Hideki Yukawa became Japan's first Nobel laureate in 1949, winning the physics prize for his prediction of mesons, the subatomic particles that mediate the strong nuclear force. His theoretical breakthrough in 1935 solved a fundamental puzzle in nuclear physics and laid the groundwork for modern particle physics.
Yukawa was born in Tokyo on 23 January 1907 and graduated from Kyoto University in 1929. He moved to Osaka University in 1933 for a PhD, and it was there that he conducted the work that would earn him the Nobel Prize. At the time, physicists were grappling with a major problem: positively charged protons repel each other through electromagnetic force, which should cause the atomic nucleus to fly apart. Yet nuclei remain stable. In 1935, Yukawa proposed a short-range, highly attractive nuclear force that overcomes this repulsion. He suggested this force is created by the exchange of massive subatomic particles between protons and neutrons, which he called mesons. Yukawa calculated that the mass of these exchange particles should fall roughly between that of an electron and a proton.
The prediction set off a search. In 1937, three separate research groups conducted cloud-chamber experiments and discovered a new particle with a mass consistent with Yukawa's prediction, but it was later identified as a muon, not the meson he had envisioned. Yukawa was awarded his PhD in 1938 and returned to Kyoto University a year later. The first evidence of the particle he had predicted came in 1947, when a research group led by Cecil Powell at the University of Bristol discovered the pi meson, or pion. Powell's team measured the particle by exposing photographic plates to cosmic rays at high altitudes, work that earned Powell the 1950 Nobel Prize in Physics.
Yukawa was awarded the 1949 Nobel Prize in Physics «for his prediction of the existence of mesons on the basis of theoretical work on nuclear forces». The news was met with tremendous joy in Japan and encouraged scientists to continue their research despite the economic conditions and the challenges of doing experimental science in postwar Japan.
In Yukawa's honour, the Japanese government built a three-storey building named Yukawa Hall in 1952. Its focus was on developing theoretical physics in Japan, and it featured lecture halls, seminar rooms, offices and a library. In 1953, the hall became home to the Research Institute for Fundamental Physics, now the Yukawa Institute for Theoretical Physics, of which Yukawa was the first director, a position he held until retiring in 1970. The institute invited research scholars from all over Japan and held symposia and seminars on particle physics. It also published the monthly English-language journal Progress of Theoretical Physics, now known as Progress of Theoretical and Experimental Physics, together with the Physical Society of Japan. Yukawa founded the journal in 1946 to publish results that had remained unpublished because of the society's financial difficulties.
Beyond the Nobel Prize, Yukawa won many other international awards and was vocal about issues in physics and society. In 1955, he joined 10 other scientists in signing the Russell-Einstein Manifesto, which called for nuclear disarmament. Yukawa died on 8 September 1981. Since he was awarded the Nobel Prize in Physics, 11 other Japanese-born physicists have gone on to win the physics Nobel, making Japan one of the most successful countries in the world in the field.
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