Steven Weinberg (3 May 1933, New York City – 23 July 2021, Austin, Texas)
Nobel laureate (1979) Steven Weinberg was Honorary Member of the International Symmetry Association (2009-). Besides the Nobel prize, he won many other prizes and distinctions. His discoveries deepened understanding of the basic forces at play in the universe.
The work for which Dr Weinberg was awarded the Nobel had a transformative impact on physics to understand and explain what happens in the subatomic world. He discovered that two of the universe’s forces are really the same and helped to lay the foundation for the development of the Standard Model, a theory that classifies all known elementary particles in the universe, making it one of the most important breakthroughs in physics in the 20th century.
Though he had the respect, almost awe, of his colleagues for his scientific abilities and insights, he also possessed a rare ability among scientists to communicate and explain abstruse scientific ideas to the public. He was a sought-after speaker, and he wrote several books in the fields of the foundations of physics and cosmology. He read a keynote lecture at the 2009 Symmetry Festival held in Budapest with the title “Accidental Symmetry”. An edited version of his paper was published in Symmetry: Culture and Science (Vol. 23, No. 1, pp. 5-16).
There are four known forces in the universe: gravity; electromagnetism; the strong force, which binds the nuclei of atoms together; and the weak force, which causes radioactive decay. The first two forces have been known for centuries, but the other two were discovered only in the 20th century. Physicists struggled long to find a theory that would account for all the forces. Though there were significant discoveries, a unified theory or model remained elusive.
He proposed that, at very high energy levels, the electromagnetic and weak forces should be one and the same. It was a step on the path to the unified theory that physicists had been searching for. Dr Weinberg published his findings in 1967 in a groundbreaking paper, “A Model of Leptons,” in the journal Physical Review Letters. The article is one of the most-cited research papers in history.
Among many groundbreaking theoretical discoveries, he developed the theory of ‘symmetry breaking’. (Not to be confused with the notion of the violation of symmetry.) Introducing the idea of a process called by him ‘spontaneous symmetry breaking’, he gave a model to explain the mass difference of the bosons that mediate between the agents of the electroweak interaction. Weinberg’s model, known as the electroweak unification theory, had the same symmetry structure as that proposed by Glashow in 1961.
Then, the electroweak theory was made consistent with a theory of the strong interactions between quarks, in one overarching theory, based on a unification of the symmetry groups characterising the individual physical interaction types. The model developed by him (and independently by A. Salam) was revolutionary, not only for proposing the unification of the electromagnetic and weak forces but also for creating a classification system of masses and charges for all fundamental particles, thereby forming the basis of the celebrated Standard Model of physics, which includes all the forces except gravity.
S. Weinberg, A. Salam and Sh. L. Glashow, an old high school classmate of Dr Weinberg’s who had resolved a critical problem with the Weinberg-Salam model, were jointly awarded the 1979 Nobel Prize “for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles” based on spontaneous symmetry breaking. His contribution to the development of the theory of physical symmetries raised him among the greatest physicists of the 20th century.
He never retired, he taught until the spring of this year. During his decades at the University of Texas at Austin he tutored many students, eight of them became full professors and five assistant professors at his university.