JULIAN SCHWINGER was an outstanding figure in post-war theoretical physics.
Although a remarkable contributor to many areas of physics, Schwinger will be primarily remembered for his work on quantum electrodynamics, the present theoretical framework in which the interaction of matter with light (or other electromagnetic radiation) is quantitatively described. 'Quantitatively' is perhaps a mild understatement: nowhere in physics is there such astonishing agreement between experimental results and theoretical predictions for the magnetic properties of the electron that agreement is to one part in 1,000,000,000,000. It is for this work that Schwinger was awarded, jointly with Richard Feynman and Shin-Itiro Tomonaga, the 1965 Nobel Prize in Physics.
Schwinger's deep involvement in physics started at a remarkably early age. To judge by his first publication, he made his debut as a professional physicist at the age of 16. He was allowed to progress rapidly through the school system of New York City (where he was born). With the assistance of Isidor Rabi, the Austrian atomic physicist who won the Nobel Prize in 1944, he transferred to Columbia University, where he completed his college education. Although his PhD thesis had been written two or three years earlier, he received his doctorate formally 'only' at 21. He subsequently spent two years at Berkeley (in close association with J. Robert Oppenheimer) and two at Purdue University, working primarily on nuclear physics problems.
From 1943 to 1945 he worked at the Radiation Laboratory at the Massachusetts Institute of Technology (MIT), solving many problems of practical importance for radar. At the end of the Second World War, he joined the permanent faculty of Harvard University, which he was to leave 20 years later for the University of California at Los Angeles.
The Harvard years were the golden years of Schwinger's activity. Not only did he develop his version of Quantum Electrodynamics there - including the first calculation of the anomalous magnetic property of the electron, very soon after its experimental discovery - but he also created a school of theoretical physics of unequalled magnitude: some 50 students got their PhDs under his guidance. His lectures, displaying both his mathematical virtuosity and his polished usage of the English language, were legendary, and extended an influence far beyond Harvard. Schwinger's and Feynman's treatments of quantum electrodynamics were developed in parallel and even had a common starting-point, but were as different as the styles of these two great men: Feynman, extroverted and gregarious, always trying to associate diagrams with his equations; Schwinger, introverted and solitary, always trying to sublimate physical reality into elegant formalism. Much has been rumoured about rivalry between them; these rumours were unfounded, since they respected each other deeply.
After concentrating on general principles, Schwinger applied a speculative approach to specific experimental problems. (As he wrote, 'Such an approach has its dangers, but it can have its rewards.') He was particularly pleased by an anticipation, early in 1957, of the existence of two different neutrinos associated, respectively, with the electron and the muon (the heavy electron), which was confirmed experimentally in 1963. Another speculation, that all weak interactions are transmitted by very massive, charged unit-spin particles, was substantially vindicated in 1983 by a famous experiment at CERN (now the European Laboratory for Particle Physics), in Geneva. It should be added that Schwinger was not unique as concerns these particular speculative proposals.
After leaving Harvard for UCLA, Schwinger abandoned the mainstream of theoretical physics - 'to follow', he said, 'my own advice about the importance of a phenomenological theory of particles'. He invented and systematically developed 'source theory', which is supposed to deal uniformly with all particles, thus providing a general approach to all physical phenomena. This approach has been described in three volumes under the title Particles, Sources, and Fields, published between 1970 and 1989.
Although not inclined to leave the ivory tower, Schwinger could be quite successful in popularising science if he decided to do so. In 1979, a BBC audience first enjoyed his television series Understanding Space and Time, produced jointly by UCLA and the Open University, which has since been rerun several times.