Bohr and Ehrenfest: transformations and correspondences in the early 1920s

We analyze the collaboration between Bohr and Ehrenfest on the quantum theory in the early 1920s (1920–1923). We focus on their reflections and developments around the adiabatic principle and the correspondence principle, the two pillars of Bohr’s quantum theory of 1922–23. We argue that the evolution of Bohr’s ideas after 1918 brought the two principles closer, subordinating the former to the latter. The examination of the weight Bohr attributed to each principle along the years illustrates very clearly the vicissitudes of Bohr’s theory before the emergence of quantum mechanics, especially with regards to its rejection/inclusion of mechanics.