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Steven Weinberg, who died on July 23, towered over theoretical physics in the second half of the 20th century. He strongly believed that, armed only with the fundamental principles of relativity and quantum mechanics, the theoretical physicist can examine all phenomena in the universe — from the smallest to the largest scales. His work transformed our understanding of every aspect of fundamental physics in startlingly deep and original ways.
Weinberg was a master of quantum field theory, a branch of physics born from applying the rules of quantum mechanics to the electromagnetic field, which sees a particle — the photon —as a “quantized” excitation of the field. He was instrumental in propelling quantum field theory to astonishing new heights in the description of nature.
The themes of unification and symmetry drove all of Weinberg’s work and led to his famous breakthrough on electroweak unification, which revealed a hidden unity between two of the universe’s four fundamental forces. At first sight, the electromagnetic and weak interactions seem utterly different: We see electromagnetic waves as light in everyday life, while the weak force — responsible for radioactivity — operates on subnuclear scales. Weinberg realized that at very high energies, the two forces should be intertwined, described by what’s known as Yang-Mills theory, whose equations have a special property called gauge symmetry. But this essential commonality is hidden by the so-called Higgs mechanism, which generates masses for elementary particles such as the electron and the W and Z particles (which mediate short-range weak interactions), all the while leaving the long-range photon massless. The model he proposed in 1967 for realizing this vision made many detailed predictions and has been triumphantly confirmed by experiments beginning in the 1970s and ’80s, capped off by the 2012 discovery of the Higgs particle. Weinberg shared the 1979 Nobel Prize in Physics with Sheldon Glashow and Abdus Salam for this work, a pillar of the Standard Model of particle physics.
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Steven Weinberg at the American Physical Society Meeting in 1977. His work in particle physics redefined our understanding of the universe. AIP Emilio Segrè Visual Archives, Weber Collection
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