Cherished friend, colleague, and collaborator
Sidney Richard Coleman, Donner Professor of Science at Harvard University, died on 18 November
2007 after a long struggle with Lewy body disease.
Sidney was born on 7 March
1937 in Chicago, where he grew up. In 1957 he received his BS degree in physics from the Illinois Institute
of Technology, then entered graduate school at Caltech. We met in 1960 while I spent a postdoctoral
year there. By then Sidney was just beginning his doctoral research under Murray Gell-Mann. We
became fast friends with common interests in science fiction and in climbing Mount Wilson on weekends
to escape the Pasadena smog. Our scientific collaboration began when Gell-Mann (and independently,
Yuval Ne'eman) devised the unitary-symmetry scheme. While most of our colleagues were put off
by the unfamiliar math, we became traveling disciples of the Eightfold Way and its sequels, such
as the eponymous Coleman–Glashow mass formula. Sidney's 1962 thesis, "The Structure of
Strong Interaction Symmetries," began with a presciently apt quotation from Justine
by the Marquis de Sade: "What we do here is nothing to what we dream of doing." So it would be!
In 1962 Sidney accepted
an instructorship at Harvard. Throughout the 1960s we often traveled together to Europe, most
frequently to Erice, Italy, to visit Antonino Zichichi's subnuclear school, where Sidney could
be counted on to present a formidable series of lectures. Many of those lectures are published in
Aspects of Symmetry (Cambridge University Press, 1985), a book that remains essential
reading to aspiring particle theorists.
To the world at large, Sidney
is hardly as well known as such popular expositors of science as Isaac Asimov, Stephen Hawking,
or Carl Sagan. But to the community of theoretical physicists he is a luminary. In the 1960s quantum
field theory had lost much of its appeal. Whatever its great success for electrodynamics, QFT seemed
unable to deal with the host of new particles being found and the puzzles they posed. Having devolved
into a formal scheme of manipulating diagrams and following rules, the underlying physics was
obscured. Sidney was, in large measure, responsible for the rebirth of QFT and thus laid the foundation
for today's standard model. Throughout the 1970s and beyond, Sidney understood quantum physics
more deeply than most of his colleagues.
Although he collaborated
with students on many of his papers, Sidney usually wrote them, as evidenced by their wit and lucidity.
Of Sidney's many significant contributions, I offer three examples.
The 1967 Coleman–Mandula
theorem showed that no conventional Lie algebra could nontrivially incorporate both spacetime
symmetries and such internal symmetries as charge conservation. That no-go theorem showed that
many then-popular speculations were senseless.
The 1973 Coleman–Weinberg
analysis of radiative corrections and spontaneous symmetry breaking, along with Sidney's Erice
lecture the following year on spontaneous symmetry breakdown and gauge fields, was an enormous
contribution to our understanding of QFT. Previously, spontaneous symmetry breaking had to be
put in explicitly via the Higgs mechanism. Sidney and Erick Weinberg showed how it could emerge
from an effective potential V(φ)
obtained by incorporating quantum radiative corrections, and, moreover, they showed that this
potential is indeed the vacuum energy density in a state for which the field has expectation value
φ.
Sidney's papers "The Fate
of the False Vacuum" parts I and II, the latter with Curtis Callan, and a third, "Gravitational Effects
of and on Vacuum Decay," with Frank De Luccia, make up another remarkable achievement. The titles
tersely and clearly summarize the papers, as do all of Sidney's sometimes whimsical titles. The
potential of a quantum theory may have more than one minimum. Sidney calculated the decay rate of
a higher (false) minimum: to lowest order in part I, with quantum corrections in part II, and in an
expanding universe in the third paper. That work is central to many subsequent developments in
both particle physics and cosmology.
Sidney's fundamental
contributions to our understanding of QFT earned him the 1990 Dirac Medal from the Abdus Salam International
Centre for Theoretical Physics and the American Physical Society's 2000 Dannie Heineman Prize.
At Harvard, Sidney was
a much-sought-after research supervisor. Most of his 40 or so thesis students are now well-known
scientists. For more than four decades, virtually all physics students at Harvard—both
graduate and undergraduate—have been inspired by Sidney's wonderfully coherent and witty
presentations of quantum mechanics and quantum field theory. Some of his legendary lectures were
recorded for posterity; others were saved as lecture notes by diligent students so that their successors
might have a taste of Sidney's inimitable style.
In 2005 Harvard hosted
a festival to honor Sidney. In attendance were many of his former students and many distinguished
physicists. Steven Weinberg remarked that "Sidney is a theorist's theorist. He has not been so
much concerned with accounting for the latest data from experiments as with understanding deeply
what our theories really mean. I can say I learned more about physics from Sidney than from anyone
else." In that connection let me recall an oft-told tale: Working late into the night as was his wont,
Sidney rarely appeared at Harvard much before noon. He once arrived so late at a seminar by Weinberg
that all he heard was Steve's uncertain response to a question from the audience. "I know the answer!"
shouted Sidney on entering the room, "What's the question?" Indeed, he often answered questions
before they were asked. Theorists who consulted him were often astounded as Sidney would patiently
explain the implications of their own ideas.
Sidney was both an incomparable
teacher and the most learned sage and sharpest critic in the world of theoretical physics: He was
Pauli's tongue in Einstein's image. We have been deprived all too soon of one of our generation's
most profound and imaginative minds.
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