The Kac Ring Model

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The Kac ring is a simple, explicitly solvable model that illustrates the process of passing from microscopic, time-reversible behavior to macroscopic, thermodynamic behavior [1]. In this model, sites are arranged around a circle, forming a one-dimensional periodic lattice. Each site is occupied by either a black ball or a white ball. A fraction
of the bonds joining neighboring sites have a "tunnel." From time
to time
each ball moves to the clockwise neighboring site, changing its color if the ball crosses a tunnel.
Contributed by: Daniel Díaz Simón and Andrés Santos (October 2016)
Open content licensed under CC BY-NC-SA
Snapshots
Details
In this model, a "microstate" at time is characterized by
quantities: the color (black or white) of balls sitting on each of the sites. A "macrostate" at time
is characterized by a single coarse-grained quantity: the difference
between the numbers of white and black balls, relative to the total number
. The microscopic dynamics are reversible [2] and recurrent [3] (the microstate repeats itself after
time steps). However, under a molecular chaos assumption (the fraction of white balls just about to cross a tunnel is assumed to be
at any time) [4], the irreversible evolution equation
is expected to be correct in the limit
.
In this Demonstration, all the balls are white initially. You can control the total number of sites , the fraction of bonds with a tunnel
and the time range to display. You can choose to follow the temporal evolution of the macrostate or the microstate.
References
[1] G. A. Gottwald and M. Oliver, "Boltzmann's Dilemma: An Introduction to Statistical Mechanics via the Kac Ring," SIAM Review, 51(3), 2009 pp. 613–635. doi:10.1137/070705799.
[2] Wikipedia. "Loschmidt's Paradox." (Oct 3, 2016) en.wikipedia.org/wiki/Loschmidt's_paradox.
[3] Wikipedia. "Poincaré Recurrence Theorem." (Oct 3, 2016) en.wikipedia.org/wiki/Poincaré_recurrence_theorem.
[4] Wikipedia. "Molecular Chaos." (Oct 3, 2016) en.wikipedia.org/wiki/Molecular_chaos.
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