Prof. Dr. Tsung-Dao Lee > Research Profile
By Roberto Lalli
Tsung-Dao (T.D.) Lee
Nobel Prize in Physics 1957 together with Chen Ning Yang
"for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles".
Along with his collaborator Chen Ning Yang, T. D. Lee was the first Chinese Nobel Prize laureate. He received the prize when he was just 31 years old, which made him the youngest scientist to receive this prestigious award after World War II. In his more than 300 papers, Lee has made momentous contributions to various branches of theoretical physics, including particle physics, quantum field theory, statistical mechanics, condensed matter physics, astrophysics and hydrodynamics. The work that gained Lee and Yang the 1957 Nobel Prize in Physics was the analysis of the status of parity conservation in weak interactions, carried out in 1956. This research led to the experimental discovery that neither parity (P) nor charge conjugation (C) is conserved in weak interactions. The words written on the inside front cover of the book celebrating the thirtieth anniversary of this discovery well summarises the significance of this endeavour: “parity nonconservation is one of the important landmarks of physics […] The close collaboration between theorists and experimentalists that led to its discovery was perhaps unprecedented and set the style of the later development of particle physics.”
Education in China and the United States
T. D. Lee was born in Shanghai, China on November 25, 1926. His father, Chun-Kang Lee was an industrial chemist and businessman who had graduated from the University of Nanking. Lee’s education was deeply affected by the outbreak and developments of the Second Sino-Japanese War (1937-1945). Lee could not conclude his secondary education in Shanghai because the school he was attending had to suspend its activities.
Nevertheless, in 1943 he was able to enrol at the Department of Chemical Engineering of the National Chekiang University (now Zhejiang University) in the Zhejiang Province of China. Some professors recognized Lee’s talent for physics and convinced him to join the Physics Department, where Lee pursued his training in the period 1943-44. Once again, the war did not allow Lee to conduct a peaceful student life. In 1944, he had to transfer to the more interior Southwest Associated University in Kunming, Yunnan Province of China. There, Lee studied under the guidance of professor Ta-You Wu, the same professor who had been the Master’s thesis supervisor of Yang a couple of years earlier. In 1946, after just two years of undergraduate training, Lee received a Chinese Government scholarship to start his PhD studies in the United States. Lee decided to enrol at the University of Chicago, which was one of the few universities willing to accept PhD students who did not have a formal degree. In 1948, Enrico Fermi selected the young Lee to become one of his doctoral students, and Lee remained strongly impressed by Fermi’s style in both research and teaching. Lee stayed in Chicago until 1950, concluding with a dissertation in theoretical astrophysics on white dwarf stars. Basing his reasoning on stability arguments, Lee demonstrated that less than one per cent of such stars could be made of hydrogen, therefore concluding that these heavenly bodies must be at the end point of stellar evolution.
Early Researches and the Beginning of the Yang-Lee Collaboration
While still a doctoral student, he contributed to an important paper written in 1949 with Yang and M. Rosenbluth. This paper, along with the researches made in the same period by other physicists, clarified the status of weak interactions. After the Bristol group formed by C. Powell, G. Occhialini, C. Lattes and others, had discovered what they believed to be two different mesons (called π-meson and µ-meson), many physicists began wondering about the relationships existing between β-decays and the phenomena concerning the newly discovered µ-particle (muon).
The Lee-Rosenbluth-Yang analysis showed that β-decay, µ-capture, and µ-e decay had very similar coupling constants, and, therefore, it was suggested that they were different manifestations of the same interaction. This and other similar analyses led to the understanding that weak interactions are universal and, consequently, that there exist four fundamental forces in nature. The three young scholars also speculated that, in analogy with the electromagnetic field, weak interactions might be transmitted by heavy bosons - an idea that was later successfully developed in the quantum field theory of electroweak interaction. This paper was also the first outcome of the collaboration and friendship between Lee and Yang, which was soon to become one of the most productive of theoretical physics.
Soon after graduation, Lee spent eight months at the Yerkes Observatory with S. Chandrasekhar working on hydrodynamics. One of Lee’s main results of this period was that turbulence couldn’t occur in a two-dimensional fluid. Later, Lee made research at the University of Wisconsin and at the University of California-Berkeley, and, in 1951, became a member of the Institute for the Advanced Study in Princeton where he stayed until 1953. J. Robert Oppenheimer had become the Institute’s director in 1947, and was determined to build a formidable environment for young theoretical physicists, who were working at the forefront of fields such as quantum field theory and quantum statistics. Yang was also working at the Institute, and Lee wrote with him two relevant papers on statistical mechanics, which solved some problems affecting the theory of phase transitions. Lee and Yang showed that one could get different thermodynamic functions (pressure, density) for different phases (gas, liquid or solid). This argument led to the formulation of the “unit circle theorem” concerning the partition functions used for ferromagnetic systems in statistical field theory. In 1953, Lee was appointed Assistant Professor at the Columbia University of New York City. Although Yang remained at Princeton, their collaboration was not disrupted, and in the next few years Lee and Yang continued to make several contributions, including the endeavour that gained them the Nobel Prize.
Towards the Non-Conservation of Parity and Charge Conjugation
The first work Lee made at Columbia concerned quantum field theory. In 1954, Lee introduced the so-called “Lee model,” which is a soluble field theoretic model; namely, a theory in which mass, charge, and wave function renormalization could be carried out exactly. This toy model had an influence on some developments of quantum field theory.
Soon afterwards, Lee’s attention was directed towards recent puzzling problems of particle physics. Since 1947, cosmic ray observations had shown the existence of several particles that exhibited a strange behaviour. The masses of these particles ranged from about 500 MeV to about 1300 MeV. One of the main problems physicists had to address was the classification of such particles according to their experimental features, including their mass, charge, life-time, and decay properties. In 1953, following the criterion of mass, physicists divided the strange particles in two groups: K-mesons, (the particles whose masses ranged between the π-meson mass and the proton mass) and hyperons (particles whose mass was intermediate between neutron and deuteron). It soon turned out, however, that the classification of strange particles was a difficult business, because it depended on the category chosen to define the particles. The most perplexing problem was known as the τ-θ puzzle, which concerned the decay properties of charged K-mesons investigated by Richard Dalitz and others. The θ particle was a K-meson defined on the basis of its decay products: θ+-> π+ + π0, which means that θ decayed into two π-mesons. The τ particle, instead, decayed into three π-mesons: τ +-> π + + π++π–. These observations might imply that θ and τ were two different particles because they had different decay products. On the other hand, θ and τ seemed to have the same life time and the same mass. In 1953, Bruno Rossi had made the important remark that particles with the same mass and life time should be considered as the same particle, even though they might have distinct decay products. According to Rossi’s criteria, θ and τ were one and the same particle that decayed in two different ways. It was soon understood that there was a serious problem, however. The two-pion decay had evidently a different parity than the three-pion decay. As a consequence, θ and τ also had to have different parities and, consequently, were two different particles unless one renounced the idea that weak interactions were invariant under mirror reflection. Although the hypothesis was informally discussed in meetings, no one was so bold as to advocate that what was considered a fundamental space symmetry was violated.
Lee and Yang’s merit was to make a deep analysis of the empirical evidence for parity conservation in weak interactions. In the paper “Question of Parity Conservation in Weak Interactions,” they demonstrated that although there was enough evidence for parity conservation in strong and electromagnetic interactions, previous experiments had not given any conclusive proof that the same held for weak interactions. In the same paper, which was submitted for publication in Physical Review in June 1956, they also proposed different experimental procedures to test parity conservation in weak interactions. Although many physicists were very sceptical about their proposals, two groups began working on the Lee-Yang suggestion by performing high precision experiments on β-decays and µ-e decays aimed at testing the validity of parity conservation in these interactions. The first experiment to be concluded was the one on the β-decay of polarized Cobalt-60, performed by the Chinese-American Chien-Shiung Wu at Columbia and her collaborators at the National Bureau of Standards. Wu was an expert on β-decay experiments and she was able to overcome the great difficulties surrounding this delicate observations. The result was that parity symmetry was in fact violated in β-decays. Wu’s surprising discovery was soon confirmed in other experiments involving µ-e decay and hyperon decay. All these confirmations were published in 1957. The very same year, Lee and Yang were awarded the Nobel Prize in Physics “for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles,” and some wondered why Wu, who had actually verified parity non-conservation, was excluded from such recognition. Later, it has been shown that Wu was not nominated for the Nobel Prize in 1957. She was first nominated for the Nobel Prize in 1958. Also, since the discovery was not published until January 1957, she was not eligible in 1957. The Statutes of the Nobel Foundation state that the publication must have taken place at the latest the year before the prize is given.
While the Nobel Prize focused on their investigations concerning parity non-conservation, the works of Lee and Yang had broader consequences on the entire understanding of discrete symmetry principles. Along with parity, charge conjugation (C) and time reversal (T) were considered fundamental invariant of physics. The C symmetry principle prescribed that natural laws should be identical if the particles were substituted by their antiparticles. The T symmetry principle implied that the equations were identical if the sign of the time was inverted. Before publishing the paper suggesting that parity might not be conserved in weak interactions, Lee and Yang circulated a preprint. Reinhard Oehme soon sent a letter with his own analysis concerning the conservation of C and T, thus inspiring Lee and Yang to consider also the status of these symmetry principles. They added a paragraph to their paper that was published in October 1956, in which it was argued that there was no empirical proof of C or T invariance in weak interactions either. This work was made even before Wu had demonstrated parity non-conservation, thus, when Wu and collaborators were publishing their paper they included an analysis of the C conservation. Following the arguments of Lee and Yang, Wu and collaborators could state that their experiment proved the violation of C invariance in weak interactions.
In the meantime, Lee and Yang, in collaboration with Oehme, developed this argument further and derived the CPT theorem: “if one of the three operators P, C, and T, is not conserved, at least one other must also be not conserved.” The theoretical and experimental work that led to the demonstration that P and C are not conserved in the weak interaction resulted in the proposal of the CP-symmetry; namely, the hypothesis that natural laws are invariant if one substitutes particles with their antiparticles and, at the same time, inverts the spatial coordinates. This idea gained momentum until experiments performed by James Cronin and Val Fitch in 1964 proved that CP was violated in the decays of neutral K-mesons. After this discovery, which gained Cronin and Fitch the Physics Nobel Prize in 1980, physicists have built consensus on the existence of a weaker symmetry that should hold for all the particle interactions: the CPT symmetry; namely, the invariance under the simultaneous transformations of parity, change conjugation and time reversal. This symmetry is still considered a fundamental principle of nature, and high-energy experiments are currently being carried out to verify its validity. All this work has been sparked by the ground-breaking ideas and analyses put forward by Lee and Yang in a period when few physicists seriously questioned the validity of the discrete symmetry principles.
Beyond Symmetry Principles: At the Forefront of Theoretical Physics
After the Nobel-rewarded research of the period 1956-57, Lee continued his fruitful collaboration with Yang until 1961, also returning for three years to the Institute for Advanced Study. In this period Lee, along with Yang and other collaborators, explored several topics, including various aspects of weak interactions and the many-body problem in quantum statistics. In the branch of weak interaction, it is worth mentioning their investigations on the mass of the hypothetical heavy bosons mediating the weak interactions. As for statistical mechanics, Lee and Yang studied the behaviour of a Bose system of quantum spheres. By formulating a method called “binary collision method,” they provided a systematic way to compute the virial coefficients of a dilute quantum gas.
In 1961, the fruitful collaboration between Lee and Yang suddenly broke up. Lee returned to Columbia University where he has stayed till his retirement after he reached Columbia’s highest academic rank becoming University Professor in 1983. Lee continued to be on the forefront of theoretical research by exploring several fields and collaborating with a number of younger scholars.
In the early 1960s, many physicists began re-exploring the idea that weak interactions were mediated by heavy vector bosons. Lee wrote a number of papers on the properties of these hypothetical particles and had a certain influence on the following development of the unified quantum field theory of weak and electromagnetic interactions, especially through the work Lee made on the Feynman rules for vector bosons. In 1964, Lee wrote a paper with Michael Nauenberg analysing the properties of zero-rest mass particles, that later became a benchmark to understand the behaviour of gluons in quantum chromodynamics. In this paper, Lee and Nauenberg provided a method for dealing with divergences that came to be known as the Kinoshita-Lee-Nauenberg theorem (KLN theorem), because it was derived independently by Toichiro Kinoshita in 1962.
In 1974, Lee and Giancarlo Wick published an important and detailed study on vacuum stability in quantum field theory and on the experimental possibilities for observing a degenerate state whose energy might be lower than that of the vacuum state. According to them, in heavy nuclei the broken symmetry of the vacuum might be restored. Their investigation proposed the existence of metastable “bubbles” in the vacuum. For fermions at high density, the total energy of the excited states would be lower than that of the normal state. A certain number of fermions might, therefore, be bound together in a stable union in a region of “abnormal” space. This paper and the subsequent elaborations made by Lee are currently being tested at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory (BNL), which began operating in 2000.
In the following years, Lee extended the Lee-Wick analysis to solitons: stable configurations without topological boundary conditions.
In his approach to theoretical physics, Lee has always been interested in building strong connections with the experimental world. The work he made with Yang on the non-conservation of parity is just an example of this attitude. In 1956, Lee collaborated directly with his Columbia colleague C. S. Wu on the testability of their theoretical proposals. In the following years, Lee continued to be strongly interested in the evolution of high-energy physics laboratories, and influenced the research that has been pursued in various facilities, including CERN and BNL. This strong attention to the connection between theoretical and experimental works made him the suitable scientist to become the first director of the RIKEN-BNL Research Center, born from the collaboration between the Institute of Physical and Chemical Research (RIKEN) in Japan and the BNL. After having acted as the Director from 1997 to 2003, Lee has become Director Emeritus of the Center, which is currently working to test several of the theories Lee has greatly contributed to.
Bibliography
Brown, L. M., Dresden, M., & Hoddeson, L. (Eds.) (2009) Pions to Quarks: Particle Physics in the 1950s. Cambridge University Press, Cambridge.
Feinberg G. (ed.) (1986) T. D. Lee: Selected Papers. 3 Volumes. Birkhäuser, Boston-Basel.
Fitch V. L., & Rosner L. (1995) Elementary Particle Physics in the Second Half of the Twentieth Century. In Brown, L., Pippard, B., & Pais, A. (Eds.). Twentieth century physics (Vol. 2). AIP, New York, pp. 635-794.
Hoddeson, L. Brown, L. M., Riordan, M. & Dresden, M., (1995) The rise of the Standard Model: Particle Physics in the 1960s and the 1970s. Cambridge University Press, Cambridge.
Novick R. (ed.) (1988) Thirty Years Since Parity Nonconservation: A Symposium for T. D. Lee. Birkhäuser, Boston-Basel.
Pais, A. (1986) Inward Bound Of Matter And Forces In The Physical World. Clarendon Press, Oxford.