As the chapter explains, the correspondence principle was gradually devised by Bohr as a constructive versatile intermediary hypothesis. It argues the principle was essential in developing and refining his full-scale model of the atom and connecting it with lower-level experimental hypotheses.
In a major work of 1918 (‘On the quantum theory of line-spectra’) Bohr reconsidered the general principles of quantum and atomic theory. Apart from reinterpreting his original theory, he introduced the correspondence principle as a crucial connection between quantum and classical physics. What was the nature of this principle and what role did it play in the understanding of atomic structure? In applying the new principle, Bohr and his assistant H. A. Kramers relied on Einstein’s radiation theory. Bohr used the correspondence principle as a selection principle, and Kramers showed how it could account quantitatively for the Stark effect. However, not all physicists appreciated the Copenhagen ‘magic wand’ and Bohr’s use of it. Sommerfeld was the most prominent of the sceptics, but not the only one. The chapter also briefly considers the role of the correspondence principle in the early phase of the new quantum mechanics.
‘Magic wand’ refers to the ‘correspondence principle’ that Bohr devised and deployed to investigate the interface between quantum and ordinary (‘classical’) physics. The chapter covers various lines of work, some inspired by his approach and some independent of it, all of which confirmed its fertility. The mobilization of the international brotherhood of physicists for the Great War gave Bohr breathing space to develop the correspondence principle with the help of Hendrik Kramers, who had come to neutral Denmark to study with him, and in friendly competition with Arnold Sommerfeld, who made important formal extensions of the theory.
The chapter explicates the experimental context in which Bohr worked and offers an account of his gradually devised conceptual approach to experimental apparatus and research in physics at the most basic level. As it explains, two distinct stages in Bohr’s vision of physics provided the foundation for his main contributions to early quantum theory and quantum mechanics: the stage of inducing the basic lower-level hypotheses from experiments and the stage of inducing higher-level theoretical hypotheses via intermediary hypotheses. These stages were also apparent in his reflections on the practice of physics and his gradual development as a physicist.
It is widely known that Paul Ehrenfest formulated and applied his adiabatic hypothesis in the early 1910s. Niels Bohr, in his first attempt to construct a quantum theory in 1916, used it for fundamental purposes in a paper which eventually did not reach the press. He decided not to publish it 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 the subsequent variations of his theory, Bohr never suppressed this principle completely. We discuss the role of Ehrenfest's principle in the works of Bohr, paying special attention to its relation to the correspondence principle. We will also consider how Ehrenfest faced Bohr's uses of his more celebrated contribution to quantum theory, as well as his own participation in the spreading of Bohr's ideas.