Chien-Shiung Wu: First Lady of Physics
Dr. Chien-Shiung Wu played an important role in nuclear and particle physics during World War II. Her accomplishments include disproving parity conservation and contributing to the development of enriched uranium for atomic bombs.
Dr. Chien-Shiung Wu (1912-1997) was a Chinese-born experimental physicist whose nuclear and particle physics work contributed to the Manhattan Project and disproved parity conservation.
Dr. Chien-Shiung Wu. Image used courtesy of American Institute of Physics
She received nicknames that recognized her impact on her field, such as “the First Lady of Physics,” “Chinese Marie Curie,” and the “Queen of Nuclear Research.”
Early Life and Education
Wu was born in 1912 in Liuhe, China, just after the official founding of the Republic of China.
Her mother was a teacher; her father, Wu Zhong-Yi, an engineer. From a young age, her father encouraged her to explore her interests and pursue higher education. Wu Zhong-Yi’s support of female education was unusual for the time, more so because it extended to other girls in their town. He opened the Ming De School, which Wu attended, and encouraged girls from rich and poor families to attend.
In 1923, Wu left Liuhe to attend the Suzhou Women’s Normal School No. 2, a boarding school that offered teacher training and regular high school classes. Wu chose to apply for the teacher training program and was accepted, having ranked ninth out of 10,000 applicants.
In 1929, she graduated top in her class, going on to teach for one year at a public school in Shanghai as a requirement of the teacher-training program. She then attended the National Central University in Nanking, where she received her degree in physics in 1934.
For two years after, Wu worked as a research assistant at Zheijiang University and then as a full researcher at the Institute of Physics of the Academia Sinica. It was here that she met Gu Jing-Wei, her research supervisor, who had earned her Ph.D. at the University of Michigan. She strongly encouraged Wu to consider getting her Ph.D. in the United States as well.
In 1936, Wu moved to the U.S. to study nuclear physics. She had been accepted to the University of Michigan and committed to attending until she learned that policies at the school were highly sexist.
Instead, she attended the University of California, Berkeley, where she met her future husband and fellow physicist, Luke Chia-Liu Yuan.
Nuclear fusion. Image used courtesy of Adobe Stock
At UC Berkeley, she worked in Ernest Lawrence’s Radiation Laboratory with Lawrence himself, J. Robert Oppenheimer, and other experimental physicists. Lawrence had just developed a brand new particle accelerator and was turning the campus into a hotspot for atomic research.
She graduated in 1940 with her Ph.D. and then moved to Massachusetts to teach physics at Smith College. The following year, Wu and her husband moved to Princeton, where she became the first female professor in the Physics Department.
The Manhattan Project
In 1944, in the middle of World War II, Wu took a new position at Columbia University, working on the classified government project, The Manhattan Project.
Initially, she worked at the Substitute Alloy Materials (SAM) Laboratories to support the gaseous diffusion (K-25) program for uranium enrichment. A few months later, her role changed to building her own instruments to monitor the radiation effect of the project’s nuclear reactor. This work led her to assist with the B Reactor, the first practical nuclear reactor.
The B Reactor was having an unexpected problem, turning off and turning back on in intervals. John Archibald Wheeler and Enrico Fermi hypothesized that Xe-135, a fission product of the reactor with a half-life of 9.4 hours, was the reason for this issue.
While completing her Ph.D. at UC Berkeley, Wu wrote her thesis on radioactive isotopes of xenon. Fermi came to Wu to ask her about her paper, which inadvertently proved that Xe-135 was, in fact, the issue.
Wu’s research showed that Xe-135 has an unexpectedly large neutron absorption cross-section, which affected the fission chain reaction.
Wu’s findings not only helped in building the standard model for producing enriched uranium for atomic bombs but also contributed to the development of Geiger counters. Her initial Ph.D. thesis was never published, but after assisting Fermi, she expanded her research and published a paper on her findings months before the bombs were used in World War II.
Ultimately, Wu distanced herself from the Manhattan Project, like many others involved, due to its destructive outcome.
Hanford Manhattan Project valve pit. Image used courtesy of Adobe Stock
In 1934, Enrico Fermi published his theory on beta decay. It was later questioned by the results of an experiment done by Luis Walter Alvarez.
Having studied beta decay at UC Berkeley, and with her success on the Manhattan Project, Wu chose to reattempt Fermi’s initial experiment.
In 1949, Wu successfully verified Fermi’s theory on beta decay. She found that the issue with other researchers’ work was the copper-64 beta-ray source; the discrepancies between Fermi’s work and others were experimental, not theoretical.
The Wu Experiment
In 1956, Wu was asked by two colleagues, Tsung-Dao Lee, and Chen Ning Yang, at Columbia to assist them with a theory about the law of conservation of parity. They were interested in the puzzle of the theta and tau particles, two very similar particles that would have been considered to be the same if not for their different decay modes and two different parity states.
Lee and Yang asked Wu to help them test their theory that parity was not conserved.
Wu, an expert in beta decay, carried out an experiment using radioactive cobalt-60 that showed the beta particles from the cobalt-60 were asymmetrically emitted, and the hypothetical law of parity conservation was invalid, proving parity was not conserved under weak nuclear interactions.
Her experiment proved that the theta and tau particles are indeed the same particle, now known as a kaon.
The discovery of parity violation had a major impact on particle physics. In 1957, Lee and Yang received the Nobel Prize for Physics for their work, but Wu’s critical contribution was omitted by the Nobel committee.
However, this experiment is now commonly referred to as The Wu Experiment. She was later awarded the inaugural Wolf Prize for her role in the discovery.
Wu was a groundbreaking physicist who made significant contributions to the field of medicine, including research on sickle-cell disease. Wu was the first woman president of the American Physical Society and received numerous awards and recognitions for her work, including the National Medal of Science.
Wu excelled in a male-dominated field and inspired future generations of women to pursue science careers. After retirement, she focused on educational programs to encourage young girls to pursue careers in the sciences. Wu passed away in 1997, and her ashes were buried in the courtyard of her father’s school in China.