Among the team of experimental physicists who developed the first atomic bomb for the U.S. government during World War II, Chien-Shiung Wu (1912-1997) spent 37 years as a leading researcher at Columbia University. She was noted for her meticulous experimental work in studying radioactive interactions. Her most famous experiment overturned what long had been considered a fundamental law of nature, the principle of conservation of parity.
Throughout her life, Wu battled the gender bias which belittled the accomplishments of women in science. Despite her remarkable achievements, boundless professional energy, and brilliant problem-solving skills, Wu was often slow to be rewarded for her work, particularly in the early stages of her career. Known as a thorough and precise experimenter, Wu was always in demand because of her trusted ability to test new theories. Many thought Wu should have won the Nobel Prize for leading the experiments which disproved the principle of conservation of parity. Instead, the prize was awarded to her two male colleagues who had proposed, but not conducted, the experiments.
Encouraged to Excel
Wu was born in Liu Ho, near Shanghai, China, on May 31, 1912. Her father, Wu Zhongyi, had been an engineer. In 1911, he abandoned that profession to take part in the revolution that overthrew the Manchu dynasty. After the revolution, he opened a school for girls in Liu Ho. His wife, Fan Fuhua, became a tutor, and the couple became known as strong advocates for education. They encouraged their daughter to put her utmost effort into academic excellence.
Wu attended her father's elementary school until she was nine, then enrolled in a teacher training program at the Suzhou Girls' School, about fifty miles from home. She soon became frustrated at the lack of science instruction there, and taught herself physics, chemistry, and mathematics using books and notes of students enrolled in other programs at Suzhou. She was active in political causes and became a class leader. Other politically minded students realized that Wu was immune to dismissal due to her stellar academic performance. She graduated as valedictorian with the highest grades in her class.
Selected to attend the National Central University in Nanjing, Wu prepared by continuing to teach herself physics. She began at the university as a mathematics major but soon switched to physics. She earned a bachelor's degree in 1934, and for two years after that taught physics at the university level and did research in X-ray crystallography.
In 1936, encouraged by her academic advisor to continue her studies in the United States, Wu left China. Her intention was to enroll at the University of Michigan, finish her doctorate quickly, and return to China. But when she reached San Francisco, she was offered an opportunity to attend the University of California at Berkeley. Faculty members in the physics department included Robert Oppenheimer, who would later lead the Manhattan Project that developed the first atomic bomb, and Ernest Lawrence, inventor of the atom-smashing machine known as the cyclotron. She enrolled at Berkeley and, in 1940, earned her doctorate in physics. Her studies and some post-doctoral work at Berkeley immediately established her as an expert in nuclear fission.
Worked on the Bomb
Wu's achievements at Berkeley clearly merited a faculty appointment there. At the time, however, there were no women teaching physics at any major American universities, and she was not offered a job. In 1942, she married physicist Luke Yuan, whom she had met at Berkeley, and they moved to the East Coast. Yuan worked on radar devices at RCA laboratories and Wu took up a teaching position at Smith College in Northampton, Massachusetts.
Wu was not entirely happy at Smith and remained there only a year. She was eager to continue her research. With a shortage of physicists due to World War II, the gender bias against women relaxed, and Wu received job offers from Columbia University, Massachusetts Institute of Technology, and Princeton. Wu accepted a job at Princeton, becoming that school's first female instructor. For a few months, she taught introductory physics to naval officers.
Wu had barely settled at Princeton before she was recruited to join Columbia University's Division of War Research. There, the U.S. Army's secret effort to develop an atomic bomb (dubbed the "Manhattan Project"), was under way. Top scientists had decided that Wu's expertise was needed, and they were right. When an atomic chain reaction stopped unexpectedly during testing, the legendary scientist Enrico Fermi was puzzled. Wu was familiar with a rare gas produced by nuclear fission that had halted the reaction. Her knowledge cleared up the problem and enabled the research to proceed. Wu also helped develop a process to enrich uranium ore to produce large quantities of uranium fuel for the bomb. Her work was vital to the historic effort.
After World War II, Wu stayed on as a senior researcher at Columbia. In 1947, her son, Vincent Wei-chen Yuan, was born; he would grow up to become a nuclear scientist himself. Wu and her husband were both offered positions at National Central University in China. They decided not to return to a country that was now Communist-ruled, and became American citizens in 1954.
At Columbia, Wu was passed over several times for faculty positions before she was finally appointed to the physics faculty in 1952. She would not become a full professor until 1958. Wu was always enthusiastic about her work and spent long days in the laboratory. Her demand for hard work and excellence from her students earned her the nickname "The Dragon Lady."
Wu became an expert in radioactive beta decay, the process by which an atom emits electrons. Her precise and thorough experiments clarified many highly technical aspects of beta theory. Her first accomplishment was to confirm Fermi's theory that most of the electrons ejected from the nucleus in beta decay traveled at extremely high velocities. Other experimenters had not been able to prove this fact because they had used radioactive films of uneven thickness. Wu's ability to solve such problems gained her a reputation as a top experimenter.
For 30 years, physicists had remained wedded to the principle of conservation of parity, which held that nature does not distinguish between left and right in nuclear reactions. Researchers made all their observations fit this theory, even though no experiments had ever solidly confirmed it. In 1956, Tsung-Dao Lee of Columbia and Chen Ning Yang of Princeton suggested that the principle might not apply to interactions between subatomic particles involving the "weak force," one of the four basic forces of nature. They approached Wu to conduct an experiment.
Wu joined forces with a team of researchers at the National Bureau of Standards in Washington, D.C., which had one of the few laboratories in the nation that could chill materials to the very low temperatures required for the experiment. Laboring tirelessly for six months in difficult conditions, Wu and her colleagues worked with cobalt 60, a radioactive isotope, cooled to minus 459 degrees Fahrenheit, measuring precisely what happened when the cobalt nuclei broke down during atomic interactions. To the surprise of Wu, who had given the radical theory only a one-ina-million chance of being confirmed, her research found that more particles flew off in the direction opposite the spin of the nuclei, like a left-handed screw. That proved parity did not apply to weak subatomic interactions, showing that beta decay is not always symmetrical.
In January 1957, Wu and her colleagues announced their startling result. The discovery changed thinking about the basic structure of the physical world, and it precipitated an avalanche of studies about the weak interactions of subatomic particles. According to biographer Ursula Allen, her finding "stunned the scientific world. … Wu's experiment was a milestone in nuclear physics." Wu later wrote of her landmark experiment: "These were moments of exhilaration and ecstacy! A glimpse of this wonder can be the reward of a lifetime. Could it be that excitement and ennobling feelings like these have kept us scientists marching forward forever?"
Yang and Lee were awarded the Nobel Prize in Physics in 1957 for their work in challenging the principle of parity. Wu did not share in the award, even though her work was essential to proving the men's theory.
As a result of the parity experiment, Wu began getting the recognition she deserved. In 1958, Wu was the first woman to get an honorary doctorate in science from Princeton. In 1964, Wu became the first woman to receive the National Academy of Science's prestigious Cyrus B. Comstock Award.
Her work included the first successful measurements of low-energy electrons emitted by beta decay. In 1963, in collaboration with Columbia research physicists Y.K. Lee and L.W. Mo, Wu's experiments proved the R.P. Feynman and Murray Gell-Mann theory of conservation of vector current in beta decay, supporting Fermi's basic theory of weak interactions in the nucleus. Wu's thorough work, Beta Decay, published in 1965, became a standard reference book for nuclear physicists.
Wu was known for the daring of her experimental ventures. One of her most creative experiments came when she tested beta decay in a 2,000-foot-deep salt mine under the city of Cleveland. She also extensively studied X-rays in the 1960s at the Brookhaven National Laboratory. Her innovative work continued for decades. In the late 1970s, though already at retirement age, Wu was doing experimental research on nuclear interactions using Columbia's cyclotron.
In 1972, Wu became a member of the American Academy of Arts and Sciences. Three years later, she became the first woman to be elected president of the American Physical Society. That same year, she received the National Medal of Science, the nation's highest award for scientific achievement.
Wu retired in 1981 and became a professor emeritus at Columbia. In her 70s, she lectured widely and taught special courses in many places, expanding her observations beyond pure science, to matters of public policy. Wu often remarked on the lack of women in sciences, maintaining that it was not the intellectual capacity or socioeconomic status of women that was the reason, but the bedrock tradition of the hard sciences that impeded their way. On February 16, 1997, she died of a stroke at a hospital near her home in Manhattan, New York.
Further Reading on Chien-Shiung Wu
Modern Scientists and Engineers, McGraw-Hill, 1980.
Notable Twentienth-Century Scientists, edited by Emily J. McMurray, Gale Research, 1995.
Notable Women in the Physical Sciences, edited by Benjamin F. Shearer and Barbara S. Shearer, Greenwood Press, 1997.
New York Times, February 18, 1997.