In 1957 the 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 émigré to the US, Chen Ning Yang, who was four years older than he was. They became Nobel laureates for physics for work that overthrew the widely accepted “parity laws” – that the forces acting on the fundamental subatomic particles are symmetric between left and right. In the popular description, they overthrew the concept of “mirror symmetry”.
Before Lee and Yang questioned this fundamental principle, it was believed that the mirror image of any process displays a sequence of events that could equally well occur in the real world. In effect, 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 being steered in one direction by electric or magnetic forces reveals a sequence that is realised in the real world simply by reversing the direction of the said forces.
In the mid-50s, physicists were puzzled by the behaviour of what were known as strange particles. Among these were two known as “tau” and “theta”, with identical masses, lifetimes, and other properties but for one: the taus decayed into two particles (pions) whereas the thetas decayed to three. In quantum theory the mathematical description of an even or odd number of pions in this situation differs; in the jargon, the quantum state has respectively “even-parity” or “odd-parity”. 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. The evidence was mounting that the tau and theta were in reality one and the same, yet the law of conservation of parity contradicted that. With hindsight, the answer was there in plain sight – tau and theta are the same and parity conservation is not a general truth. Nonetheless, the axiom of parity conservation, or mirror symmetry, was regarded as so far beyond question that in the ensuing debate only Richard Feynman was prepared to ask: “Does nature have a way of distinguishing right or left handedness uniquely?”
Yang and Lee started poring over data and verified that while there was conclusive proof that mirror symmetry is good for electromagnetic and strong forces, the weak force responsible for some forms of radioactivity had not been sufficiently well tested to either confirm or deny parity conservation. And it was this force that was responsible for the decays of tau and theta. They raised the possibility that the tau and theta are indeed but one, and that the decays to two or three are linked, via quantum field theory arguments, 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 sceptics was Wolfgang Pauli, who quipped: “I do not believe that the Lord is a weak left-hander.”
Yang was based at the Institute for Advanced Study at 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 following a template proposed in their paper. This involved measuring the directional properties of the electrons emitted in the decays of radioactive cobalt-60.
This 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 mirror 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, this will appear different in a mirror, and by implication parity be violated. The only way for parity symmetry to be true would be for there to be no correlation at all.
After several months of work, the highly sensitive experiment proved conclusively 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 distinguish left from right absolutely. This became a founding property of today’s standard model of particles and forces. Among the many applications of parity violation, spanning now seven decades, has been 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 completing this 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 to mark his retirement from Columbia, in 2012, he compared his work to gardening: “You may not be cultivating new species, but you can still keep the old beautiful things going on.”
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.
• Tsung-Dao Lee, physicist, born 24 November 1926; died 4 August 2024
