Small systems with fluctuations appear to show that the second law of thermodynamics may be split into two contributions.

Small systems are continuously subjected to fluctuations from their environment and must be treated statistically. Recently discovered fluctuation theorems indicate that trajectories producing a given dissipation are exponentially more likely than those for processes of equal magnitude but opposite sign.

Writing in Physical Review Letters, Massimiliano Esposito of Université Libre de Bruxelles and Christian Van den Broeck of Hasselt University, both in Belgium, combine an abstract approach with a physical model to provide deeper insight into the uniformity behind fluctuation theorems. For a system driven by either nonequilibrium boundary conditions or an external time-dependent driving force, they find three fluctuation theorems—one for the total entropy production and one each for the two contributions into which this is resolved (adiabatic and nonadiabatic).

These results seem to suggest that the second law could be split into two parts. Possible consequences for the symmetries of the response coefficients or efficiencies of engines remain to be explored. – Yonko Millev

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