The Moment Method in Relativistic Radiative Transfer

Abstract

The moment method of Grad is applied to the problem of radiative transfer in a medium with relativistic differential motions. If a mean absorption coefficient is used, the method readily leads to a closed system of equations. The first approximation gives the relativistic analog of the classical Eddington approximation. In the limit of small photon mean free path, the Eddington approximation does not reproduce Thomas's radiative-viscosity terms which were derived from the exact transfer equation. To recover Thomas's results it is necessary to go to the second approximation. It is suggested that this second approximation will be of use when scales of interest are not optically thick, although in general, such problems may be fairly complicated.