(1918-1994) American Theoretician, Quantum Field Theorist
Julian Schwinger, one of the great theoretical physicists of his time, won the 1965 Nobel Prize in physics for his role as a prime architect of quantum electrodynamics (QED), the study of the interaction of electrons and electromagnetic radiation. Working concurrently with richard phillips feynman and sin-itiro tomonaga, Schwinger laid the foundations of relativistic QED and set the stage for freeman dyson’s work, which reconciled his mathematical formalism with Feyn-man’s diagrammatic formulation of QED.
Schwinger was born on February 12, 1918, in New York City, into a middle-class Jewish family, the younger of two sons. His father, Benjamin, who had immigrated to the United States in 1880, was a successful designer of women’s clothing. His mother, Belle, had immigrated as a child from Lodz, then in eastern Poland. The family lived in Jewish Harlem, a well-to-do neighborhood, and later in the prosperous environs of Riverside Drive. A prodigy who discovered his obsession with physics at an early age, Schwinger attended Townsend Harris High School, a renowned institution affiliated with the City College of New York (CCNY), which he entered in 1934.
At the age of 16, he wrote his first paper (never published), “On the Interaction of Several Electrons,” an insightful analysis of the electron field within the context of the quantum mechanics of the electromagnetic field. He was from the beginning an autodidact, who sat in the college’s library teaching himself modern physics from original papers. He read all the papers of paul adrien maurice dirac, who he later said was “by far the overwhelming influence” in his thinking. However, his failure to attend classes led to a mediocre record at CCNY.
Then, through the intervention of isidor isaac rabi, whom he instantly impressed with his theoretical insights, Schwinger was admitted to Columbia University, where, despite an F in a chemistry course, he was elected to Phi Beta Kappa. At the age of 19, he published his first paper, “The Magnetic Scattering of Neutrons,” which contained the core of his soon-to-be-completed dissertation, in Physical Review in 1937. He hid not receive a Ph.D. until two years later, however, since, refusing to attend classes, he had trouble fulfilling the formal requirements for the degree. For the next two years he was at the University of California at Berkeley, first as a National Research Council Fellow and then as an assistant to j. robert oppenheimer.
When World War II began, he was teaching elementary physics to engineering students at Purdue University, where, in 1942, he was made an assistant professor. The following year he was given a leave of absence by Purdue and went to work at the Radiation Laboratory at the Massachusetts Institute of Technology (MIT), Cambridge. He was soon sent to the University of Chicago’s Metallurgical Laboratory, which was involved in the atom bomb project. During that summer he worked on improving the design of the Hanford nuclear reactor. His colleagues at Chicago found working with him to be a challenge, since Schwinger had a two-handed blackboard technique and when excited would be simultaneously solving two equations. Dissatisfied with this type of work, Schwinger drove back to Boston and was reinstated at the Radiation Lab; he worked in George Uhlenbeck’s group, in which he became the prime force in the development of radar waveguides.
Later, he would say of his work at the Radiation Lab, “I first approached radar problems as a nuclear physicist, but soon I began to think of nuclear physics in the language of electrical engineering that would eventually emerge as the effective range formulation of nuclear scattering.”
Having become aware of the large magnitude of microwave powers available, he began to think about electron accelerators, which led to the question of radiation by electrons in magnetic fields. He was especially impressed by the fact that at the classical level, the reaction of the electron’s field alters the properties of the particle, including its mass. This property would be significant in the future development of QED. During this period he also developed variational techniques that produced major advances in several fields of mathematical physics.
In 1945, when the war ended, Schwinger resigned from his position at Purdue to become associate professor at Harvard. Two years later he was promoted to full professor and married Clarice Carrol of Boston. This was the beginning of a legendary period of physics in Cambridge. Schwinger’s friendship with the eminent theorist Victor Weisskopf, who was at MIT, forged the theoretical physicists at Harvard and MIT into a close-knit community. Schwinger’s brilliant lectures and mentoring of outstanding graduate students helped Harvard rapidly become one of the most important training grounds for theoretical physicists. The notes from one of Schwinger’s courses were compiled by John Blatt as “Advanced Theoretical Nuclear Physics” and had a tremendous influence on graduate students in the late 1940s and 1950s.
Julian Seymour Schwinger was a prime theoretical architect of relativistic quantum electrodynamics (QED), the study of the interaction of electrons, positrons, and electromagnetic radiation.
While Schwinger was thriving at Harvard, in 1947, willis eugene lamb, working in Rabi’s laboratory at Columbia University, began experimental investigations that would have a profound impact on the formulation of QED. Applying the art of spectroscopy with unprecedented precision, he shone a beam of microwaves onto a hot wisp of hydrogen gas blowing from an oven. He found that two fine structure levels in the next lowest group, which should have coincided with the Dirac theory, were in reality shifted relative to each other by a certain quantity (the Lamb shift). He measured it with great accuracy and later made similar measurements on heavy hydrogen. Lamb’s announcement of his news to the participants of the 1947 Shelter Island (New York) Conference had an electrifying effect. Schwinger, who was present, commented:
Everybody was highly euphoric. . . . The facts were incredible—to be told that the sacred Dirac theory was breaking down all over the place.
Another flaw in the Dirac theory was found that same year in the same Columbia lab, by polykarp kusch, who discovered a tiny discrepancy from what the theory predicted when he made highly accurate measurements of the magnetic moment of the electron.
Schwinger and other quantum theorists such as hans albrecht bethe, richard phillips feynman, and sin-itiro tomonaga began to realize that what was missing from Dirac’s theory was a proper interpretation of the unwieldy concept of the self-interaction of the electron, which by its very nature contains infinities, thus preventing a straightforward physical interpretation. When the electromagnetic field is quantized, according to the rules of quantum mechanics, particles of light called photons are generated. At the heart of the quantum electrodynamic process is the quantum exchange force by which different electrons interact by exchanging photons; in this context an electron can also exchange a photon with itself.
How were physicists to deal with this self-interaction? QED, as it was formulated in the mid-1940s, was not considered to be a relativistically covariant formalism (that is, it was not formally compatible with the rules of special relativity). This lack of relativistic covariance prevented a unique mathematical interpretation of the physical effects of self-interaction. Schwinger changed all this when he discovered a relativis-tically covariant form for QED, by introducing the concept of renormalization, which allowed a consistent mathematical interpretation of the self-energy infinities.
On the physical level, renormalization implied that physical particles are surrounded by a cloud of virtual particles, that is, ghostly particles that exist within the context of the uncertainty principle, whose energy, momentum, and charge modify the physical appearance of the bare original particle. In applying the method of renormalization, Schwinger found that the self-energy infinities could be subtracted out. This led to a fully consistent relativistic theory of QED that explained the Lamb shift as due to the virtual particle modification of the Coulomb force between the electron and the proton in the hydrogen atom. Using his new relativistically covariant QED formalism with renormalization, Schwinger was also able to calculate the anomalous magnetic moment of the electron.
After his groundbreaking work on QED, Schwinger concentrated on general theoretical questions, rather than specific topics of immediate experimental interest. Early in 1957, he anticipated the existence of two different neutrinos, associated, respectively, with the electron and the muon. This was later confirmed experimentally by leon m. lederman. A related and somewhat earlier speculation, that all weak interactions are transmitted by heavy, charged, unit-spin particles, was also later confirmed by decisive experimental tests. Schwinger’s habit of finding theoretical value in experimentally unknown particles led him to a revived concern with the possible existence of magnetically charged particles called magnetic monopoles, which later were found to be involved in the understanding of strong interactions.
In his later years, Schwinger backed away from his earlier work on quantum field theory and worked on a phenomenological theory of particles, which he called “source theory,” which deals uniformly with strong interacting particles, photons, and gravitons, thus providing a general approach to all physical phenomena. He described this work in his two-volume Particles, Sources, and Fields. The theory’s modest scientific goal was to move from solid knowledge of phenomena at accessible energies to that at higher energies. He perceived it as a sound and simple mathematical description of laboratory practice, without the difficulties that occur in the standard quantum field operator formalism. It incorporated no infinities and thus needed no renormalization; no new constants would appear, because all parameters were fixed when the class of phenomena under examination was fixed.
However, the physics community, which was evolving into the realm of the unified gauge theories of elementary particles, was less than enthusiastic about Schwinger’s new development. In 1965, Physical Review Letters returned his submissions with scathing comments. In protest, he resigned from the American Physical Society. Schwinger grew increasingly isolated as he pursued his new theory. Nonetheless, his views influenced what is now known as the effective field theory (EFT) approach to particle processes. The rationalizing of EFT has produced a resurgence of interest in Schwinger’s legacy, even if its long-term effects are currently unpredictable.
In 1972, Schwinger left Harvard and from then until his death in 1994, he taught at the University of California, Los Angeles. An enormously respected and highly gifted lecturer, he supervised numerous gifted graduate students, including three future Nobel Prize winners. His books include Discontinuities in Waveguides, with D. Saxon (1968); Quantum Kinematics and Dynamics (1970); Einstein’s Legacy: The Unity of Space and Time, (1987) and Particles, Sources, and Fields (1989). He died on July 16, 1994, in Los Angeles at the age of 76.
Schwinger’s career spanned an unusual arc— from early recognition for his work on QED to post-Nobel Prize ostracism for his work on the source theory of elementary particles. Nonetheless, he was a legendary figure in mid-20th-century physics, who will be remembered for his reformulation of QED. His concept of renormal-ization made possible the first relativistically self-consistent framework for the quantization of fields from which physical consequences could be extracted and experimentally verified.
