Why hyperbolic theories of dissipation cannot be ignored: Comments on a paper by Kostadt and Liu

TL;DR

This paper argues that hyperbolic theories of dissipation are experimentally distinguishable from parabolic ones and that non-negligible relaxation times do not mean the system is outside hydrodynamics.

Contribution

It clarifies the experimental distinguishability of hyperbolic and parabolic dissipation theories and emphasizes the relevance of relaxation times within hydrodynamic regimes.

Findings

Experiments can differentiate hyperbolic from parabolic dissipation theories.

Non-negligible relaxation times do not exclude hydrodynamic behavior.

Hyperbolic theories are relevant and should not be ignored in dissipation studies.

Abstract

Contrary to what is asserted in a recent paper by Kostadt and Liu ("Causality and stability of the relativistic diffusion equation"), experiments can tell apart (and in fact do) hyperbolic theories from parabolic theories of dissipation. It is stressed that the existence of a non--negligible relaxation time does not imply for the system to be out of the hydrodynamic regime.