Chen Ning Yang (22 September 1922–18 October 2025) was a Chinese theoretical physicist whose research encompassed particle physics, statistical mechanics, and quantum field theory. He shared the 1957 Nobel Prize in Physics with Tsung-Dao Lee for their investigation of parity symmetry in elementary-particle interactions. With Robert Mills, he developed the non-Abelian gauge framework known as Yang–Mills theory, which subsequently became a foundation of the Standard Model. (nobelprize.org)
Early life and education
Yang was born in Hefei, Anhui, China, and grew up partly on the campus of Tsinghua University, where his father taught mathematics. During the Second Sino-Japanese War, he studied in Kunming at the National Southwest Associated University, an institution formed by universities displaced by the war. He received his bachelor's degree in 1942 and a master's degree from Tsinghua University in 1944. His early academic work concerned group theory, molecular spectra, and statistical theories of ordering transitions. (tsinghua.edu.cn)
He went to the United States in 1945 on a Tsinghua fellowship and entered the University of Chicago in January 1946. There he was strongly influenced by Enrico Fermi and completed his doctorate in 1948 under Edward Teller. After a year as an instructor at Chicago, he joined the Institute for Advanced Study in Princeton in 1949, becoming a professor in 1955. (tsinghua.edu.cn)
Parity violation and the Nobel Prize
Parity is the transformation that reverses all spatial coordinates. Parity conservation means that the laws governing a process remain unchanged under this transformation, commonly described as equivalence between a process and its mirror image. In 1956, Lee and Yang examined whether this symmetry had actually been established experimentally for the weak interaction. They found that the available evidence did not establish it and proposed experiments capable of testing it directly. (nobelprize.org)
The decisive experiment was performed by Chien-Shiung Wu and collaborators at the US National Bureau of Standards. They aligned cobalt-60 nuclei at very low temperature and measured the directions of electrons emitted in beta decay. The unequal emission relative to the nuclear spin direction demonstrated that the weak interaction violates parity. Results announced in January 1957 overturned the assumption that left–right symmetry applied universally to fundamental interactions. (nist.gov)
Lee and Yang received the 1957 Nobel Prize for the theoretical investigation that led to this discovery. Their contribution was distinct from the experimental demonstration by Wu and her collaborators, who did not share the prize. The award was not for Yang–Mills theory or for the Lee–Yang theorem in statistical mechanics. (nobelprize.org)
Yang–Mills theory
In a paper published in October 1954, Yang and Robert Mills extended gauge theory to local transformations associated with a noncommutative symmetry group. Their starting point was isotopic spin, or isospin, used to describe relationships between nuclear particles. Requiring invariance under independently chosen isospin transformations at different points led them to introduce a new field governed by nonlinear equations. (journals.aps.org)
The essential distinction from electromagnetism is that the non-Abelian field's force carriers can themselves carry the charges to which the field couples. This allows interactions among the force carriers, unlike the electrically neutral photon in electromagnetic theory. Yang and Mills supplied the general framework, rather than the complete modern theory of elementary particles. Questions concerning quantization and the generation of particle masses required substantial subsequent work by other physicists. (ias.edu)
Non-Abelian gauge theories became central to the Standard Model, including quantum chromodynamics. Their mathematical structure also helped establish connections between physics and differential geometry, particularly through the interpretation of gauge fields in terms of fiber bundles. (journals.aps.org)
Statistical mechanics and exactly solvable systems
Yang's collaboration with Lee also produced a mathematical approach to phase transitions. Their 1952 work related the behavior of macroscopic systems to the zeros of the partition function when its parameters are extended to complex values. The Lee–Yang circle theorem established that, for a specified class of ferromagnetic systems, these zeros lie on a circle in an appropriate complex variable. Their papers also demonstrated an equivalence between lattice-gas models and the Ising model. The circle result depends on its stated assumptions; it is not a universal property of every statistical system. (journals.aps.org)
In 1967, Yang obtained exact results for a one-dimensional quantum many-body system with repulsive contact interactions, extending the Bethe ansatz method. In particular, he reduced the ground-state problem for spin-one-half fermions to an integral equation. This research was associated with the equation now called the Yang–Baxter equation, which became an important tool in the study of exactly solvable physical models. (journals.aps.org)
Academic career and scientific exchange
In 1966, Yang moved to Stony Brook University as Albert Einstein Professor of Physics and founding director of its Institute for Theoretical Physics. He directed the institute until 1999; it was subsequently named the C. N. Yang Institute for Theoretical Physics. His work there included particle theory, exactly solvable systems, and the geometric interpretation of gauge theories. (tsinghua.edu.cn)
Yang visited China in 1971 and subsequently participated in scientific and educational exchanges between China and the United States. He became honorary director of Tsinghua University's newly established Center for Advanced Study in 1997 and a professor there in 1999. He moved back to China in 2003 and continued activities connected with theoretical physics and science education. He died in Beijing on 18 October 2025, aged 103. (stonybrook.edu)
References
- Chen Ning Yang – Factsnobelprize.org
- Chen Ning Yang – Biographytsinghua.edu.cn
- Nobel Prize in Physics 1957 – Presentation Speechnobelprize.org
- A ‘Forever’ Stamp and a Discovery That Changed Physics Forevernist.gov
- Conservation of Isotopic Spin and Isotopic Gauge Invariancejournals.aps.org
- APS Journals – 50 Years of QCDjournals.aps.org
- C.N. Yangstonybrook.edu
- Statistical Theory of Equations of State and Phase Transitions. II. Lattice Gas and Ising Modeljournals.aps.org
- Statistical Theory of Equations of State and Phase Transitions. II. Lattice Gas and Ising Modelfig.if.usp.br
- Some Exact Results for the Many-Body Problem in one Dimension with Repulsive Delta-Function Interactionjournals.aps.org