Hideki Yukawa | Biography

The discovery of the neutron by James Chadwick in 1932 resolved an important problem in atomic physics: the discrepancy between atomic weight and atomic number. But it raised a new problem. If the nucleus consists only of positively-charged protons and neutral neutrons, how can it stay together? It would seem that the force of repulsion among protons would tear the nucleus apart. The resolution to this problem was suggested by the Japanese physicist, Hideki Yukawa, in 1935.

Yukawa reasoned that some very strong force must exist within the nucleus to hold protons and neutrons together. The force must act over only very short distances, the size of the nucleus (about 10-15 m), but dissipate rapidly over distances greater than that. To describe the nature of this force, Yukawa looked to the electromagnetic force for an analogy.

Scientists knew that charged particles interact with each other through the interchange of a virtual particle, the photon. The mass of the photon, and of any other virtual particle, has an inverse relationship to the distance over which it acts. Since the electromagnetic force acts over an infinite distance, its mass must be zero.

In contrast, the very short distance over which the strong force acts means that its virtual particle must have a significant mass. Yukawa determined that mass to be about 200 times that of the electron and about a tenth that of the proton or neutron. Because of its intermediary mass, the particle eventually became known as a meson.

Apparent confirmation of Yukawa's prediction appeared within a year when Carl David Anderson discovered a particle with a mass 200 times that of the electron in cosmic ray showers. However, it soon became evident that Anderson's meson (known as the mu meson or muon) was not the expected particle. Although it had the correct weight, it did not interact with protons and neutrons as required.

In fact, it was not until 1947 that Yukawa's prediction was finally confirmed. In that year, Cecil Powell discovered another type of meson, the pi meson or pion, also in cosmic ray showers. Powell showed that the pion had the correct weight (285 times that of an electron) and that it did interact with protons and neutrons, as predicted. Two years later, Yukawa was awarded the Nobel Prize for physics for his work on nuclear forces.

Yukawa made a second important contribution to nuclear physics in 1936. He predicted that, under certain circumstances, a nucleus could capture an electron from the first (K) orbit of the atom. Yukawa's prediction of K-capture was confirmed less than two years later.

Yukawa was born in Kyoto on January 23, 1907, the son of a professor of geology at Kyoto University. He received his bachelor's degree from Kyoto in 1929 and, after teaching there for four years, accepted a position at Osaka University. He received his doctorate from Osaka while still on the teaching staff.

In 1939, Yukawa accepted an appointment as professor of theoretical physics at Kyoto University. Except for two visits to the United States in 1948-1949 (at the Institute for Advanced Studies) and 1949-1951 (at Columbia), Yukawa remained at Kyoto for the remainder of his academic career. He died in Kyoto on September 8, 1981.