Chien-Shiung Wu

Chinese American physicist

• Born: May 31, 1912; Liu Ho, China
• Died: February 16, 1997; New York, New York

Nuclear physicist Chien-Shiung Wu disproved the principle of conservation of parity and proved the law of vector current in beta decay. Wu was the first woman elected president of the American Physical Society and honored with Israel’s Wolf Prize.

Primary field: Physics

Specialties: Nuclear physics; atomic and molecular physics; quantum mechanics

Early Life

Chien-Shiung Wu was born in Liu Ho, China, near Shanghai, on May 31, 1912. At this time, Chinese girls were educated at home, if at all, but Wu’s father, Wu Zhong-Yi, was progressive. With Wu’s mother, he operated a local school for girls, the first school of its kind in China. Wu attended this institution until she was nine years old. When Wu had completed her studies at the girls’ primary school, her father encouraged her to go to Suzhou to attend a private high school. Studying under leading scholar Hu Shi, Wu learned English and became politically active. She graduated as valedictorian of her class.

Wu decided she wanted to become a physicist, and with the encouragement of Hu Shi, she entered the National Central University in Nanjing in 1930. By this time, it was not unusual for women to study science. Although Wu was studying during the early years of the Sino-Japanese War, and Japan had invaded China, she managed to complete her degree in physics in four years.

In 1936, Wu traveled to the United States, where she pursued her graduate work in physics at the University of California at Berkeley. One of her most influential professors was Ernest O. Lawrence, winner of the Nobel Prize in Physics in 1939 for inventing the particle accelerator known as the cyclotron. Wu was inducted into the Phi Beta Kappa academic honor society in recognition of her outstanding work as a graduate student.

Wu received her PhD in 1940, and accepted a job as a research assistant at Berkeley. During the early years of World War II, she taught at Smith College in Northampton, Massachusetts, and then Princeton University in New Jersey, where she was the first woman to teach in the Physics Department. In 1942, she married Luke Chia-Liu Yuan, also a physicist.

Life’s Work

In the early 1940s, Wu began conducting experiments to test Enrico Fermi’s 1934 theory of beta decay, the radioactive transformation of an atom from one atomic number to another, caused by the emission of beta particles from the nucleus. In March 1944, Wu joined Columbia University in New York City, where she would remain until her retirement. Her first position was on the scientific staff of the Division of War Research. This group was working on the Manhattan Project to develop the atomic bomb. Specifically, Wu worked on radiation detection. In this capacity, she developed a gaseous diffusion process for separating radioactive uranium-235 from common uranium-238. She also helped to develop a more sensitive Geiger counter. After the war, Wu stayed on at Columbia as a research associate and teacher. Her son, Vincent, was born in 1948. In 1952, she was made associate professor of physics.

In the 1950s, theoretical physicists Chen Ning Yang and Tsung-Dao Lee were looking for an experimental physicist to conduct research that would disprove the principle of conservation of parity. Lee was a colleague of Wu’s on the Columbia faculty, so he knew of her expertise in beta decay, and asked for her help.

It had long been known that physical properties of particles—such as mass, energy, momentum, and electrical charge—remain unchanged after a nuclear reaction. Parity refers to the property of symmetry of the physical laws. Parity, like other physical properties, had been believed since 1924 to be conserved in nuclear reactions. In other words, particles emitted during a nuclear reaction should be emitted in all directions equally. However, in 1956, Yang and Lee found theoretical evidence that parity is not always conserved in nuclear reactions, specifically in certain weak interactions, such as those occurring in beta decay. But they needed experimental results to prove their hypothesis.

Wu’s team consisted of a group of top scientists from the National Bureau of Standards. In 1957, the team tested Yang and Lee’s hypothesis with the radioactive material cobalt-60. First, they cooled the cobalt to 0.01 degree above absolute zero, approximately -459 degrees Fahrenheit. This was to minimize any random thermal movements of the nuclei, so that the scientists could record the disintegration of the radioactive atoms without the interference of other effects. When the electrons emitted during the cobalt’s decay moved in the direction opposite that of the magnetic field that Wu had set up, they contravened the law of parity.

Worldwide recognition followed for Wu, Lee, and Yang. Scientists at Columbia and the University of Chicago conducted similar experiments utilizing other weak reactions, and their results confirmed Wu’s research. This disproved forever the law of conservation of parity. When Yang and Lee received the Nobel Prize in Physics in 1957 for disproving the conservation of parity, many scientists were disappointed that Wu was not included in the award.

Wu did, however, receive numerous other awards and honors. For instance, in 1957, Wu was named a full professor at Columbia University. She was made a member of the National Academy of Sciences in 1958; at that time, she was only the seventh woman so honored and was also the first Chinese American to be made a member. Also in 1958, Wu became the first woman to receive the Research Corporation Award, given annually to outstanding scientists. In her acceptance speech, Wu remarked on the uniqueness of winning the award for destroying a law rather than for establishing one. Also that year, Wu received an honorary doctorate of science degree from Princeton University. She was the first woman to receive this honor from Princeton.

While continuing to research and teach, Wu and her husband edited the book Nuclear Physics (1961). In 1963, Wu’s experiments provided evidence to confirm the theoretical law of vector current in beta decay. This theory had been proposed in 1958 by physicists Murray Gell-Mann and Richard P. Feynman. Wu’s research culminated in her publication of Beta Decay in 1965. The text became a standard reference for physicists. That same year, she was awarded the Chi-Tsin Achievement Award from Taiwan’s Chi-Tsin Culture Foundation.

Wu also performed experiments to confirm the theory that electromagnetic radiation is emitted when an electron and a positron collide. Her success in measuring low-energy electrons emitted by beta decay supported Fermi’s theory of weak interactions. Later, Wu conducted research in ultra-low-temperature physics, muonic and pionic X-rays, and spectroscopic examinations of hemoglobin. She coedited, with Vernon W. Hughes, a three-volume text called Muon Physics (1975–1977).

The honors that Wu received for her research included a number of firsts. In 1975, the American Physical Society honored Wu with the National Medal of Science, the Tom Bonner Prize, and election to the presidency of the society. Wu was the first woman appointed to this role and the first woman honored with Israel’s Wolf Prize in 1978. She was also the first scientist to receive the Wolf Prize in the category of physics. An asteroid was named for Wu in 1990; she was the first living scientist to receive that honor.

Wu retired from her position at Columbia University in 1981, after thirty-seven years, and was named professor emeritus. She died of a stroke on February 16, 1997, in New York City.

Impact

Though Wu never won a Nobel Prize, she was considered one of the foremost female physicists during her lifetime. She is remembered primarily for her research in nuclear forces and structure, which helped disprove the principle of conservation of parity. Until 1957, the conservation of parity, related to symmetry, was considered a basic law of nuclear physics. Wu showed that although parity might be conserved in strong, electromagnetic interactions, the same does not necessarily hold true in weak interactions of subatomic particles. The discovery of this lack of symmetry in parity was hugely influential to the scientific community, since the idea of symmetry has been tied to the laws of physics for centuries.

Wu’s findings in nuclear physics led to further research by other influential physicists, including Feynman, Gell-Mann, Robert Marshak, and George Sudarshan. While Wu’s work disproved the law of conservation of parity, it did not explain why this was so. Feynman and Gell-Mann were later able to explain it with the V-A theory of weak interactions.

Bibliography

Bertulani, Carlos A. Nuclear Physics in a Nutshell. Princeton: Princeton UP, 2007. Print. Discusses the atomic nucleus and explains the theories of the physicists who have studied it. Includes Wu’s disproving of the principle of conservation of parity.

Cooperman, Stephanie H. Chien-Shiung Wu: Pioneering Physicist and Atomic Researcher. New York: Rosen, 2004. Print. Describes Wu’s life, career, and legacy.

Garwin, Richard L., and Tsung-Dao Lee. “Chien-Shiung Wu.” Physics Today 50.10 (1997): 120–21. Print. An obituary for Chien-Shiung Wu, with biographical data and information regarding her scientific career.