= Microscopic reversibility
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For an equilibrium process, a path and its physically time-reversed path have equal probability after including their equilibrium starting weights. Conditional on their endpoints, this means
$$
\frac{P_F}{P_B}=\exp[-(F_2-F_1)/(k_BT)].
$$
This is the path version of <detailed balance>. A time-even <order parameter> is read backward without changing sign, while a transport flux changes sign. The <Onsager--Machlup path probability> implements this reversal in a coarse-grained stochastic description.
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