We argue that entropy production in hydrodynamics can be understood via a superspace inflow mechanism. Our arguments are based on a recently developed formalism for constructing effective actions for Schwinger-Keldysh observables in quantum field theories. The formalism explicitly incorporates microscopic unitarity and the Kubo-Martin-Schwinger thermal periodicity conditions, by recasting them in terms of topological Becchi-Rouet-Stora-Tyutin symmetries of the effective action.

• Received 6 April 2018
• Revised 29 June 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.051602

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Published by the American Physical Society