Advent Calendar of Science Stories 23: Parity

For the penultimate advent calendar of science stories post, we'll turn to a great experimentalist with a great biography. This story also appears in Eureka: Discovering your Inner Scientist, but it's too good not to re-use.

Chien-Shiung Wu was born in china in 1912, at right around the time education of women was first legalized. Her father founded a school for girls so he could teach her, then at around the age of 10 she went off to a boarding school, and then the best universities in the country, where she distinguished herself as one of the finest math and physics students in China. At that time, however, China did not have universities granting Ph.D.'s in physics, so Wu was obliged to travel abroad for further study, earning an invitation to work with Ernest Lawrence at Berkeley. Lawrence had recently invented the cyclotron, and his lab was arguably the finest nuclear physics facility in the world during this period. Wu moved to California in 1936, and never saw her family in China again.

She built up a fine reputation working with Lawrence, and during WWII is believed to be the only Chinese scientist invited to join the Manhattan Project. The physics legend has it that Enrico Fermi was struggling to produce plutonium in significant quantities, and another physicist told him that if he wanted the problem solved, he should "ask Miss Wu." He did, and she found the problem, which was related to the buildup of an isotope of xenon produced during fission, which tended to absorb the neutrons needed to sustain the reaction. In short order, the plutonium production line was up and running, thanks to Wu's input. Later in the war, she formally joined the project, and worked on uranium enrichment at Columbia.

After the war, she wound up as faculty at Columbia University, and that's where the most important part of her story comes in. As part of her research there, she studied beta decay, where an unstable nucleus decays by spitting out an electron; this process had a number of odd aspects compared to the other radioactive decays, which led Wolfgang Pauli to propose the existence of the neutrino, which Enrico Fermi used to put together a theory of the weak nuclear interaction in the mid-1930's. There were some minor discrepancies between Fermi's theory and the best experiments at the time, though, which Wu realized had to do with the loss of energy as the electrons made their way out of large chunks of radioactive material. At Columbia, she used new technology developed during the war to deposit extremely thin films of radioactive material, and was able to show that the beta decay energy spectrum did, in fact, agree with Fermi's theory.

This helped establish her as one of the world's leading expert in beta decay-- her 1965 book on the subject was the definitive reference for decades. So when one of her Columbia colleagues, Tsung-Dao Lee, had a question about the process, it was natural that he would turn to her. She pointed him to the appropriate references, and he and his collaborator Chen Ning Yang pored through them to determine that their crazy idea had not, in fact, been ruled out: that the weak interaction did not need to respect parity symmetry.

"Parity" is the jargon term for a special symmetry in space, and up until 1955 or so, everybody believed that it was an absolute physical law: that switching the signs of all the coordinates describing a physical system should not change its behavior. Lee and Yang had noticed that this didn't have to be true for the weak interaction described by Fermi and involved in beta decay. This would be a profound change in fundamental physics, and would have experimental consequences for beta decay-- a radioactive nucleus undergoing beta decay would be more likely to spit out electrons along the direction of its spin than in the opposite direction.

The asymmetry would be a clear signature of parity violation, but it's an extremely tricky measurement to make. In fact, Wu was one of the very few people in the world who had any chance of doing it, and this is the reason why I said above that she never saw her family in China again. She and her husband had been planning a trip back there for some time, and were scheduled to leave in mid-1956, but after hearing of Lee and Yang's results, Wu scrapped the trip to do the parity violation experiment.

This was a tour de force of experimental nuclear physics, requiring the preparation of thin films of cobalt-60 at cryogenic temperatures in a large magnetic field (to align the nuclear spins). In collaboration with a team at the National Bureau of Standards in DC, though, Wu pulled it off, seeing a clear signature of parity violation right around Christmas of 1956, and completing the measurement in early January. It's a spectacular piece of work. Not too long afterwards, a different team measured a similar violation in the decay of muons, and parity symmetry was definitively dead.

(Lee and Yang got the 1957 Nobel Prize for the theory side of this, but Wu didn't. There are a whole host of stupid biases behind this gross oversight.)

So, take a moment today to appreciate "Madame Wu," one of the great inspirational figures in 20th century physics. Her work was essential to transforming our understanding of how the universe operates, and stands as a counterexample to a number of pernicious and stupid ideas about who is capable of doing science at the highest levels.


(Part of a series promoting Eureka: Discovering Your Inner Scientist, available from Amazon, Barnes and Noble, IndieBound, Powell’s, and anywhere else books are sold.)

(Featured image above from Wilkimedia)


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