The Wu experiment was a particle and nuclear physics experiment conducted in 1956 by the Chinese American physicist Chien-Shiung Wu in collaboration with the Low Temperature Group of the US National Bureau of Standards. The experiment's purpose was to establish whether or not conservation of parity (P-conservation), which was previously established in the electromagnetic and strong interactions, also applied to weak interactions. If P-conservation were true, a mirrored version of the world (where left is right and right is left) would behave as the mirror image of the current world. If P-conservation were violated, then it would be possible to distinguish between a mirrored version of the world and the mirror image of the current world.
The experiment established that conservation of parity was violated (P-violation) by the weak interaction, providing a way to operationally define left and right without reference to the human body. This result was not expected by the physics community, which had previously regarded parity as a symmetry applying to all forces of nature. Tsung-Dao Lee and Chen-Ning Yang, the theoretical physicists who originated the idea of parity nonconservation and proposed the experiment, received the 1957 Nobel Prize in physics for this result. While not awarded the Nobel prize, Chien-Shiung Wu's role in the discovery was mentioned in the Nobel prize acceptance speech of Yang and Lee, but was not honored until 1978, when she was awarded the first Wolf Prize.
History
In 1927, Eugene Wigner formalized the principle of the conservation of parity (P-conservation), the idea that the current world and one built like its mirror image would behave in the same way, with the only difference that left and right would be reversed (for example, a clock which spins clockwise would spin counterclockwise if a mirrored version of it were built).
This principle was widely accepted by physicists, and P-conservation was experimentally verified in the electromagnetic and strong interactions. However, during the mid-1950s, certain decays involving kaons could not be explained by existing theories in which P-conservation was assumed to be true. There seemed to be two types of kaons, one which decayed into two pions, and the other which decayed into three pions. This was known as the puzzle.
Theoretical physicists Tsung-Dao Lee and Chen-Ning Yang did a literature review on the question of parity conservation in all fundamental interactions. They concluded that in the case of the weak interaction, experimental data neither confirmed nor refuted P-conservation. Shortly after, they approached Chien-Shiung Wu, who was an expert on beta decay spectroscopy, with various ideas for experiments. They settled on the idea of testing the directional properties of beta decay in cobalt-60. Wu realized the potential for a breakthrough experiment and began work in earnest at the end of May 1956, cancelling a planned trip to Geneva and the Far East with her husband, wanting to beat the rest of the physics community to the punch. Most physicists, such as close friend Wolfgang Pauli, thought it was impossible and even expressed skepticism regarding the Yang-Lee proposal.
Wu had to contact Henry Boorse and Mark W. Zemansky, who had extensive experience in low-temperature physics, to perform her experiment. At the behest of Boorse and Zemansky, Wu contacted Ernest Ambler, of the National Bureau of Standards, who arranged for the experiment to be carried out in 1956 at the NBS' low-temperature laboratories. After several months of work overcoming technical difficulties, Wu's team observed an asymmetry indicating parity violation in December 1956.
Lee and Yang, who prompted the Wu experiment, were awarded the Nobel prize in physics in 1957, shortly after the experiment was performed. Wu's role in the discovery was mentioned in the prize acceptance speech, but was not honored until 1978, when she was awarded the inaugural Wolf Prize. Many were outraged, from her close friend Wolfgang Pauli, to Lee and Yang, with 1988 Nobel Laureate Jack Steinberger labeling it as the biggest mistake in the Nobel committee's history. Wu did not publicly discuss her feelings about the prize, but in a letter she wrote to Steinberger, she said, "Although I did not do research just for the prize, it still hurts me a lot that my work was overlooked for certain reasons."