Li Zhengdao

Li Zhengdao the Chinese-American physicist (1926-2024) who shared the 1957 Nobel Prize in Physics with Chen-Ning Yang for the discovery that parity is not conserved in weak interactions, one of the most surprising and consequential results of twentieth-century physics.

Early Life and Education

Li Zhengdao, known in the West as Tsung-Dao Lee, was born on 24 November 1926 in Shanghai into a family of scholars and merchants. He showed exceptional mathematical ability as a child and entered university at a very young age, but his studies were interrupted by the war, which forced him and his classmates to flee the advancing armies.

He completed his undergraduate education during the wartime migration of the Chinese universities and then went to the United States in 1946 to continue his studies. He entered the University of Chicago, where he worked under the theoretical physicist Enrico Fermi and received his doctorate in 1950 for a thesis on the hydrogen content of white dwarf stars.

The Chicago department was then one of the most brilliant centres of physics in the world, and the combination of Fermi's insistence on clear physical reasoning and the ferment of postwar theoretical work shaped Li's approach to research. He moved afterwards to the Institute for Advanced Study and to Columbia University, where he spent most of his career.

The Parity Question

In the mid-1950s physicists were troubled by a puzzle concerning the decay of K-mesons, particles that seemed to decay in two different ways into states of opposite parity. The accepted assumption was that parity, a symmetry relating a phenomenon to its mirror image, was conserved in all interactions, and many tried to save that assumption by other means.

Li Zhengdao and Chen-Ning Yang examined the experimental evidence and concluded that parity conservation had in fact been verified only for the strong and electromagnetic interactions, and never for the weak interaction responsible for decay. They proposed that parity might be violated in weak processes and suggested a series of experiments that could settle the question.

Their paper of 1956 was at first received with scepticism, but a group led by Chien-Shiung Wu soon confirmed their prediction by observing an asymmetry in the beta decay of polarised cobalt nuclei. The result was announced in early 1957 and caused a sensation, overturning a symmetry that had been regarded as almost self-evident.

The Nobel Prize and Later Recognition

The discovery of parity non-conservation earned Li Zhengdao and Chen-Ning Yang the Nobel Prize in Physics in 1957, only a year after their paper appeared and just a few years after Li had completed his doctorate. At the age of thirty-one he was one of the youngest Nobel laureates in the sciences, and the award drew great attention to the achievements of Chinese physicists.

The recognition also highlighted the contributions of Chien-Shiung Wu, whose meticulous experiment had confirmed the theory, and the episode became a much-discussed case in the history and ethics of science concerning credit and collaboration. The insight reshaped the understanding of the fundamental symmetries of nature and opened the study of the discrete symmetries of particle physics.

Li continued to work on the theory of weak interactions and on the wider problems of elementary particle physics. His name is associated with the Lee model, with work on the renormalisation of quantum field theory, and with early speculations on the structure of the strong interaction that influenced later developments.

Statistical Mechanics and Field Theory

Beyond particle physics Li Zhengdao made important contributions to statistical mechanics, the theory of many-particle systems. He studied the behaviour of turbulence and of the theory of phase transitions, and he helped to develop methods for summing the complicated series of terms that arise in such problems.

He worked extensively on the renormalisation group and on lattice field theory, the formulation of quantum field theories on a discrete grid that allows them to be studied by numerical computation. He collaborated with many colleagues in the United States and abroad and supervised a large number of doctoral students.

In later years he became interested in the possibility of describing strongly interacting systems and the structure of the vacuum through novel mathematical methods, and he pursued the question of whether the symmetry of the laws of nature might be hidden rather than manifest, an idea that resonated with his earlier work.

Influence and Legacy

Li Zhengdao was a tireless advocate for science and for international cooperation. He helped to build bridges between Chinese and American physicists, encouraged the development of research facilities in China, and supported programmes that brought young Chinese scientists to study abroad and then to return home.

He founded and directed research centres and associations devoted to physics and to the promotion of scientific exchange, and he was a frequent lecturer and adviser. His contributions to the training of a generation of physicists, in China and in the United States, were regarded as being as valuable as his own discoveries.

He died on 4 August 2024 in San Francisco at the age of ninety-seven. He is remembered as one of the most original physicists of his time, a man whose questioning of a supposedly settled principle revealed a new and unexpected feature of the physical world and changed the course of particle physics.

See also

• Chen-Ning Yang
• Parity violation
• Nobel Prize in Physics
• Chien-Shiung Wu