At its heart, physics is the study of symmetries. In the 1950s Chien-Shiung Wu made a fundamental discovery about how under certain circumstances a fundamental symmetry of the particle world, known as parity symmetry, could be broken – but she never got full recognition for the achievement.
Symmetry is important because by observing how, under certain circumstances, processes always work in the same way – and the circumstances under which they work differently – we can begin to draw up precise mathematical laws about how the world works. Parity symmetry says that particle processes – collisions, decays – should happen in the same way if all the positions and orientations of the particles involved are flipped in the mirror.
By the mid-20th century, the field of particle physics was booming with the construction of new particle accelerators, and there were various hints that parity symmetry might not always hold. In 1956, theorists Tsung-Dao Lee and Yang Chen-Ning showed that it was valid for interactions involving electromagnetism and the strong nuclear force. But they found no evidence for it holding in interactions involving the weak nuclear force, the one responsible for processes including radioactive beta decay. They also drew a rough design of an experiment for testing the idea of “parity symmetry breaking” out in the laboratory.
Enter Chien-Shiung Wu. Born in China in 1912, she moved to the US to do a PhD in 1936. She never saw her parents again: the intervention of the second world war and the accession of the communists to power in China meant Wu didn’t return to her birth country until 1973, and then only as tourist.
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Throwing herself into her work the University of California, Berkeley, home of a pioneering particle accelerator, she became an authority on beta decay, and worked on the wartime Manhattan project to build the first nuclear bomb.
In 1956, Wu designed and built an experiment at the US National Bureau of Standards in Maryland to test Lee and Yang’s idea. It involved cooling atoms of beta-radioactive cobalt-60 atoms to cryogenic temperatures in a constant magnetic field to ensure they were all lined up the same way. If these cobalt atoms then emitted beta particles asymmetrically in space, that was a cast-iron indication that parity conservation was indeed violated.
And so it proved to be. This fundamental discovery about the way the world works has huge ramifications – not least, it could provide a hint for how a breaking symmetry caused the universe to be filled with matter in the first place. Parity violation was subsequently confirmed by many experiments, led to Lee and Yang sharing the 1957 Nobel prize for physics – while Wu got nothing.
It seems she was the victim of a sexism very prevalent in physics then and probably even now. Wu went on to do much more seminal work, based at Colombia University in New York. She became an outspoken critic both of gender discrimination in science and the repressive policies of the Chinese government. She died in New York City in 1997 at the age of 84.
