Yang Chen-Ning showed nature is not perfectly mirror-symmetric
Physicists long assumed that any process and its mirror image obey the same laws. In 1956 Yang Chen-Ning and Tsung-Dao Lee proposed that the weak nuclear force breaks that rule. Chien-Shiung Wu's experiment confirmed it within months, and the pair won the 1957 Nobel Prize, Yang aged just 35.
The clue was the tau-theta puzzle: two particles seemed identical, yet one decayed into two pions and the other into three, outcomes with opposite mirror symmetry. Lee and Yang suggested they were one particle, now called the kaon, and that parity simply is not conserved in weak interactions. Two further teams, at Chicago and Columbia, independently confirmed the result, all publishing in early 1957.
Yang's deepest legacy may be the non-abelian gauge theory he developed with Robert Mills in 1953 and 1954 while sharing an office at Brookhaven. Mills, then a novice, later said the key ideas were Yang's. Yang–Mills theory now describes the nuclear forces in the Standard Model, and at least ten physics laureates, including Steven Weinberg and Peter Higgs, cited Yang's work in their Nobel lectures. Freeman Dyson ranked him with Einstein and Dirac as a supreme stylist of 20th-century physics, and credited him with the insight that symmetry can dictate how forces interact.
Born in Hefei on October 1, 1922, the son of a mathematician, Yang studied at a wartime university relocated to Kunming after Japan invaded China. A scholarship funded by Boxer Rebellion indemnity money brought him to the University of Chicago in 1946, where he studied under Edward Teller and then assisted Enrico Fermi. He spent twenty months in an accelerator lab before admitting he was poor at experiments and returning to theory.
At Princeton's Institute for Advanced Study he and Lee wrote 32 papers together. In 1965 he became founding director of Stony Brook's Institute for Theoretical Physics, now named for him. Other work included the Yang–Baxter equation and, with Tai Tsun Wu, a magnetic monopole without the troublesome string of Dirac's version. After retiring in 1999 he settled at Tsinghua University in Beijing, where he died on October 18, 2025, aged 103.
Source: Yang Chen-Ning
