Chien-Shiung Wu and the Demise of Parity

"It is shameful that there are so few women in science... There is a misconception
in America that women scientists are all dowdy spinsters
- Chien-Shiung Wu


This actually wasn't the Astronomical Returns article I was planning to write this week, but recently there's been a spike in terrible hate crimes against Asian Americans across the US amid the coronavirus pandemic, so I decided this would be the perfect story to share, especially since I myself am Asian American. In the realm of cosmology and theoretical physics, icons like Albert Einstein and Stephen Hawking are household names, and those of us who can remember our high school physics classes may recognize others like Heisenberg, Schrodinger, and Planck. But few people have heard of Chinese-American experimental physicist Chien-Shiung Wu, whose discovery of parity violation was a major contribution to the development of the Standard Model of Particle Physics. So who was Dr. Wu, and what does parity violation even mean?

Chien-Shiung Wu in the 1940s, shortly before joining the Manhattan Project

Chien-Shiung Wu was born in Jiangsu Province, China in 1912, and attended various institutions across China, including the National Central University in Nanjing and Zhejiang University in Hangzhou, initially studying math but later transferring to physics. While doing graduate level research, her supervisor encouraged her to apply for a doctorate abroad at the University of Michigan like he did, but although she was accepted, she was extremely put off by the fact that women weren't even allowed to use the front entrance at Michigan, so she attended UC Berkeley instead. Her thesis there laid the foundation of her expertise in beta decay (the emission of an energetic electron or positron from an atomic nucleus) - she studied the radioactive isotope phosphorus-32 for potential use as a radioactive tracer for cancer treatment, as well as the radioactive isotopes of xenon produced by uranium fission

Wu and her husband moved to the East Coast following her PhD, and with the outbreak of WWII and the rush to build the atomic bomb, Wu's experience in nuclear physics was very much in demand. She joined the Manhattan Project's Substitute Alloy Materials Laboratories at Columbia and was instrumental in developing a method of uranium-235 enrichment by gaseous diffusion. And after the war ended, she remained at Columbia continuing her research in beta decay

Chien-Shiung Wu in 1963 at Columbia University

So now we've come to this question about parity. At the time, physicists operated under the assumption of "the law of conservation of parity", which essentially states that the laws of the Universe are the same regardless of any sense of "direction". In other words, if you were to take a perfect mirror image of the world, it would be indistinguishable from the Universe that we actually live in. Now in the context of particle physics, the assumption of parity means that subatomic particles should behave exactly the same as their mirror image counterparts. Wu was aware of this assumption, and she knew that of the 4 fundamental forces of nature, parity had already been proven for electromagnetic interactions and strong nuclear interactions (gravity is negligible at the subatomic level). However, parity had not yet been proven for the weak nuclear force, the one that mediates radioactive decay, so in 1956 she assembled a team including fellow Chinese-American physicists Tsung-Dao Lee and Chen Ning Yang to test it out

For electromagnetism and the strong nuclear force, particles behave exactly the same as their mirrors. What about the weak nuclear force?

The Wu Experiment, as it later became known, took a cryogenic sample of Cobalt-60, a radioactive isotope that undergoes beta decay, and used an extremely uniform magnetic field from the National Bureau of Standards to align the spins of all the atomic nuclei in the same direction. If the law of conservation of parity holds true for the weak nuclear force, then the byproducts of the radioactive decay should be emitted equally in the "up" and "down" directions, regardless of whether the atoms in the sample are spun "clockwise" or "counterclockwise". Instead, they found that beta particles tended to fly out in the direction opposite the atom's spin! Thus, it would be possible to distinguish between the cobalt sample and its mirror image, breaking the law of conservation of parity!

"Nature, it seems, is a semi-ambidextrous southpaw" 

Wu's discovery has major implications in the field of cosmology; to this day, one of the greatest unsolved mysteries in science is the question of why there's more matter than antimatter in the Universe. Known formally as baryon asymmetry, the idea is that the Big Bang theoretically should've produced equal amounts of matter and antimatter, which would've instantly annihilated and left the Universe a matterless sea of radiation. The fact that matter seems to have won the primordial battle for dominance implies that the laws of physics must've acted differently on the two, tipping the scales in favor of matter. Physicists have defined these circumstances (known as the Sakharov conditions), and one of them is a violation of charge-parity symmetry**. So while we don't have a solution to baryon asymmetry yet, we know Wu's discovery will play a huge role in the ultimate explanation! 

** Charge-parity symmetry essentially combines Wu's notion of parity (mirror images) with the notion of charge (particle vs antiparticle). From Wikipedia: "CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle (C symmetry) while its spatial coordinates are inverted ("mirror" or P symmetry)". CP-violation was discovered in 1964, just a few years after Wu's P-violation. And if you've read Stephen Hawking's A Brief History of Time, you'll recall he discusses the acronym CPT-symmetry in the Universe: charge, parity, and time

Illustrating CP-symmetry, and it's potential impact on the matter-antimatter distribution of the early universe | Credit: Antimatter Matters

Finally, it's worth noting that although Wu's fellow researchers Lee and Yang won the 1957 Nobel Prize for their theoretical work on parity, Wu's experimental design was snubbed and she wasn't included in the award (though Lee and Yang did give her credit in their acceptance speech, and Wu subsequently won the Wolf Prize in 1978). Also, in the decades after Wu first left China to begin her doctoral studies, she became separated from her family back home, who endured enormous suffering in the wake of WWII, the Communist takeover following the Chinese Civil War, and the ensuing chaos of the Cultural Revolution. Wu initially found it very difficult to travel, since her passport had been issued by the now exiled Kuomintang government. Later on she became a US citizen, but the State Department had imposed restrictions on Americans traveling to Communist countries. Not until Nixon's groundbreaking 1972 visit to China, when Chinese-American relations began to improve, was Wu able to return to China, but by then her parents, uncle, and two brothers had all perished

Dr. Chien-Shiung Wu's contributions to experimental physics and cosmology were enormous, and although she passed away in 1997, she remains a pioneer and an icon for Asian Americans


  1. This is a great story - Dr Wu is an inspiration!

    1. I agree, I wish more people know about her. I'm glad you enjoyed it :)