  Using the effective Lagrangian for the low energy photon-photon interaction
the lowest order photon self energy at finite temperature and in
non-equilibrium is calculated within the real time formalism. The Debye mass,
the dispersion relation, the dielectric tensor, and the velocity of light
following from the photon self energy are discussed. As an application we
consider the interaction of photons with the cosmic microwave background
radiation.
