🇨🇳Chien-Shiung
Wu

The law of conservation of parity had the force of philosophical certainty behind it. It held that the laws of physics were symmetric — that any physical process and its mirror image were equally valid, that nature made no distinction between left and right at the fundamental level. For decades, no physicist had found any reason to doubt it. Then in 1956, two theoretical physicists — Tsung-Dao Lee and Chen-Ning Yang — proposed that parity might be violated in the weak nuclear force, the interaction responsible for radioactive decay. No one had ever actually tested conservation of parity in weak interactions; it had simply been assumed. Lee and Yang told Chien-Shiung Wu about their idea. She designed an experiment to test it, spent months executing it under conditions of extreme technical difficulty, and proved it correct. Lee and Yang received the 1957 Nobel Prize in Physics. Chien-Shiung Wu did not.

She was born on May 31, 1912, in Liuhe, a small town near Shanghai, Jiangsu province, China. Her father, Wu Zhongyi, founded the first school in the region that admitted girls and encouraged his daughter to pursue education beyond the traditional expectations of Chinese society at the time. She attended a teacher training school in Suzhou, then the National Central University in Nanjing, graduating in 1934 with a degree in physics. Her professors recognized her as exceptional and urged her to pursue advanced study. In 1936 she sailed for the United States, initially intending to study at the University of Michigan. She visited Berkeley instead, met physicist Ernest Lawrence, and enrolled at the University of California, Berkeley, where she completed her PhD in 1940 under Emilio Segrè.

Her doctoral work, on the products of uranium fission and the emission spectra of xenon, put her among the best-informed researchers in the world on nuclear fission at exactly the moment the Manhattan Project was being assembled. During the war, she worked first at Smith College and then at Princeton, teaching naval officers who needed physics instruction for the war effort. She joined Columbia University in 1944 and contributed to the Manhattan Project's electromagnetic isotope separation program. She remained at Columbia for the rest of her career, eventually becoming the Michael I. Pupin Professor of Physics — the first woman to hold a named chair in Columbia's physics department.

The Wu experiment was a masterpiece of experimental design executed under conditions of extraordinary difficulty. To test parity conservation in the weak force, Wu chose cobalt-60, a radioactive isotope that undergoes beta decay — one of the processes governed by the weak nuclear force. She cooled the cobalt-60 to 0.01 degrees above absolute zero, using equipment at the National Bureau of Standards in Washington, to prevent thermal fluctuations from scrambling the alignment of atomic spins. She then applied a magnetic field to align the nuclear spins of the cobalt-60 atoms, and measured the direction of beta particle emission. If parity were conserved, beta particles would be emitted equally in both directions relative to the nuclear spin axis. They were not. The beta particles were emitted preferentially in one direction. Nature, it turned out, was not left-right symmetric. Parity was violated.

The results were confirmed in January 1957, announced at a Columbia physics seminar, and published in Physical Review in February. The physics community was stunned. Wolfgang Pauli, who had been certain parity would be conserved, wrote: "I cannot believe that God is a weak left-hander." Niels Bohr described it as one of the greatest surprises in the history of physics. Lee and Yang's theoretical paper had predicted the possibility; Wu's experiment had proven the fact. The Nobel Committee moved faster than usual: Lee and Yang were awarded the prize in October 1957, less than a year after Wu's results were published. The prize citation did not include Wu. She was nominated repeatedly in subsequent years and never won. She received instead the Wolf Prize in Physics in 1978, the first Comstock Prize ever awarded to a woman, the National Medal of Science, and the recognition of her peers as the greatest experimental physicist of her generation.

Wu spent the remainder of her career at Columbia, making additional important contributions to understanding beta decay and the electromagnetic current structure of the weak force. She was an inspiring teacher and mentor, and a vocal advocate for gender equity in science at a time when it was professionally costly to be so. She gave a famous speech at MIT in 1964 in which she challenged the academic physics community directly on its treatment of women scientists. She was awarded an honorary doctorate by Princeton University in 1958 — the first woman so honored. She was elected to the National Academy of Sciences in 1958. She served as president of the American Physical Society in 1975, the first woman to hold that office. She died in New York City on February 16, 1997, at eighty-four. The Nobel she deserved was never given. Her experiment remains one of the most elegant in the history of physics.