by Howard Georgi
Theoretical physics lost one of its most brilliant, thoughtful, influential and durable practitioners when Steven Weinberg died in July at the age of 88. Steve came to MIT in 1969 after ten very productive years at Berkeley that established him as one of the world’s leading quantum field theorists. He joined the Harvard faculty in 1973 as the Higgins Professor of Physics, taking over the chair held by Julian Schwinger and Percy Bridgeman before him. Legend has it that when Steve moved into Schwinger’s elegant wood-paneled office on the third floor of Lyman he found a large pair of shoes. There may never be another Julian Schwinger, but Steve did an admirable job of filling those shoes.
Steve’s time in Cambridge from 1969 to 1983 was an exciting period for particle physics during which the standard model was built and largely established. But it didn’t start out that way! When Weinberg wrote the paper that is sometimes hailed as the birth of the standard model of particle physics, “A Model of Leptons” in 1967, quantum field theory (QFT) was in the doldrums. While many of the pieces of the standard model were developed in the ‘60s and even the late ‘50s, nobody could put them all together. Steve’s paper did little to change that. It was not obvious that calculations with the model made sense, and the paper was largely ignored—even by Steve himself. So too was the prediction of the charmed quark at Harvard by Glashow, Iliopoulos and Maiani.
The situation changed dramatically at the beginning of the ’70s because of seismic shifts in both theory and experiment. A Dutch theory student, Gerard ‘t Hooft, made sense of spontaneously broken non-Abelian gauge QFTs in general and Weinberg’s model in particular. And experiments to look inside protons at the great accelerator laboratories using electrons and muons began to see evidence of the underlying QFT. Few were convinced immediately, but Steve and the Harvard group and a few other centers of QFT research realized that the game was afoot and built the standard model. The final pieces of the puzzle emerged after just a few frenetic years with the discovery of asymptotic freedom by David Politzer at Harvard and others and the experimental discovery of states involving the charmed quark.
Eventually, the rest of the world caught up, and the Nobel Prize in 1979 to Glashow, Salam and Weinberg put a suitable crown on the most exciting period in QFT since the work of Feynman, Schwinger and Tomonaga in the late ‘40s. In the process, Weinberg, along with Ken Wilson, ‘t Hooft and a few others, radically changed our (and his) view of QFT.
QFT is only a part of Steve’s enormous contribution to physics. His breadth and productivity were astonishing. He wrote over twenty books, some learned tomes such as the three-volume The Quantum Theory of Fields and Gravitation and Cosmology, but also many books for a general audience, both on physics (e.g., The First Three Minutes) and on pressing societal issues (e.g., Glory and Terror: The Growing Nuclear Danger).
Steve was an unusual character even by the standards of theoretical physicists! I collaborated with him on two papers, one important one with Helen Quinn was one of the two pillars of grand unification. He and I interacted pretty much every day while he was at Harvard because we were interested in all the same issues. He was the only person I ever met who didn’t need examples. He almost always started with generalities. One of the great ironies is that he got his Nobel primarily for a specific model.
Perhaps one of Steve’s greatest strengths was that he thought deeply, sometimes philosophically about issues, but didn’t let this get in the way of progress. He could change his mind. One of my favorite examples comes from his 1972 text on Gravitation and Cosmology where he wrote “At one time it was even hoped that the rest of physics could be brought into a geometric formulation, but this hope has met with disappointment, and the geometric interpretation of the theory of gravitation has dwindled to a mere analogy, which lingers in our language in terms like ‘metric,’ ‘affine connection,’ and ‘curvature,’ but is not otherwise useful. The important thing is to be able to make predictions about images on the astronomers' photographic plates, frequencies of spectral lines, and so on, and it simply doesn't matter whether we ascribe these predictions to the physical effect of gravitational fields on the motion of planets and photons or to a curvature of space and time.” Less than 15 years later, he returned to Harvard from Texas as a Loeb Lecturer, giving talks on string theory and the geometry of extra dimensions.
When I think of Steve’s long career, what comes to mind are words like “brilliant”, “deep”, “thoughtful”, but even more "noblesse oblige." He knew he had special talents and was compulsively driven to use them to the fullest. He continued to work hard on his physics almost to the very end. Very few have ever done more.