Radiative bulk viscosity

Viscous resistance to changes in the volume of a gas arises when different degrees of freedom have different relaxation times. Collisions tend to oppo se the resulting departures from equilibrium and, in so doing, generate ent ropy. Even for a classical gas of hard spheres, when the mean free paths or mean flight times of constituent particles are long we find a non-vanishin g bulk viscosity. Here we apply a method recently used to uncover this resu lt for a classical rarefied gas to radiative transfer theory, and derive an expression for the radiative stress tensor for a grey medium with absorpti on and Thomson scattering. We determine the transport coefficients through the calculation of the comoving entropy generation. When scattering dominat es absorption, the bulk viscosity may be much larger than either the shear viscosity or the thermal conductivity.