Prologue

The development of quantum mechanics in the years 1925 and 1926 had produced rules for the description of systems of microscopic particles, which involved promoting the fundamental dynamical variables of a corresponding classical system into operators with specified commutators. By this means, a system, described initially in classical particle language, acquires characteristics associated with the complementary classical wave picture. It was also known that electromagnetic radiation contained in an enclosure, when considered as a classical dynamical system, was equivalent energetically to a denumerably infinite number of harmonic oscillators. With the application of the quantization process to these fictitious oscillators, the classical radiation field assumed characteristics describable in the complementary classical particle language. The ensuing theory of light quantum emission and absorption by atomic systems marked the beginning of quantum electrodynamics…