Tomonaga, Sin-Itiro (physicist)

(1906-1979) Japanese Quantum Field Theorist, Particle Physicist

Sin-Itiro Tomonaga was a great Japanese theoretician, who laid the foundations of a relativistic quantum electrodynamics (QED), the study of the quantum mechanical interaction of electrons, positrons, and photons. He shared the 1965 Nobel Prize in physics with richard phillips feynman and julian seymour schwinger, who, working concurrently, achieved the same result, using different approaches.

He was born in Tokyo, Japan, on March 31, 1906, the eldest son of Sanjuro and Hide Tomonaga. When he was seven, the family moved to Kyoto, where his father was appointed a professor of philosophy at Kyoto Imperial University. He was a sickly boy, who did poorly in athletics and was teased by his peers as a “crybaby.” He discovered his passion for science early and spent his free time with a friend building simple electric circuits with parts they had found in junkyards. After graduating from the renowned Third Higher School, he entered Kyoto Imperial University in 1923. After earning their rigakushi (bachelor’s degrees) in physics in 1929, Tomonaga and his good friend, another future Nobel Prize winner, hideki yukawa, traveled to Tokyo to listen to a series of lectures by werner heisenberg and paul adrien maurice dirac, which made a strong impression on them.

He remained at Kyoto for three years of graduate studies, after which he was taken on as a research associate by Yoshio Nishina, one of Japan’s leading physicists at the Institute for Physical and Chemical Research (known as Riken) in Tokyo. Nishina, who introduced research on quantum mechanics to Japan, was a role model and father figure for the young Tomonaga, who later reminisced:

The Nishina laboratory in those days was full of freshness. All the members were young; even our great chief Nishina was still in his early forties. We all got together for lunch every day, an eager group of people discussing various matters, not only physics, but also such things as plans for beer parties, excursions and so on.

In this stimulating atmosphere, Tomonaga made his first contribution to QED in a 1933 paper, coauthored with Nishina, on photoelectric pair creation. With the other members of the theoretical group, he studied Dirac’s work, translating his textbook on quantum mechanics into Japanese.

In the mid-193 0s, Tomonaga’s interests shifted to nuclear physics. From 1937 to 1939, he was in Leipzig, Germany, studying nuclear physics and quantum field theory under Heisen-berg. With Heisenberg’s encouragement, he attacked the problem of the description in the Bohr liquid-drop model of the heating of a nucleus when it absorbs a neutron. He had hoped to continue working with Heisenberg, but when war broke out on September 1, 1939, he returned to Japan. The results of his Leipzig research formed a large part of his doctoral thesis, which he submitted in December 1939 at Tokyo Imperial University. The following year, he married Ryoko Sekiguchi, with whom he would have two sons and a daughter.

Sin-Itiro Tomonaga shared the 1965 Nobel Prize in physics with Richard Phillips Feynman and Julian S. Schwinger for his work in quantum electrodynamics (QED), the study of the quantum mechanical interaction of electrons, positrons, and photons.

Sin-Itiro Tomonaga shared the 1965 Nobel Prize in physics with Richard Phillips Feynman and Julian S. Schwinger for his work in quantum electrodynamics (QED), the study of the quantum mechanical interaction of electrons, positrons, and photons.

With his country at war, Tomonaga, who, in 1941, became professor of physics at Tokyo University of Education (Bunrika), managed to press on with his fundamental research on QED. He addressed himself to the problem that, whereas every physical theory must obey the rules of special relativity, QED, in its early Dirac formulation, was not fully relativistic. Some physicists believed that the infinities (also called divergences) that QED yielded, which were physically nonsensical, could be dealt with more realistically if the theory could be made relativis-tically covariant, that is, compatible with the rules of special relativity. In work done between 1941 and 1943, Tomonaga proposed a relativis-tic formulation of quantum field theory, in which the concepts of the quantum state vector (the quantum field theoretic equivalent of the Schrodinger wave function) and its equation of motion, concepts having relativistic spacetime meaning, were generalized so as to be relativisti-cally covariant.

In 1943, Tomonaga published this work, developed in isolation from his Western counterparts, who would not learn of it until four years later, just as he was forced to turn away from basic research to problems related to military needs. Until the end of the war, he worked on the properties of magnetrons and the behavior of microwaves in waveguides and cavity resonators, developing a unified theory of the systems consisting of waveguides and cavity resonators.

In the midst of Japan’s postwar devastation, Tomonaga and his family found themselves homeless and hungry. Nonetheless, there was a sense of communal responsibility for national reconstruction among the group of young scientists who gathered around Tomonaga in Tokyo. In 1946, he returned to the problems of quantum field theory, with the goal of summarizing and applying the covariant field theory to actual physical systems. He was sure that the divergence problems in QED could be overcome by finding a way to handle the infinite mass and charge due to field reaction. Elimination of the divergences was to be accomplished by a renor-malization procedure that first identified the divergent terms according to their relativistic and gauge transformation properties, then showed that these divergent contributions could be absorbed in a redefinition of the mass and charge parameters entering the original formulation of the theory. If this could be done, then the renormalized formalism could make predictions about observable phenomena.

In 1947, Tomonaga learned about the Lamb shift and read hans albrecht bethe’s article in Physical Review, presenting his nonrelativistic Lamb shift calculation. He recognized its importance and went on to work out his mass renor-malization method, substituting the experimental mass for the fictive mechanical mass that appeared in the equations of QED. He performed a similar renormalization of the electric charge. He also deduced a correct formula for the Lamb shift, which agreed with measurements. In 1948, he wrote about his work to j. robert oppen-heimer, who urged him to submit a summary to Physical Review for publication.

In a scientific community only slowly overcoming the barriers imposed by global war, Tomonaga’s discovery did not influence QED research in the United States. Schwinger independently made the same advances and Feynman was unaware of his Japanese colleague’s work. However, Tomonaga did resume his collaboration with Western physicists when, in 1949, he was invited to the Institute for Advanced Study, Princeton; there he turned his attention to nuclear physics, investigating a one-dimensional fermion system. In so doing he clarified the nature of collective oscillations of a quantum mechanical many-body system and opened a new frontier of theoretical physics, the modern many-body problem. In 1955, he published an elementary theory of quantum mechanical collective motions.

Tomonaga was a leader in establishing the Institute for Nuclear Physics, Tokyo, in 1955. From 1956 to 1962, he was president of the Tokyo University of Education. In 1963, he became president of the science council of Japan and director of the Institute for Optical Research at the Tokyo University of Education. He held many key positions on government committees dealing with scientific research and policy making.

In 1965, Tomonaga shared the Nobel Prize in physics with Schwinger and Feynman but was prevented by an accident from being present in Stockholm.

He published widely in scientific journals on QED, the meson theory, nuclear physics, cosmic rays, and the many-body problem. His book Quantum Mechanics was published in Japan in 1949; an English-language edition appeared in 1962. His memoir, Development of Quantum Electrodynamics: Personal Recollections, became available in English translation in 1966.

Tomonaga, who died on July 8, 1979, in Tokyo, was a physicist of great intellectual powers, an extraordinary teacher in the Socratic tradition whose dictum was that “if you formulate the problem correctly, that is, if you ask the right question, the answer emerges spontaneously.” He possessed keen aesthetic sensibilities and a love of communing with nature. He will be remembered as a strong advocate of nonprolifer-ation of nuclear weapons and the peaceful uses of atomic energy.

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