CLASSICAL PHYSICS AND EARLY QUANTUM THEORY: A LEGITIMATE CASE OF THEORETICAL UNDERDETERMINATION

In 1912, Henri Poincaré published an argument which apparently shows that the hypothesis of quanta is both necessary and sufficient for the truth of Planck's experimentally corroborated law describing the spectral distribution of radiant energy in a black body. In a recent paper, John Norton has reaffirmed the authority of Poincarés argument, setting it up as a paradigm case in which empirical data can be used to definitively rule out theoretical competitors to a given theoretical hypothesis. My goal is to dispute Norton's claim that there is no theoretical underdetermination problem arising between classical physics and early quantum theory. The strategy I use in defending my view is to adopt a suggestion made by Jarrett Leplin and Larry Laudan on how to assess the relative merits of competing theoretical alternatives, where each alternative has an equal capacity to ‘save the phenomena’. In the course of the paper, I distinguish between two branches of classical physics: classical mechanics and classical electromagnetism. The former is claimed by Norton and Poincaré to be determinately ruled out by the black body evidence; and it is the former that I argue is compatible with this evidence.