Nonequilibrium fluctuation theorems from equilibrium fluctuations

A system can be driven to nonequilibrium behavior in the presence of thermodynamic imbalances in its environment, such as temperature or chemical potential differences. In derivations of far-from-equilibrium fluctuation theorems, such imbalances are typically modeled as fixed, externally imposed thermodynamic forces. Here, we argue that fluctuation theorems can instead be understood in terms of the equilibrium dynamics of a larger supersystem, containing both the system of interest and its thermal surroundings. To this end, we consider rare fluctuations that spontaneously produce imbalances in the surroundings. In the aftermath of such fluctuations, the system of interest transiently behaves as though it were in the presence of an externally applied thermodynamic force. By applying the principle of detailed balance to these rare events, we recover the fluctuation theorem in both its transient and steady-state formulations.