Biographical Memoirs Volume 90 (2009) / Chapter Skim
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JULIAN SCHWINGER
JULIAN SCHWINGER
Pages 332-353
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From page 332... ...
Photograph courtesy of Clarice Schwinger.
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From page 333... ...
It began "According to Julian Schwinger" and invoked "the Green's function expression for …". References to unpublished Schwinger lecture notes and some classic Schwinger papers followed.
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. His knowledge of quantum electrodynamics is certainly equal to my own, and I can hardly understand how he could acquire that knowledge in less than two years and almost all by himself." Bethe concludes that "Schwinger will develop into one of the world's foremost theoretical physicists if properly guided, i.e., if his curriculum is largely left to his own free choice.
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His analysis for j = 1/2 remains the prototype for all discussions of transitions in two-level systems by "Rabi flipping." During the spring of 1937, he and Edward Teller studied coherent neutron scattering by hydrogen molecules, showing how the spin-dependent, zero-energy, neutron-proton-scattering amplitudes could be determined from the experimental data. This topic was the theme of his doctoral thesis.
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From page 336... ...
But this generalization of the Dirac equation to particles with spin 3/2, and the study of its invariances when the particles are massless, has been recalled by theorists who postulate a gravitino, a spin-3/2 fermion supersymmetric partner of the graviton. Notwithstanding a ticker tape parade for Albert Einstein, theoretical physics held little fascination for the American public or major American universities prior to the Second World War.
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From page 337... ...
recalled that in February 1942, "We had to spend the whole time trying to cheer Julian up" at his 24th birthday party "because he had not yet made the great discovery expected of him." Along with physicists at Cornell University and the University of Rochester and with colleagues at Purdue, Schwinger spent the first year and a half of World War II working on the properties of microwave cavities. The work was coordinated with and supported by MIT Radiation Laboratory research projects.
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From page 338... ...
Schwinger and Marcuvitz appreciated the value of integral equation formulations of waveguide theory that incorporate the boundary conditions accompanying partial differential equation formulations and can be cast in the engineering language of transmission lines and networks. The isolation of complex internal properties of components and the characterization of these components through a small set of parameters provided valuable insights -- insights that would later prove valuable in characterizing nuclear phenomena via effective range theory, scattering matrices, and new formal approaches to complex scattering processes.
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From page 339... ...
During this period, Schwinger also designed a novel accelerator, later named the minotron. In addition to work on other aspects of synchrotron radiations, notepads in his desk drawers at that time included studies of neutron scattering in a Coulomb field, and a group-theory-free approach to the properties of angular momentum that expresses angular momentum operators in terms of oscillator creation and annihilation operators.
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When the academic year ended, Schwinger and 22 other physicists headed off to the Shelter Island conference on the foundations of quantum physics, where the electrodynamic origin of the spectral lineshift measured by Willis Lamb and Robert Retherford was discussed. Legend has it that Weisskopf and Schwinger proposed that in the Dirac theory compensating effects of electrons and positrons could lead to a cancellation of divergences, and that Hans Bethe -- on his way home from the conference -- recognized that the bulk of the effect could be estimated nonrelativistically.
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From page 341... ...
Each time he offered a course he carefully reworked and honed his ideas, methods, and examples, presenting them in a new way, a way that differed from his earlier versions circulating in others' articles and lecture notes, often without attribution. Significant portions of many classic texts on nuclear physics, atomic physics, optics, electromagnetism, statistical physics, quantum mechanics, and quantum field theory can be traced to one or another version of his lectures.
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From page 342... ...
Not until September 1947 did Schwinger begin to work on the electrodynamic effects responsible for deviations of experimental observations from values predicted by the Dirac equation. Hyperfine structure measurements of hydrogen, deuterium, and tritium by John Nafe, Edward Nelson, and Rabi indicated a 0.12 percent error in the electron's magnetic moment, and measurements by Lamb and Retherford displayed a splitting of about 1050 megacycles between states of the hydrogen atom with degenerate Dirac energies.
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From page 343... ...
the propagation and interaction of quantum fields. This approach opened the way for major conceptual and computational advances in quantum electrodynamics.
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Schwinger's investigations of quantum field theory continued through the 1950s. Relativistic invariance and gauge invariance constrain the formally divergent expressions appearing in quantum electrodynamics calculations.
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The nonperturbative properties of a Dirac field coupled to a prescribed external electromagnetic field, first derived in this paper, are still widely used and admired. Schwinger saw that many ambiguities associated with interacting quantum fields lay in the treatment of formal expressions for composite operators such as currents.
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From page 346... ...
He also suggested that vacuum expectation values of scalar fields could provide a way of breaking symmetries and giving fermions their masses. Schwinger's 1957 paper on particle symmetries appeared at a time of rapid progress and great confusion, between the discoveries of parity violation and the V-A nature of the weak interactions.
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Today we recognize the key roles such terms play in particle physics and statistical mechanics. In the late 1960s Schwinger directed much of his attention to his source theory.
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From page 348... ...
The publicized experiments might be flawed, he would observe, but fundamental physical principles do not rigorously exclude the possibility that without tokamaks and high-temperature plasmas, somehow, in some way, in some material, the energy required for fusion might be coherently concentrated and transferred from atoms to nuclei. One of Schwinger's last papers is a 1993 talk titled "The Greening of Quantum Field Theory: George and I, Lecture at Nottingham, July 14, 1993." It contains the count of references to Green in Discontinuities in Waveguides mentioned earlier and a recital of a multitude of the linkages with George Green of Schwinger's research on field and particle theory, statistical mechanics, through to work on the Casimir effect and sonoluminescence.
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Two features shared by Schwinger's professional offspring are striking: the diversity of their specialties and the consistently high regard and great debt they express for his mentorship. The group includes leaders in particle theory, nuclear physics, astrophysics, gravity, space physics, optics, atomic physics, condensed matter physics, electromagnetic phenomena, applied physics, mathematics, and biology.
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While too reserved to savor media stardom, he enjoyed presenting relativity to a wide audience in a popular book and on BBC television. He was always willing to lend his name and support to worthy causes.
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72:742. 1948 On quantum electrodynamics and the magnetic moment of the electron.
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1966 Magnetic charge and quantum field theory.
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Free man and Co. 1996 The Greening of Quantum Field Theory: George and I, Lecture at Nottingham, July 14, 1993.
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