Photon mass and blackbody radiation

Electromagnetic radiation in a cavity is used to investigate the question of the photon mass. Bounds on this quantity are calculated from the spectrum and the radiance obtained under the assumption of a nonzero mass for light, and they turn out to be several orders of magnitude worse than the limits from nonthermal measurements. Considering the correction to the Planck formula due to the size and shape of the cavity, one is led to the conclusion that thermal methods cannot improve on already existing bounds: This would either require enormous cavities or unreasonably small errors in measurements of the cosmic background radiation.