Planck’s and Einstein’s Pathways to Quantization

Planck’s and Einstein’s steps toward quantization are discussed, including a historical comparison of these two very different thinkers, their motives and heuristics. Sect. 2.2 studies Albert Einstein’s arguments up to the 1905 paper and how the many important publications from this annus mirabilis and shortly afterwards until 1909 are interconnected. Max Planck’s second quantum theory 1909–13 serves as a contrast: with it Planck attempts to retract his hesitantly introduced quantization of energy from 1900 by blaming it on the material resonators inside an idealized radiator at equilibrium (a ‘black body’) and thereby uphold the theory of a continuous radiation field satisfying Maxwell’s equations. Other papers by Einstein banished this interpretative option by showing that Planck’s formula for the distribution of radiation already presupposes quantized energy in the radiation field itself. Sect. 2.5 discusses the variety of terms used by Einstein and his contemporaries during this work-in-progress. Most of the alternatives considered then, such as ‘light corpuscles’ or ‘energy projectiles’, have been forgotten since. Sect. 2.6 statistically retraces the slow rise of the term ’photon’ in common usage generally attributed to the physical chemist Gilbert N. Lewis (1926).