Restoring time-reversal covariance in relaxed hydrodynamics

Abstract

In hydrodynamics, for generic relaxations, the stress tensor and $U(1)$ charge current two-point functions are not time-reversal covariant. This remains true even if the Martin-Kadanoff procedure happens to yield Onsager reciprocal correlators. We consider linearised relativistic hydrodynamics on Minkowski space in the presence of energy, $U(1)$ charge and momentum relaxation. We then show how one can find the minimal relaxed hydrodynamic framework that does yield two-point functions consistent with time-reversal covariance. We claim the same approach naturally applies to boost agnostic hydrodynamics and its limits (e.g. Carrollian, Galilean and Lifshitz fluids).