Hideki Yukawa

A boy from Kyoto who read the Doctrine of the Mean

He was born Hideki Ogawa on the 23rd of January, 1907, in Tokyo, and grew up in Kyoto in a household of seven children. His father was a geologist who would later move the family back to the old capital and take a chair at Kyoto Imperial University. The young Hideki was given a classical Confucian education by his grandfather, drilled on the Doctrine of the Mean before he was old enough to argue back, and later turned on his own initiative to Lao Tzu and Chuang Tzu. The habit of holding two opposing ideas in mind without forcing them to a quick resolution would, he later said, prove very useful when he tried to imagine a particle that nobody had ever seen.

His father almost sent him to technical college instead of university, on the quiet grounds that he was “not as outstanding a student as his older brothers.” His middle-school principal intervened. The boy had a high potential for mathematics, the principal said, and the father relented. Hideki himself nearly went into pure maths until a teacher marked one of his proofs wrong simply because he had not used the expected route. He drifted to physics instead, and then away from experimental physics when he discovered, in a college lab, that he was hopelessly clumsy at glassblowing. Theory was what was left, and theory turned out to be what he was for.

Hideki Yukawa was born on 23 January 1907 in Tokyo, Japan, and grew up in Kyoto with two older brothers, two older sisters, and two younger brothers. He read the Confucian Doctrine of the Mean, and later Lao-Tzu and Chuang-Tzu. His father, for a time, considered sending him to technical college rather than university since he was "not as outstanding a student as his older brothers." However, when his father broached the idea with his middle…

Osaka, 1935: an eleven-page paper

In 1929 he graduated from Kyoto Imperial University and stayed on as a lecturer. In 1932 he married Sumi Yukawa and, following the Japanese custom for families with many sons and a father-in-law with none, was adopted into her family and took her surname. He moved to Osaka Imperial University in 1933 as a lecturer and assistant professor.

Two years later, at the age of 28, he published On the Interaction of Elementary Particles in the Proceedings of the Physico-Mathematical Society of Japan. It was eleven pages long. It argued that the force binding protons and neutrons together inside an atomic nucleus could be modelled by analogy with electromagnetism, with one essential difference: the messenger particle that carried the force had to be heavy. From the known short range of the nuclear force, Yukawa calculated that this particle should weigh roughly 200 times the mass of the electron. He called it the U-quantum. The paper closed with the honest admission that no such object had ever been observed.

For most of a decade the rest of physics paid little attention. Japan was politically isolated, the journal had a small foreign readership, and the prediction seemed exotic. Yukawa kept teaching. In 1938 Osaka awarded him a doctorate for the work. In 1939, the year the war began in Europe, he was called back to Kyoto as Professor of Theoretical Physics.

A particle in a cosmic-ray plate, 1947

Then in 1947 Cecil Powell, working at Bristol with photographic plates flown to mountain altitudes in the Pyrenees and the Bolivian Andes, found tracks that matched Yukawa’s particle: short-lived, heavier than an electron, lighter than a proton. He named it the pi meson, or pion. The measured mass came out at about 270 electron masses, close enough to Yukawa’s 1935 estimate that everyone understood at once what they were looking at. Giuseppe Occhialini and Cesar Lattes shared the detection. Powell got the 1950 Nobel for finding the particle.

In 1935, Yukawa published his theory of mesons, which explained the interaction between protons and neutrons at Osaka Imperial University, and was a major influence on research into elementary particles. In 1938, Yukawa received a doctorate from Osaka Imperial University for his predictions regarding the existence of mesons and his theoretical work on the nature of nuclear forces. These research achievements were the reason he was later awarded…

Yukawa got his Nobel a year earlier. In 1949 the committee in Stockholm awarded him the Physics prize “for his prediction of the existence of mesons on the basis of theoretical work on nuclear forces.” He was the first Japanese citizen to win any Nobel in any field. The country had lost the war four years earlier, was still under American occupation, and was rebuilding from the ruins of two atomic strikes. The prize arrived as a public event larger than the physics. Yukawa spent that year as a visiting professor at Columbia.

After the pion: institutes, journals, manifestos

The pion was eventually revealed to be a composite particle, not a fundamental one. The deepest layer of the nuclear force, we now know, is gluon exchange between quarks. But the style of Yukawa’s argument (a finite-range force comes from the exchange of a massive particle, with the range fixed by the Compton wavelength of that particle) became the universal template of the Standard Model. The W and Z bosons of the weak force are Yukawa particles in exactly his sense.

In 1946 he founded Progress of Theoretical Physics, the journal that became Japan’s window onto international physics. In 1953 he opened the Research Institute for Fundamental Physics in Kyoto, now called the Yukawa Institute for Theoretical Physics, and directed it until 1970. In 1955 he joined Bertrand Russell, Albert Einstein, and nine other signatories on the Russell-Einstein Manifesto calling for nuclear disarmament. He spent the rest of his life on that work and on a nonlocal field theory that never quite came together. He died of pneumonia and heart failure at his home in Sakyo-ku, Kyoto, on the 8th of September, 1981, at the age of 74. The W boson was detected at CERN sixteen months later.

What he means to the quantum story

Yukawa is the moment when quantum theory leaves the atom and starts to describe the forces inside it. Every Feynman diagram in this book descends from his 1935 picture of two nucleons trading a heavy particle across the gap between them. The specific particle changed. The grammar did not.