Tuesday, 13 May 2014

Is ergodicity a reasonable hypothesis? Understanding Boltzmann's ergodic hypothesis

Ergodic vs. non-ergodic
trajectories (Wikipedia)
Many undergraduate Physics students barely study Ergodic Hypothesis in detail. It is usually manifested as ensemble averages being equal to time averages. While the concept of the statistical ensemble maybe accessible to students, when it comes to ergodic theory and theorems,  where higher level mathematical jargon kicks in, it maybe confusing for the novice reader or even practicing Physicists and educator what does ergodicity really mean. For example recent pre-print titled "Is ergodicity a reasonable hypothesis?" defines the ergodicity as follows:
...In the physics literature "ergodicity" is taken to mean that a system, including a macroscopic one, visits all microscopic states in a relatively short time...[link]
Visiting all microscopic states is not a pre-condition for ergodicity from statistical physics stand point. This form of the theory is the manifestation of strong ergodic hypothesis because of the Birkhoff theorem and may not reflect the physical meaning of ergodicity.  However,  the originator of ergodic hypothesis,  Boltzmann, had a different thing in mind in explaining how a system approaches to thermodynamic equilibrium. One of the best explanations are given in the book of J. R. Dorfman, titled An introduction to Chaos and Nonequilibrium Statistical Mechanics [link], in section 1.3, Dorfman explains what Boltzmann had in mind:
...Boltzmann then made the hypothesis that a mechanical system's trajectory in phase-space will spend equal times in regions of equal phase-space measure. If this is true, then any dynamical system will spend most of its time in phase-space region where the values of the interesting macroscopic properties are extremely close to the equilibrium values...[link]
Saying this, Boltzmann did not suggest that a system should visit ALL microscopic states.  His argument only suggests that only states which are close the equilibrium has more likelihood to be visited.

Postscript (June 2022)

The sufficiency of Sparse Visits: Physical states are rarely fine-grained

A requirement for attaining ergodicity is visiting all possible states or regions due to the ergodic theorems of Birkhoff and von Neumann. This requirement is not correct for Physics. The key concepts here are coarse-graining and the sufficiency of sparse visits. Most of the physical systems have equally likely states.

The generated dynamics would rarely need to visit all accessible states or regions. Physical systems are rarely fine-grained and have a degree of sparseness, reducing their astronomically large number of states to a handful. In summary, visiting all physical states or regions in time averages is not strictly needed for the physics definition of ergodicity.

A collection of regions or multiple states with a higher probability will need to be covered to achieve thermodynamic equilibrium. A concept of “sufficiency of sparse visits”. This approach makes physical experiments possible over a finite time consistent with thermodynamics.

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