Thermodynamically Reversible Processes in Statistical Physics (Extended Version)
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Equilibrium states are used as limit states to define thermodynamically reversible processes. When these processes are implemented in statistical physics, these limit states become unstable and can change with time, due to thermal fluctuations. For macroscopic systems, the changes are insignificant on ordinary time scales and what little there is can be suppressed by macroscopically negligible, entropy-creating dissipation. For systems of molecular sizes, the changes are large on short time scales and can only sometimes be suppressed with significant entropy-creating dissipation. As a result, at molecular scales, thermodynamically reversible processes are impossible in principle, even as approximations, when we account for all sources of dissipation.