  During the 1920s and 1930s, Italian physicists established strong
relationships with scientists from other European countries and the United
States. The career of Bruno Rossi, a leading personality in the study of cosmic
rays and an Italian pioneer of this field of research, provides a prominent
example of this kind of international cooperation. Physics underwent major
changes during these turbulent years, and the traditional internationalism of
physics assumed a more institutionalised character. Against this backdrop,
Rossi's early work was crucial in transforming the study of cosmic rays into a
branch of modern physics. His friendly relationships with eminent scientists
-notably Enrico Fermi, Walther Bothe, Werner Heisenberg, Hans Bethe, and Homi
Bhabha- were instrumental both for the exchange of knowledge about experimental
practises and theoretical discussions, and for attracting the attention of
physicists such as Arthur Compton, Louis Leprince-Ringuet, Pierre Auger and
Patrick Blackett to the problem of cosmic rays. Relying on material from
different archives in Europe and United States, this case study aims to provide
a glimpse of the intersection between national and international dimensions
during the 1930s, at a time when the study of cosmic rays was still very much
in its infancy, strongly interlaced with nuclear physics, and full of
uncertain, contradictory, and puzzling results. Nevertheless, as a source of
high-energy particles it became a proving ground for testing the validity of
the laws of quantum electrodynamics, and made a fundamental contribution to the
origins of particle physics.
