Ryogo Kubo in his formative years as a physicist

The Japanese theoretical physicist Ryogo Kubo made remarkable contributions to statistical mechanics and condensed matter physics, amongst which his name is most widely associated with the linear response theory. Despite his importance in the history of modern physics, however, historians have paid him little attention. Using his unpublished manuscripts in a newly organized archive, this paper examines his studies and research up to the end of World War II. Influenced by his brother Masaji Kubo, a physical chemist, and the milieu at Tokyo Imperial University, he became interested in theoretical approaches to properties of matter and worked on dipolar gases and resistance in metals. After graduation, he studied three different phenomena—relaxation, melting, and rubber elasticity—by applying the method of eigenvalue problems. He was also involved in wartime research on noctovision and worked on photoemission in semiconductors. This paper also identifies two distinct focuses in his early research that persisted in his work after the war: solid-state physics and statistical mechanics in today’s terminology. Reconstructing Kubo’s formative years is instrumental for constructing a historiography of a key aspect of modern Japanese physics, namely, how the science of matter evolved before and during the war.