Long-range correlation of thermal radiation

A general theory is presented for the spatial correlations in the intensity of the radiation emitted by a random medium in thermal equilibrium. We fin d that a nonzero correlation persists over large distances, compared to the transverse coherence length of the thermal radiation. This long-range corr elation vanishes in the limit of an ideal black body. We analyze two types of systems (a disordered waveguide and an optical cavity with chaotic scatt ering), in which it should be observable. [S1050-2947(99)50501-5].