern interpenetration is Many Worlds. In this interpenetration, there is no collapse. So when we observe the cat, the universe is in a superposition of us having observed a dead cat, and us having observed a living cat. When we run time backwards, we end up at the point where we put the cat into the box, as you would expect. If we run time forwards again, we arrive at the same superposition of observing a living cat, and observing a dead cat.

Without looking at the math, the ability to go from a superimposed state to a pure state may seem like it presents an asymmetry, but the equations allow (and we have experimentally done) this in forward time already. As it turns out, the mechanism that allows for this to happen (interference) is the essence of quantum computation.

[0] The standard phrasing is is "gets measured". I read this as "gets large enough" because we could, in principle, consider the measurement device to be part of the system.

Re: Time’s Almost Reversible Arrow

#57

Post by effie »

trhway wrote: post #55
effie wrote: post #51

Earlier quoted context omitted.

> 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 result unmix itself again, even though the interactions between the individual molecules are perfectly reversible. Entropy does not always increase in time, because entropy is rarely a function of time. The most common meaning of the word is a therm…

>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.

Re: Time’s Almost Reversible Arrow

#58

Post by effie »

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?

If the model is Hamiltonian and the phase space finite, then any state will get eventually reapproached. For systems with infinite phase space (infinite volume suffices) or non-Hamiltonian systems, such conclusion may not be true.

Re: Time’s Almost Reversible Arrow

#59

Post by nickpeterson »

jberryman wrote: post #54
nickpeterson wrote: post #43

This is an honest question, how are we so certain time exists? Does anyone have a few quick references to books or articles that would explain it to the layman? I always found the concept of time completely artificial, created by observation of cyclical systems.

"created by observation of cyclical systems". Can you elaborate on that? I'm not sure what you mean, but it sounds interesting. It seems to me we believe time "exists" because of causality; things that happen at "this moment" affect things that happen "in the future" but not things that happen "in the past". We've also predicted and measured certain weird things about time, such as how it can go faster or slower.

So, you could condense my knowledge of physics to an index card, but here we go...time seems like it's always based on the measurement of a regular thing. Say the oscillation of a quartz crystal. So we have no proof there of time, just that certain phenomena can be very regular in intervals/cycles.

I've heard things like, "if you go the speed of light, or at least really fast, time will move differently." But that seems like it would be a side effect of that local system moving at a rate that interferes with its physical interaction relative to what we consider normal.

There are probably tons of holes in that. But my point is that we tend to think of time almost as an invisible "thing", and I really have trouble wrapping my brain around that. It just feels like things happen and we call it time, but really its ordered intervals of effects.

Re: Time’s Almost Reversible Arrow

#60

Post by jsprogrammer »

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…

Yes. This is a known result.

If you want to talk about "truly random", please say more.

Some theories rule out what you may be calling "truly random".

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