Julian Schwinger (12 February 1918 - 16 July 1994)
In
the
post-quantum-mechanics era, few physicists, if any, have matched Julian
Schwinger in contributions to and influence on the development of
physics. A towering giant in theoretical physics, Schwinger left his
indelible mark on diverse fields such as quantum mechanics, quantum
field theory, electrodynamics, nuclear physics, statistical mechanics,
atomic physics, elementary particle physics, gravity, and mathematical
physics. He shared the Nobel
Prize in Physics 1965 with Sin-Itiro
Tomonaga and Richard
P. Feynman for "their
fundamental work in quantum electrodynamics, with deep-ploughing
consequences for the physics of elementary particles".
Rare is the theoretical physicist who
makes repeated and varied
contributions apart from the throng; rarer still one who not only
contributes but sets standards and priorities single-handedly. Julian
Schwinger was such an individual. His ideas,
discoveries, and techniques pervade all areas of theoretical physics.
Fiercely independent, the
teen-age Schwinger taught himself physics and
mathematics by reading books and journals, mostly at the New York
Public Library. With his exceptional gift in
grasping the profound ideas of physics, he debuted as a theoretical
physicist at the age of sixteen, to be judged by his 'zeroth' publication.
With I.I.
Rabi
as his discoverer and mentor, he
received his Ph.D. in physics from Columbia
University at the age of twenty-one in 1939.
For the next two years he did
postdoctoral studies at the University of California, Berkerly
, first as a
National Research Fellow and then as assistant to J.R.
Oppenheimer. After the
outbreak of the Pacific War, he worked on radar for the allied war
effort at the Radiation Laboratory at the
Massachusetts Institute of
Technology in Cambridge. He first approached electromagnetic radar
problems with methods pertaining to
nuclear physics, but later thought of nuclear physics in the
language of electrical engineering. That would eventually emerge as the
effective-range formulation of nuclear scattering. Then, being aware of
the large microwave powers available, Schwinger thought about electron
accelerators, which led to the question of
radiation by electrons in magnetic fields. In studying the latter
problem he was reminded, by classical physics, that the reaction of
the electron's field alters the properties of the particle, including
its mass. This would be important in the intensive developments of
renormalized quantum electrodynamics(QED), on which Schwinger did his
most famous research work and for which he was awarded the Nobel Prize
in physics in 1965.
Earlier he shared the first Einstein
Prize with the mathematician Kurt
Gödel in 1951, and was awarded the National Medal of Science in
1964. The theory of
QED has withstood the test of time; it is arguably the most accurate
theory ever devised by mankind. As
C. N. Yang
observed, "Renormalization was one of the
great peaks of the development of fundamental physics in this century.
Scaling this peak was a difficult enterprise. Many many people can
climb the peak now. But, the person who first conquered the peak was
Julian Schwinger."
In addition to QED, Schwinger made
fundamental contributions to many
areas of physics. He laid down the foundation for much of quantum field
theory as we understand it today. He introduced operator and
functional techniques, Euclidean and finite temperature field theories,
proper-time methods and strong field techniques, and he discovered
the anomalies of quantized fields. His Quantum Action Principle
summarizes concisely all kinematics and
dynamics of quantum mechanics. Schwinger was also the first to note
that there must be other neutrinos besides the electron neutrino,
which is but one of the many steps he took in preparing the ground for
the electroweak unification that was completed by his student
S.L.
Glashow and others.
Julian Schwinger joined the faculty
of Harvard University shortly after
WW II and was quickly promoted to the youngest Full Professor at the
time. His work on Source Theory began in Harvard in 1965 but most of
the later development was carried out at UCLA, where he moved to in
1970 and retired from in 1988. Source theory represented Schwinger's
effort to replace the prevailing operator field theory, to which he had
contributed so richly and fundamentally, by a philosophy and
methodology
that eliminated all infinite quantities. Not only did he succeed in
constructing an infinity-free formalism, which led to the introduction
of new experimental information and new theoretical ideas, he and his
source theory group, Kim Milton, Lester de Raad and Wu-Yang Tsai, also
made many applications to high-energy physics and showed that
source theory was a
very effective calculational tool.
Apart from source theory, two of
his important projects at UCLA are an extensive
study on multi-electron atoms carried out in collaboration with B.-G.
Englert and
the experiments on sonoluminescence, stimulated by Seth Putterman.
One significant result of the first project became known as the
Schwinger
conjecture and was proven by the mathematicians
Charles Fefferman
and
Louis Angel Seco.
In the second project, Schwinger speculated that the emitted light is
dynamic
Casimir radiation and studied this radiation by source-theory
techniques. The
analysis of this complicated problem, which involves the interaction
between hydrodynamic and electromagnetic effects, was not completed
until Schwinger's death, though he continued working on this problem
even during his fatal illness.
Clarice Carrol, Julian's wife for 47
years, is now the President of the
Julian Schwinger Foundation for Physics Research, which he founded
with his Last Will.
4. P. C. Martin and S. L. Glashow.
Julian Schwinger 1918-1994.
Biographical Memoir, National Academy
of Sciences. 2008.
The biographical sketch above uses material, sometimes
quoted
literally,
compiled by Y. Jack Ng in Julian Schwinger: The Physicist, the
Teacher and the Man (World Scientific, Singapore 1996) and
S. Treiman's review thereof in the October 1996 issue of Physics Today,
and also biographical material available at the website of the Nobel
Prize
Foundation.
