Productive conservatism: Planck's second theory of radiation

Abstract

Since the first months of 1910, Max Planck began to think that his theory of radiation developed in 1900, albeit confirmed by all experimental measurements, was “plagued [. . . ] by a remarkable flaw”.1 After two years of intense work, changes of tack, doubts, and rethinking, Planck completed his second theory of radiation, which was published in a mature form on the pages of Annalen der Physik exactly one hundred years ago.2 In the introduction to the paper, Planck summarized the requirements that a satisfactory theory of radiation should fulfill: it must be (1) free of internal contradictions and (2) as close to the core of classical electrodynamics as allowed by the necessity of the quantum hypothesis. As Planck’s 1900 theory is mostly valid still today and it is at the very foundations of quantum physics, the modern reader cannot help being baffled by Planck’s effort. To understand why his celebrated first theory did not comply the requirements given above, we have to move a step backwards.