Chen Ning Yang

The Chinese-born American physicist Chen Ning Yang (born 1922) codiscovered the nonconservation of parity in weak interactions.

Academically inclined from childhood Chen Yang was born September 22, 1922, in Hotei, Anhwei, in China, enjoying what he later categorized as "a tranquil childhood that was unfortunately denied most of the Chinese of my generation." His father was a professor of mathematics at Tsinghua University, where Yang came to do post-graduate study after earning his bachelor's degree in 1942.

In 1944 Yang completed his master's degree, after which he taught in a Chinese high school for a time and then traveled to the United States on a fellowship. Determined to benefit from direct contact with Enrico Fermi (the 1938 Nobel laureate who later built the world's first nuclear reactor), Yang enrolled at the University of Chicago in 1946. He completed his doctoral degree in less than two years, his thesis being supervised by Edward Teller.

Soaring to the Top

Yang remained a year at the University of Chicago as instructor in physics, and in 1949 went to the Institute for Advanced Study in Princeton; in 1955 he became one of the very small number of professors on the institute's permanent staff. In 1950 he married Chih Li Tu, a former student of his in China, who was studying in Princeton. The Yangs had two sons and one daughter.

Although Yang consistently made very significant contributions to statistical mechanics and symmetry principles, he is best known for his and Lee's joint work demonstrating the limitations of the principle of conservation of parity. This principle, although tacitly assumed for centuries to be valid, took on entirely new significance with the birth of relativity, with an empirical rule discovered by O. Laporte in 1924, the advent of quantum mechanics, and E.P. Wigner's 1927 proof that Laporte's rule follows from the "right-left symmetry" of the electromagnetic forces in the atom. Conservation of parity, along with, for example, conservation of energy and momentum, seemed to be a general law of nature. Indeed, its consequences—for example, that the same experiment carried out on an object and its mirror image should yield the same result—seemed to be so obviously true that its universal validity was unquestioned.

Lee and Yang came to feel otherwise in the course of their attempts to understand what was known as the "thetatau puzzle"; two particles, the theta meson and the tau meson, having the same mass and lifetime, would ordinarily have been considered to be one and the same particle, except that while the former decayed into two pions ("even parity"), the latter decayed into three ("odd parity")—and the same particle decaying into two states of different parity would constitute a violation of conservation of parity. Only after intensive study and an extensive survey of the relevant experimental evidence did Lee and Yang conclude in early 1956 that for interactions like the ones described—the so-called "weak interactions," in contrast to the "strong" nuclear interactions, the electromagnetic interactions, and the gravitational interactions—there was absolutely no conclusive experimental basis for conservation of parity. This astonishing fact had escaped all their contemporaries. Indeed, in the words of O.B. Klein, it had been revealed to Lee and Yang only as a consequence of their "consistent and unprejudiced thinking."

But nonconservation of parity in weak interactions was still only a hypothesis. Lee and Yang therefore suggested a number of specific experimental tests for it, all of which yielded positive results. Perhaps the most famous of the tests was the cobalt-60 beta-decay experiment carried out by Madame C.S. Wu of Columbia University and her National Bureau of Standards collaborators. Since Lee and Yang's discovery led to a reexamination of all of the conservation laws, it shook the very foundations of physics, opening up entirely new and unanticipated vistas. For it, they received a number of honors, the highest of which was their shared Nobel Prize for 1957.

A New Life at Stony Brook

In the summer on 1965 Yang was invited, as he often was, to spend a summer worrking at the Brookhaven National Laboratory near the new Stony Brook-based State University of New York. He met many of the physics faculty there, and the following year he was appointed Albert Einstein Professor of Physics and Director of the Institute of Theoretical Physics, which was established especially to attract other top physicists who wished to work with Professor Yang.

By 1997 Yang had been at Stony Brook for 31 years. A man with a strong moral conscience, he had done whatever he could to promote friendship between his adopted home-land and his native China. In 1971 he beecame the first President of the National Association of Chinese Americans, following up with encouragement in every possible quarter to establish the diplomatic relationship between the two countries that finally came to pass in 1979. He also raises an ongoing fund of money, that allows scholars from China to visit the Stony Brook campus for study purposes. Most of them are aware of his many trailblazine papers, which were collected together in a book called Selected Papers 1945-1980, published in 1983 by W.H. Freeman. In honor of Professor Yang's 70th birthday, another volume, Chen Ning Yang: A Great Physicist of the Twentieth Century, was produced by several former students. Obviously, all of them have great respect and affection for him. As many of the pieces in the book note, it is Yang's warmth and sensitivity as much as his reputation which have elevated the Physics Department of the State University of New York to great heights in the world of scientific research.

Further Reading on Chen Ning Yang

Yang discussed his and Lee's discovery in his Nobel lecture, reprinted in Nobel Foundation, Nobel Lectures in Physics, vol. 3 (1967). A detailed account of the discovery is also in Jeremy Bernstein, A Comprehensible World (1967).

Additional Biography Sources

Selected Papers 1945-1980 W. H. Freeman, 1983.

Liu, C. S., ed. Chen Ning Yang: A Great Physicist of the Twentieth Century.