Chen-Ning Yang made important contributions to the theory of solvable models in statistical mechanics, including generalizations of the Bethe Ansatz, magnetization in the Ising model, the Lee-Yang circle theorem, and the Yang-Baxter equation. Most famously, Yang made transformative contributions to the current Standard Model of elementary particle interactions. The proposal of Yang and T. D. Lee, that left-right symetry (parity) is violated in weak particle decays, established that the primary currents involved in weak interactions are left handed. The work of Yang and R. L. Mills gave a framework for force carriers coupling to these currents that are non-Abelian generalizations of the electromagnetic photon, which unlike the electrically neutral photon, carry ``charges'' to which they self-couple . Two decades of work by others on quantization and mass-generation mechanisms then culminated in the Standard Model.
guys."'. Recounting his early education, Yang writes "In 1929 when I was seven years old, my father became a professor of mathematics at Tsinghua University in. ..(now Beijing), and we lived in a house on the campus for eight years." (II-239) These were idyllic years. Then, "The War of Resistance Against Japan started in July 1937. We first moved back to Heifei, and then after the Japanese reached Nanjing, took a long journey... reaching finally Kunming in March 1938". (II-230). After a few months in eleventh grade, Yang "skipping one year, entered the Southwest Associated University...in the fall of that year." The National Southwest Associated University (SAU) was formed by the wartime incorporation in Kunming of National Peking University, National Tsihghua University, and National Nankai University. Kunming, at an altitude of 6,234 feet on the Yunnan-Guizhou Plateau, was during World War II a Chinese military center, transport terminus for the Burma Road, and home of the Flying Tigers. It was subject to constant air raids: "On September 30, 1940, the house that my family rented in Kunming received a direct hit, reducing most of our meager possessions to rubble.
Fortunately, every member of the family was in some shelter and no one was wounded." (I-3) Moreover “…there was the constant threat of inflation. My father was a professor at the Southwest Associated University, and his savings were totally wiped out….. To feed and clothe a family of seven, my mother, a woman of great will power and self-discipline, toiled from dawn to night, year after year, with calm dignity. The family survived the war intact -lean, very lean, but healthy.” (I-4) “I had not studied high school physics, so to prepare for the entrance examination [to SAU] I borrowed a copy of a standard high school physics textbook and read it through in several weeks…I concluded that physics was the subject that I liked … [and] Despite this unpleasant experience, Yang wrote “Yes, there were things that held me back. Yet I knew that America had been most generous to me. I had come very well equipped, but America had allowed me the opportunity to develop my potential. I knew there was no country in the world that was as generous to immigrants. I also realized that.. my roots here were deepening..”. Yang’s most famous work, however, was in applications of symmetries to particle physics. From reading starting in Kunming, Yang “was very much impressed with the idea that charge conservation was related to the invariance of a theory under phase changes…I was even more impressed with the fact that gauge invariance determined all electromagnetic interactions. When in Chicago, I tried to generalize this to isotopic spin interactions” [rotations in charge space such as from proton to neutron] but initial attempts “led to a mess, and I had to give up” (I-19). However, the general idea became an “obsession” (I-19) to which Yang returned repeatedly. During a 1954 sabbatical from the IAS,“while at Brookhaven I returned once more to the idea of generalizing gauge invariance,” this time working with office-mate Robert L. Mills. (I-19) They tried to eliminate the “mess” by adding polynomial terms to the relation between field strength and gauge potentials. “We decided to first try a quadratic polynomial. If that did not work we would try a cubic one…Fortunately, we rapidly found that if we [added a simple quadratic term] the subsequent calculation became increasingly simple. Thus we knew that we had uncovered a great treasure!!!” (II-319) This was written up as classic papers, and Yang-Mills theory, or in technical terms non-Abelian gauge theory, became a foundation for the subsequent unification of particle forces. The questions of mass, emphasized in critique by Pauli (who had not published his own similar ideas), and of quantization, took two decades of work by a score of brilliant theorists to resolve.
In 1956 Yang, while on summer leave at Brookhaven, working with T. D. Lee, made a comprehensive study of whether experiments to date had established parity (spatial reflection symmetry) as a valid symmetry of beta type weak particle decays. This was motivated by the so-called thetatau puzzle, the fact that particle(s) of the same mass decayed into even and odd parity final states of pions. “The result was that, in all these processes, previous experiments did not yield any information about whether there was only [one type of] interaction or there were both types of interaction [i.e., mixtures of even and odd parity terms]. In other words, all previous beta decay experiments were irrelevant as far as the question of parity conservation for beta decay was concerned.” (I-28). The underlying reason was that “terms proportional to [products of the two types] in the calculations must be ‘pseudoscalars’. Since all previous experiments did not measure a pseudoscalar, they had therefore no bearing on parity conservation in beta decay.” (I-28,29).
Experiments measuring pseudoscalars that involved particle spins were soon done, and gave the sensational result that parity is not conserved in the weak interactions. This was front page news in the NY Times, and two years later Lee and Yang shared the 1957 Nobel Prize for their incisive analysis. They were the first Chinese to win a Nobel Prize. Yang’s parents were living then, and Yang was in regular touch with them, but China was still very isolated and his parents were not recorded as present at the ceremony in Stockholm.
Other important symmetry related work included a 1957 paper with T. D. Lee and Reinhard Oehme studying parity (P), charge conjugation (C), and time reversal (T) symmetries, which included an early analysis of the neutral K meson-anti K meson system. In 1964, after violation of the product symmetry charge conjugation times parity (CP) was discovered experimentally in this system, Yang in collaboration with his younger protege T. T. Wu decided to avoid the rush towards premature speculations as to its origin. Instead, “With our tendency toward restraint, Wu and I decided to make a phenomenological analysis of kaon-antikaon decay….It provided the framework within which subsequent experiments… were analyzed.” (I-58,59) The collaboration with T. T. Wu continued, and a decade later on Yang and Wu wrote important papers on formal aspects of gauge theories and their relationship to the mathematics of fiber bundles.
Shortly after the CP discoveries, Oppenheimer decided to retire as IAS Director, and told Yang that “he would propose to the Trustees that I be appointed his successor…I thought it over and some days later wrote him…It is quite uncertain that I shall make a good Director, while it is quite certain that I shall not enjoy the life of a Director.” (I-60) Likely Yang had in mind in writing this the ongoing acrimony between some of the Mathematics Faculty and the Director. Not that Yang was incapable of the occasional sharp remark. In his roast of Gell-Mann at Murray’s 80th birthday conference in Singapore mentioned above, Yang said “Anyone who had had contact with Murray cannot fail to be impressed by his catholic interests in many things, by his knowledge, by his humor, but also by his sometimes overbearing self-confidence.” (II-295) In terms of having Murray as a colleague at the iAS this last tipped the scale. “In the late 1950s, at a physics faculty meeting at the Institute for Advanced Study, Oppenheimer said casually he was thinking of asking Murray to join the Institute. At the next meeting, I said Murray is great, but if he comes to the Institute, I shall leave. Oppenheimer never mentioned the subject again.” . The move back to China also led to a big change in Yang’s personal life. On November 7, 2004 Yang announced his engagment to Miss Fan Weng, 52 years his junior, and they were married December 24, 2004. She was Yang’s constant companion for the final two decades of his life, “God’s benevolent last gift To give my old soul A joyous rejuvenating lift.” (II-279).
Although Yang expressed deep appreciation for the “many, many papers on gauge theories in the 1960s and 1970s” (I-67), he remained aloof from these efforts that led to the current Standard Model. “But I continue to believe that fundamental new ideas are still missing. For example, the introduction of a field to break symmetry cannot be the final story, although it may be a good temporary development, perhaps not unlike Fermi’s theory of beta decay.” (II-67) The mystery of the Higgs field is still a deep topic for investigation by current and future high energy theory and experiment.
In the Preface to volume (II) of Yang’s Selecta, he wrote that while the first volume “covered the years when my main interests were in physics research, the present one covers later years when my interests gradually shifted to history of physics.” (II-v) Many of these articles are reprinted in II, and Yang’s astute assessments of the great physicists who were his predecessors make fascinating reading. After completion of II, the published record ceases, like a footpath that has petered out in a dense forest. Yang’s summation “On Reaching Age Ninety” suffices: “Mine has been, A promising life, fully fulfilled, A dedicated life, with purpose and principle, A happy life with no remorse or resentment, and long life ….Traversed in deep gratitude.”
