Japanese physicist (1907–1981)

Yukawa was born Hideki Ogawa at Kyoto in Japan, the son of the professor of geography at the university there; he assumed the name of his wife, Sumi Yukawa, on their marriage in 1932. He was educated at the university of Kyoto and at Osaka, where he joined the faculty in 1933 and where he completed his doctorate in 1938. In the following year Yukawa was appointed professor of physics at Kyoto University, a position he continued to hold until his retirement in 1970.

Yukawa was concerned with the force that binds the neutrons and protons together in the nucleus. At first sight, any nucleus containing more than one proton should be unstable since positively charged particles repel each other; squeezing a number of positively charged protons into the nucleus of an atom should generate powerful repulsive forces. The obvious answer is that there must be another, attractive, force that operates only at short range and holds the nucleons together. Such a force became known to physicists as the ‘strong interaction’.

Yukawa sought to find the mechanism of the strong force and used the electromagnetic force as an analogy. Here the interaction between charged particles is seen as the result of the continuous exchange of a quantum or unit of energy carried by a ‘virtual particle’ – in this case the photon. So, just as electrons and protons interact by exchanging photons, the nucleons interact by exchanging the appropriate particle. Yukawa could predict its mass from quantum theory as the range over which a particle operates is inversely proportional to its mass. The massless photon is thus thought to operate over an infinite distance; as the strong force operates over a distance of less than 10^–12 cm it must be mediated by a particle, Yukawa predicted, with a mass of about 200 times that of the electron.

Yukawa made his prediction in 1935 and when two years later Carl Anderson found signs of such a particle in cosmic-ray tracks physicists took this as supporting Yukawa's hypothesis and named the particle a mu-meson (now called a muon). However, although the muon had the appropriate mass it interacted with nucleons so infrequently that it could not possibly be the nuclear ‘glue’. Yukawa's theory was saved, however, by the discovery in 1947 by Cecil Powell, once more in cosmic-ray tracks, of a particle with a mass of 264 times that of the electron and of which the muons were the decay product. The pi-meson, or pion as it became known, interacted very strongly with nucleons and thus filled precisely Yukawa's predicted role.

For this work Yukawa was awarded the Nobel Prize for physics in 1949, the first Japanese person to be so honored.

The Japanese physicist Hideki Yukawa (1907-1981) was one of the world's most highly-respected theoretical physicists. His most visible contributions to science were in the field of particle physics.

Hideki Yukawa was born in Tokyo on Jan. 23, 1907. His father was a professor of geology at Kyoto University, and Yukawa grew up in an academically-oriented household which focused his attention on science from his early years. He entered Kyoto University in 1926, and, showing his intelligence early, graduated only three years later with a master's degree. Setting out on the long journey to academic achievement, Yukawa spent the next ten years continuing his education and teaching. First came three years of research, which was followed by a 1934 appointment as a physics lecturer at Kyoto University. Next, there was a move to a lecturer's post at Osaka University, where Yukawa completed his doctorate in physics in 1938.

Scientific Achievements

By this time, Yukawa was already immersed in the study of sub-atomic particles that be his focus for the rest of his life. His first paper, "On the Interaction of Elementary Particles," met with a lukewarm reception when he presented it in Osaka at the 1934 meeting of the Physico-Mathematical Society. Nevertheless, he chose to publish it the following year, in the Society's Proceedings, Yukawa's paper postulated that, as an analogy to the way in which a particle of light may be exchanged between two charged particles in the "electromagnetic interaction," a new particle (later termed the meson) might be exchanged between two nucleons in the "nuclear interaction." The meson was envisioned by Yukawa to be a particle providing the "glue" for holding together the various other particles making up the nucleus of the atom.

The theory caused great interest in scientific circles, especially after such a particle was discovered in cosmic radiation by Carl Anderson, a 1936 Nobel laureate from the California Institute of Technology. Used together by researchers, the work of Yukawa and Anderson provided a noteworthy overture to the 1939 discovery of nuclear fission. Now an eminent scientist, Yukawa took his place in 1939 as Professor of Physics at Kyoto University. He taught there during World War II, which ended abruptly for Japan in 1945 with the shattering atomic destruction of Hiroshima and Nagasaki.

The Scientist's Real Responsibility

In 1948 he was invited to spend a year at Princeton University's Institute for Advanced Study. It was here that Yukawa met Albert Einstein, with whom he remained friendly for the rest of the older scientist's life. Quoted in Yukawa's own obituary in The Bulletin of the Atomic Scientists was an excerpt from his graceful epitaph for Einstein: "I feel very strongly that we have to take up his search and striving for world peace," a mission which Yukawa himself took extremely seriously.

His feeling that at least some of the responsibility for preventing war must rest with the scientists who produce its technology was strongly expressed once again in 1962, at the Kyoto Conference of Scientists. "The results of physics are inevitably connected with the problems of humanity through their application to human society," he warned. In 1975, at the 25th Pugwash International Symposium held in Kyoto, he took this theme even further. "Usually it has been thought that, particularly in pure science, it is desirable for its progress not to include any value criterion other than true-or-false. We physicists, by experience, have realized that the advent of nuclear weapons dealt a great blow to the above-mentioned way of thinking," he noted.

Yukawa's strong feelings on science's duty to humanity had begun to crystallize by 1949, when he won the Nobel Prize. He chose to donate most of his award money to several institutions in Japan, including the Research Institute for Fundamental Physics at Kyoto University, to which he returned in 1953 after three years at Columbia University. It was time, he said, to return to Japan to train "new faces." He remained there until 1970, when he retired.

Yukawa died at his home in Kyoto in 1981.

Further Reading

Yukawa discussed his contributions to meson theory in his Nobel lecture, reprinted in Nobel Lectures in Physics, vol. 3 (1964). Niels H. de V. Heathcote, Nobel Prize Winners, 1901-1950 (1953), contains important biographical material. For additional insights into his work see Rudolf Ernst Peierl's, The Laws of Nature (1956); Richard M. Harbeck and Lloyd K. Johnson's, Physical Science (1965); and Samuel A. Morantz's, Physics (1969).

Additional Sources

Bulletin of the Atomic Scientists, October 1982.

New York Times, September 10, 1981.

(hē'dĕkē yūkä'wä) , 1907–81, Japanese physicist, grad. Kyoto Univ., 1929, Ph.D. Osaka Univ., 1938. He was professor of physics at Kyoto Univ. from 1939 to 1970. He received the 1949 Nobel Prize in Physics for predicting (1935) the existence of the meson. After further developing the meson theory of nuclear forces, he began (1947) work on his “nonlocal field” theory for elementary particles. In 1948 he came to the United States, where he spent a year at the Institute for Advanced Study and was (1949–53) visiting professor at Columbia.

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