Corpuscles of Light
In the following paper, translated from German by Arons and Peppard in 1965, Einstein extends Planck’s quantum concept, claiming that not only the emission and absorption of radiant energy, but even light itself is quantized. Essentially, Einstein treats light not as a classical electromagnetic wave, but rather as a gas of corpuscles, each of which carries a discrete amount of energy that depends on its wavelength. At the outset of the reading selection, Einstein introduces his paradoxical new concept of light. Then in the next several sections, Einstein motivates and explains this concept by applying Boltzmann’s principle of entropy to a gas of light corpuscles. These sections are rather technical, so feel free to skim over them for now; you might come back and re-read these after having studied a bit of quantum theory, variational calculus and statistical mechanics. In the mean time, be sure to carefully study Sect. 16.2.8, wherein Einstein describes how his concept of the light quantum may be used to understand the photoelectric effect. This curious emission of cathode rays (electrons) from illuminated metal bodies had been observed and noted by Heinrich Hertz in the 1880’s. Einstein’s novel theory of the photoelectric effect was originally published in 1905; it earned him a Nobel Prize in physics 16 year later.