Einstein 1905: From “Energy quanta” to “Light quanta”

Abstract

“The energy of a light ray spreading out from a point source is not continuously distributed over an increasing space [wave theory of light] but consists of a finite number of energy quanta which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.” With these words Albert Einstein (1879-1955) introduced his “heuristic point of view toward the emission and transformation of light” which was presented in his first Annus Mirabilis paper published in 1905, a year that Einstein himself referred to as “very revolutionary”. Einstein introduced the concept of “light quanta”, an indivisible packet, although it was not until 1926 when the term “photon” (coined by Gilbert Lewis (1875-1946) in an article published in Nature) substituted Einstein’s “light quanta” forever.