Imagine an experience and its perfect image in a mirror. Do physical laws always impose the same behavior on both? For a long time, physicists assumed this symmetry preserved in all subatomic processes. In 1956, a theoretical proposal was made to verify what had sometimes been accepted without a decisive test.
Chien-Shiung Wu works with a team from the National Bureau of Standards. The researchers cool a sample of cobalt 60 to an extremely low temperature and orient its nuclei through a magnetic field. They then measure the direction of the electrons emitted during decay. The device must separate a real effect from a simple disordered agitation.
Decisive observations arrived at the end of 1956: emissions showed a preference incompatible with the expected symmetry. The preservation of parity is therefore not universal. The result does not mean that all laws cease to function; it reveals that a rule held to be general had a more limited field of validity.
What we know
The result here is beta decay, linked to the weak interaction.
To go to the source
References consulted on September 21, 2026. Linked pages may be in another language.
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