Parity Violation

Parity violation is the discovery that the weak nuclear interaction, unlike electromagnetism and the strong force, does not treat a physical process and its mirror image as equally probable. Physicists had assumed parity conservation was a universal symmetry until 1956, when Tsung-Dao Lee and Chen-Ning Yang, investigating an apparent contradiction in the decay of certain mesons known as the tau-theta puzzle, proposed that the weak interaction might violate it and suggested experiments that could test the idea. Chien-Shiung Wu tested the prediction within months, cooling radioactive cobalt-60 nuclei to align their spins and showing that far more electrons were emitted opposite the direction of nuclear spin than along it, proof that nature does, in specific circumstances, distinguish left from right. The result reshaped particle physics and won Lee and Yang the 1957 Nobel Prize in Physics, though Wu, whose experiment made the confirmation possible, was not included.