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Time’s Almost Reversible Arrow

quantamagazine.org

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Re: Time’s Almost Reversible Arrow

#71

Post by tyho »

charlieflowers wrote: post #42
blaze33 wrote: post #12

Earlier quoted context omitted.

As I understand it, the universe actually started in a heat-death state (very small temperature differences, homogeneous and isotropic), it's just that it is a very hot heat death state, not the usual cold one where everything freezes. Then it is the universe expansion that made the entropy per volume decrease, actually allowing complex systems to evolve. The expansion still has to occur faster than the entropy incre…

Or whole universe was in a hot dense state. Then, nearly 14 million years ago, expansion started. Wait ... The earth began to cool.

*billion

Re: Time’s Almost Reversible Arrow

#72

Post by wildmusings »

wildmusings wrote: post #63
effie wrote: post #57

Earlier quoted context omitted.

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.…

My response here is actually overly-simplistic and somewhat wrong. effie is right about classical thermodynamics only defining a measure of entropy (and various other properties of a system) for idealized states of equilibrium. There's been some attempts to define entropy for non-equilibrium systems, but our understanding is incomplete. It's partially a philosophical debate too.

Re: Time’s Almost Reversible Arrow

#73

Post by effie »

d4rkph1b3r wrote: post #64
effie wrote: post #53

Earlier quoted context omitted.

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.

We don't know whether what we observe is more coherent than the majority of universes, because we have never observed other kind of universe. The very fluctuation hypothesis and the argument against it have no solid basis in science.

Re: Time’s Almost Reversible Arrow

#74

Post by effie »

wildmusings wrote: post #72
wildmusings wrote: post #63

Earlier quoted context omitted.

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.…

My response here is actually overly-simplistic and somewhat wrong. effie is right about classical thermodynamics only defining a measure of entropy (and various other properties of a system) for idealized states of equilibrium. There's been some attempts to define entropy for non-equilibrium systems, but our understanding is incomplete. It's partially a philosophical debate too.

Admittedly, the concept of entropy density has been used somewhat successfully for description of continuum close to equilibrium. But there are problems with extending this to general non-equilibrium case. I think a mix of oil with water is rather awkward to describe thermodynamically, because it is unstable and it is hard to introduce thermodynamic description for a fluid consisting of many drops of oil of different sizes scattered in water. Second law of thermodynamics can be phrased in terms of entropy, but it does not really involve time at all. All it says is after process involving no heat transfer is finished, entropy of final equilibrium state is not lower than entropy of initial equilibrium state.

Re: Time’s Almost Reversible Arrow

#75

Post by effie »

trhway wrote: post #62
effie wrote: post #57

Earlier quoted context omitted.

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 t…

Of course, gravitational potential energy of the system is higher when the oil is scattered temporarily in the water, because the water is higher than it would be in equilibrium state. However, I do not see how this has anything to do with the concept of entropy. The non-equilibrium state cannot be easily assigned thermodynamic entropy, because there is no obvious thermodynamic state variable that could describe the state of mixed water and oil. If you know how to introduce such a variable and corresponding entropy, then please explain it.

Re: Time’s Almost Reversible Arrow

#76

Post by charlieflowers »

tyho wrote: post #71
charlieflowers wrote: post #42

Earlier quoted context omitted.

Or whole universe was in a hot dense state. Then, nearly 14 million years ago, expansion started. Wait ... The earth began to cool.

*billion

Oh crap, that typo makes me sound like a Young Earth crackpot :)

Re: Time’s Almost Reversible Arrow

#77

Post by trhway »

effie wrote: post #75
trhway wrote: post #62

Earlier quoted context omitted.

>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 t…

Of course, gravitational potential energy of the system is higher when the oil is scattered temporarily in the water, because the water is higher than it would be in equilibrium state. However, I do not see how this has anything to do with the concept of entropy. The non-equilibrium state cannot be easily assigned thermodynamic entropy, because there is no obvious thermodynamic state variable that could describe the…

>However, I do not see how this has anything to do with the concept of entropy.

in short - entropy of the system with lower gravitational potential is higher (https://en.wikipedia.org/wiki/Entropic_gravity). When oil is mixed into water, the thermodynamic entropy "St,mix" of oil/water system is higher than "St,unmixed" while the entropy "Sg,mix" of the gravitational system of oil/water/Earth is lower when mixed than the "Sg,unmixed" of the unmixed case. In case of bad solubility, like oil in water, we have ["St,mix" - "St,unmixed" < "Sg,unmixed" - "Sg,mix"] and thus system on its own will evolve toward unmixed state.

Re: Time’s Almost Reversible Arrow

#78

Post by effie »

trhway wrote: post #77
effie wrote: post #75

Earlier quoted context omitted.

Of course, gravitational potential energy of the system is higher when the oil is scattered temporarily in the water, because the water is higher than it would be in equilibrium state. However, I do not see how this has anything to do with the concept of entropy. The non-equilibrium state cannot be easily assigned thermodynamic entropy, because there is no obvious thermodynamic state variable that could describe the…

>However, I do not see how this has anything to do with the concept of entropy. in short - entropy of the system with lower gravitational potential is higher ( https://en.wikipedia.org/wiki/Entropic_gravity ). When oil is mixed into water, the thermodynamic entropy "St,mix" of oil/water system is higher than "St,unmixed" while the entropy "Sg,mix" of the gravitational system of oil/water/Earth is lower when mixed tha…

Entropic explanation of gravity force is not an accepted physics theory. Regardless of whether it is right or not, the concept of entropy in common thermodynamics I was referring to above is different from that in entropic gravity. In thermodynamics, in contrast with theories of entropic forces, entropy is assigned only to states of thermodynamic equilibrium that can be achieved by exchanging heat reversibly. State that is not a state of mechanical equilibrium (such as state of accelerating bodies, unmixing water+oil) is not such a state.

Re: Time’s Almost Reversible Arrow

#79

Post by trhway »

effie wrote: post #78
trhway wrote: post #77

Earlier quoted context omitted.

>However, I do not see how this has anything to do with the concept of entropy. in short - entropy of the system with lower gravitational potential is higher ( https://en.wikipedia.org/wiki/Entropic_gravity ). When oil is mixed into water, the thermodynamic entropy "St,mix" of oil/water system is higher than "St,unmixed" while the entropy "Sg,mix" of the gravitational system of oil/water/Earth is lower when mixed tha…

Entropic explanation of gravity force is not an accepted physics theory. Regardless of whether it is right or not, the concept of entropy in common thermodynamics I was referring to above is different from that in entropic gravity. In thermodynamics, in contrast with theories of entropic forces, entropy is assigned only to states of thermodynamic equilibrium that can be achieved by exchanging heat reversibly. State t…

the thermodynamics (and specifically the notion of "heat") is just statistical/aggregate description of the interaction between the forces which are subject to the same entropic description like the entropic gravity and obey the same 2nd law. It is not surprising once you see that "minimal action principle" is just the 2nd law ("entropy evolves along the gradient") expressed using the language of Lagrangian in mechanics. Thus all the known forces are subject to it, ie. in addition to gravity electromagnetic forces is subject to the same, and the Schrodinger too. That allows to calculate and use entropy for the whole system along its whole evolution path.

Re: Time’s Almost Reversible Arrow

#80

Post by effie »

trhway wrote: post #79
effie wrote: post #78

Earlier quoted context omitted.

Entropic explanation of gravity force is not an accepted physics theory. Regardless of whether it is right or not, the concept of entropy in common thermodynamics I was referring to above is different from that in entropic gravity. In thermodynamics, in contrast with theories of entropic forces, entropy is assigned only to states of thermodynamic equilibrium that can be achieved by exchanging heat reversibly. State t…

the thermodynamics (and specifically the notion of "heat") is just statistical/aggregate description of the interaction between the forces which are subject to the same entropic description like the entropic gravity and obey the same 2nd law. It is not surprising once you see that "minimal action principle" is just the 2nd law ("entropy evolves along the gradient") expressed using the language of Lagrangian in mechan…

Your statements are incompatible with classical physics developed for centuries and enjoying high level of acceptance. The concept of thermodynamic entropy brings nothing to understanding of mechanics. There is no time in thermodynamic theory, thermodynamic entropy is not a function of time so there is no possibility of deriving equations of motion of mechanics from the laws of thermodynamics. You may need to study mechanics and thermodynamics and their history from several books before you realize that.
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