How did Lee and Yang prove parity violation?
Let us consider a simple case first. In 1956, Chen Ning Yang and I began by examining the existing experimental evidence for parity conservation in weak interactions. We found that while strong and electromagnetic interactions had been tested, weak interactions had no such verification. We proposed several experiments, notably involving the beta decay of cobalt-60 nuclei. The key was to align the nuclei in a magnetic field and observe the direction of emitted electrons. If parity were conserved, electrons would be emitted symmetrically in opposite directions. But our calculations predicted an asymmetry. Chien-Shiung Wu and her team at the National Bureau of Standards performed the experiment in 1957, and indeed, more electrons were emitted opposite to the nuclear spin. This confirmed that the universe does not care about our assumptions—parity is violated in weak interactions. It was a moment when symmetry itself became the door to deeper understanding.
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