Chen-Ning Yang thought the unthinkable and won the 1957 Nobel Prize in physics: Yang and his coworker Tsung-Dao Lee showed that parity – a property that physicists had believed was always conserved – like energy, momentum and electric charge – need not be conserved.
Yang also worked with Robert Mills to produce Yang-Mills theory, which today lies at the heart of the Standard Model in physics.
Early Life and Education
Chen-Ning Franklin Yang was born on September 22, 1922 in the city of Hefei, China.
His family moved to Beijing when he was young after his father, Wu-Chih, became a Professor of Mathematics at Tsinghua University. His mother, Meng-hua was a housewife.
Yang was schooled in Beijing until 1937, when the Japanese invasion of China forced his family to return to Hefei, and then, a year later, move to the city of Kunming. The Japanese Army did not reach Kunming in the south-west of China, although it was bombed by the Japanese Air Force.
Yang enrolled at the National Southwestern Associated University in Kunming and was awarded a bachelor’s degree in physics in 1942.
In 1944 he was awarded a master’s degree in physics for his work in statistical mechanics. He was awarded his degree by Beijing’s Tsinghua University, which had relocated to Kunming.
Yang worked as a teacher until he won a United States government scholarship in 1946, which took him to the University of Chicago. There his doctoral advisor was Edward Teller, the father of the hydrogen bomb.
In 1948 Yang was awarded a Ph.D. in physics for his work on nuclear reactions.
Chen-Ning Yang’s Research Work
After the award of his Ph.D., Yang stayed at Chicago for a year, working with one of the giants of 20th century physics, Enrico Fermi.
In 1949 he was invited to become a theoretical physics researcher at the Institute for Advanced Study in Princeton.
The Institute had been founded in 1930 with the goal of employing the best mathematicians and physicists in the world; Albert Einstein was there from 1933 until his death in 1955.
During the 1950s, increasingly complex results had been coming out of particle accelerators and cosmic ray detectors, causing increasing confusion among physicists.
The accelerators were pushing ions and other particles to enormous speeds, then smashing them into one another. Physicists hoped the debris from the collisions would reveal more about what matter is and how it behaves.
Cosmic rays – high energy particles reaching Earth from the sun and the stars – also produced interesting debris.
The debris from both accelerators and cosmic rays contained subatomic particles, which are generally unstable, quickly decaying into other particles.
The Meson Problem
Two unstable particles, the theta-meson and the tau-meson, were causing a lot of head-scratching.
In some senses, the theta-meson and the tau-meson looked as if they might be the same particle: their masses and the average time they took to decay into other particles seemed to be the same. The theta-meson and the tau-meson both decayed into pi-mesons, usually known as pions.
BUT, the theta decayed to produce two pions, while the tau decayed to produce three pions.
Most physicists took it as a fundamental law of the universe that when any particle decayed, its parity stayed the same.
Parity must never be broken. This meant, in a very simplified way, that the same particle could not possibly decay sometimes into two pions, and at other times into three pions. Physicists believed there was a fundamental symmetry in the universe. If parity were broken, the fundamental symmetry they believed in would also be broken.
Physicists regarded parity as a property that was conserved in the same way that energy, momentum, and electric charge are always conserved.
Yet the only difference physicists could find between the theta-meson and the tau-meson was that they decayed differently. Otherwise these mesons seemed identical.
A Daring Hypothesis: Broken Parity
What if there really were only one meson – a meson that sometimes decayed into two pions and sometimes into three pions?
Most physicists thought the idea was ludicrous; if there was one thing they could rely on Mother Nature to do, it was to preserve parity and symmetry.
Enter Yang and Lee
At the Institute for Advanced Study, Yang had started working with Tsung-Dao Lee. They had actually first met in China at the National Southwest University.
Yang was now a full professor of theoretical physics, having been promoted in 1955.
In summer 1956, Yang and Lee thought the unthinkable. What if parity really could be broken? At this time, Yang was 34 and Lee was 29 years old.
The meson decay they were looking at involved the weak nuclear force – the force responsible for nuclear fission and beta particle emission from atomic nuclei.
The two physicists read everything they could and carried out a large number of calculations; they wanted to see if there truly was a fundamental physical law preventing parity being broken for interactions involving the weak nuclear force. There was already good evidence that parity could not be broken for interactions involving the strong nuclear force.
They published their work late in 1956, showing they could find nothing to stop parity being broken for weak interactions and they described experiments they had devised which could prove whether parity was broken.
The Unthinkable is True = Nobel Prize
A team of physicists at the Cryogenics Physics Laboratory at the National Bureau of Standards in Washington carried out one of the experiments designed by Yang and Lee, cementing Yang and Lee’s place in the history of science.
In 1957 Yang and Lee won the Nobel Prize in Physics: they had thought the unthinkable, their calculations showed the unthinkable was possible, and they had devised experiments that had established that the unthinkable was actually true: the theta and tau mesons were actually the same particle and Mother Nature did not preserve parity. At a deep level, this means that nature can tell the difference between left and right.
In the more somber words of the Nobel Prize Committee, Yang and Lee’s prize was for their:
“penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles.”
Even in the face of the theta-tau puzzle, most physicists had not seriously contemplated the possibility of parity breaking. Physics giant Richard Feynman was pleased that at one point he gave the odds of parity breaking being discovered as low as 1 in 50!
Particle physics had been held back for years by the incorrect assumption that parity could not be broken in weak interactions. Yang and Lee set particle physics free again.
Prior to his Nobel Prize winning work, Yang studied the fundamental forces in particle physics and how they relate to one another.
The first unification of forces in physics had happened in the 19th century, when James Clerk Maxwell unified the electric and magnetic forces; he showed they were actually manifestations of a single force: the electromagnetic force. In doing so, Maxwell established that light is an electromagnetic wave which carries energy between electric charges.
Maxwell’s work shook physics to its core.
Ever since Maxwell set the ball rolling, physicists have dreamed of uniting all of the forces of nature into one fundamental theory: a theory of everything.
In 1954 Yang was doing some work at Brookhaven National Laboratory, where he shared an office with Robert Mills, another young physicist.
Bouncing ideas off one another, they developed a new generalization of Maxwell’s equations, now called Yang-Mills theory.
The theory produces Maxwell’s equations as a special case. In addition to explaining electromagnetic forces, Yang-Mills theory also explains interactions between nuclear particles – in doing so, it carries physics closer to a theory of everything.
Yang-Mills theory now lies at the heart of the Standard Model of particle physics. The Standard Model tries to tie together the electromagnetic force, the weak nuclear force, the strong nuclear force, and all of the subatomic particles into a single consistent system – a theory of everything.
Yang–Mills theory is one of the seven Millennium Prize Problems in mathematics. The Yang-Mills millennium problem asks scientists to rigorously establish quantum Yang-Mills theory and to solve a further Yang-Mills issue known as the mass gap. There’s a prize of $1 million for the solution.
Today, more than fifty years after it was born, Yang–Mills theory is a very active research field in physics.
Yang was married to Chi-Li Tu from 1950 until she died in 2003. He has three children from this marriage. In 2004, he married Weng Fan.
Although he has been an American citizen since 1964, he now lives in China, where he is an honorary director of Tsinghua University, Beijing – his father’s old university, and the university where he studied for his master’s degree.
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