Steven Weinberg

Steven Weinberg (born 1933) shared the 1979 Nobel Prize in Physics with two other scientists for their work in the field of elementary-particle forces.

Steven Weinberg was born on May 3, 1933, in New York City. He graduated from the Bronx High School of Science in 1950; one of his classmates was Sheldon Lee Glashow, with whom Weinberg would share the Nobel Prize in 1979. Weinberg received his B.A. from Cornell (1954) and then for a year went to the Institute for Theoretical Physics (now the Niels Bohr Institute) in Copenhagen before returning to the United States to complete his Ph.D. at Princeton (1957). Weinberg taught at Columbia University (1957-1959), the University of California at Berkeley (1959-1966), the Massachusetts Institute of Technology (1969-1973), and Harvard University (1973-1982). In 1982 he became Josey Professor of Science at the University of Texas in Austin. He met his wife, Louise, while an undergraduate at Cornell, and they were married in 1954; their only child, Elizabeth, was born in Berkeley in 1963.

Weinberg was awarded the Nobel Prize in recognition of his contributions to the unification of elementary-particle forces. "Unification" refers to the process by which scientists succeed in describing apparently disparate phenomena in terms of a few simple principles. Success in unification often goes hand-in-hand with progress in science. Isaac Newton's demonstration in the 17th century that the forces that pull objects to the ground were the same as those that keep the planets in their orbits was an example of unification. Likewise, James Clerk Maxwell's discovery two centuries later that electricity and magnetism are but different manifestations of the same phenomenon, electromagnetism.

When Weinberg entered Princeton as a graduate student in 1955, four fundamental forces (also called interactions) were known: gravitation, electromagnetism, weak forces, and strong forces. The latter three are called elementary-particle forces because they govern the behavior of the subatomic realm. Although periodic attempts had been made to unify them, the efforts had been unsuccessful. Of the three forces, the theory describing electromagnetism was the most elaborately developed and was couched in a mathematical language known as quantum field theory. According to quantum field theory, a force is carried by a type of particle called a vector or "spin-one" boson; the vector boson carrying the electromagnetic force, for instance, is the photon.

In the early 1960s Weinberg began exploring a version of quantum field theory called gauge theory and wondered whether it could also be used to describe the strong force in a manner analogous to the already successful description of the electromagnetic force. But, if written as a gauge theory, the strong force would have to be carried by massive vector bosons while the photon was massless, making the attempt appear hopeless, because in a unified theory the bosons would have to be described symmetrically. Weinberg tried to overcome the apparent discrepancy by utilizing a new type of symmetry principle called broken symmetry. He was thoroughly familiar with broken symmetry, having exploited it in inventing the successful modern theory of the low-energy interactions of the particles known as hadrons (particles that feel the strong interaction). In the context of gauge theories, however, the application of symmetry breaking generated a new problem, for it seemed to entail the postulation of a kind of particle already known not to exist. Weinberg tried for years to find a loophole in the apparent requirement without success.

"A Model of Leptons"

One day while driving to his Massachusetts Institute of Technology office, he suddenly realized that he had been applying the right idea to the wrong problem. The mathematical apparatus involving broken symmetry that he had been trying to fit to the strong interaction would work when applied to the weak. This involved a major shifting of conceptual gears, for whereas Weinberg's models previously had involved hadrons, they would now have to involve another set of particles called leptons, which only experience the weak and electromagnetic interactions. The result was "A Model of Leptons," which was published in Physical Review Letters in November 1967. This short paper, only two-and-a-half pages long, crystallized years of effort and represents the work for which Weinberg would receive his Nobel Prize.

"Leptons interact only with photons, and with the intermediate bosons that presumably mediate weak interactions," the paper began. "What could be more natural than to unite these spin-one bosons into a multiplet of gauge fields?" Weinberg then acknowledged that the attempt would immediately run into the same problem that he had faced in his models of the strong interaction of the mass differences between photons and the vector bosons of the weak interaction. Furthermore, attempts to use broken symmetry to finesse the problem would create unwanted bosons. Weinberg's paper then proposed a solution to the problem involving a spontaneously broken model that avoids the troublesome particles by introducing the photons and intermediate bosons as gauge fields.

In retrospect, the model described in the paper was a major step forward in the unification of elementary-particle interactions. It is the most frequently cited paper on elementary-particles physics in the last half-century. But this was hardly apparent at the time. The model had two serious problems. One was that the gauge theory that Weinberg used contained certain inconsistencies (it was apparently not "renormalizable"), and though the paper asserted the difficulty could be eliminated, the claim was unsubstantiated. A second problem was that the model implied that so-called "neutral" weak interactions, in which no charge was exchanged, ought to exist. Thus far none had been detected.

These two problems were soon overcome. In 1971 a Dutch theorist, Gerard't Hooft, showed that Weinberg's hunch was correct, and that the scheme was indeed renormalizable. Around the same time several theorists, including Weinberg, demonstrated that if a fourth quark existed, the rate of neutral weak interactions would be less than the existing observational limit.

A similar model was also proposed by Weinberg's school colleague Sheldon Glashow and by the Pakistani physicist Abdus Salam. The electroweak theory, as it is now called, made several important predictions that were confirmed one by one throughout the 1970s: neutral weak interactions at the reduced rate (1973), the existence of the fourth quark (1974), and an effect known as atomic parity violation (1978). Weinberg's Nobel Prize came the following year, shared with Glashow and Salam. The electroweak theory forms a major part of what has come to be known as the standard model of elementary-particle physics. This provides a comprehensive picture of the basic units of matter and their behavior and explains virtually all the experimental data physicists have been able to obtain.

Tying High-Energy Physics to Cosmology

Meanwhile, Weinberg had already been at work on other important steps in the drive toward unification. In 1974 he co-authored a paper describing how the coupling constants, or measures of strength, of the electromagnetic, weak, and strong interactions would converge at extremely high energies, such as existed in nature only fractions of a second after the Big Bang. This result gave further impetus to a growing convergence of interests between high-energy physicists and cosmologists. Three years later Weinberg wrote a book, The First Three Minutes (1977), which awakened many scientists and nonscientists to the importance of cosmology in understanding the present-day universe.

Though his Nobel Prize was for work in unification, Weinberg made significant contributions in a wide range of areas in particle physics and even in plasma physics. Among colleagues he was known more for versatility than mathematical strength. One consequence of the stunning success of the standard model was that it outran the ability of experimental physicists to produce data that will enable theoretical physicists to make further advances. Concerned by this fact, Weinberg was one of the staunchest proponents of the superconducting supercollider (SSC), an ill-fated particle accelerator that would have been able to produce data whose implications reached beyond the standard model. However, that project was killed by the U.S. Congress in the fall of 1993.

Weinberg remains at the University of Texas, Austin, where he serves as the Josey-Welch Foundation Chair in Science at the school's College of Natural Sciences Physics Department. He is also working at Harvard again as professor of physics.

At the University of Texas Weinberg became a senior statesman in the field of physics and a champion for the further development of elementary-particle physics. By 1994 The First Three Minutes had been translated into 22 foreign languages. He continued writing and made notable contributions to both the scientific and general literature. Writing for a general audience in Dreams of a Final Theory (1994), he argued the case for the SSC, reminding readers that "science has always had its enemies throughout history." Included among numerous theoretical publications, papers, and presentations were Unbreaking Symmetries (1995), Pion Scattering Lengths (1996), Theories of the Cosmological Constant (1996), and Precise Relations between the Spectra of Vector and Axial Vector Mesons (1997). He also wrote the highly regarded three-volume textbook The Quantum Theory of Fields: Foundations (1995) and The Quantum Theory of Fields: Applications (1997), and The Quantum Theory of Fields: Supersymmetry (2000). His latest work is Facing Up: Science and Its Cultural Adversaries (2001).

Weinberg's colleagues consider him the foremost champion of the value and dignity of the scientific enterprise, an attitude of which the concluding line of The First Three Minutes offers a typical expression: "The effort to understand the universe is one of the very few things that lifts human life a little above the level of farce, and gives it some of the grace of tragedy."

Further Reading

• Aside from scientific articles, Weinberg has written numerous books, including The Theory of Subatomic Particles, Gravitation, and Cosmology: Principles and Applications of the General Theory of Relativity (1990) and Dreams of a Final Theory: The Search for the Fundamental Laws of Nature (1993). The story of the drive toward unification, with a long description of Weinberg's role, is contained in The Second Creation: Makers of the Revolution in Twentieth Century Physics by Robert P. Crease and Charles C. Mann (1986). Reviews of general and scientific works can be found in publications such as Science and Physics Review .