Chapter V

Abstract

Already early on the principle of correspondence had been a guide for Bohr’s research on the radiation properties of atoms, with respect to which it had an exact, technical meaning. Later on, after Bohr had introduced his ideas of complementarity, it was used in a more general and broader sense, as we shall see in a subsequent chapter. The correspondence principle is a generalization of the fact that results derived from Bohr’s theory of hydrogen in the limiting region of high quantum numbers coincide approximately with those yielded by classical electrodynamics. The principle states that such a coincidence must hold generally in all cases involving high quantum numbers; hence it became the methodological principle which guided Bohr’s research in his endeavour to establish a coherent quantum theory during the 1910s and 1920s. However, although Bohr and others little by little were able to explain many spectroscopic data on the basis of the correspondence principle, they never made a decisive breakthrough. On the contrary, at the beginning of the twenties Bohr’s theory of atomic structure was confronted with serious problems, such as that concerning the determination of the energy states of any atom other than hydrogen atoms and the anomalous Zeeman effect, problems with which it failed to cope. In the end it was not Bohr himself but Werner Heisenberg, his young assistant, who in 1925 articulated the foundation of a coherent quantum theory for which Bohr had been searching for so long. Nevertheless, he saw Heisenberg’s theory as “a precise formulation of the tendencies embodied in the correspondence principle”.¹