Yoichiro Nambu, Nobel-winning theoretical physicist, 1921-2015

“Nambu was one of the great theoretical physicists of the latter half of the 20th century. Much of the current theory of elementary particles revolves around seminal contributions by Nambu,” said Peter Freund, professor emeritus in physics at UChicago.

“Yoichiro Nambu arrived at the University of Chicago at the very end of the Enrico Fermi era and quickly established himself as a key force in maintaining the world-renowned vitality of our physics department and our Enrico Fermi Institute,” said Rocky Kolb, dean of the Physical Sciences Division. “We will remember him for his quiet dignity, his modesty and his deep scientific acumen.”

Colleagues highlight three of Nambu’s contributions as especially important: spontaneous symmetry breaking, the mechanism that gives mass to the Higgs boson and other elementary particles; color gauging, the means by which quarks bind to matter in atomic nuclei; and string theory. Though not yet experimentally tested, string theory is the leading candidate for a unified theory of matter and forces in nature.

“Each of these fundamental theories owe their existence to Nambu’s deep insights,” said physics Professor Emil Martinec, director of the Enrico Fermi Institute. Nambu was the Harry Pratt Judson Distinguished Service Professor Emeritus in Physics and the Enrico Fermi Institute.

“With his passing, we lose one of the giants of 20th-century theoretical physics, who laid the foundations of his science and set the direction of research for decades to come,” said Jeffrey Harvey, the Enrico Fermi Distinguished Service Professor in Physics at UChicago.

Discovery of spontaneous broken symmetry

Nambu shared his Nobel Prize with Makoto Kobayashi of the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, and Toshihide Maskawa of Kyoto University in Japan. Nambu was cited “for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics.”                                      

Nambu developed his Nobel-Prize-winning concept of spontaneous symmetry breaking, or SSB, while studying superconductivity in the early 1960s. Superconductivity is the nearly complete disappearance of electrical resistance at extremely low temperatures. He spent two years in determining that spontaneous symmetry breaking contributes to the explanation of how superconductivity works, then quickly turned that theory around and in two brilliant papers with the Italian physicist Giovanni Jona-Lasinio, then a research associate at Chicago, applied it to particle physics.

In his Nobel Prize acceptance speech, Nambu explained how spontaneous symmetry breaking is a special law of physics that applies to large numbers of subatomic particles. “SSB arises from a kind of group mentality, group psychology, among the constituents.”

By analogy, Nambu noted that when a group gathers in a large, open area, people usually look in a variety of directions. But sometimes, when one person begins looking in one direction, those in the crowd do likewise. “That’s a broken symmetry,” Nambu said, a behavior that subatomic particles also display.

In 1965 Nambu, in collaboration with Moo-Young Han, now of Duke University, developed the forerunner of the modern theory of quantum chromodynamics, which accounts for the nuclear forces that bind protons and neutrons into atomic nuclei. Shortly thereafter Nambu and two others, acting independently, showed that physicists needed to abandon their concept of point-like elementary particles and replace it by a dynamics of strings acting in accordance with relativity theory.

“Once again, Nambu’s deep insight led to a vast scientific enterprise—one which continues even today, nearly half a century after his original papers,” Harvey said.

‘I liked it so much, I ended up staying’

Nambu was born Jan. 18, 1921 in Tokyo. His interest in science developed in high school after his father showed him some science magazines. In college, one of his professors was Hideki Yukawa, a leading particle physicist of his day. He received his bachelor’s degree in 1942 from the University of Tokyo. He was then drafted into the Japanese army and was assigned to follow the radar work of Sin-ichiro Tomonaga, the leading theoretical physicist for one of the competing military services.

Nambu married Chieko Hida in 1945. He became an associate professor at the newly created Osaka City University at the age of 29 in 1950. He completed his doctorate in 1952 from the University of Tokyo.

Nambu retained his appointment at Osaka City University until 1956, but went to the Institute for Advanced Study in Princeton, N.J., in 1952 upon the recommendation of Tomonaga to study with J. Robert Oppenheimer. Freund told the Chicago Tribune in 2008 that while at the Institute for Advanced Study, Nambu summoned the courage to introduce himself to Albert Einstein, whom Oppenheimer had tried to insulate from visits by junior researchers.

“Einstein was just happy that finally someone had come to talk with him,” Freund said.

Nambu joined the University of Chicago as a research associate in 1954, becoming associate professor in 1956 and professor in 1958. He obtained U.S. citizenship in 1970. He had not intended to spend his entire career at UChicago, “but I liked it so much, I ended up staying,” Nambu said in 2008. He found that even in the rarefied intellectual atmosphere of the University’s physics department, “everybody treated everybody else as members of a big family.”

Nambu served as chairman of the physics department from 1974 to 1977. He was named the Harry Pratt Judson Distinguished Service Professor in 1977, and retired as professor emeritus in 1991.

His many honors included membership in the National Academy of Sciences, and honorary membership in the Japan Academy. He also was a fellow of the American Academy of Arts and Sciences, and had received J. Robert Oppenheimer Prize, the Wolf Prize in Physics, the Benjamin Franklin Medal, and the Order of Culture from the Japanese government.

Nambu is survived by his wife, Chieko, and his son, John.