Chen-Ning (Frank) Yang 1922-2025

Chen-Ning Yang (often known in the US as “Frank”) passed away in Beijing yesterday at the age of 103. He was the last of the great figures of 1950s high-energy particle physics still with us.

His name is associated with two of the central ideas of what was to become the Standard Model.

• In 1953, together with Robert Mills, he wrote down the extension of the theory of quantum electrodynamics from the the case of gauge group U(1) to that of SU(2).
• In 1956, together with T.D. Lee, he argued that the weak interaction theory should (very unexpectedly) be one that violated parity conservation.

The original Yang-Mills theory was supposed to describe the strong interactions, with SU(2) isospin symmetry. This idea doesn’t work, but later on the SU(2) Yang-Mills theory became the foundation of the unified electroweak theory. Extending from SU(2) to SU(3), one gets the rest of the Standard Model, QCD.

The problem with Yang-Mills theory in the 1950s was that, treated perturbatively, the photon of QED gets replaced by a triplet of spin 1 massless particles, but there is no such thing in nature. Unlike the photon case, this triplet will have self-interactions, and the hope from the beginning was that these would explain away the lack of massless particles. One way to get rid of the massless particles is the Anderson-Higgs mechanism, first described by Anderson in 1962. For Anderson and others working on this, the hope was to use a spontaneously broken SU(2) gauge theory to describe the strong interactions. Only later, in 1967, was this idea applied to unified electroweak models.

The other way to get rid of the massless particles is to realize that while the quantum Yang-Mills theory is asymptotically free at short distances (where massless states are relevant), at long distances the self-interactions become large and the spectrum of the theory has only massive confined states. Moving from SU(2) isospin symmetry to SU(3) color symmetry, one finally got (1973) a very successful Yang-Mills type theory of the strong interactions.

While the Yang-Mills proposal for the strong interactions took 20 years to come to fruition, things happened very differently with the Lee-Yang proposal of parity non-conservation. Lee and Yang submitted their paper about this on June 22, 1956. At the time Yang was visiting Brookhaven for the summer, coming into New York to work with Lee. A good place to read about this is here (with more here), which has:

Yang did not remember precisely how the idea of examining the previous experiments on weak interactions came to him and Lee, but he remembers when and where. It was early in May, and it occurred just after he had driven in to New York from Upton to visit Lee. He had picked up Lee at his Columbia office and was having difficulty finding a parking place near Columbia. He and Lee were driving around looking for one and talking. They finally parked the car temporarily in front of a Chinese restaurant near the corner of Broadway and 125th Street. The restaurant was not yet open, so they went into the White Rose Café nearby. They sat down at a table and resumed their conversation, and it was then that the idea struck them. It was suddenly clear to them that the results of one weak-interaction experiment after another had to be examined to see if they gave any information on parity non-conservation.

A Columbia colleague, Chien-Shiung Wu, soon started an experiment using cobalt-60 and had experimentally demonstrated parity non-conservation by December. Another Columbia group with a muon beam quickly showed parity non-conservation in that context. Not much more than a year after their paper, Lee and Yang received the physics Nobel Prize for 1957. This was definitely a high point for the Columbia physics department.

In 1966 Yang left Princeton for Stony Brook, where he founded the Institute for Theoretical Physics. By the time I arrived there much later as a postdoc (1984), Yang was very much an elder statesman of the field. I only remember talking to him a couple times, both not about science but about administrative issues.

The ITP shared a building with the math department, which had become a world center for geometry starting with the arrival of Jim Simons in 1968. It’s fascinating to hear (see here) the account by Simons and Yang of their attempts to understand each other’s language and their slow realization that the gauge fields of Yang-Mills theory were the geometer’s connections on a principal fiber bundle. Yang wrote a paper with Wu giving what became known as the “Wu-Yang” dictionary relating the two subjects. Singer learned of this when visiting Stony Brook, then brought the news to Atiyah in Oxford, starting an explosive development during the late 1970s.

Yang retired in 1999 and returned to Beijing. In recent years he has been involved in debates over whether China should build a new large collider, taking a rather skeptical view. For an extensive discussion of this, see here. Already in 1980 (see here) Yang had come to the conclusion that particle physics was not going to again see anything like what had happened in 1956: “the party’s over”.

So, on the spur of the moment, I said to Marshak, “Yes, I will say something, if you promise not to publish it.”

He said OK, and he stuck to his word later.

So I said, “In the next ten years, I think the title of the panel was either the future or the next ten years of high-energy physics,” I said, “In the next ten years, the most important discovery in high-energy physics is that ‘the party’s over’.” After I said that, there was general silence. Nobody said a word, and then Marshak declared the panel was finished.

I fear that developments since 1980 have very much proved Yang right. While the party’s over, his passing provides a good opportunity to recall what a fine time it was.