Planck's radiation law: a many body theory perspective

An exposition of Planck's Law of Radiation is presented within the context of modern quantum many body theory. In particular the generality of the Planck radiation law is demonstrated to be Valid to all orders of perturbation theory for an interacting system, regardless of the underlying statistics for the particles comprising the matter field and the precise nature of the interaction mechanism. This exposition justifies the use of Planck's law with minor modifications via the refractive index, in the optical properties of semiconductors and insulators. We conclude by clarifying when Planck's law is likely to fail.