A panoply of Schwinger-Keldysh transport

TL;DR

This paper classifies all possible relativistic fluid transport relations using Schwinger-Keldysh effective action, revealing new constraints from unitarity that go beyond classical hydrodynamics.

Contribution

It introduces Schwinger-Keldysh positivity constraints, a novel set of conditions on transport coefficients derived from microscopic unitarity.

Findings

Identification of new unitarity-based constraints on transport coefficients

Explicit examples of allowed constitutive relations

Demonstration that classical laws are insufficient to capture all constraints

Abstract

We classify all possible allowed constitutive relations of relativistic fluids in a statistical mechanical limit using the Schwinger-Keldysh effective action for hydrodynamics. We find that microscopic unitarity enforces genuinely new constraints on the allowed transport coefficients that are invisible in the classical hydrodynamic description; they are not implied by the second law or the Onsager relations. We term these conditions Schwinger-Keldysh positivity and provide explicit examples of the various allowed terms.