Historical remarks

The theory of quantized fields has its origin in the early days of quantum mechanics. For accounts on the early history of quantum field theory see [1.1, 1.2, 1.3, 1.4, 1.5]. After the seminal work of Heisenberg [1.6] the subsequent article of Born and Jordan [1.7] already contains a section in which the quantization of the electromagnetic field is sketched. In the following ‘Drei-Mäanner-Arbeit’ [1.8] the quantization of an arbitrary number of degrees of freedom is worked out in more detail and is applied to a calculation of the thermal and quantal fluctuations of the electromagnetic field. These parts were entirely due to P. Jordan.

In the following years the idea of quantizing classical fields was pursued further by him. But he did not only think of the quantization of the electromagnetic field, which was, apart from the gravitational field, the only known fundamental classical field at that time. He also aimed at a representation of matter by quantized fields. In this case the Schrödinger waves were considered as classical fields. Therefore he called this procedure ‘second quantization’. In this way he attempted a unified description of matter and radiation, in which both were treated on equal footing. Such a formulation should form the framework for a mathematical realization of the duality between waves and particles. Despite the criticisms of many colleagues progress was made along these lines.