Chen-Ning Yang, the Chinese American theoretical physicist, who has died aged 103, won the Nobel prize in physics in 1957.<\/p>\n
It was during a period at the Institute for Advanced Study at Princeton, New Jersey, in 1950 that Yang (also known as CN Yang, or Frank Yang) befriended another young Chinese \u00e9migr\u00e9, Tsung-Dao Lee<\/a>. They shared the Nobel prize for work that overthrew the widely accepted \u201cparity laws\u201d \u2013 that the forces acting on fundamental subatomic particles are symmetric between left and right. In the popular description, they overthrew the concept of \u201cmirror symmetry\u201d. Although the Nobel was for this work, performed in 1956, the most far-reaching of Yang\u2019s many contributions to theoretical physics had come earlier.<\/p>\n An idea conceived by Yang in 1953 was developed with the assistance of a doctoral student, Robert Mills, with whom he was sharing an office during a visit to Brookhaven National Laboratory on Long Island, New York. This brief encounter led to Mills becoming immortalised by inclusion in the moniker \u201cYang-Mills theories\u201d. Today Yang-Mills theories<\/a> underpin fundamental particle physics, successfully describing the weak and the strong nuclear forces. Initially, however, the reaction of the acerbic theorist Wolfgang Pauli<\/a> was so damning that Yang\u2019s nascent career was in jeopardy.<\/p>\n Yang\u2019s inspiration came from the successful demonstration in 1947 that quantum electrodynamics \u2013 QED \u2013 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 throughout space and time \u2013 a quantum property known as \u201clocal gauge invariance\u201d. In the jargon, QED is known as a \u201cgauge theory<\/a>\u201d.<\/p>\n Yang, left, with his fellow Nobel laureate Tsung-Dao Lee. Photograph: Science History Images\/Alamy<\/p>\n Gauge invariance is profound and places severe restrictions on what is possible. One such is that all electrons in the universe have the same sign and magnitude of electric charge.<\/p>\n The response of an electron to some stimulus, such as a magnetic field, cannot depend on whether the experiment is done in Europe, America or on the moon.<\/p>\n A consequence is that there necessarily exists some connection linking the various electrons, allowing us to compare the situation at the different locations.<\/p>\n In quantum field theory this connection consists of particles with a sense of direction \u2013 \u201cvector\u201d particles. The connection must act over very large distances, which in quantum theory equates to the particle having no mass. In QED this is the photon, a quantum bundle of light; the photon is thus a necessary consequence of local gauge invariance.<\/p>\n In 1953 Yang attempted to build a theory describing the nuclear force between protons and neutrons by using the same ideas that had proved successful for QED. The proton is positively charged, whereas the neutron is neutral. The strong nuclear force can transfer electric charge between these, converting a neutron into a proton while a neighbouring proton is transformed into a neutron, balancing the books.<\/p>\n Yang and Mills\u2019s equations took account of this flow of electric charge, and the resulting theory of the nuclear force produced a testable consequence: there exist three distinct varieties of massless vector particle, one of the trio being electrically neutral like the photon, the others being electrically charged, one positive and one negative.<\/p>\n On 23 February 1954, Yang spoke about this theory at the weekly seminar at Princeton\u2019s Institute for Advanced Study. He later recalled the occasion with anguish.<\/p>\n In the audience was Pauli, a Nobel laureate who did not suffer fools gladly, and regarded many of his colleagues as belonging to that camp. Yang had hardly begun to speak when Pauli interrupted: \u201cWhat is the mass of these vector particles?\u201d Pauli intuitively realised they had to be massless, and that no such things exist. Yang hedged, saying he did not know.<\/p>\n Pauli sulked for a while and then repeated the question. Yang said he and Mills had thought about this, but it was complicated, and they had come to no definite conclusion. Pauli leaped in for the kill: \u201cThat is not sufficient excuse.\u201d Yang, taken aback, sat down, unable to continue until J Robert Oppenheimer<\/a>, chair of the seminar, asked him to do so. Pauli said no more, but the following day Yang received a note: \u201cDear Yang, I regret you made it almost impossible for me to talk with you after the seminar. All good wishes, W Pauli.\u201d<\/p>\n Yang in Beijing, 2021. Photograph: Xinhua\/Shutterstock<\/p>\n Yang was right: it is \u201ccomplicated\u201d, and not until 1964, thanks to the work of Peter Higgs<\/a> and others, was it understood that in the presence of the \u201cHiggs field\u201d, these vector particles can have mass. Today these massive, charged particles are the empirically confirmed W bosons, agents of the weak nuclear force, responsible for some radioactive decays.<\/p>\n The strong nuclear force is also described by Yang-Mills theory \u2013 quantum chromodynamics, QCD. The \u201ccharge\u201d in this case is known as \u201ccolour\u201d, which is carried by quarks, the constituents of neutrons and protons \u2013 a deeper layer of matter yet unknown in 1953. The force between quarks is carried by massless vector particles known as gluons. Their existence and the validity of QCD was confirmed in the 1970s.<\/p>\n Thus, Yang\u2019s insight of 1953 is today the foundation of theories that successfully describe electromagnetic strong and weak forces. The one missing ingredient, at the time of Yang\u2019s ill-fated seminar, was the \u201cmass mechanism\u201d, finally confirmed in 2012 with the discovery of the Higgs boson<\/a>. When combined with Yang\u2019s work on the violation of mirror symmetry, we have the foundation of today\u2019s standard model of particles and forces.<\/p>\n 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 National Southwestern Associated University in Yunnan in 1942.<\/p>\n After two years of graduate studies, he won a scholarship to do a PhD in the US. He entered the University of Chicago in 1946, and under the guidance of Edward Teller<\/a> completed his PhD in 1948.<\/p>\n