Yukawa , Hideki

(1907–1981) Japanese physicist

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^–12cm 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.