Enric Pérez Canals and Blai Pie Valls, University of Barcelona, Spain
Ehrenfest’s adiabatic hypothesis in Bohr’s quantum theory

It is widely known that Paul Ehrenfest formulated and applied his adiabatic hypothesis in the early 1910’s. Niels Bohr, in his first attempt to construct a quantum theory in 1916, used it for fundamental purposes in a paper which he decided not to publish after having received the new results by Sommerfeld in Munich. Two years later, Bohr published On the Quantum Theory of Line Spectra. There, the adiabatic hypothesis played an important role, although it appeared with another name: the principle of mechanical transformability. In subsequent variations of his theory, Bohr never suppressed this principle completely; in the final version before the rise of Quantum Mechanics, in a paper of 1924, it was called principle of the existence and permanence of the quantum numbers.

In our paper we will describe and analyze the role of Ehrenfest’s principle in the work of Bohr, before the emergence of Quantum Mechanics. We will also consider how Ehrenfest faced Bohr’s uses of his most celebrated contribution to quantum theory, as well as its wide distribution after Bohr’s intervention.