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We explore the viscosities of a photon gas by means of the Euler-Heisenbergeffective theory and quantum electrodynamics at zero electron chemicalpotential. We find parametric estimates that show a very large shear viscosityand an extremely small bulk viscosity (reflecting the very weak couplingsimultaneously with a very approximate dilatation invariance). The system is ofsome interest because it exemplifies very neatly the influence of the breakingof scale invariance on the bulk viscosity.

Shear and bulk viscosities of a photon gas at low temperature