Scientist of the Day - Chien-Shiung Wu

Chien-Shiung Wu in the lab, photograph, undated (scientificwomen.net)

Chien-Shiung Wu, a Chinese-American experimental physicist, was born May 31, 1912, in Jiangsu province, China.  She received a good education in China, thanks in part to her father, who founded the first school she attended, and who believed strongly that women deserved equal opportunities for education. She became interested in physics, and was told that she would have to come to the United States to get a proper advanced degree.  She had planned to attend the University of Michigan, but when she landed in San Francisco in 1937 and visited the Berkeley campus of the University of California, she decided to study there.  It was a wise choice for a would-be experimental physicist, as Berkeley housed the cyclotrons of E. O. Lawrence, and was also the academic home of Emilio Segré.  Wu planned to return to China after receiving her PhD, but then Japan invaded China and the War broke out, and China subsequently underwent its own revolution.  As it turned out, she would not get back to China until the 1970s, and she never saw her parents again after saying goodbye in 1937.

Emilo Segré was an expert on beta decay, an interest he inherited from his own mentor, Enrico Fermi, who discovered beta decay in 1934.  Beta decay is the result of what is called the weak interaction, where an atomic nucleus emits an electron (a beta particle) and decays into another element.  Wu made beta decay her own specialty, and continued to pursue it when she moved to Columbia University in 1945.

Another photograph of Chien-Shiung Wu in the lab, undated (physicsworld.com)

One of the fundamental tenets of theoretical physics in the 1950s was the conservation of parity.  Parity is a physicist’s word for "handedness”.  Our world seems to be left-handed and right-handed.  Some molecules twist to the right, some magnetic fields bend particles to the left – there are many ways in which handedness is manifest in the world.  The conservation of parity says that nature has no preference for left or right.  For every left-handed molecule, there is a corresponding right-handed one; for every particle with a right-handed spin, there is one that spins to the left.   And nature does not play favorites.  Left and right are equally probable.  If there were two worlds, each a mirror of the other, with left and right interchanged, you would not be able to do any experiment that told you which world you were in. Or so said the law of the conservation of parity.

Many physicists had their suspicions about the validity of the conservation of parity, and in 1956, two theoretical physicists, Tsung-Dao Lee and Chen Ning Yang, were looking to test whether it was true of the weak interaction, of beta decay.  They had an idea for a beta-decay experiment to test parity.  Wu by that time had established a reputation as one of the most adept experimental physicists in the United States, and Lee knew Wu personally, since both were at Columbia. They asked Wu if she would do the experiment; she was interested and accepted the challenge.

The experiment involved lining up radioactive cobalt-60 atoms using a magnetic field, so that they were all pointing in the same direction, and then measuring the directions of the electrons emitted when the atoms decayed. If the law of conservation of parity were true, there would be no preferred direction.  The difficulty was in lining up the atoms, which required reducing their temperature to near absolute zero so they would hold still. Wu, with the help of the lab at the National Bureau of Standards, was able to do this.

Wu found that there was a preferred direction - the electrons had a 60% chance of emerging in one direction and 40% in the other.  This means that if you wondered about the handedness of your world, you could do the cobalt-60 experiment, find out which way most of the electrons went, and determine which way was "right."  So nature does not conserve parity.  Such was the inescapable consequence of the Wu experiment of 1956, as it is still called.  Many physicists did not believe the results, so ingrained was the conservation of parity, but Wu's experiment was repeated in other labs and confirmed. Lee and Yang built upon the experiment to announce that parity was not conserved in beta decay, and for this, they received the Nobel Prize in 1957.  Wu did not share in the award and was not mentioned in the citations.  Some have pointed out that she was not eligible for a 1957 award, since her own paper did not appear until 1957. But many feel that a great injustice was done, and that Wu should have received a Nobel prize, if not in 1957, then in 1958 or one of the many other years that ensued before her death in 1997.  It is hard to disagree with those sentiments.

U.S. postage stamp honoring Chien-Shiung Wu, issued 2021 (usps.com)

Perhaps Wu's greatest contribution to science is the inspiration she has provided to young women of color, who have found Wu to be a beacon of hope in the white-male dominated world of science.  Just last week, NPR’s Short Wave program ran a two-part interview.  Wu was the subject of both.  The interviewer was a Chinese-American woman journalist, and the two being interviewed were a Chinese-Latina-American woman physicist, and the granddaughter of Chien-Shiung Wu.  It was inspirational to hear what a life-changing event it was for the physicist when as a girl she learned that a physicist who looked just like herself had been a major player in the science of the 1950s, and the granddaughter was equally transformed when she learned, when she grew up, that her grandmother was world famous, and why.  You can hear the first interview here, and the second one here.  Both are about 15 minutes long.  My guess is that if you were to ask any Chinese-American woman physicist who her hero is, the answer would be Chien-Shiung Wu.  That is a considerable legacy, all by itself.

William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor emeritus, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to ashworthw@umkc.edu.