“C.S. Wu’s great discovery inaugurated the golden age of particle physics.” –James Watson Cronin, nuclear physicist
Top Ten Women in Physics: see also our other blogs about Lise Meitner, Sally Ride, and Emmy Noether.
Who is C.S. Wu?
If you were asked to point your finger at the top female experimental physicist, many people would select Chien-Shiung Wu (C.S. Wu). She created one of the most precise and groundbreaking, paradigm-shifting experiments in physics. Her work had a huge impact on theoretical physics, and she is famous as an experimentalist. She performed experiments for Lise Meitner and James Chadwick (who discovered the neutron), but her most famous experiment proved the theory proposed by theoretical physicists T.D. Lee and C.N. Yang.
Early Years and Education
A book for kids ages 9-12 about C.S. Wu
C.S. Wu was born in China in 1912. Her father was an engineer who also directed a school for girls. He believed that girls should be educated. In 1936, Wu immigrated to the United States. She earned her Ph.D. by 1940 from the University of California, Berkeley and her work was admired by Oppenheimer, which later led to an invitation to work on the Manhattan Project at Columbia University. After WWII, Wu continued to work at Columbia as a research scientist. She is honored as one of 250 Remarkable Columbians.
Understanding the Experiment: Chirality
To understand the work of Chinese-American physicist C.S. Wu, you must understand first the concept of chirality. Imagine you are walking up a spiral staircase. As viewed from above, are you walking clockwise or counterclockwise? To answer that question is to describe the chirality of the staircase. A more common example is the turning of the screw. As you turn the screw, your thumb is probably pointed downward—this is the chirality of most screws, which are said to be “right-handed.” Chirality is important in the microscopic world, specifically in relation to electrons and nucleons (protons and neutrons), because they spin with specific chirality.
The question addressed by C.S. Wu was whether the laws of physics would be symmetric under reflection. For example, if you held a mirror up to a ballerina, the reflection would spin in the opposite direction. On this scale, no one doubts that the laws of physics are symmetric. In the past, this belief was extended to the scale of the nucleus, without testing the belief. However, C.S. Wu performed a nuclear experiment that demonstration a violation of the parity. She worked in the field of nuclear physics, within quantum mechanics.
Chinese theoretical physicists C.N. Yang and T.D. Lee proposed the hypothesis of parity violation and asked C.S. Wu, who was at the time working at Columbia University in New York, if she could test the hypothesis. Wu designed and carried out an experiment where she artificially magnetized radioactive Cobalt-60 crystals (which must be kept at low temps or cryogenically frozen) and observed that a majority of the radioactive beta particles were ejected in the direction of the spin of the nucleus. The prediction based on the erroneous parity assumption was that the particles should be randomly scattered--however, they were not during the experiment. Through her experiment, she discovered parity violation: the laws of physics are NOT symmetric on the atomic scale. Thus, she proved the hypothesis of Yang and Lee.
In 1957, Yang and Lee were awarded the Nobel Prize in Physics and they were the first Chinese Nobel Laureates. T.D. Lee stated of Wu that “In the field of beta-decay, she has no equal.” However, Wu was snubbed and did not receive the prize, but her work was essential. The experiment that upsets the established physics is as important as the ideas proposed by Yang and Lee. She should have shared in the prize.
Without her contribution, no one would have believed in parity violation. Wu was a respected physicist, often consulted for her experimental work. She was the only non-citizen to work on the Manhattan Project, where she worked on radiation detectors. Some call her the “First Lady of Physics," and she was the first female president of the American Physical Society. Wu has authored or co-authored books such as Beta Decay and Nuclear Physics.
According to Griffiths, “the overthrow of parity had a profound effect on physicists—devastating to some, exhilarating to others.” --from Introduction to Elementary Particles by David Griffiths.
Later, the work was extended and the explanation of parity violation as explained through the chirality of neutrinos, which cause beta decay. It turns out that all neutrinos* have left-handed spins (*however, recent research or theorizing seems to point toward the possible existence of some right-handed neutrinos in the universe).
A True Paradigm Shift
The paradigm-shifting work of C.S. Wu is valuable because it reveals yet another surprise of the quantum mechanical world—another way this mysterious world does not match the macroscopic world and breaks with the assumptions and laws of physics in the macroscopic world. Wu stated of her work: "The sudden liberation of our thinking on the very structure of the physical world was overwhelming."
According to this webpage about Wu at UCLA, Wu had many "firsts," including first female physics instructor at Princeton University, first woman to receive an honorary doctorate from Princeton University, and first woman to be elected President of the American Physical Society.
Read more at:
National Women's History Museum: https://www.nwhm.org/education-resources/biography/biographies/chien-shiung-wu/
and in this book:
An article about the writing process for her biography by T.C. Chiang.