## Schwinger, Julian

## Schwinger, Julian

# SCHWINGER, JULIAN

Julian Schwinger was born in New York City on February 12, 1918, to a middle-class Jewish family. His father immigrated to the United States in 1880 when he was in his early teens and later became a successful designer of women's clothing. The family of Schwinger's mother came to the United States when she was very young. Julian was a very precocious and talented child, but it was Harold, Julian's older brother, who was considered the bright child in the family since he won many prizes at school. Like Harold, Schwinger attended Townsend Harris High School, then one of the best secondary schools in the United States, located on the campus of the City College of New York (CCNY). Schwinger was fourteen when he entered the school in 1932. He graduated from Townsend Harris in 1934 and entered CCNY in the fall of that year as a physics major.

Schwinger's precocity and ability in physics had made him a living legend even while in high school. However, he did not do well at CCNY. He spent most of his time in the library reading advanced physics and mathematical texts and rarely went to his classes. His grades reflected his erratic class attendance. The matter became serious enough for Lloyd Motz, one of his physics instructors at CCNY, to bring Schwinger's problems to Isidor Rabi's attention. Motz was aware of Schwinger's talents because Schwinger had given him a copy of a paper he had written as a freshman entitled "On the Interaction of Several Electrons." Additionally, as a sophomore, Schwinger had collaborated with Motz and had calculated the lifetime of the neutron in the Konopinski-Uhlenbeck version of Enrico Fermi's beta-decay theory. Similarly, even as a freshman at CCNY, Schwinger regularly attended the weekly theoretical seminar that Rabi and Gregory Breit ran at Columbia on Wednesday

evenings. The 16-year-old undergraduate published a joint paper with Otto Halpern on the problem of the polarization of electrons in double scattering experiments, the young Julian having done extensive and difficult calculations.

Largely through Rabi's efforts, Columbia offered Schwinger a scholarship, and Schwinger became an active participant in Rabi's research activities. During his senior year, Schwinger worked on the problem of the magnetic scattering of slow neutrons by atoms. In early January 1937 he sent a manuscript entitled "The Magnetic Scattering of Neutrons" to the *Physical Review.* The characteristics that distinguished Schwinger's subsequent works are present in this paper: an important physical problem is addressed; the solution is elegant; the methods used are powerful; contact is made with experimental data; and suggestions for empirical tests are given. Edward Teller, who was visiting Columbia in the spring of 1937, suggested that Schwinger's research on the scattering of neutrons be further developed for his Ph.D. thesis. Schwinger worked with Teller and showed that the scattering of neutrons by orthoand para-hydrogen could yield information about the spin dependence and the range of neutron-proton interaction. That Schwinger had written his Ph.D. dissertation before receiving his bachelor's degree is indicative of his remarkable talents.

After receiving his B.S. from Columbia in 1936, Schwinger continued his graduate studies there. But shortly thereafter, Rabi arranged for Schwinger to receive a traveling fellowship from Columbia for the academic year 1937–8. The plan was for Schwinger to spend six months in Wisconsin studying with Gregory Breit and Eugene Wigner, and then to go on to Berkeley for another six months to work with Robert Oppenheimer. As it turned out, he remained at Wisconsin for the entire year, and there developed his characteristic working habits: staying up at night and sleeping during the day. Thereafter, Schwinger did go to Berkeley for two years: spending the first as a National Research Council (NRC) fellow and the second as a research associate to Oppenheimer. His stay was enormously productive. He collaborated extensively and worked on a wide range of subjects. An analysis of the electromagnetic properties of the deuteron when tensor forces are present led him to predict the existence of the deuteron's quadrupole moment—before it had been measured by Jerome Kellogg, Norman Ramsey, Isidor I. Rabi, and Jerrold Zacharias.

Schwinger left Berkeley in the summer of 1941 to accept a position as instructor at Purdue. An active program in semiconductor research to develop better rectifiers for the detection of radar was being carried out there by Karl Lark-Horovitz for the Radiation Laboratory (Rad Lab). In 1942 Schwinger and several other theorists at Purdue were asked to join a Rad Lab project on the propagation of microwave radiation under Hans Bethe's direction.

When Los Alamos was organized in early 1943 to build an atomic bomb, Robert Oppenheimer invited Schwinger to join the laboratory, but he declined. However, since many leading theorists were leaving their academic posts to go to Los Alamos, Schwinger was offered a full-time position at MIT, which he started in the fall of 1943. In his work at MIT Schwinger indicated how to set up and solve a wide variety of microwave problems. In a memorial lecture for Sin-itiro Tomonaga delivered in 1980, Schwinger commented that his waveguide investigations showed the utility of organizing a theory to isolate those inner structural aspects that are not probed under the given experimental circumstances. That lesson was subsequently applied to the effectiverange description of nuclear forces. It was this viewpoint that would lead to the quantum electro-dynamic concept of self-consistent subtraction or renormalization. Schwinger also worked on the problem of the radiation emitted by fast electrons traveling in synchrotron orbits. The formulation of this problem taught him the importance of describing relativistic situations covariantly, that is, without specialization to any particular coordinate system.

In 1944 universities began competing with one another for the outstanding talent in physics, and Schwinger was courted by a number of academic institutions and, in particular, by Harvard. In the fall of 1945 Schwinger accepted an appointment there as an associate professor. A year later he was offered a full professorship at Berkeley, and Harvard promptly promoted him. This same year Schwinger married Clarice Carrol of Boston. Harvard provided him with outstanding graduate students, and he became the thesis adviser to many of them. They, together with many of MIT's graduate students and postdoctoral fellows and a fair number of the Harvard and MIT physics faculty, formed the audience for Schwinger's brilliant lectures. It is difficult to exaggerate the impact of these lectures—and of the widely circulated notes based on them—on the generation of physics graduate students in the late 1940s and 1950s. Many of today's texts on nuclear physics, electromagnetic theory, quantum mechanics, quantum field theory, and statistical mechanics have incorporated the approaches, techniques, and examples that Schwinger discussed in his lectures.

During his stay at Harvard, Schwinger's contributions and that of his students to physics were numerous and profound. In the late 1940s he reformulated quantum electrodynamics (QED) in terms of a manifestly covariant formalism which—using the concepts of mass and charge renormalization— allowed him to unambiguously extract the corrections to the magnetic moment of the electron that the theory implied. Similarly, he was able to calculate the level shifts predicted by QED for the energy levels of a hydrogen atom described by the Dirac equation. His covariant formulation, when amended with the notions of renormalization, was the first self-consistent framework in quantum field theory (QFT) from which physical consequences could be extracted and checked with experiments. For this work, Schwinger shared the Nobel Prize in Physics in 1965 with Richard Feynman and Sin-itiro Tomonaga. Schwinger's 1948 formulation of QED could not be easily extended to calculate higher-order effects. He thereafter developed increasingly powerful calculation techniques.

In 1951 in an eight-page-paper published in the *Proceedings of the National Academy of Sciences,* Schwinger gave a concise presentation of his formulation of the equations for Green's functions of quantum fields. He there introduced the use of "sources"—classical sources for Bosonic fields and Grassmann anticommuting sources for Fermionic fields—as functional variables. To this same period belong his formulation of the Schwinger action principle and the use of temperature-dependent many-particle Green's functions for addressing equilibrium and nonequilibrium problems in condensed-matter physics.

In the mid-1960s Schwinger started reformulating the foundations of fundamental physics and expressing these within a new framework: source theory. Source theory represented Schwinger's efforts to replace the prevailing operator field theory by a philosophy and methodology that eliminated all infinite quantities. Schwinger's objections to operator field theory arose at the pragmatic level from the fact that it seemed impossible to incorporate the strong interactions within its framework and at the philosophical level that from the fact it made implicit assumptions about unknown phenomena at inaccessible, very high energies to make predictions at lower energies. Source theory, on the other hand, began with robust knowledge about known phenomena at accessible energies to make predictions of physical phenomena at higher energies. But, Schwinger's insistence on basing his theories on phenomenology led him to reject the quark model of hadrons and quantum chromodynamics. His pursuit of source theory in the early 1970s, at the very time quantum field theory was resurging in the aftermath of the successes of the Glashow-Salam-Weinberg electroweak theory and of the proof by Gerardus 't Hooft that the Yang-Mills theory with a Higgs mechanism for breaking symmetry and giving masses to particles is renormalizable, alienated him from his community and drove him out of the mainstream of modern physics. This alienation was further aggravated when he left Harvard in February 1971 to accept a position at UCLA. He thus had to establish ties to a new community with interests somewhat different from those in Cambridge.

As a young Harvard professor, Schwinger had been the person who set the agenda for the field theory and high-energy community. While at Harvard, he directed some seventy doctoral theses and became an important influence on at least four generations of active, and later influential, theoretical physicists. However, when he left the mainstream of particle physics and challenged the foundations on which numerous theoretical investigations were being carried out, his new endeavors were contemptuously dismissed by the community as mistaken or irrelevant. His research papers, in turn, were rejected in a dismissive manner by *Physical Review Letters* and other leading journals. His response was to resign both as a member and as a fellow of the American Physical Society. The hostility toward source theory that he had experienced probably contributed to his involvement in such fringe projects as cold fusion.

Starting in the 1980s, after teaching a course in quantum mechanics, Schwinger began writing a series of papers on the Thomas-Fermi model of atoms, and together with Berthold-Georg Englert he elaborated on the approach. These contributions have been deemed extremely important by the atomic physics community. His last scientific endeavor before his death in 1994 was an attempt to explain sonoluminescence.

Schwinger's work extended to almost every frontier of modern theoretical physics. He made farreaching contributions to nuclear, particle, and atomic physics, to statistical mechanics, to classical electrodynamics, and to general relativity. Many of the mathematical techniques that he developed are to be found in every theorist's toolkit. He was one of the prophets and pioneers in the use of gauge theories. The influence of Julian Schwinger on the physics of his time was profound.

*See also:*Feynman, Richard; Quantum Electrodynamics; Quantum Field Theory; Tomonaga, Sinitiro

## Bibliography

Flato, M.; Fronsdal, C.; and Milton, K. A.; eds. *Selected Papers (1937–1976) of Julian Schwinger* (Reidel, Dordrecht, Netherlands, 1979).

Mehra, J., and Milton, K. A. *Climbing the Mountain: The Scientific Biography of Julian Schwinger* (Oxford University Press, New York, 2000).

Milton, K. A., ed. *A Quantum Legacy: Seminal Papers of Julian Schwinger* (World Scientific, Singapore, 2000).

Schweber, S. S. *QED and the Men Who Made It* (Princeton University Press, Princeton, 1994).

Schwinger, J. *Particles, Sources and Fields,* 3 Vols. (Addison-Wesley, Redwood City, CA, 1989).

*Silvan S. Schweber*