Chen-Ning Yang

Chen-Ning Yang the Chinese-American physicist (born 1922) who shared the 1957 Nobel Prize in Physics for the theory of parity non-conservation in weak interactions and who helped to found the Yang-Mills theory, a cornerstone of the modern understanding of elementary particles.

Early Life and Education

Chen-Ning Yang was born on 1 October 1922 in Hefei, Anhui province, the son of a mathematician who taught at various universities. The family moved frequently, and the boy grew up amid an academic environment that encouraged curiosity and careful reasoning. He received a strong grounding in both the classical Chinese tradition and modern science.

He studied at National Southwest Associated University during the war years and graduated in physics in 1942, remaining there for a master's degree. The university, formed from the merger of several institutions displaced by the Japanese invasion, produced a remarkable number of scientists despite the hardships of the period.

In 1945 he travelled to the United States on a scholarship and entered the University of Chicago, where he studied under Enrico Fermi and Edward Teller. He received his doctorate in 1948 for a thesis on the angular distribution of nuclear reactions, and he then joined the Institute for Advanced Study, beginning a long and productive association with American theoretical physics.

The Yang-Mills Theory

In 1954 Chen-Ning Yang and Robert Mills published a paper proposing a generalisation of the theory of electromagnetism in which the symmetry transformations are allowed to vary from place to place in a more general, non-commuting way. The resulting gauge theory described the interaction of particles through fields that obey a sophisticated mathematical symmetry.

The theory was at first regarded as a mathematical curiosity because it predicted massless particles that were not observed in the form the authors had assumed. But it supplied the framework within which the weak and strong nuclear forces were later understood, and it became one of the most important structures in all of theoretical physics.

Gauge theories of the kind Yang and Mills constructed now underlie the standard model of elementary particles, the account of matter and its interactions that has been confirmed by decades of experiment. Their paper is often cited as one of the most influential in the history of physics, and its ideas extended far beyond the problem that originally prompted it.

Parity and the Nobel Prize

In 1956 Yang and Li Zhengdao examined the evidence for the conservation of parity in the weak interaction and concluded that it had never actually been tested. They proposed experiments that could decide the matter and predicted that the symmetry would be found to fail. Their bold suggestion was soon confirmed by experiment.

The discovery that the weak interaction distinguishes between a process and its mirror image overturned a principle that had been almost universally assumed, and it transformed the study of the fundamental symmetries of nature. Yang and Li were awarded the Nobel Prize in Physics in 1957, and the result made both men internationally famous.

Yang continued to work on symmetries and on the structure of the weak interaction, making contributions to the theory of beta decay and to the understanding of the conservation laws that govern elementary processes. His ability to find deep questions in the data and to frame them precisely was widely admired.

Statistical Mechanics and Mathematics

Beyond particle physics Yang made important contributions to statistical mechanics. With his collaborators he solved the two-dimensional Ising model of magnetism in nonzero magnetic field, one of the rare exactly solvable models of a phase transition, and he studied the theory of the many-body problem and the behaviour of systems at low temperature.

He also worked on problems at the boundary between physics and mathematics, including the theory of fibre bundles and the relations between gauge fields and geometry. He recognised early that the mathematical structures required by modern physics had much in common with the pure mathematics of topology and differential geometry.

His papers on these subjects were influential in both disciplines and helped to bring physicists and mathematicians into closer contact. He was a master of the elegant calculation and the illuminating approximation, and his work is studied by students in both fields for its clarity and depth.

Influence and Legacy

Chen-Ning Yang spent much of his career at the Institute for Advanced Study and at the State University of New York at Stony Brook, where he founded and led a distinguished research group in theoretical physics. He trained many graduate students and postdoctoral fellows who became leading figures in the field.

From the 1970s he worked actively to strengthen scientific ties between the United States and China, lecturing widely in his homeland and helping to establish institutions and programmes that supported young Chinese scientists. He was also a notable historian and commentator on physics, and his essays on the work of Einstein and others are widely read.

He was elected to the National Academy of Sciences and to many other learned bodies, and he received numerous honorary degrees and prizes. He has remained active in scientific life into extreme old age, and he is regarded as one of the central figures in the physics of the twentieth century.

See also

• Li Zhengdao
• Yang-Mills theory
• Gauge theory
• Parity violation