The comprehensive theory of light

TL;DR

This paper reviews the historical development of light theories from Newton to quantum mechanics, emphasizing the unification of classical and quantum descriptions through Sudarshan's diagonal representation.

Contribution

It highlights the comprehensive framework that unifies classical and quantum light descriptions, resolving longstanding conceptual schisms.

Findings

Classical states of light are subsumed within the quantum description.

Sudarshan's diagonal representation provides a complete formalism for light.

Classical and quantum light theories are fundamentally compatible.

Abstract

Modern attempts to understand light go back to Newton who considered light to be particles, the so called corpuscular theory, and the other school of Huygens, Young and others. Huygens and Young viewpoint emphasised the wave property. This difference of opinions persisted for close to two centuries till Maxwell theory solidly established light as a wave phenomenon associated with Electromagnetism. A serious schism so to speak was introduced into the theory of light with the understanding of "Light gas", the so called Black Body radiation. Einstein's photon hypothesis and Bose's successful derivation of the Planck spectrum put the corpuscular theory back in place, while classical description continues to be used as well. One may then wonder if this being a classical limit may have to be corrected when dealing with quantum phenomena. To most people's surprise, the classical description was in fact a subset of the full quantum description, and the classical states of light could be shown to be subsumed within the fully quantum description without having to take the limit of Planck's constant h going to zero. This was the resolution provided by E. C. G. Sudarshan's Diagonal representation as the most general formalism for dealing with light.