Chien-Shiung Wu

Chien-Shiung Wu: The Woman Who Broke Parity

Over the winter of 1956 and into January 1957, in a cryogenics laboratory at the National Bureau of Standards, Chien-Shiung Wu chilled a sample of cobalt-60 to within a whisper of absolute zero, lined up the nuclei with a magnetic field, and counted the electrons coming off. More came out against the direction of the nuclear spin than with it. The asymmetry meant something no physicist had expected to be true: a fundamental particle and its mirror image are not always identical, and the universe can tell left from right. Ten months later the Nobel Prize in Physics went to the two theorists who had suggested she look. Her name was not on it.

The School Her Father Built

Wu was born on 31 May 1912 in Liuhe, in Taicang, Jiangsu province. Her father, Wu Zhong-Yi, was an engineer and a social progressive who founded the Ming De School for girls at a time when educating daughters was still an argument that had to be won. Wu took her primary education there. Her mother, Fan Fu-Hua, was a teacher. At eleven she went as a boarder to the Suzhou Women's Normal School No. 2, having placed ninth out of roughly ten thousand applicants.

She entered National Central University in 1930 intending to read mathematics and switched to physics, taking part in student protests along the way. She graduated in 1934, did research at the Institute of Physics at Academia Sinica, and in August 1936 sailed for the United States with her friend the chemist Dong Ruo-Fen.

Berkeley, and the Bomb

At Berkeley she took her doctorate in 1940 under Ernest Lawrence, working closely with Emilio Segrè on beta decay and on radioactive xenon. J. Robert Oppenheimer sat on her doctoral committee and was struck by her command of neutron absorption cross sections. Luis Alvarez, less usefully, called her "the most talented and most beautiful experimental physicist I have ever met." She married the physicist Luke Chia-Liu Yuan on 30 May 1942 at Robert Millikan's house, taught at Smith College, and became the first woman on the physics faculty at Princeton, where she taught naval officers.

In 1944 she joined the Substitute Alloy Materials Laboratories at Columbia and the Manhattan Project, working on gaseous diffusion for uranium enrichment, building separation models for the Oak Ridge K-25 plant and refining Geiger counters. Her most consequential contribution was knowledge nobody else had. When the B Reactor at Hanford kept mysteriously shutting itself down, Enrico Fermi sought her out: her unpublished doctoral work on xenon-135 identified it as the neutron poison responsible. She is thought to have been the only Chinese person on the project. She later distanced herself from it because of what she called its destructive outcome.

Columbia: Beta Decay and a Forgotten First

Wu stayed at Columbia for the rest of her career, and between 1949 and 1963 did the painstaking work that put Fermi's 1934 theory of beta decay on solid experimental ground, tracing earlier disagreements to an artefact of thick copper sulfate films in other people's apparatus. Her 1966 textbook *Beta Decay*, written with Steven Moszkowski, became the standard reference.

In November 1949, in the basement of Pupin Hall, she and her graduate student Irving Shaknov did something whose importance took decades to register. Using copper-64 as a positron source and two gamma-ray detectors built from photomultiplier tubes and anthracene scintillators, they measured the angular correlation of the photon pairs produced by electron-positron annihilation, holding one detector fixed and setting the other at four azimuthal angles across thirty continuous hours of counting. The measured asymmetry of 2.04, against a theoretical 2.00, was the first conclusive experimental verification of photon entanglement, arriving fifteen years after the Einstein-Podolsky-Rosen paper. The 2022 Nobel Prize in Physics was awarded for work built on that foundation.

The Prize She Did Not Get

In 1956 Tsung-Dao Lee and Chen-Ning Yang proposed that parity, the assumed symmetry between a process and its mirror image, might not be conserved in weak interactions. It was a theory nobody knew how to test. Wu designed the test, cancelling a planned trip to do it, and ran it with a team drawn from Columbia and the National Bureau of Standards. The result was unambiguous and it rewrote the standard picture of the weak force.

Lee and Yang took the 1957 Nobel Prize. Wu did not share it. She was nominated repeatedly afterwards, by one accounting at least a dozen times and by another twenty-three times between 1958 and 1974 by eighteen different physicists, and never won. Lars Brink, a former chair of the Nobel Committee for Physics, has said her parity experiment was Nobel class and described her parity and entanglement work together as "two gems in a box of pearls." Brink also reported that Yang himself said Wu should have been a co-recipient in 1957.

First Lady of Physics

She became an American citizen in 1954, a tenured professor in 1952 as the first woman in her department, a full professor in 1958, and the first holder of the Michael I. Pupin chair in 1973. She confirmed the conserved vector current hypothesis of Feynman and Gell-Mann in 1962-63, hunted double beta decay in a salt mine two thousand feet beneath Lake Erie, ran Bell inequality tests with her students Leonard Kasday and John Ullman, and turned Mössbauer spectroscopy on the molecular structure of sickle cell anaemia. She was called the First Lady of Physics, the Chinese Marie Curie and the Queen of Nuclear Research; her students, who found her exacting, called her the Dragon Lady. She insisted on being addressed as Professor Wu, not Professor Yuan. At an MIT symposium in 1964 she asked the room whether "the tiny atoms and nuclei" have "any preference for either masculine or feminine treatment."

Why Chien-Shiung Is Called a Genius

Wu's genius is of a kind the public reflexively undervalues: it is experimental. The parity theory was Lee and Yang's. What Wu supplied was the far harder question of how you would ever know, and the answer required polarising cobalt nuclei at temperatures barely above absolute zero and holding the whole delicate arrangement stable long enough to count a statistically decisive asymmetry, at a time when nobody had done anything comparable. That is not a technician's contribution. It is design, and design of that quality is rare enough that her colleague Herwig Schopper reported the profession's working rule: "if the experiment was done by Wu, it must be correct." She was, in Physics Today's assessment, "known for her ingenious and precise designs" and stood "among the foremost experimentalists of her time."

The honest counter-case is that the conceptual leap of 1956 was not hers, and that a career of superb measurement, however decisive, sits differently in the history of physics from a career of theory. Wu did not propose parity violation; she killed the alternative. But the same charge would demote Michelson and Morley, and it ignores 1949, where the intellectual initiative was entirely her own and fifteen years ahead of the field's ability to appreciate it. The fairest statement is that Wu was a genius of the apparatus and the decisive test, and that the Nobel committee's failure to recognise it is now widely regarded as among its gravest errors.

Legacy

Wu received the Comstock Prize in 1964 as its first woman, the National Medal of Science in 1975, and the inaugural Wolf Prize in Physics in 1978. She was the first woman to preside over the American Physical Society. Asteroid 2752 carries her name, as does a street at CERN, granted only once before to a woman, Marie Curie. She died on 16 February 1997 in New York after a stroke, aged eighty-four, and her ashes were buried in the courtyard of the Ming De School in Liuhe, the school her father built so that girls could learn.

Achievements

• Designed and conducted the Wu experiment (1956–57), proving the violation of parity conservation
• First recipient of the Wolf Prize in Physics (1978)
• First woman elected president of the American Physical Society
• Contributed to the Manhattan Project, improving uranium enrichment and the chain reaction