  An isotropic electromagnetic field in a plasma of thermalized electrons
undergoes changes in energy as a result of Compton scattering and an
Einstein-Hopf drag force on the electrons, eventually approaching a
Bose-Einstein photon distribution at the electron temperature. The rate of
change of feld energy due to the combined effects of Compton scattering and the
drag force is shown to be described by the Kompaneets equation for photon
diffusion in frequency space. A similarity is noted between this approach and
Einstein's derivation of the Planck spectrum based on the recoil of atoms as
they absorb and emit radiation.
