The enrichment of physics and astrophysics: The legacy of Victor Hess

The Third Victor F. Hess Memorial Lecture traces the evolution of the cosmic-ray discipline from Hess' discovery in his balloon flights of 1911. Serendipitously, this led to the genesis of two major fields of research, high-energy elementary-particle physics and cosmic-ray astrophysics. The subsequent development of the latter field is traced from the early decades when its promise was not widely appreciated. Epochal discoveries in the 1930's and 1940's stimulated widespread interest among physicists. Identification of the positron, the «soft component», muons, pions, and the elaboration of the electromagnetic theory of shower production were among the major advances. The nuclear character of the main primary component was demonstrated in the forties. Symbiosis with radioastronomy, notably the understanding of the synchrotron radiation emitted from the Crab Nebula, revealed the role of supernova explosions in providing the energy for the galactic cosmic radiation. Fermi's use of magnetohydrodynamics in his acceleration mechanisms, and their subsequest realization in shock-wave theory are sketched. Detailed investigations of composition—especially measurements of the Li−Be−B and of the3He components—yielded knowledge of the propagation and transformations of the relativistic nuclei in passing through the interstellar medium. This, in turn, made possible the evaluation of path length distributions, predictions of the arriving isotopic composition, the source composition, and cosmic-ray age. Finally, some tantalizing problems that remain are mentioned, notably those of the highest-energy and the lowest-energy cosmic rays.