An Unintentional Consequence: The Quantum in the Framework of the 19th Century Physics

There is an ingrained tendency to see the emergence of the quantum hypothesis almost exclusively from the human side. The deep break with the pre-quantum physics has been often described in terms of an “act of desperation” (echoing Planck’s famous expression in a letter to Robert Wood) and Planck himself has been declared an “unwilling revolutionary”. While these metaphors correctly stress the element of contingency intrinsic in this story, they run the risk to underestimate the structural features of the 19th century physics that prepared the discovery of the quantum. In this paper, I argue that the quantum was the, admittedly unintentional, consequence of two major features. On the one hand, the fin-de-siècle physics provided Planck with a number of conceptual and formal resources, primarily the Fourier series and combinatorial techniques, which he creatively adapted to his approach to radiation theory. On the other, thermodynamics, statistical mechanics, and electromagnetic theory imposed a series of constraints through which Planck had to navigate in order to construct a consistent theory. Ultimately, it was the extreme difficulty of integrating these multiple elements that made the introduction of the quantum a desperate measure.