Chen-Ning Yang

Chen-Ning Yang was born in Hefei, Anhui province, in 1922, the son of a mathematics professor at Southwest Associated University (Xinan Lianda) — the wartime-evacuation merger of Tsinghua, Peking, and Nankai universities that operated in Kunming during the Sino-Japanese War. Yang took his BSc at Lianda in 1942 and his MSc in 1944, then went to the University of Chicago on a Boxer Indemnity Scholarship, taking his PhD in 1948 under Edward Teller with a thesis on angular distributions in nuclear reactions. His Chicago classmate and lifelong collaborator T.D. Lee arrived a year after him, and the two became inseparable scientific partners.

Yang joined the Institute for Advanced Study at Princeton in 1949 (overlapping with Einstein, Gödel, von Neumann, and Pauli) and remained for sixteen years. In the summer of 1953, while visiting Brookhaven, Yang and the postdoctoral fellow Robert Mills wrote the paper *Conservation of Isotopic Spin and Isotopic Gauge Invariance* (published 1954). The construction generalised Weyl's 1929 gauge principle from the abelian U(1) phase rotation of QED to non-abelian groups (specifically SU(2), the symmetry of nuclear isospin), and the resulting field equations were highly non-trivial because the gauge fields themselves carried charge under the gauge group — the field strength tensor acquired a self-interaction term g f^{abc} A^b_μ A^c_ν that was absent in the abelian case. Pauli, who had independently considered the same construction, raised an objection at Yang's 1954 IAS seminar that the resulting gauge bosons would have to be massless and therefore long-range — contradicting the short range of the strong force the construction was meant to describe. Yang famously had no good answer, and the paper sat largely unused for a decade until the development of the Higgs mechanism and dimensional regularisation in the 1960s gave it the missing ingredients. The construction is now the mathematical backbone of the entire Standard Model: the weak force is an SU(2) gauge theory, the strong force is an SU(3) gauge theory, and the electroweak unification is an SU(2)×U(1) gauge theory.

In 1956, Yang and Lee proposed that parity — the symmetry under spatial reflection — was *not* conserved in the weak interaction, in defiance of forty years of physical intuition. Chien-Shiung Wu's experiment at Columbia in late 1956 confirmed the prediction within months; Yang and Lee shared the 1957 Nobel Prize (the fastest Nobel ever awarded after the experimental confirmation). Yang moved to Stony Brook in 1965 to found the Institute for Theoretical Physics there, retired from Stony Brook in 1999, and returned permanently to Tsinghua University in Beijing in 2003. He has been an active physicist into his late nineties; the 2019 Nobel ceremony in Stockholm noted that Yang-Mills theory remains the most-cited single result in modern theoretical physics.