The laws of physics always treat a process and its mirror image identically.

Imagine filming a physical experiment and then watching its mirror image. Parity symmetry says that the mirrored experiment should still obey the same laws: nature should not have a built-in preference for left over right.

By the mid-twentieth century, this symmetry had worked so reliably that physicists treated parity conservation as a fundamental rule. But in 1956, Tsung-Dao Lee and Chen-Ning Yang noticed an important gap. Parity had been tested in electromagnetic and strong interactions, but not directly in the weak interaction, which is involved in radioactive beta decay.

Chien-Shiung Wu and collaborators performed the crucial test with cobalt-60. They aligned the radioactive nuclei and measured the directions in which electrons were emitted during beta decay. If parity were conserved, the mirrored arrangement would be equally likely. Instead, the electrons showed a preferred direction.

The weak interaction really does distinguish left from right. That did not make parity useless: electromagnetic and strong interactions still preserve it to extremely high precision. What disappeared was the belief that mirror symmetry had to hold for every fundamental interaction.