A mirror test for radioactive decay
A mirror image reverses left and right. If a physical process respects parity symmetry—the idea that nature treats a process and its mirror image alike—then mirroring the setup should not change what happens. Physicists once expected this symmetry to hold in subatomic interactions, too.
Chien-Shiung Wu and her collaborators tested that expectation with cobalt-60, a radioactive form of cobalt. They cooled the material and used a magnetic field to align the nuclei’s spins. Spin is an intrinsic quantum property that gives a nucleus a direction, much like a tiny arrow. As cobalt-60 decayed, it emitted electrons; the team measured whether those electrons tended to travel along or against the aligned spins.
The direction that gave the answer
The electrons showed a preference: they were emitted more often opposite to the nuclei’s spin direction. In a mirror image, the electron’s direction reverses, but spin—an angular momentum—does not reverse in the same way. The mirrored decay would therefore favor the opposite relationship between electron motion and spin. It would not match the original pattern.
That mismatch showed that beta decay, a radioactive process governed by the weak nuclear force, violates parity symmetry. The weak force does not treat mirror-related directions as interchangeable. The result did not mean that every subatomic process has a preferred direction; it revealed that this particular interaction has a built-in handedness.
A discovery with a divided credit
Wu’s experiment supplied the decisive test of a proposal by physicists Tsung-Dao Lee and Chen-Ning Yang that parity might fail in weak interactions. Lee and Yang received the 1957 Nobel Prize in Physics for their work on that proposal. Wu was not included, despite leading the experiment that demonstrated the effect. The practical consequence of her result was profound: mirror symmetry could no longer be assumed as a universal rule for the subatomic world.
