Chen-Ning Yang obituary | Particle physics

Chen-Ning Yang, Chinese-American theoretical physicist, who died at the age of 103, won the Nobel Prize in physics in 1957.

It was during a period at the Institute for Advanced Study in Princeton, New Jersey, in 1950, that Yang (also known as C. N. Yang or Frank Yang) became friends with another young Chinese émigré, Tsung-Dao Lee. They shared the Nobel Prize for their work that overturned the widely accepted “laws of parity” – according to which the forces acting on fundamental subatomic particles are symmetrical between left and right. In popular description, they overturned the concept of “mirror symmetry”. Although this work won the Nobel Prize in 1956, the most important of Yang’s many contributions to theoretical physics was earlier.

An idea conceived by Yang in 1953 was developed with the help of a doctoral student, Robert Mills, with whom he shared an office while visiting Brookhaven National Laboratory on Long Island, New York. This brief encounter resulted in Mills being immortalized by his inclusion in the nickname “Yang-Mills theories.” Today, Yang-Mills theories underpin fundamental particle physics, successfully describing weak and strong nuclear forces. But initially the reaction from the acerbic theorist Wolfgang Pauli was so overwhelming that Yang’s burgeoning career was threatened.

Yang’s inspiration came from the successful demonstration in 1947 that quantum electrodynamics – QED – could be a viable theory of the interaction between electrically charged particles and light. A key feature of QED is that its equations apply uniformly across space and time – a quantum property known as “local gauge invariance”. In the jargon, QED is known as “gauge theory”.

Gauge invariance is profound and imposes severe restrictions on what is possible. One is that all electrons in the universe have the same sign and magnitude of electric charge.

The response of an electron to a stimulus, such as a magnetic field, cannot depend on whether the experiment is carried out in Europe, America or on the Moon.

A consequence is that there necessarily exists a connection between the different electrons, making it possible to compare the situation at different locations.

In quantum field theory, this connection is made up of particles with a sense of direction – “vector” particles. The connection must act over very large distances, which in quantum theory is equivalent to a massless particle. In QED, this is the photon, a quantum beam of light; the photon is therefore a necessary consequence of local gauge invariance.

In 1953, Yang attempted to develop a theory describing the nuclear force between protons and neutrons using the same ideas that had proven successful for QED. The proton is positively charged while the neutron is neutral. The powerful nuclear force can transfer an electric charge between these, converting a neutron into a proton while a neighboring proton turns into a neutron, thus balancing the accounts.

The Yang and Mills equations took into account this flow of electric charge, and the resulting nuclear force theory produced a testable consequence: there are three distinct varieties of massless vector particles, one of the three being electrically neutral like the photon, the others being electrically charged, one positive and one negative.

On February 23, 1954, Yang spoke about this theory at the weekly seminar of the Institute for Advanced Study in Princeton. He would remember it later with anguish.

Among the spectators was Pauli, a Nobel Prize winner who did not suffer fools kindly and who considered many of his colleagues to belong to this camp. Yang had barely started speaking when Pauli interrupted: “What is the mass of these vector particles?” Pauli intuitively understood that they must be massless and that such things do not exist. Yang covered himself, saying he didn’t know.

Pauli sulked for a moment then repeated the question. Yang said he and Mills thought about it, but it was complicated and they didn’t come to a definitive conclusion. Pauli rushed to kill: “This is not a good enough excuse.” Yang, taken aback, sat down, unable to continue until J Robert Oppenheimer, president of the seminary, asked him to. Pauli said no more, but the next day Yang received a note: “Dear Yang, I regret that it was almost impossible for me to speak to you after the seminar. Best wishes, W Pauli.”

Yang was right: it is “complicated”, and it was only in 1964, thanks to the work of Peter Higgs and others, that it was understood that in the presence of the “Higgs field”, these vector particles could have mass. Today, these massive, charged particles are the empirically confirmed W bosons, agents of the weak nuclear force, responsible for certain radioactive decays.

The strong nuclear force is also described by the Yang-Mills theory – quantum chromodynamics, QCD. The “charge” in this case is known as “color,” which is carried by quarks, the constituents of neutrons and protons – a deeper layer of matter still unknown in 1953. The force between quarks is carried by massless vector particles called gluons. Their existence and the validity of QCD were confirmed in the 1970s.

Thus, Yang’s 1953 vision today forms the basis of theories that successfully describe strong and weak electromagnetic forces. The only missing ingredient, at the time of Yang’s ill-fated seminar, was the “mass mechanism”, finally confirmed in 2012 with the discovery of the Higgs boson. Combined with Yang’s work on mirror symmetry violation, we have the foundations of the current standard model of particles and forces.

The eldest of five children of Yang Wu-zhi, a mathematician, and Luo Meng-hua, Chen-Ning was born in Hefei, China, and went to school in Beijing. He graduated from the National Associated University of Southwestern Yunnan in 1942.

After two years of higher education, he obtained a scholarship to do a doctorate in the United States. He entered the University of Chicago in 1946 and, under the supervision of Edward Teller, completed his doctorate in 1948.

A year as Enrico Fermi’s assistant was followed by an invitation to the Institute for Advanced Study, where Yang met TD Lee. In 1965, Yang joined Stony Brook University as the Albert Einstein Professor of Physics, where he remained until his retirement in 1999, then returned to China to Tsinghua University in Beijing.

In addition to his work in particle physics, Yang made fundamental contributions to statistical mechanics and condensed matter physics. In his later years, he played a major role in establishing large-scale research centers in China. He has won numerous awards besides the Nobel Prize, including the American National Medal of Science (1986) and the Albert Einstein Medal in 1995.

In 1950 he married Chih-li Tu and they had two sons, Franklin and Gilbert, and a daughter, Eulee. After Chih-li’s death in 2003, Yang married Weng Fan. She survives him, as do her children.