Understanding Poincare's recurrence theorem

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    Recurrence Theorem
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Discussion Overview

The discussion centers around Poincare's recurrence theorem, particularly its implications for the behavior of particles in a closed system and its relationship with the second law of thermodynamics. Participants explore the counter-intuitive nature of the theorem and its potential conflicts with established thermodynamic principles.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants express confusion over the implications of Poincare's recurrence theorem, particularly regarding the expectation that particles would continue to move randomly without returning to their original state.
  • It is suggested that particles will eventually return to a similar state by chance, although this could take an extraordinarily long time, potentially exceeding the age of the Universe.
  • Concerns are raised about whether this implies a breakdown of the second law of thermodynamics, with some arguing that the theorem suggests a return to a state of zero change in entropy (ΔS = 0).
  • Others clarify that thermodynamics deals with statistical properties and macro states, which may not apply to deterministic scenarios as described by the theorem.
  • Participants discuss the calculation of the time it takes for particles to return to their original state, noting that it depends on the desired proximity to the original state in phase space.
  • One participant mentions that while the laws of thermodynamics could theoretically break down, such occurrences are extremely rare in practice.
  • A question is raised regarding the meaning of the term "not fundamental" in relation to the laws of thermodynamics.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the implications of Poincare's recurrence theorem on the second law of thermodynamics, with some asserting that it does not violate the law while others remain unconvinced. The discussion remains unresolved as differing interpretations and clarifications are presented.

Contextual Notes

Participants note that the time required for recurrence can be extraordinarily long, which may influence interpretations of thermodynamic laws. The discussion also highlights the dependence on definitions and assumptions related to statistical mechanics and phase space.

JD_PM
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The Poincare's recurrence theorem :

PoincareTheorem.png


This theorem implies the following:

Suppose a container is divided in two by a wall. Half of it contains particles and the other none. If you were to remove the wall and wait a very very long time, the particles would eventually be found in the same half of the container.

To me this statement is counter-intuitive. I would expect the particles to jiggle around forever.

Why am I wrong?

I have read the proof but I would rather discuss the theorem to understand it.
 
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JD_PM said:
Summary:: I want to understand the idea of Poincare's recurrence theorem

To me this statement is counter-intuitive. I would expect the particles to jiggle around forever.
They ”jiggle around” randomly forever. This means that they will eventually come back to the original state (or very close to it) by pure chance. The time for this to happen can be orders of magnitude longer than the age of the Universe.
 
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Thanks for your answer.

Orodruin said:
This means that they will eventually come back to the original state (or very close to it) by pure chance.

Doesn't this mean that the second law of Thermodynamics breaks down? Theoretically, if all particles eventually come back to the original state means that ##\Delta S =0##. This is actually what shocks me about this theorem.

Orodruin said:
The time for this to happen can be orders of magnitude longer than the age of the Universe.

Actually my book justifies that the second law is not violated because very long time means ##10^{20}## years.

To me that is not a good reason because, in the end we end up with ##\Delta S =0## so we still have the same issue.

By the way, how is this time calculated?
 
JD_PM said:
Doesn't this mean that the second law of Thermodynamics breaks down?
No. Thermodynamics is about the statistical properties of the system and the resulting macro states. It is not describing the fully deterministic situation. Looking at statistical mechanics, you have the different statistical ensembles that will allow you to derive the thermodynamic properties of the system.

JD_PM said:
By the way, how is this time calculated?
This depends on how close you want to get to the original state in phase space. Take a characteristic time to move out of that volume and multiply by the ratio of the total phase space volume accessible and the phase space volume you want to move back to. This gives you an upper estimate.
 
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JD_PM said:
Doesn't this mean that the second law of Thermodynamics breaks down?

The laws of thermodynamics are not fundamental, and they can break down in theory, but these are so rare that there is not any breakdown in practice.
 
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atyy said:
The laws of thermodynamics are not fundamental

What do you mean saying 'not fundamental'?
 

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