Intrinsic ambiguity in second order viscosity parameters in relativistic hydrodynamics

TL;DR

This paper reveals an inherent ambiguity in second order viscosity parameters within relativistic hydrodynamics, showing that some parameters can be eliminated or hiddenly conformal, affecting the interpretation of hydrodynamic theories.

Contribution

It demonstrates the intrinsic ambiguity in second order viscosity parameters and how certain theories can be reinterpreted as conformal due to this ambiguity.

Findings

Two viscosity parameters can be removed via energy-momentum tensor improvements

Hidden conformal invariance can exist in seemingly non-conformal hydrodynamics

Ambiguity affects the classification of hydrodynamic theories

Abstract

We show that relativistic hydrodynamics in Minkowski space-time has intrinsic ambiguity in second order viscosity parameters in the Landau-Lifshitz frame. This stems from the possibility of improvements of energy-momentum tensor. There exist at least two viscosity parameters which can be removed by using this ambiguity in scale invariant hydrodynamics in (1+3) dimension, and seemingly non-conformal hydrodynamic theories can be hiddenly conformal invariant.