Causal UV completions of relativistic hydrodynamics

TL;DR

This paper demonstrates that relativistic hydrodynamics must include transient UV modes to be causal and explores how these modes emerge and are constrained within causal UV completions.

Contribution

It proves the inherent acausality of standalone hydrodynamic EFTs and introduces a framework for their causal UV completions with transient modes.

Findings

Hydrodynamic EFTs are inherently acausal without UV modes.

Exponential decay of dissipative hydrodynamics enables causal descriptions.

Constraints on non-hydrodynamic modes in UV completions.

Abstract

Relativistic hydrodynamics successfully provides an effective field theory description for the low energy regime of many out-of-equilibrium systems. On the other hand, in this paper we proof that any stand-alone hydrodynamic EFT is inherently acausal and therefore requires the addition of transient UV modes in order to restore causality. This is made possible by the exponential decay of dissipative hydrodynamics in a majority of the lightcone, allowing the possibility of a causal description that still reduces to the hydrodynamic one at late timescales. We then investigate the emergence and possible restrictions of the non-hydrodynamic modes in these causal UV completions.