On 5 July 2015, at the age of 94, Yoichiro Nambu, one of the truly great theoretical physicists of our time, died in Osaka, Japan, due to an acute myocardial infarction.
Most of the important physics theories of the second half of the 20th century contain a seminal contribution by Yoichiro. We mention but three: spontaneous symmetry breaking, color gauging, and string theory, which all owe their existence to Yoichiro’s deep insights. Indeed, when an idea is introduced in particle physics, it often turns out to have been already developed by Yoichiro years earlier. He was the recipient of virtually all major physics prizes, including the Nobel Prize in Physics in 2008 and the Wolf Prize in 1995. But such was the modesty of the man that one always thought of him as the creator of this or that fundamental theory and not as a Nobel or other prize laureate.
Yoichiro was born in Tokyo on 18 January 1921. His youth was affected by World War II. He served in the Japanese Army and was assigned to keep an eye on Sin-itiro Tomonaga, who was developing radar for one of the other military services. That assignment brought Yoichiro in contact with Tomonaga’s physics ideas, and after the war, though not a member of his group, Yoichiro kept up to date with its work. He set out on his own to calculate the electron’s anomalous magnetic moment and obtained the famous α/2π correction. He was not aware of similar work elsewhere because Douglas MacArthur, to make sure the Japanese did not develop nuclear weapons of their own, forbade the import of US physics journals to Japan. Instead, he encouraged the Japanese to read Time magazine. It was in Time that Yoichiro read an article about Julian Schwinger’s calculation of the electron’s anomalous magnetic moment, which reached the same result as the one he had obtained and which thereby made Yoichiro’s work no longer publishable.
Two years before receiving his PhD from Tokyo Imperial University in 1952, Yoichiro was appointed an associate professor at Osaka City University. In that capacity he published two papers whose results are often quoted under the names of physicists who rediscovered them. In one, Yoichiro derived the quantum field theoretic bound-state equation usually known as the Bethe–Salpeter equation. In another, with Kazuhiko Nishijima and Yoshio Yamaguchi, he proposed the mechanism of associated production of strange particles a year before Abraham Pais did.
Yoichiro realized that the center of worldwide theoretical physics research was in the US, and in 1952 he went to the Institute for Advanced Study in Princeton, New Jersey. In 1954 he moved to the University of Chicago, where he would spend the rest of his career. He caught the tail end of Chicago’s Enrico Fermi era.
In the mid 1950s, dispersion theory was center stage, and with Geoffrey Chew, Marvin Goldberger, and Francis Low, Yoichiro wrote the influential CGLN papers on meson scattering and photoproduction. From an analysis of the Stanford nucleon form-factor data, he predicted in 1957 the existence of the isospin-zero vector meson ω, which was confirmed in 1961.
At the University of Illinois in Urbana, the brilliant Bardeen-Cooper-Schrieffer theory of superconductivity was being developed. In an important paper, Yoichiro solved the problem of the theory’s apparent lack of gauge invariance, and in the fundamental papers he wrote with Giovanni Jona-Lasinio, he transported the basic idea of that work into relativistic quantum field theory. Those contributions marked the birth of the fundamental theory of spontaneous symmetry breaking and also led to the Brout-Englert-Higgs mechanism by which gauge fields acquire mass.
Yoichiro made the fundamental observation that whereas the laws of nature exhibit all kinds of exact symmetries, the ground state—the vacuum—can violate those symmetries, and that by itself can result in all the effects we normally associate with symmetry breaking. The signature of the spontaneous symmetry breaking mechanism is the appearance of a massless particle, the so-called Nambu–Goldstone boson, which interacts in a characteristic manner with other particles.
In 1965, with Moo-Young Han, Yoichiro set up a model of strong interactions based on a gauge treatment of a color symmetry similar to that in Wally Greenberg’s quark parastatistics. Both color gauging and spontaneous symmetry breaking are crucial to the standard model of particle physics.
Four years later Yoichiro and, independently, Holger Bech Nielsen and Leonard Susskind showed that the Veneziano four-point amplitudes and their N-point (N > 4) generalizations call for abandoning the picture of point-like elementary particles and replacing it by extended one-dimensional objects, strings. That work has led to a vast scientific enterprise still going strong today.
Yoichiro approached physics with his characteristic deep and creative curiosity and took great pleasure in his work. His keen insights were driven by a marvelous and unique form of intuition. His reasoning was clear and convincing, but it was hard to find out how those superb ideas arose in his thinking.
In a typically Japanese manner, Yoichiro was unable to use the word “no.” Even if a preposterous request was made of him, he would finally “agree” to it, but the more preposterous he found the request, the longer the time he took before saying yes. A “yes” delivered after an infinite pause was his version of the word “no.” That led to some funny situations while he was chair of the University of Chicago department of physics.
At a personal level, Yoichiro was a kind and understanding colleague, who established a pleasant and cordial atmosphere at the Enrico Fermi Institute. With his passing, we lose one of the few dominant figures who set the direction in which theoretical physics is evolving.
