Sin-Itiro Tomonaga

Japanese theoretical physicist Sin-Itiro Tomonaga resolved key problems with the theory of quantum electrodynamics (QED) developed by Paul Dirac in the late 1920s through the use of a mathematical technique he referred to as renormalization.

TomonagaTomonaga’s work did not change the basic physical foundation of Dirac’s theory, which described the relationships between electrically charged particles and the electromagnetic field, but rather refined QED in order to make it consistent with the theory of special relativity and to show that the theory agrees quantitatively with results obtained experimentally to a great degree of accuracy. In 1965, Tomonaga received a portion of the Nobel Prize in Physics for his contributions to quantum electrodynamics. He shared the award with American physicists Julian Schwinger and Richard Feynman, who similarly worked out the difficulties inherent in QED as described by Dirac at about the same time as Tomonaga, albeit through different means.

The son of a philosophy professor, Tomonaga was born in Tokyo in 1906. He spent his early childhood there, but in 1913 his family relocated to Kyoto. Tomonaga attended Kyoto Imperial University (where his father taught), earning the Japanese equivalent to a bachelor’s degree in physics in 1929. He then began graduate studies. At the time physics was being revolutionized by such scientists as Dirac, Werner Heisenberg, and Wolfgang Pauli, who were forming the backbone of quantum mechanics. At first Tomonaga carried out graduate research in the laboratory of Kajuro Tomaki in Kyoto, but later he transferred to the Science Research Institute in Tokyo, where he worked under Yoshio Nishina. He focused on the nascent field of quantum electrodynamics, which would retain his attention throughout his career.

In the late 1930s, Tomonaga had the opportunity to visit the University of Leipzig. In Germany he worked with Heisenberg’s group and produced a paper on the nucleus of the atom that served as his dissertation when he returned to Japan in 1939 and received his doctoral degree. In 1940 he married Ryoko Sekiguchi, with whom he would have three children, and in 1941 he accepted a physics professorship at Bunrika University, which later became incorporated into the Tokyo University of Education. He taught and held various departmental posts until 1956, when he became university president.

Shortly after he joined Bunrika University, Tomonaga began his groundbreaking work in quantum electrodynamics. However, due to World War II, he was not in contact with western physicists. Tomonaga’s key papers describing his use of renormalization and his adaptation of QED theory appeared in Japan in 1943, but were not translated into English until 1948. By that time, Schwinger and Feynman had independently completed similar work. The American physicists, however, based their approaches partly on experiments, whereas Tomonaga considered QED from a purely theoretical standpoint. Together their work saved QED, which some scientists had contemplated abandoning because of the incongruity between theoretical predictions and experimental findings.

Following the war, Tomonaga’s contributions to quantum electrodynamics gained him wide recognition. In 1949, he went to Princeton University for two years as a visiting scholar, then returned to his position in Japan.

Tomonaga stepped down as president of Tokyo University in 1962, but was appointed director of the school’s Institute of Optical Research the following year. Also in 1963 he became president of the Science Council of Japan. In 1969 he retired from both posts. Ten years later, on July 8, 1979, Tomonaga died in Tokyo. The great physicist received many honors over the course of his career. In addition to the Nobel Prize, he accepted the Japan Academy Prize, the Order of Culture of Japan and the Lomonosov Gold Medal, bestowed by the Russian Academy of Sciences. Throughout his life, Tomonaga dedicated himself to stemming the proliferation of nuclear weapons while encouraging the use of nuclear energy for peaceful purposes. His efforts proved critical in the foundation of the University of Tokyo’s Institute for Nuclear Studies.