A mirror had seemed like a safe assumption
In the winter of 1956–1957, Chien-Shiung Wu and a low-temperature team at the US National Bureau of Standards cooled cobalt-60 until many nuclear spins pointed preferentially along one direction. They then counted the electrons emitted in beta decay. More electrons emerged opposite to the spin direction. Reversing the nuclear orientation reversed the preference. The weak decay distinguished two arrangements that a mirror would exchange.
The question followed Tsung-Dao Lee and Chen-Ning Yang’s 1956 analysis. Physicists had strong reasons to trust mirror symmetry in several interactions, but the evidence for weak interactions had not actually tested it in the needed way. Lee and Yang proposed measurements that could expose a preference. Wu recognized that a polarized radioactive nucleus would provide a particularly direct test.
A spinning nucleus gives the decay a reference direction
Ordinary radioactive material contains nuclei pointing every way. Any preference relative to an individual nucleus averages away when all those directions are mixed. The experiment therefore had to prepare nuclear polarization: an unequal population of spin orientations, so the sample as a whole supplied a reference direction.
Spin is intrinsic angular momentum. For this test, a useful picture is the direction of rotation of a tiny gyroscope, although a nucleus is a quantum object. Place an ordinary rotating wheel in a mirror and follow its apparent rotation carefully. The spin, an axial direction defined by the rotation, transforms differently from the arrow describing a particle’s travel. Under spatial inversion, momentum reverses and spin does not. A correlation that favours electrons along or against spin can therefore distinguish a process from its mirror.
That is why turning a detector left or right would not by itself test parity. The experiment compared the electron’s motion with a physically prepared spin direction. The question was whether those two directions had a preferred relative arrangement in the decay law.
How the mirror test differs from reversing the source
There is a helpful way to keep the geometry straight. Reverse every spatial coordinate of the whole imagined experiment, including the electron’s path. The electron then travels the opposite way. A spin direction is built from two ordinary spatial directions, position and momentum, so reversing both leaves that axial direction unchanged. The relative preference therefore flips under the parity transformation. A physical reversal of the nuclear orientation is a separate laboratory control that checks whether the counting asymmetry follows the source.
Cooling made the tiny magnetic preference visible
Thermal agitation usually overwhelms the small energy preference between nuclear spin orientations in a magnetic field. The National Bureau of Standards team used magnetic cooling methods to reach temperatures of a few thousandths of a kelvin. The cobalt source was prepared in a suitable crystal, and gamma-ray measurements provided information about its nuclear orientation.
The electron detector recorded a large asymmetry while the nuclei were oriented. As the sample warmed and the orientation weakened, the electron asymmetry disappeared. Reversing the magnetization reversed the electron preference. Those changes were central: a fixed detector bias could make one side seem more efficient, but would not naturally follow the nuclear orientation and its loss on warming.
Wu worked with Ernest Ambler, Raymond Hayward, Dale Hoppes and Ralph Hudson. Their paper reached Physical Review in January 1957 and appeared in February. It connected a demanding piece of low-temperature preparation to a very plain observable, an unequal number of electrons travelling along two opposed directions.
The weak interaction had its own handedness
The result overturned parity conservation for this weak decay. Parity is the name physicists give to the spatial mirror transformation in the underlying description. A parity-conserving law would give the mirror-related arrangements equal probabilities. The cobalt measurement found a term in the decay pattern that changed under that transformation.
Other weak-decay experiments quickly reinforced the new picture. Lee and Yang received the 1957 Nobel Prize for their investigation of parity laws; Wu’s team supplied the cobalt beta-decay test. The discovery changed how weak interactions had to be described and made the relation between spin and motion a central clue to their structure.
The experiment remains understandable without following the later theory. Prepare a direction inside the source, watch the outgoing electrons, then reverse and erase the preparation in turn. The observed preference follows the source’s nuclear orientation. That sequence is what connects a counter’s unequal clicks to a symmetry of nature.