Shear and bulk viscosities of a photon gas at low temperature

TL;DR

This paper investigates the shear and bulk viscosities of a photon gas at low temperatures using quantum electrodynamics and effective field theory, revealing a very high shear viscosity and an extremely small bulk viscosity.

Contribution

It provides the first parametric estimates of photon gas viscosities highlighting the effects of scale invariance breaking on bulk viscosity.

Findings

Shear viscosity is very large.

Bulk viscosity is extremely small.

Scale invariance breaking influences bulk viscosity.

Abstract

We explore the viscosities of a photon gas by means of the Euler-Heisenberg effective theory and quantum electrodynamics at zero electron chemical potential. We find parametric estimates that show a very large shear viscosity and an extremely small bulk viscosity (reflecting the very weak coupling simultaneously with a very approximate dilatation invariance). The system is of some interest because it exemplifies very neatly the influence of the breaking of scale invariance on the bulk viscosity.