In 1957, Chinese-American theoretical physicist TD (Tsung-Dao) Lee, who has died aged 97, became the second youngest scientist to win a Nobel Prize. He did this with another Chinese immigrant to the US, Chen Ning Yangwho was four years older than him. They became Nobel laureates in physics for work that overturned the generally accepted “parity laws” – that the forces acting on the fundamental subatomic particles are symmetrical between left and right. In the popular description, they overturned the concept of “mirror symmetry”.
Before Lee and Yang questioned this fundamental principle, it was believed that the mirror image of any process displayed a sequence of events that could equally well occur in the real world. In reality, there is no way to tell whether you are viewing a real event or its mirror image. This was well established in the familiar case of electromagnetic forces and the strong force that binds atomic nuclei. For example, the mirror image of an electrically charged particle driven in one direction by electric or magnetic forces reveals an order realized in the real world simply by reversing the direction of said forces.
In the mid-50s, physicists were puzzled by the behavior of what were known as strange particles. Among these, two were known as “tau” and “theta”, with identical masses, lifetimes and other properties, except for one: the taus decayed into two particles (pions), while the thetas decayed into three. In quantum theory, the mathematical description of an even or odd number of pions differs in this situation; in the jargon the quantum state has “even-parity” or “odd-parity” respectively. The belief that parity is preserved implied that the tau and theta must be different particles, the tau having even parity and the theta, odd.
At a conference in April 1956, Yang discussed the tau-theta puzzle. Evidence mounted that the tau and theta were in fact one and the same, but the law of conservation of parity contradicted this. In retrospect, the answer was there in sight – tau and theta are the same and parity conservation is not a general truth. Nevertheless, the axiom of parity conservation, or mirror symmetry, was regarded as so far indisputable that only in the subsequent debate Richard Feynman was prepared to ask, “Does nature have a way of uniquely distinguishing right-handedness or left-handedness?”
Yang and Lee began to examine data and verified that while there was conclusive evidence that mirror symmetry held good for electromagnetic and strong forces, the weak force responsible for some forms of radioactivity had not been tested well enough to either confirm or deny parity conservation . And it was this force that was responsible for the decay of tau and theta. They raised the possibility that the tau and theta are indeed only one, and that the decay to two or three, via quantum field theory arguments, is linked to a violation of mirror symmetry.
Their paper Question of Parity Violation in Weak Interactions was published in October 1956 and took the physics community by storm. Among many skeptics were Wolfgang Pauliwho scoffed, “I don’t believe the Lord is a weak left-hander.”
Yang was based at the Institute for Advanced Study in Princeton, New Jersey, and Lee at Columbia University, New York. Within days, a colleague of Lee’s at Columbia, Chien-Shiung Wu, set up an experiment in which she tested parity conservation in weak interactions according to a template proposed in their paper. This involved measuring the directional properties of the electrons released in the decay of radioactive cobalt-60.
It built on a key observation by Lee and Yang, that nuclei of cobalt-60 atoms carry angular momentum, which behaves differently from ordinary momentum under specular reflection. So measure the correlation between the directions – momenta – of the decay products (electrons) and the angular momentum of the cobalt nucleus. If there is any correlation, it will appear differently in a mirror, and by implication parity is violated. The only way parity symmetry can be true is for there to be no correlation.
After several months of work, the highly sensitive experiment conclusively proved that there is a correlation and therefore parity symmetry is not a general property. The news was sensational: the weak interaction cracks the mirror; nature can absolutely distinguish left from right. This has become a fundamental property of today’s standard model of particles and forces. Among the many applications of parity violation, now spanning seven decades, was the identification of the Higgs boson.
Born in Shanghai, TD was the third of six children of Ming-Chang Chang and Tsing-Kong Lee, a merchant. After high school in Shanghai, he studied physics at Chekiang (now Zhejiang) University (1943-44) and later at the National Southwest Associated University in Kunming.
In 1946, he won a Chinese government fellowship to do a PhD in the United States. under the guidance of Enrico Fermi at the University of Chicago. After graduating in 1950, he did postdoctoral work at the University of California, Berkeley, and the Institute for Advanced Study, Princeton, and in 1953 joined Columbia University as an assistant professor. Three years later, at the age of 29, he became Columbia’s youngest ever full professor.
Lee published several books on science, and in addition to the Nobel, he won the Albert Einstein Award in 1957. At a reception celebrating his retirement from Columbia, in 2012, he compared his work to gardening: “You may not grow new species. , but you can still keep the old beautiful things going.”
In 1950 Lee married Jeannette Hui-Chun Chin. She died in 1996, and he is survived by his sons, James and Stephen, a brother and a sister.
