The transition from classical to quantum physics is one of the major revolutions of physics. Considering the quantum theory as a discontinuous break with classical physics would, however, disregard the complex entanglement of classical and quantum theory in this revolution. In my talk I will show how this entanglement was part of the correspondence principle and its uses, extensions and transformations within the old quantum theory and how it became fruitful in the development of quantum theory.
Using the correspondence principle enabled physicists to translate between the continuous radiation theory of classical electrodynamics and the discontinuous quantum theory. The search for such a translation did not remain restricted to the classical limit of quantum theory, but also spread into the realm where classical theory failed. In this realm, the correspondence principle led to the construction of a "state in between;" a classical tool which was not part of Bohr’s atomic model but in intimate connection with it. Appearing as one of the first extensions of the correspondence principle, this “state in between” remained rather unnoticed and became a silent ingredient of the quantum revolution in 1925.
It was this "state in between" that allowed to deal with the otherwise ineffable "quantum jumps" of Bohr's theory on the basis of classical reasoning. By means of the “state in between”, the correspondence principle acted as the interface in which the knowledge embedded in the classical formalism and its models took a new quantum theoretical form.