Yang Chen-Ning

• 1. Introduction

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  Yang Chen-Ning, the theoretical physicist also known as C.N. Yang and Franklin Yang, died in Beijing on the 18th of October 2025. He was 103 years old. The day after his death was announced, people gathered at Tsinghua University and waited in line to pay their respects. Few theorists receive a farewell like that. Yang worked on statistical mechanics, integrable systems, gauge theory, particle physics and condensed matter physics. At least ten Nobel laureates in physics mentioned his work in their prize speeches. How did a student from wartime China come to shape so much of modern physics? What did he build with a junior colleague who shared his office? And why did this celebrated scientist argue against a giant particle collider in his own country?

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• 2. Wartime Classrooms

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  Yang was born in Hefei, Anhui, on the 1st of October 1922. His father, Ko-Chuen Yang, was a mathematician who lived from 1896 to 1973. His mother was Luo Meng-hua. Yang went to elementary and high school in Beiping, now Beijing. In the autumn of 1937 the Japanese invaded China, and the family moved to Hefei. A year later they moved again, to Kunming in Yunnan, where National Southwestern Associated University was based. Yang passed its entrance examination in 1938. He earned a Bachelor of Science in 1942 under Ta-You Wu, with a thesis applying group theory to molecular spectra. Two more years of graduate work followed under Wang Zhuxi, also known as J.S. Wang, on statistical mechanics. National Tsing Hua University had relocated to Kunming during the Sino-Japanese War, and it awarded Yang a Master of Science in 1944. Next came a scholarship from the Boxer Indemnity Scholarship Program. The United States government had created it from part of the money China was forced to pay after the Boxer Rebellion. Yang's departure was delayed by a year. He spent that year teaching in a middle school and studying field theory. He entered the University of Chicago in January 1946 and studied with Edward Teller. At first he spent twenty months in an accelerator laboratory. He concluded that he was not good at experimental physics and returned to theory. Even so, his doctoral thesis, finished in 1948, described an atomic beam apparatus for measuring the nuclear quadrupole resonance of sodium. His earliest papers, from 1944 and 1945, grew out of his master's thesis on statistical physics. By 1949 he was on to particle theory. The Fermi-Yang model of that year treated the pion as a bound pair made of a nucleon and an antinucleon.

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• 3. The Yang-Mills Idea

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  During the academic year 1953-54, Yang visited Brookhaven National Laboratory. There he shared an office with Robert Mills, then a novice scientist. Mills later recalled that Yang was generous to physicists starting their careers. Yang described his plan to generalize gauge invariance, and the two talked it through at length. Mills said he helped with the quantization procedures and, to a small degree, with the formalism. In his words, "the key ideas were Yang's." The result is known as Yang-Mills theory, a non-abelian gauge theory. It describes the nuclear forces in the Standard Model of particle physics. In 1980 Yang and Mills shared the Rumford Prize for it. Freeman Dyson, writing in 2009, called this Yang's greatest contribution. He explained that in such theories the possible interactions between fields become unique, so symmetry dictates interaction. Dyson compared Yang to a bird "flying high over the rain forest of little problems in which most of us spend our lives." Yang returned to gauge theory in the 1970s. With Tai Tsun Wu he worked out the topological properties of these theories. Together they described the Wu-Yang monopole, a kind of magnetic monopole. Unlike the Dirac monopole, it has no singular Dirac string. Their 1975 paper, the Wu-Yang dictionary, helped link physics with differential geometry.

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• 4. Breaking Mirror Symmetry

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  In 1956 Yang and Tsung-Dao Lee took on the tau-theta puzzle. A particle called theta decayed into two pions, while a particle called tau decayed into three pions. The two decays had different parity symmetry, yet the particles could not be told apart. Lee and Yang proposed that they were one particle, now known as the kaon K+. The catch was that the weak interaction would then have to violate parity. Parity conservation had been observed to hold in every other physical process. The weak interaction governs nuclear reactions that emit beta particles. To test the idea, Lee contacted the team of Chien-Shiung Wu at the National Bureau of Standards in Washington. The Wu experiment confirmed the theory, and the results were announced early at a press conference. Two independent experiments agreed. One was by Valentine Telegdi and Jerome Isaac Friedman at the University of Chicago. The other was by Richard Garwin and Leon M. Lederman at Columbia University. All three published in early 1957, and Yang and Lee won the 1957 Nobel Prize in Physics that year. Yang kept pulling on this thread. With Lee and Reinhard Oehme he explained how parity, charge and time-reversal symmetries relate, now called CPT symmetry. In 1960 a paper by Lee and Yang proposed using high energy neutrinos to study the weak interaction, and it promoted a range of neutrino experiments.

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• 5. Thirteen Faces of a Cube

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  At Yang's 90th birthday celebrations in 2012, the Chinese University of Hong Kong held a scientific conference and a dinner banquet. The evening announced the creation of the CN Yang archive, and Kenneth Young opened the ceremony. Tsinghua University gave Yang a black cube with 13 of his seminal contributions engraved on its faces. The cube also carried an ancient Chinese poem that Yang had used in his 2013 Selected Papers and Commentaries. Statistical mechanics supplies four of the thirteen. In 1952 the Lee-Yang theory and theorem described the liquid-gas phase transition from microscopic properties. In 1957 the Lee-Huang-Yang correction, with Kerson Huang, explained the stability of Bose-Einstein condensates and superfluids. The Yang-Baxter equation of 1967 gave a consistency condition for one-dimensional scattering systems and revived interest in the Bethe ansatz. The Yang-Yang thermodynamics of 1969 was developed with Yang's brother Chen-Ping Yang, who lived from 1930 to 1918 by no account but 2018. Ultracold atoms demonstrated its predictions in 2008. Condensed matter physics adds two more. The Byers-Yang theorem of 1961, with Nina Byers, concerned flux trapped in a superconducting ring. In 1962 Yang developed off-diagonal long-range order, which characterizes macroscopic quantum phenomena such as superconductivity and superfluidity. In particle physics the cube also marks a 1964 paper on CP violation written with T.T. Wu. Field theory rounds out the list with a 1974 integral formalism that gave meaning to the nonintegral phase factor in gauge theories. The cube also recognizes the parity, gauge theory and Wu-Yang dictionary work already described.

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• 6. Institutions and Honors

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  After Chicago, Yang stayed a year as an assistant to Enrico Fermi. In 1949 he was invited to the Institute for Advanced Study in Princeton, New Jersey. There he began his fruitful partnership with Lee, and the pair published 32 papers together. He became a permanent member of the Institute in 1952 and a full professor in 1955. Princeton University Press published his textbook Elementary Particles in 1963. In 1965 he moved to Stony Brook University as the Albert Einstein Professor of Physics. He also became first director of the newly founded Institute for Theoretical Physics. In 1998, the year before he retired, it was renamed the C. N. Yang Institute for Theoretical Physics. A residence hall and activity center, C. N. Yang Hall, was dedicated there in 2010. On his retirement in 1999, Dyson called Yang "the pre-eminent stylist" of twentieth-century physics, alongside Albert Einstein and Paul Dirac. Honors piled up. Yang was named one of the Ten Outstanding Young Americans in 1957. He received the National Medal of Science in 1986, the Albert Einstein Medal in 1995 and the King Faisal International Prize in 2001. The Marcel Grossmann Awards of 2015 praised him for deepening Einstein's geometrical approach to physics. The ten laureates who cited his work in their Nobel speeches include Steven Weinberg, Sheldon Glashow, Gerard 't Hooft, Yoichiro Nambu, Peter Higgs and François Englert.

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• 7. Return to China

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  In 1971 Yang visited the Chinese mainland for the first time, after the thaw in China-US relations. The research atmosphere there had eroded under the political movements of the Cultural Revolution. He worked afterward to help the Chinese physics community rebuild it. In 1986 Shiing-Shen Chern, the inaugural director of the Chern Institute of Mathematics, asked him to help set up its Theoretical Physics Division. Yang agreed. He had also been the first president of the Association of Asia Pacific Physical Societies when it was established in 1989. In 1997 the association created the C. N. Yang Award, which highlights young researchers. After leaving Stony Brook he went back to Beijing as an honorary director at Tsinghua University. There he held the first Huang Jibei-Lu Kaiqun Professorship at the Center for Advanced Study. He was also a founding member of the Shaw Prize and a Distinguished Professor-at-Large at the Chinese University of Hong Kong. Late in life Yang opposed building the Circular Electron Positron Collider, a particle collider 100 km in circumference meant to study the Higgs boson. He called the project "guess" work with no guaranteed results. Yang also said that "even if they see something with the machine, it's not going to benefit the life of Chinese people any sooner."

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• 8. Loyalties and Family

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  In 1950 Yang married Tu Chih-li, a teacher. They had two sons and a daughter. Her father was the Kuomintang general Du Yuming. Their son Guangnuo became a computer scientist, Guangyu an astronomer, and their daughter Youli a doctor. Tu died in October 2003. Yang had met Weng Fan, then a university student, at a physics seminar in 1995. They reestablished contact in February 2004, when Yang moved to China to join Tsinghua University. They married in January 2005. Weng was 54 years his junior, and Yang called her his "final blessing from God." His American citizenship carried its own weight. According to the state-run Xinhua News Agency, Yang said taking it was painful because his father never forgave him. Xinhua and other mainstream Chinese media reported that he formally renounced it on the 1st of April 2015. Yang said the United States was a beautiful country that gave him good opportunities to study science. In his view, though, China had offered the best secondary and undergraduate institutions since his youth, while the US had the top graduate programs. By the turn of the century, he said, circumstances had shifted in favor of China's growth.

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