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Doctoral studies > PhD dissertations UAB

Xavier Roqué
D'aplicacions marginals a paradigmes. Les fórmules de Klein-Nishina, Møller i Bhabha en els inicis de l'electrodinàmica quàntica (1928-1947)
Supervisor/s: Manuel García Doncel
Date of defense: 30-06-1993

Quantum electrodynamics (QED) faced severe problems for almost twenty years after its inception in the mid-twenties. We have some historical understanding of the theoretical developments that made QED worth pursuing, but almost none of the role experiment played in the face of the theoretical drawbacks. Some of the elementary processes of QED were successfully approached during the early stage of the theory. Three of them, Compton, Møller and Bhabha scattering, are properly considered today as paradigmatic applications of the theory. Yet, were they regarded thus in its early period, as has been assumed? This thesis consists in a historical analysis of the original condition of the scattering processes.

The thesis is in two parts and an epilogue. In Part I, the original deduction of the scattering formulas is analysed with a view to unveil their conceptual basis and their relationship with QED. As a main conclusion, it is stressed that the Klein-Nishina, Møller and Bhabha formulas were originally deduced without regard to early quantum electrodynamics, on the basis of the notion of correspondence. Though the scattering formulas were soon recovered from QED, they remained essentially neglected applications of the theory during the 1930s, to judge by the scarce texts devoted to it before renormalization.

Part II considers the attempts to test the formulas during the 1930s.  Klein-Nishina's displaced previous scattering formulas with ease, and was widely used in the analysis of gamma-ray absorption; by the mid 1930s, the formula was part and parcel of the absorption picture. The Møller and Bhabha formulas, on the contrary, were not tested during the thirties: Champion's experiments, the one exception that might be considered, were conceived without knowledge of the formula. The significance of the scattering formulas for the physics of nuclei and cosmic radiation explains this unequal reception.

In the epilogue it is argued that the scattering formulas, though deduced and in some instances tested in the pre-war period, only became applications of QED after the war, once the theory had recovered from its theoretical disease. Scattering processes were only retrospectively and rhetorically seen as exemplary applications of the theory, at a time when it no longer really needed their support. As to the causes of this belated recognition, one should mention the theoretical incongruencies that beset QED, which cast their shadow over some successful applications, and the outstanding role that scattering processes assumed in Feynman's quantum electrodynamics.