Parity, in physics, concerns the symmetry between an event and its reflection in a mirror. The idea of parity is a useful tool in quantum mechanics. Physicists say that parity is conserved when an event and its mirror image both satisfy identical laws of nature. In this case, an observer cannot tell the difference between the event and its reflection. The same laws apply to the event and its image, and give the observer no clue by which to identify one or the other. Parity is conserved in all ordinary mechanical and electrical systems.
Physicists once thought that the conservation of parity was a natural law that applied to all events. But in 1956, two Chinese-born physicists, Tsung Dao Lee and Chen Ning Yang, suggested a number of experiments which proved otherwise. The experiments showed that parity was not conserved in a type of nuclear event called a weak interaction. An example of such an event is the emission of an electron by a radioactive nucleus.
The first such experiment was performed at the United States National Bureau of Standards (now the National Institute of Standards and Technology) by Chien-Shiung Wu of Columbia University and E. Ambler, R. W. Hayward, D. D. Hoppes, and R. P. Hudson of the Bureau, who used atoms of the radioactive cobalt-60. The result of their experiment showed that parity conservation is not a universal law of nature.
