Non-equilibrium Stefan-Boltzmann law

We study thermal radiation outside equilibrium. The situation we consider consists of two bodies emitting photons at two different temperatures. We show that the system evolves to a stationary state characterized by an energy current which satisfies a law similiar to the Stefan-Boltzmann law. The magnitude of this current depends on the temperature of the emitters expressed through the difference of the fourth power of these temperatures. The results obtained show how the classical laws governing the thermal radiation at equlibrium can be generalized away from equilibrium situations.