Chien-Shiung Wu (May 31, 1912–February 16, 1997) was a Chinese American experimental physicist specializing in nuclear physics. She is best known for leading the experiment that demonstrated that parity, the symmetry associated with spatial inversion, is not conserved in beta decay. This finding established that the weak interaction can distinguish between otherwise mirror-related physical processes. Her research also included precise measurements of beta-ray spectra and wartime work on the Manhattan Project. (home.nps.gov)
Early life and education
Wu was born in Liuhe, near Shanghai, China. Her father, Wu Zhongyi, founded the Mingde School, which admitted girls, and encouraged her interest in science and mathematics. After attending a girls’ boarding school in Suzhou, she entered National Central University in Nanjing in 1930. She initially studied mathematics before changing to physics, graduating in 1934. She subsequently conducted research in X-ray crystallography at Academia Sinica under the physicist Gu Jingwei. (nasonline.org)
In 1936, Wu traveled to the United States and enrolled at the University of California, Berkeley. She received her doctorate in 1940. In 1942, she married the physicist Luke Chia-Liu Yuan. After teaching at Smith College, she joined Princeton University in 1943 as the first woman instructor in its physics department. (nasonline.org)
Columbia and wartime research
Wu moved to Columbia University in 1944 to work on the Manhattan Project during World War II. Her work included radiation-detection instrumentation and research supporting the gaseous-diffusion process used to separate uranium isotopes. She remained at Columbia after the war, making it the principal institutional base of her scientific career. (home.nps.gov)
She became a full professor in 1958 and was appointed the first Michael I. Pupin Professor of Physics in 1973. Her laboratory concentrated on experimental nuclear physics, particularly beta decay, a process in which a nucleus changes its charge while emitting an electron or positron and a neutrino or antineutrino. (c250.columbia.edu)
Beta-decay measurements
An important part of Wu’s work concerned the energy spectra of electrons emitted in beta decay. Early measurements had appeared inconsistent with the theory developed by Enrico Fermi. Wu identified experimental distortions, including electrons losing energy through scattering in thick radioactive samples and errors associated with iron-core magnetic spectrometers. Improved source preparation and instrumentation helped resolve the discrepancies. (nasonline.org)
This expertise made Wu a central participant in the experimental investigation of weak interactions. Her book Beta Decay, published in 1965, brought together the theory and experimental study of the subject. Her later research also included molecular changes in hemoglobin associated with sickle-cell anemia. (c250.columbia.edu)
The parity experiment
The theoretical question
In 1956, Tsung-Dao Lee and Chen-Ning Yang reexamined the assumption that all fundamental interactions conserve parity. They found that evidence supporting parity conservation in other interactions did not establish its validity for weak processes. They proposed experiments capable of testing it directly, including measurements of electron emission from oriented radioactive nuclei. (nobelprize.org)
Parity conservation would require a physical process and its spatially inverted counterpart to obey the same laws. A correlation between nuclear spin and the direction of electron emission could reveal a failure of that symmetry. The question was therefore not whether a particular apparatus looked symmetrical, but whether the underlying decay process treated mirror-related configurations equivalently. (nobelprize.org)
Design and observations
Wu organized what became known as the Wu experiment in collaboration with Ernest Ambler, Raymond W. Hayward, Dale D. Hoppes, and Ralph P. Hudson at the National Bureau of Standards in Washington, D.C. The bureau supplied expertise in cryogenics, while Wu contributed her knowledge of beta-particle detection and measurement. (nist.gov)
The team used cobalt-60 nuclei cooled to extremely low temperatures and oriented with a magnetic field. They measured the directions in which electrons emerged during decay. Instead of equal emission along and against the nuclear orientation, they observed a pronounced preference for one direction. The result demonstrated parity nonconservation in cobalt-60 beta decay. (nist.gov)
The team obtained decisive evidence in late December 1956 and performed further checks in early January 1957. Columbia announced the discovery on January 15, 1957. The paper, “Experimental Test of Parity Conservation in Beta Decay,” appeared in Physical Review on February 15, 1957, with Wu and the four National Bureau of Standards collaborators as authors. (nist.gov)
Recognition and professional leadership
The 1957 Nobel Prize in Physics was awarded jointly to Lee and Yang for their investigation of parity laws. Wu and her experimental collaborators were not recipients. The Nobel presentation speech nevertheless explicitly described their experiment as the first of the proposed tests to be carried out and explained its decisive result. (nobelprize.org)
Wu was elected to the United States National Academy of Sciences in 1958. Her honors included the National Medal of Science in 1975 and the inaugural Wolf Prize in Physics in 1978. In 1975, she became the first woman to serve as president of the American Physical Society. (c250.columbia.edu)
Later life
Wu retired from Columbia in 1981. She continued to participate in educational and scientific activities, including programs in China and Taiwan. She died in New York City on February 16, 1997. Her ashes were returned to China and buried at the Mingde School where she had received her early education. (home.nps.gov)
References
- Chien-Shiung Wuc250.columbia.edu
- Dr. Chien-Shiung Wu, The First Lady of Physicshome.nps.gov
- The Reversal of Parity Law in Nuclear Physicsnist.gov
- A ‘Forever’ Stamp and a Discovery That Changed Physics Forevernist.gov
- The Nobel Prize in Physics 1957nobelprize.org
- Nobel Prize in Physics 1957 — Presentation Speechnobelprize.org
- History of the APS Presidential Lineaps.org