Sheldon Lee Glashow

The theoretical work of American physicist Sheldon Lee Glashow (born 1932) made an important contribution to the unification of elementary particles and forces. He shared the 1979 Nobel Prize in Physics.

Sheldon Lee Glashow was born on December 5, 1932, in the northern tip of Manhattan in New York City. He was the youngest of three children of two Russian immigrants, Lewis Gluchovsky, a plumber, and Bella Rubin. He graduated from the Bronx High School of Science in 1950; one of his classmates was Steven Weinberg, with whom Glashow later shared the Nobel Prize. He received his B.A. from Cornell (1954) and Ph.D. from Harvard (1958). A post-doctoral fellow at CERN (European Council for Nuclear Research) and at the Niels Bohr Institute in Copenhagen from 1958 to 1960, Glashow taught at Stanford (1961-1962) and Berkeley (1962-1966) before assuming a professorship at Harvard in 1966, where he remained into the 1990s. Beginning 1979 he was the Higgins Professor of Physics there. In 1972 he married Joan Alexander, with whom he had two children, Bryan and Rebecca, and two step-children, Jason and Jordan.

Glashow's principal contribution was to the unification of elementary-particle forces. "Unification" refers to the process by which scientists learn to describe apparently disparate phenomena as different manifestations of the same thing. Lightning, static electricity, the aurora borealis, and St. Elmo's fire, for instance, are different forms of electricity. Significant progress in science is often tied up with acts of unification. When in the 17th century Isaac Newton showed that the force pulling objects to the ground and the one keeping planets in motion around the sun is the same force, gravitation—it was an act of unification. So was James Clerk Maxwell's discovery, in the 19th century, that electricity and magnetism were different aspects of one phenomenon, electromagnetism. Subsequently, numerous attempts were made to achieve further unifications, such as Einstein's failed attempt to link gravity and electromagnetism, but when Glashow arrived at Harvard as a graduate student in 1954, little progress had been realized.

Elementary Particle Forces

At that time, four fundamental forces (or interactions) were known—gravitation, electromagnetism, and the strong and the weak forces—and every other force had been shown to be a manifestation of one of these. The last three are called elementary-particle forces, because they govern the behavior of the subatomic world, and a number of physicists had given thought to their unification. A prerequisite, however, for unification was the existence of a common mathematical "language," and the mathematics in which each was then couched were very different.

Glashow was put on the track of elementary-particle unification by his mentor at Harvard, Julian Schwinger. In the late 1940s Schwinger had helped solve certain extremely troublesome problems that beset the mathematical language, called quantum field theory, in which electromagnetism was then couched. For this work he was awarded the 1965 Nobel Prize in Physics. According to quantum field theory, a force is carried by a type of particle called a vector boson; electromagnetism, for instance, is carried by the photon. Schwinger conveyed to Glashow his intuition that electromagnetism might be unified with the weak force if the latter, too, could be written in the language of quantum field theory. Schwinger tried to do this by assuming the weak force to be carried by two vector bosons, one positively, one negatively charged. The scheme, however, was plagued with difficulties.

Glashow examined vector bosons in his Ph.D. thesis, and then, between 1958 and 1960 while on a post-doctoral fellowship abroad, studied their possible role in the weak interaction. In March of 1960, in Paris, he met another advocate of unification, Murray Gell-Mann, and accepted the latter's invitation to become a research fellow at the California Institute of Technology. At Caltech, Glashow composed the paper that first set forth the ideas on which his Nobel Prize is based. "At first sight," the paper began, "there may be little or no similarity between electromagnetic effects and the phenomena associated with weak interactions. Yet certain remarkable parallels emerge with the supposition that the weak interactions are mediated by unstable bosons." The paper developed these parallels, proposing that not two but three vector bosons—positive, negative, and neutral—carried the electroweak force. This proposal solved many problems Schwinger's theory had not and provided a scheme for unifying the weak electromagnetic interactions. Glashow's "electroweak" unification, however, had two embarrassing features. One was that the version of quantum field theory on which the scheme was based, called gauge theory, contained certain inconsistencies (it was apparently not "renormalizable"). The other was that the scheme implied that so-called "neutral" weak interactions (in which no charge was exchanged) ought to exist, and they had not been seen.

Refining the Theory

The first of these two problems was solved in 1971 by the work of two Dutch theorists, Gerard Hooft and Martinus Veltman. The second problem was solved by Glashow himself, through another important contribution to particle physics. This one was based upon an idea of Gell-Mann. In 1964 Gell-Mann proposed that most types of particles, including protons and neutrons, consisted of different combinations of a type of particle that came in three varieties called a "quark." In the summer of 1964 Glashow proposed (with colleague James Bjorken) the existence of a fourth variety of quark, whimsically named "charm." This created a symmetry between the two fundamental families of elementary particles: leptons (which include electrons and muons) and quarks. Incorporated into Glashow's unified scheme of weak and electromagnetic forces, the charmed quark also eliminated the problem of neutral weak interactions, reducing them to a much smaller rate.

Glashow's electroweak scheme (similar schemes were also proposed by Glashow's school colleague Steven Weinberg and Pakistani physicist Abdus Salam) made certain predictions, which were confirmed in short order: neutral weak interactions at the reduced rate (1973), the existence of a charmed particle (1974), and an effect known as atomic parity violation (1978). Glashow's Nobel Prize came the following year, 1979, shared with Weinberg and Salam. Their work forged a major part of what has come to be known as the standard model of elementary particle physics, which is a comprehensive picture of the basic units of matter and their behavior.

Meanwhile, Glashow had already been at work on the next logical step, the unification of the electroweak and strong forces. In 1974, in collaboration with Harvard colleague Howard Giorgi, he proposed a "Grand Unified Theory" that used a gauge theory to unify the electroweak and strong interactions. This theory, however, had some disquieting implications, cosmologically speaking. The most notable one was that protons, the basic building blocks of atomic nuclei, were unstable and would ultimately all decay—that matter, in short, was mortal. So far, experiments have failed to detect proton decay, which may only mean, however, that more complex versions of Grand Unification are involved. Glashow's work in the late 1980s and early 1990s was concerned with cosmology and in particular with the nature of the so-called "dark" matter in the universe.

Glashow spoke out as an advocate of mathematics in 1995 when the University of Rochester, New York proposed a plan to dismantle the math graduate program and reduce it to a "mere service facility." In a letter to the president of the University, Glashow stated that, "Americans, whether college educated or not, often lack the critical quantitative skills fostered by the study of mathematics." He worried that the action by the University would send the message that math played no significant role in education.

Polemical, witty, and rarely without a trademark cigar, Glashow was a striking character among contemporary physicists. More than many, he loved collaborations and did much of his best work with others, including Murray Gell-Mann and his fellow Nobel laureates Abdus Salam and Steven Weinberg. Though a theorist concerned with the fundamental structure of matter, Glashow was temperamentally and philosophically inclined toward areas of physics that have experimental consequences and devoted much time to examining how high-energy processes could be made experimentally observable.

Further Reading on Sheldon Lee Glashow

Aside from scientific articles, Glashow wrote a number of popular articles, as well as a book, Interactions (1988), a potpourri of tales, charts, cartoons, and poems about physics and physicists. Glashow was the subject of an extensive profile in the Atlantic Monthly in August 1984. A narrative account of the drive toward unification, with a long description of Glashow's role, is provided in The Second Creation: Makers of the Revolution in Twentieth Century Physics, by Robert P. Crease and Charles C. Mann (1986).

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