(1933- ) American Theoretical Physicist, Quantum Field Theorist, Particle Physicist, Astrophysicist
Steven Weinberg is a giant of contemporary physics, a theorist with broad research interests whose most notable work has been in unified field theory. In 1967, he hypothesized that quantum electrodynamics can be generalized into a form that allows the electromagnetic and weak forces to be unified into a single electroweak quantum field theory at extremely high energy levels. When his theory was confirmed by particle accelerator experiments in 1983, physics moved significantly closer to the goal of finding “the theory of everything,” a single quantum field theory to describe nature’s basic forces. For this work, Weinberg shared the 1979 Nobel Prize in physics with sheldon lee glashow and abdus salam, who independently developed similar versions of the same ideas.
Steven was born on May 3, 1933, in the Bronx, New York City, to Frederick Weinberg and Eva Weinberg, who had lost much of her family in Germany during the Holocaust. Encouraged by his father, who was a court stenographer, and by his teachers at the Bronx High School of Science (where Sheldon Glashow was his close friend) to follow his innate interest in science, he already knew at age 16 that he was heading for a career in theoretical physics. Far from one-sided, however, he grew up listening to classical music, a love he would retain all his life.
Weinberg attended Cornell University, where he met his future wife, Louise, another Cornell undergraduate, who would eventually become a lawyer. The couple was married in 1954, the year Weinberg graduated. He began his life as a researcher the following year, at what is now the Niels Bohr Institute in Copenhagen, working with David Frisch. After his return to the United States, he enrolled at Princeton University to complete his graduate studies. Working under Sam Treiman, he wrote his doctoral thesis on the application of renor-malization theory to the effects of strong interactions in weak interaction processes and received his Ph.D. in 1957.
His first position was at Columbia University, where he stayed for only two years before moving to the University of California, Berkeley. In 1963, his daughter, Elizabeth, was born. For the next few years, he worked on a broad spectrum of problems, including high-energy behavior of Feynman graphs, second-class weak interaction currents, broken symmetries, scattering theory, and muon physics. Highly studious and self-disciplined, Weinberg writes that, in many cases, he chose a problem “because I was trying to teach myself some area of physics.” In the early 1960s, he first became interested in astrophysics; he wrote papers on the cosmic population of neutrinos and began his book Gravitation and Cosmology, which he would complete in 1971. Late in 1965, he began to work on current algebra and the application to strong interactions of the idea of spontaneous symmetry breaking.
The following year, Weinberg left Berkeley on what was to be a leave of absence but turned out to be a final break. From 1966 to 1969, he was Loeb Lecturer at Harvard University and then visiting professor at the Massachusetts Institute of Technology (MIT), where, in 1969, he accepted a professorship in the physics department under the chairman Victor Weis-skopf. In 1967, at MIT, he did his groundbreaking work, turning his previous studies of broken symmetries, current algebra, and renormaliza-tion theory in the direction of the unification of weak and electromagnetic interactions.
Weinberg’s starting point was the innovative work of chen ning yang and Robert Mills. In 1950, they had shown that the quantum elec-trodynamic (QED) formalism developed earlier by julian seymour schwinger, richard phillips feynman, and sin-itiro tomonaga could be generalized to include internal dynamic symmetries that were more general than the standard spacetime C (charge), P (parity), and T (time-reversal) symmetries. In terms of these so-called non-Abelian Yang-Mills gauge theories with internal symmetries, Weinberg’s quest was to find the hidden symmetry of the apparent asymmetries that occurred in particle physics. He would later write:
Nothing in physics seems so hopeful to me as the idea that it is possible for a theory to have a very high degree of symmetry, which is hidden from us in ordinary life. The physicist’s task is to find this deeper symmetry.
He was fascinated by the fact that nature was replete with so-called broken symmetries— asymmetric relations that have spontaneously arisen from the functioning of symmetrical laws (e.g., the asymmetric crystal structure of ice that freezes out from the symmetric liquid structure of water when the temperature becomes low enough).
Weinberg was aware that in the early 1960s Peter Higgs had published papers demonstrating that spontaneous symmetry breaking events could create new kinds of force-carrying particles, some of them massive. This led Weinberg to speculate that if the virtual particles that carry the electromagnetic and weak forces (known collectively as the intermediate vector boson W and Z particles) were related by a broken symmetry, these new theoretical ideas might make it possible to estimate their masses in terms of the unified, more symmetrical force from which the two forces were thought to have arisen.
Steven Weinberg discovered that quantum electrodynamics could be generalized into a form that allowed the electromagnetic and weak forces to be unified in a single electroweak quantum field theory at extremely high energy levels.
First Weinberg tried, unsuccessfully, to apply the new theoretical ideas of symmetry breaking to the strong force, but he soon realized that the descriptions emerging from his equations—one set massless, the other massive—resembled nothing related to the strong force but fit perfectly with the particles that carry the weak and electromagnetic forces. The massless particle was the photon, carrier of electromagnetism; the massive particles were the W’s and the Z’s, carriers of the weak force. By accident, Weinberg had found a unified quantum field theory of electromagnetic and weak interactions (i.e., a quantum electroweak theory) that could make a verifiable prediction of the approximate masses of the required triplet W and the singlet Z particles needed to describe the weak interactions. The following year, Salam independently made the same finding.
However, over the next four years, Wein-berg’s unified theory attracted scant attention since, unlike quantum electrodynamics, his electroweak theory had not yet been shown to be renormalizable, that is, capable of being altered by a mathematical procedure that cancels the unwanted infinities in a quantum field theory by introducing the appropriate renor-malization constants. But, in 1971, the Dutch physicist gerard ‘t hooft used computer algebra techniques to prove that Weinberg’s elec-troweak theory was indeed renormalizable. After this development, the attention of the physics community shifted to testing the elec-troweak theory. In 1973, Weinberg accepted Schwinger’s recently vacated chair as Higgins Professor of Physics at Harvard, together with an appointment as Senior Scientist at the Smithsonian Astrophysical Observatory. During the 1970s, he worked primarily with the implications of the unified theory of weak and electromagnetic interactions, development of the related theory of strong interactions known as quantum chromodynamics, and the unification of all interactions.
Although decisive experimental confirmation would not be found until 1983, Weinberg, Salam, and Glashow shared the 1979 Nobel Prize for their theoretical breakthrough. Two particle accelerator teams at the European Center for Nuclear Research (CERN) and the Fermi Laboratory in Batavia, Illinois, raced to test the predictions of the electroweak theory. The CERN team under Paul Musset found the neutral currents associated with the singlet Z particle, but these results were inconclusive, since other competing theories of the weak interactions could predict the existence of these particles.
Finally, in 1981, carlo rubbia, working with Simon Van der Meer, Guido Petrucci, and Jacques Gareyte at CERN, did the decisive experiment and found the triplet W and singlet Z particles characteristic of Weinberg’s electroweak theory. His team used a new type of colliding beam machine built with intersecting storage rings in which a beam of protons and antiprotons counterrotate and collide head-on. The team developed techniques for creating antiprotons, confining them in a concentrated beam and colliding them with an intense proton beam.
Hundreds of scientists were involved in these experiments in teams throughout the world. The first collisions were observed in 1981. In 1983, Rubbia and collaborating teams of scientists announced the discovery of the triplet W and singlet Z particles, which was based on signals from detectors specially designed for this purpose.
In 1982, Weinberg moved to the physics and astronomy departments of the University of Texas at Austin, where he currently holds the Josey Regental Chair of Science, and founded its theory group. His wife, Louise Weinberg, also joined the university, as a professor of law. He was extremely active in the lobbying campaign for a multibillion-dollar Superconducting Supercollider to be located near Waxahachie, Texas, which Congress killed in 1993.
Weinberg is a prolific author and fine prose stylist, who was recently in 2000 awarded the Lewis Thomas Prize, given to the researcher who best embodies “the scientist as poet.” His books for general readers include the prize-winning The First Three Minutes (1993), which has been translated into 22 languages; The Discovery of Subatomic Particles; Dreams of a Final Theory (1993); and most recently Facing Up: Science and Its Cultural Adversaries (2001). He has also written many books for specialists, such as The Quantum Theory of Fields, volume 1, Foundations, and volume 2, Modern Applications (2000), and Elementary Particles and the Laws of Physics, with Richard Feynman.
An outspoken atheist, Weinberg was awarded the 1999 Emperor Has No Clothes Award from the Freedom from Religion Foundation and the 2002 Humanist of the Year Award from the American Humanist Foundation. For Weinberg, a redemption of sorts lies not in religion, but in the scientific endeavor:
The effort to understand the universe is one of the very few things that lifts human life above the level of farce, and gives it some of the grace of tragedy.
Weinberg’s discovery of the electroweak theory became the precursor to what is known today as the Standard Model of the strong and electroweak interactions, which uses a combination of three symmetry principles to unify the electromagnetic, weak, and nuclear forces into one renormalizable quantum field theoretic formalism. Weinberg is today’s foremost proponent of the idea that physicists are moving toward generalizing the Standard Model into the formulation of the long-sought “final theory” that will unify all particles and fundamental forces of nature in the context of a single universal symmetry principle.
