Autocorrelation functions of the blackbody radiation in cavities with walls of finite conductivity

Electromagnetic equilibrium fluctuations in cavities with non ideal metallic walls are considered. On the boundary the electric and magnetic fields are connected by Leontovich's boundary condition. With the aid of Maxwell's equations and Leontovich's relation the response of the EM-field to a polarization field is calculated. The correlation functions of the EM-field are then given by the fluctuation dissipation theorem. For cube shaped cavities explicit considerations are made. Expressing the surface impedance by a frequency independent conductivity the volume averaged autocorrelation functions of the EM-field and the total energy density are calculated. Besides the contributions due to the normal modes an additional term is found which characterizes the contributions due to the evanescent waves existing near the internal surface of the cavity. As a consequence of the frequency dependence of the impedance this term shows aT^7/2 dependence.