Chien-Shiung Wu

Chien-Shiung Wu the Chinese-American experimental physicist (1912-1997) who disproved the conservation of parity in the weak nuclear interaction and became one of the foremost experimentalists of the twentieth century, honoured for the precision of her measurements and for her advocacy for women in science.

Origins and Move to America

Chien-Shiung Wu was born on 31 May 1912 in Liuhe, Jiangsu province, China, the daughter of an educator who founded a school for girls and encouraged her scientific ambitions. She studied physics at the National Central University in Nanjing, graduating in 1934, and then taught and did research for two years before sailing to the United States in 1936.

She entered graduate study in physics, first at the University of Michigan and then at the University of California, Berkeley, where she completed her doctorate in 1940. Berkeley was then a powerhouse of nuclear physics, and Wu absorbed both the theory and the experimental technique that would define her career. She held positions at several institutions before joining Columbia University in 1944.

At Columbia she took part in the wartime research on the separation of uranium isotopes, where she helped to resolve a problem of reactor poisoning that threatened the production of fissile material. She became a professor at Columbia and remained there for the rest of her working life, establishing a laboratory renowned for the exactness of its results.

The Parity Experiment

The central achievement of Wu's career came in 1956, when Tsung-Dao Lee and Chen Ning Yang raised the possibility that the weak nuclear force might violate parity, the symmetry that relates a physical process to its mirror image. Parity conservation had been assumed for decades, and testing the proposal required an experiment of extraordinary difficulty.

Wu devised and performed the experiment with a team at Columbia and the National Bureau of Standards. They placed radioactive cobalt-60 in a strong magnetic field at temperatures near absolute zero, aligning the spins of the nuclei, and observed that the electrons emitted in beta decay came out preferentially in one direction. The mirror symmetry was broken, and parity was shown not to be conserved in the weak interaction.

The result, announced in early 1957, was a sensation and one of the pivotal discoveries of twentieth-century physics. Lee and Yang were awarded the Nobel Prize in Physics in 1957, but Wu was not included, a decision that has been widely criticised and that fuelled debate about the recognition of experimental work and of women in science.

Beta Decay and Nuclear Physics

Wu's mastery lay above all in the study of beta decay, the process by which a nucleus emits an electron and a neutrino. She made measurements of unprecedented accuracy of the shape of the beta spectrum, of the angular correlation of the emitted particles and of the role of the nucleus in the decay, testing the theory of the weak interaction against experiment.

Her book on beta decay, published in 1965, became the definitive account of the subject and trained a generation of physicists. She also investigated the double beta decay of nuclei, the structure of the nucleon and the decay of strange particles, and she contributed to the understanding of the internal constitution of matter at the deepest level then accessible.

Her laboratory became a magnet for visiting physicists who sought to test their theories against her measurements. Colleagues recalled that a result from Wu was trusted, and her insistence on the elimination of systematic error set a standard that raised the quality of experimental physics across the field.

Awards and Public Role

Wu received the Comstock Prize of the National Academy of Sciences, the first woman to be so honoured, and the National Medal of Science, among many other distinctions. She was elected to the National Academy of Sciences and became the first woman president of the American Physical Society, a recognition of her standing in the profession.

She used her prominence to speak about the barriers facing women and members of minority groups in science. In widely quoted addresses she criticised the assumption that talent was the monopoly of any group and urged institutions to judge scientists by the quality of their work alone. Her example opened doors for those who followed.

She also remained engaged with science in China, visiting and lecturing there and supporting the training of young researchers. Her family background and her early education in China remained a source of pride, and she encouraged exchanges between Chinese and American physicists even in periods of political tension.

Influence and Legacy

Chien-Shiung Wu is remembered as one of the greatest experimental physicists of her time and as the scientist who demonstrated that a fundamental symmetry of nature could be violated. Her experiment reshaped theoretical physics and remains a standard subject in the teaching of the subject, cited in every account of the weak interaction.

Her refusal to accept second-class status for women in science made her a model and a campaigner, and her influence is felt in the increased participation of women in physics and in the wider recognition of experimental contributions to discovery. Awards in her name now honour scientists who exemplify her rigour.

She died on 16 February 1997 in New York. Her ashes were returned to China and buried at the school her father had founded. She is honoured on both sides of the Pacific, a Chinese-American physicist whose work belongs to the whole of science.

See also

• Wu Jianxiong
• Parity
• Weak interaction
• Columbia University