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Who Was Dr. Chien-Shiung Wu, Nuclear Physics Pioneer?

U.S. Forever Stamp of Chien-Shiung Wu issued February 11, 2021

U.S. Forever Stamp of Chien-Shiung Wu issued February 11, 2021

On February 11, 2021, the sixth International Day of Women and Girls in Science, the U.S. Postal Service issued a new Forever Stamp to honor Dr. Chien-Shiung Wu, one of the most influential nuclear physicists of the 20th century.

I was surprised to learn about her accomplishments. In America she has the nicknames "First Lady of Physics", the "Chinese Madame Curie" and the "Queen of Nuclear Research". She is known widely on the Chinese mainland today as Wu Zhenxiong. Yet this is the first time I have heard of her.

Her legacy is worthy of a Hollywood biopic such as Hidden Figures, the film about the female African-American mathematicians who served a vital role in NASA during the early years of the U.S. space program.

Dr. Chien-Shiung Wu led a life of many firsts: the first female president of the American Physical Society, the first woman hired by Columbia Physics Department as a tenured faculty member, and the first living scientist to have an asteroid named in her honor. She is now the first Chinese American physicist to be honored by the U.S. Postal Service with a commemorative postage stamp.

“Chien-Shiung Wu was one of the most influential physicists of the 20th century,” said William Gicker Jr., director of stamp services at USPS. “During a career that spanned 40 years in a field dominated by men, she established herself as an authority in the physical sciences and made enormous contributions to the field of nuclear physics, altering modern physical theory forever.”


Wu was born on May 31, 1912 in a small fishing town Liuhe, Taicang (near Shanghai) in Jiangsu province. It was a year after the Qing Dynasty (1644-1911) was overthrown in a revolution led by Dr. Sun Yat-sen, who founded China's first republic.

She attended Mingde Women's Vocational Continuing School, an all-girls elementary school founded by her father, an engineer, who encouraged her love of science and math.

It was the family’s tradition that all the boys in the generation have the character “Chien” in their first names, followed by characters from the phrase “Ying-Shiung-Hao-Jie”, which means “heroes”. Her parents believed their daughter should be treated equally, so they named her “Chien-Shiung” and raised their daughter to become a hero.

Her father, Wu Zhong-Yi, believed in education for girls, despite it being an uncommon belief at that time. He encouraged women's equality and became a notable activist during the revolution led by Dr. Sun Yat-sen (Sun Zhongshan) that modernized the country. Her father supported the revolt due to his modern ideals. He even led a local militia that wiped out local bandits and completely modernized the little town of Liuhe, while seeking girls from rich and poor families to join his new school.

Wu left her hometown at the age of 11 to go to the Suzhou Women's Normal School No. 2. According to the governmental regulations of the time, "normal school" (teacher-training college) students wanting to move on to the universities needed to serve as schoolteachers for one year. Hence, in 1929 Wu went to teach in the Public School of China, founded by Hu Shih in Shanghai.

After graduating a top student from Suzhou Women's Normal School No. 2 in 1929, she enrolled in the National Central University from 1930-1934 to study mathematics and then physics.

In 1934, Wu earned her Bachelor of Science degree in physics. She graduated from National Central University with top honors at the head of her class.

For two years after graduation, she did graduate-level study in physics and worked as an assistant at National Chekiang (Zhejiang) University in Hangzhou, the capital of Zhejiang province. She became a researcher at the Institute of Physics of the Academia Sinica in Nanjing where she conducted her first experimental research in X-ray crystallography (1935-1936). Her supervisor was Dr. Jing-Wei Gu, a female professor, who had earned her PhD abroad at the University of Michigan and encouraged Wu to do the same. She became an important role model to the young Wu.

Wu was accepted by the University of Michigan, and her uncle, Wu Zhou-Zhi, provided the necessary funds. On August 1936 Wu and her female friend, Dong Ruofen (a chemist from Taicang), boarded the steamship President Hoover and voyaged from China to the United States. The two women arrived in San Francisco, where Wu's plans for graduate study changed after visiting the University of California, Berkeley. It was there she met Professor Ernest Lawrence, who was responsible for the first cyclotron particle accelerator and who later won a Nobel Prize in nuclear physics. She also met her future husband, another Chinese physics student, Luke Chia-Liu Yuan, who influenced her both to remain at Berkeley and obtain her PhD.

Wu was shocked at the sexism in American society when she learned that at Michigan women were not even allowed to use the front entrance, and decided that she would prefer to study at the more liberal Berkeley in California. Wu's achievements earned her an offer to study under Ernest Lawrence. Wu's graduate work focused on a highly desirable topic of that era: uranium fission products. She completed her PhD in 1940.

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She married Luke Chia-Liu Yuan on May 30, 1942. The two moved to the East coast where Yuan worked at Princeton University and Wu worked at Smith College. After a few years she accepted an offer from Princeton University as the first female instructor ever hired to join the faculty.


Her work

In 1944, Dr. Wu became the only Chinese-American person invited to join the top-secret Manhattan Project, an initiative begun during WWII to develop the atomic bomb. She worked at the Substitute Alloy Materials (SAM) Laboratories, a Columbia University-based facility focusing on the gaseous diffusion program for uranium enrichment. She was instrumental in discovering how to enrich uranium to produce large quantities of fuels, and her experiments improved the ability of Geiger counters to detect radiation. Her doctoral research on radioactive isotopes was critical to the success of the B-reactor in Hanford. In September 1944, the reactor began unexpectedly starting up and shutting down again. Dr. Wu helped them figure out that xenon-135, a product of fission, was the culprit.

Dr. Wu, like most involved physicists in their later years, distanced herself from the Manhattan Project due to its destructive outcome. She recommended to the Taiwanese president Chiang Kai-shek in 1962 to never build nuclear weapons.


Beta decay

After the end of the war in August 1945, Dr. Wu accepted an offer of a position as an associate research professor at Columbia University. She would remain at Columbia until her retirement in 1980, and was first named associate professor in 1952, which made her the first woman to become a tenured physics professor in university history. She was named an associate professor in 1952, full professor in 1958, and the first Pupin Professor of Physics in 1973. In 1975, her pay as a professor was raised to be equal to that of her male colleagues.

At Columbia, she began investigating beta decay, the mysterious transformation of one type of element into another, which is the basis of nuclear reactions. Among her important contributions was the first confirmation of Enrico Fermi’s 1933 theory of beta decay, the process by which radioactive atoms become more stable.

Enrico Fermi had published his theory of beta decay in 1934, but an experiment by Luis Walter Alvarez had produced results at variance with the theory. Dr. Wu set out to repeat the experiment and verify the result. Dr. Wu was already heavily invested in working on beta decay as she took on the subject at UC Berkeley. In the year 1949, Dr. Wu completely established Fermi's theory and showed how beta decay worked, especially in creating electrons, neutrinos, and positrons. Supposedly, most of the electrons should come out of the nucleus at high speeds.

After careful research, Dr. Wu suspected that the problem was that a thick and uneven film of copper(II) sulfate (CuSO4) was being used as a copper-64 beta ray source, which was causing the emitted electrons to lose energy. To get around this, she adapted an older form of the spectrometer, a solenoidal spectrometer. She added detergent to the copper sulfate to produce a thin, even film. She then demonstrated that the discrepancies observed were the result of experimental error; her results were consistent with Fermi's theory. The speeds of the electrons that were commonly produced in experiments were now shown to be significantly slower. Thus by analyzing radioactive materials used by previous researchers, she proved that this was the cause of the problem and not from theoretical flaws. Dr. Wu thus established herself as the leading physicist on beta decay. Her work on beta decay became hugely beneficial to her later research and to modern physics in general.


The Wu experiment

In 1956, Dr. Wu was approached by theoretical physicists Tsung Dao Lee and Chen Ning Yang who knew about her expertise in beta decay. They asked her to devise an experiment to prove their theory that the law of conservation of parity did not hold true during beta decay. The law of parity states that all objects and their mirror images behave the same way, but with the left hand and right hand reversed.

She designed and carried out the famous “Wu Experiment” based on the beta decay of the radioactive atom cobalt-60. She precisely measured the small particles released from the atom, and found that they were jettisoned in an asymmetrical manner. This proved her colleagues’ theory that parity is not conserved for beta decay, leading to them winning the Nobel Prize in 1957.

But Dr. Wu's work was not acknowledged. Other female scientists during this time were excluded from the Nobel Prize. Many in the scientific community believe Dr. Wu was overlooked because of gender, while others say it was less a snub than a rigid adherence to committee rules and procedures. If Dr. Wu was disappointed about missing out on the Nobel, she never spoke about it, at least not to her son (Vincent Wei-chen Yuan, a nuclear physicist).

Before the Wu experiment, there was no unambiguous way to describe left and right without basically pointing at some visible or previously defined object and saying “that’s the left side, that’s the right side.”

This discovery, lauded as one of the most important developments in the field of atomic and nuclear physics, established that the laws of nature are not always symmetrical.

Brian Greene, professor of physics and mathematics at Columbia, said Dr. Wu’s discovery that some events in nature can violate parity conservation, as the mirror symmetry is known, stunned physicists and played a crucial role in the advancement in atomic science.

“After Madame Wu’s experiment, we had to discard completely the idea that the universe is mirror symmetric,” Greene said. “Only a handful of physicists have made contributions that have radically changed our perspective on reality, and she is one of them.”


Remembered in China

Dr. Wu was unable to visit China until 1973, a year after the historic trip there by Richard Nixon. In Beijing, she was greeted by then-premier Zhou Enlai.

Her name and stories became widely known in China after she made more trips there in later years. She was professor emeritus at Nanjing University, Peking University and University of Science and Technology of China and elected one of the first foreign academicians of the Chinese Academy of Sciences in 1994.

Dr. Wu would spend most of her time in her later years visiting China, Taiwan, and different American states. She became renowned for her steadfast promotion of teaching STEM subjects to all students regardless of gender or any other discriminating cause.

Dr. Wu suffered a stroke on February 16, 1997, in New York City. An ambulance rushed her to St. Luke's-Roosevelt Hospital Center where she was pronounced dead.

In accordance with her wishes, her ashes were buried in the courtyard of the Mingde School that her father had founded and that she had attended as a girl. On May 31, 2012, a ceremony was held there to celebrate her 100th birthday. It was attended by her son Vincent Wei-chen Yuan and several other family members.

President Gerald Ford presenting the 1975 National Medal of Science to Dr. Wu in the East Room at the White House on October 18, 1976

President Gerald Ford presenting the 1975 National Medal of Science to Dr. Wu in the East Room at the White House on October 18, 1976

Accolades and awards

Dr. Wu was honored with many accolades throughout her career.

In 1958, her research helped answer important biological questions about blood and sickle cell anemia.

She was the first woman to receive a Sc.D. from Princeton University (1958), and was awarded many honorary degrees.

In 1959, she was the first woman to earn the Research Corporation Award and the seventh woman elected to the National Academy of Sciences.

She also received the John Price Wetherill Medal of the Franklin Institute (1962), the National Academy of Sciences Cyrus B. Comstock Award in Physics (first woman to receive this award, 1964), the Bonner Prize (1975), the National Medal of Science (1975), and the Wolf Prize in Physics (inaugural award, 1978) – one of its criteria considered those who were thought deserving to win a Nobel Prize without receiving one.

Her book 'Beta Decay', published in 1965, is still a standard reference for nuclear physicists.

In 1974 she was named Scientist of the Year by Industrial Research Magazine and in 1976, she was the first woman to serve as president of the American Physical Society.

In 1990 the Chinese Academy of Sciences named Asteroid 2752 after her (she was the first living scientist to receive this honor) and five years later, Tsung-Dao Lee, Chen Ning Yang, Samuel C. C. Ting, and Yuan T. Lee founded the Wu Chien-Shiung Education Foundation in Taiwan for the purposes of providing scholarships to young aspiring scientists.

Nanjing University dedicated The Chien-Shiung Wu Library and a scholarship named after her. At the Southeast University, also in Nanjing, the Chien-Shiung Wu Memorial Hall hosts an extensive exhibition of her lifetime achievements that continue to inspire many Chinese.

In 1998 Dr. Wu was inducted into the American National Women’s Hall of Fame a year after her death.

She was included in TIME's 100 Historical ‘Women of the Year’ (a project in 2020 to celebrate International Women's Day).

Art by Jennifer Dionisio for TIME magazine

Art by Jennifer Dionisio for TIME magazine


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