Introduction

Study of the formation and propagation of S-matrix theory in the 1940s helps one to understand the rapid development, under radically different scientific and sociological conditions, of the theory of elementary particles in the fifties. The S matrix is a two-dimensional array whose elements are the transition amplitudes between states of atomic or elementary particle systems. According to a standard textbook of the midfifties, “Such a scattering matrix was first introduced by Wheeler (1937) in connection with the problems of nuclear structure and scattering. It has been reinvestigated in great detail by Heisenberg (1943) in connection with the theory of elementary particles.” During World War II and the first postwar years, few people were interacting on problems of pure physics; only on rare occasions, often accidentally, did they meet for discussions, or even exchange letters. Despite these obstacles, they did interesting and original work that led to the resumption of particle physics by a larger group after World War II. The S matrix played a comparatively large role in the discussions around 1945, but was abandoned in the early fifties. Toward the end of the fifties, however, several developments led to renewed interest in it.

The origin of the S-matrix theory of elementary particles

In a paper submitted in the summer of 1937, John A. Wheeler proposed to derive the properties of light nuclei, such as energy levels and transition probabilities, with the help of a unitary “scattering matrix.” The elements of this matrix were connected with the transition from incoming to outgoing groups, consisting of a few protons and neutrons within the nuclei in question.