Chien Shiung Wu – The Parity Warrior

Chien Shiung Wu, stood against the laws of physics and society alike, and they bowed down to her.

When I started this podcast, my plan for the first series was to talk about the women in science who have inspired me but whose contributions have been overlooked by society. I wanted to create do three episodes and then move on to other topics. I did not expect that during my research, I would find so many others who have been ignored by history, or did not receive the recognition that they deserve. For my benefit, and for the benefit of others who these stories might inspire, I have decided to continue this series.

“I wonder whether the tiny atoms and nuclei, or the mathematical symbols, or the DNA molecules have any preference for either masculine or feminine treatment” Chien-Shiung Wu said at a symposium on women in science. By this point in her life, she had been key member of the Manhattan Project, author of a standard read book for Nuclear Physics, and ‘broken’ one of the ‘Laws of Physics’. Chien-Shiung Wu and her experiments revolutionized our understanding of nuclear physics.

Wu was born in 1912, in a Liuhe, a small town near Shanghai, in Jiangsu province of China. She was the second child with two brothers. They were all named using a family tradition involving a phrase which can be translated to ‘heroes and outstanding figures’. Her name can be translated to ‘A strong hero’ Wu. Her father was an Engineer and an Educator. He was involved in the Republican revolution of 1911 and revolt against Yuan Shikai, the first president of the new republic in 1913. He founded the first school for girls in the region to try to improve education among girls. He and his wife visited families asking them to allow their daughters to study. He encouraged her to read and study mathematics from a young age. Wu received her elementary school education at a school for girls founded by her father. She graduated from the school and at the age of 11 moved to a boarding school. There she graduated top of her class, majoring in Mathematics. She attended Shanghai Gong Xue Public School for one year, where she met renowned scholar, Professor Shi Hu who became one of her long-term mentors.

She moved on to National Central University in Taiwan. Wu studied Mathematics but changed her major in second year to Physics. She served as a student leader during 1930-1934, during which she led several demonstrations urging the govt to take stronger actions against Japanese aggression. She led an occupation of the presidential courtyard which gained them an audience with the President. In college, Wu especially enjoyed taking classes with Professor Shi Shiyuan, who had returned from her role model, Marie Curie’s lab in Paris in 1933 after his PhD under her. He used to tell her stories about Curie and her perseverance in a field dominated by men. Wu did her senior thesis with Shi concerning crystal structure and Bragg’s law on x-ray diffraction. In a crystal, all atoms are at nearly equal distances. If the wavelength of light falling on it matches with the distance between the atoms, you get constructive interference. You can use this effect to find the distance between atoms and study the structure of crystals. She graduated in 1934 with top honours at the top of her class and earned a B.S. degree.

She worked as a teaching assistant for a year at Physics Department of the National Chekiang University in Hangzhou. Wu then moved to a research assistant position in the Academia Sinica’s Institute of Physics in Shanghai. There she worked on x-ray crystallography under Jinghui Gu (Zing Whai Ku), who had received her PhD in physics from the University of Michigan. With financial support from her family and encouragement from Gu, she decided to do her PhD from University of Michigan, same as Gu. She learned some English and boarded the ship President Hoover, bound for San Francisco, USA.

Shortly after reaching San Francisco she visited the University of California at Berkeley. There she met Luke Chia-Liu Yuan, who had arrived from China just a few weeks before her. She later learned that Luke was the grandson of Yuan Shikai, the president who her father had protested back in 1913. Luke gave her a tour of the campus and introduced her to Professor Ernest Lawrence. Her interest the Radiation Lab at the Physics Department, coupled with Prof. Lawrence and Luke’s persuasion, convinced her to enrol there. Additionally, she had also heard of the discriminatory policies of University of Michigan, such as their rule against women in student unions and against women using the front entrance which made her decision easier.

Wu’s official advisor was Prof. Lawrence while she worked under direct supervisor of Prof. Emilio Serge and J. Robert Oppenheimer. Her PhD thesis involved studies of fission products of uranium and their effects on nuclear reactions. She completed two separate experiments for her PhD thesis from 1938 to 1940. Her first experiment suggested by Prof Lawrence, was regarding ‘breaking radiation’ the radiation from a charged particle being deaccelerated (due to another charged particle or a field). Per suggestion by Dr. Enrico Fermi, it was a comparative study of internal and external x-ray radiation from electrons during beta decay. Beta decay is a type of radioactive decay in which a beta particle, which is a fast-moving high-energy electron or positron is emitted from the nucleus. Internal radiation here refers to the x-rays emitted by electrons when they come out of nucleus during beta decay, and External radiation refers to x-rays emitted when they get deaccelerated moving through the nucleus’ electromagnetic field.

Wu’s experiments were some of the first to confirm theories regarding these effects. A few other researchers also conducted similar experiments and found contradicting results. Wu successfully defended her results by repeating the experiments and finding causes for the conflicting results. These experiments marked her entrance into the field of Beta Decay, a field she was about to make her own. The second part of her thesis was reports on her experiments under Prof. Emilio Serge. She researched production of Radioactive Xenon gas from Iodine as a product of Uranium Fission. This marked her entry into the field of nuclear fission research.

Application of her research helped resolve the ‘Xenon-Poisoning problem` in Plutonium Producing reactors at Hanford, Washington. Enrico Fermi, who had been asked to explain the fluctuations in the reactor, postulated that Wu’s doctoral thesis ‘Identification of Two Radioactive Xenons from Uranium Fission’ was relevant to this problem. Xenon gas, or specifically Xenon 135 is produced as a direct result of Uranium fission but also formed by decay of Iodine 135 which is one of the primary products of fission with a half-life of 6.6 hours. Xenon 135 readily captures neutrons; the density of neutrons is essential to controlling any nuclear reactor. Delayed build-up of Xenon as a result of Iodine decay and immediate production of it when powering up causes the peculiar fluctuations.

In 1940 at the age of 28, she received her PhD. She stayed at Prof. Lawrence’ lab for two more years. Despite being making contributions at the frontier of her fields, and endorsement from Prof. Lawrence and Prof. Serge, Wu, being a Chinese woman, could not find a job in USA.

Chien-Shiung Wu assembling an electro-static generator at Smith College Physics Laboratory.
© AIP Emilio Segre Visual Archives

Wu married Luke on 30th May 1942, in California Institute of Technology where Luke had transferred and did his PhD from. They moved to East Coast of USA together when Luke got a job at RCA labs in New Jersey. Wu meanwhile got a job as an Assistant Professor at Smith College in Massachusetts. Disappointed by the lack of opportunities for research, she moved to Princeton University in 1943 as a Physics Instructor for Naval Officers. A few months later, she was recruited by the Division of War Research at Columbia University in New York into the Manhattan Project to develop radiation detectors. 

The end of World War in 1945 was a good year for her. She learned her family was safe in China and had survived the Japanese invasion. She also received an offer for the position of a senior scientist with a lab of her own at Columbia University. There she spent the rest of her career, at the Department of Physics. She remained loyal to the Columbia University through her life despite offers and recognition by many other institutes. Luke meanwhile found a position designing accelerators at Brookhaven National Lab on Long Island. Her son Vincent Wei-Chen Yuan was born in 1947 who later became a Physicist. Her intentions to return to China were delayed, at first by the Chinese Civil War, then China-US relations cut-off in 1949, and finally the Korean War in 1950. In 1954 she and her husband decided to become naturalized U.S. Citizens. A lot of Chinese scholars settled in US during this period out of similar concerns.

Wu gave her full weight as an experimental researcher to research into Weak Interactions and Neutrinos after this. Fundamental Forces or Fundamental Interactions are interactions which can not be reduced to more basic interactions. Four such interactions are known to exist. Gravitational and Electromagnetic, the ones which we are more familiar with, have long range effects that we see every day. The other two are Strong and Weak interactions, these work at subatomic distances and are involved in nuclear interactions. These forces can be described as fields, gravity is attributed to curvature of spacetime, as per General Relativity, and other three are discrete quantum fields. Interactions on these three fields are mediated by elementary particles called bosons described in the Standard Model.  An example can be how photon is an elementary particle of electromagnetic field.

Enrico Fermi’s work in Statistical Mechanics is one of the pillars of modern physics. His theory of beta decay was elegant and made very specific predictions, but its predictions were being contradicted by some existing experiments. So far as per the observations done, the electrons produced during beta decay were lower in energy as compared to energy spectrum shape defined by Fermi’s theory of beta decay. Wu understood and realized the problems in the existing experiments. She realized that the samples used for these experiments were thick and the electrons were being slowed because of traveling through the material. She also realized that magnetic spectrometers with Iron cores were causing disturbances in the magnetic field. She devised a better experiment taking these concerns into account. Her results agreed with the predictions from Fermi’s theory. These experiments settled all theoretical and experimental disagreements of the theory. She also investigated different types of beta decay and ‘allowed’ and ‘forbidden’ transitions to test Fermi’s theory. These and other experiments by her during this time established her as a recognized authority in the field of beta decay.

She shifted her focus from Beta Decay to other problems briefly, but her interest was rekindled when she learned of a new puzzle. Two newly discovered mesons, which are subatomic particles, called theta and tou were identical in mass, spin, and lifetime. Yet one of them decayed into two pions which is another meson, and another decayed into three pions. Despite repeated and accurate measurements, both theta and tou seemed identical or the same particle, which would mean a violation of the law of conservation parity.

When you look into a mirror, you see an identical self except few things in inverse, your left hand being right hand and right being left. Due to this, there is no way to tell if you are in this world or in the mirror world. Similarly, if the universe was reflected in the mirror, most laws of physics would be identical. Things would behave in the same way if you simply flip the special dimensions. Some properties such as mass, time, energy would remain the same, and some properties like magnetic flux, velocity, and positions would simply be inverted to conserve this identical behavior. This property is defined as parity, and is expressed as 1 or -1 to represent no inversion or an inversion of the property. Parity is multiplicative and must be conserved across any interaction, much like the law of conservation of momentum or energy. Since the two different decays produce an even and odd number of pions, and pions are intrinsically -1 parity, it would result in +1 and -1 parity, which seems like a violation of law of conservation of parity.

This puzzle had led to much debate about validity of parity as a quantum number, Professor Tsung Dao Lee of Columbia was studying the data presented on the production and decay of strange particles (actual name). The statistics available were not enough to establish a firm explanation and validity of parity in these reactions. Professor Lee and Professor Chen-Ning Yang noted that the experimentation evidence and data for parity symmetry existed for strong interactions, but the validity of symmetry had not been tested for weak interactions. The best studied weak interactions decays are beta decay processes, and since most accurate experiments back then were done by Wu, they approached her.

Wu devised a way to test parity conservation or non-conservation in weak interactions. She designed an experiment using Cobalt 60 which has a simple decay pattern and would yield an unambiguous result. Cobalt 60 can be polarised accurately using magnetic fields at very low temperatures. She worked in collaboration with National Bureau of Standards in Washington to assemble a cryostat in which the crystals of Cobalt were kept at a low temperature. Two sensors were placed at opposite ends outside to detect electrons given off by the sample and two to detect gamma photons as control. Gamma photons are produced by electromagnetic interaction as opposed to electrons which are produced by weak interaction.

Imagine placing a fan in front of a mirror such that it faces downwards. The fan in your world is rotating counter-clockwise, and since parity for direction of spin is -1, the one in the mirror world would be rotating clockwise from your perspective. And yet, both would be blowing air downwards. This is the expected result, so you cannot tell which world you are in as the one in your world is rotating counter-clockwise for you and blowing air downwards. The cobalt 60 atoms give off more electrons in one direction which is dependent on the spin. Thus, in a mirror world they would give off electrons in opposite direction and you can tell, which world you are in. This was a rather well-crafted experiment which would lead to direct observation of conservation of parity. She spent the summer of 1956 testing and recording observations. In December 1956 she found reproducible results of non-conservation of parity (P-symmetry) in beta decay.

Tsung-Dao Lee and Chen-Ning Yang received the Nobel Prize in Physics for this result in 1957. She was mentioned in the award acceptance speech for being the first to perform experimental verification of this phenomenon. Wu however, had to wait till 1978 to be appropriately credited for this, the very first Wolf Prize was awarded to her for this result. She later went on to say that while she never conducted research for prizes, the omission from Nobel Prize disappointed her.

Wu however quickly moved on to the next puzzle. After the magnitude and implications of her famous ‘Wu Experiment’ into parity were ascertained, Leon Lederman, Richard Garwin, and Marcel Weinrich at the Columbia Nevis cyclotron laboratory were able to detect the asymmetry in muon decay. This confirmed the universality of the concept of parity non-conservation in weak decays other than beta decay. Wu’s experiment’s results went much further than verification of parity asymmetry, the results implied that the law of ‘invariance under charge conjugation’ was violated as well. Similar to parity, say you invert the charges of all particles and invert all the electromagnetic fields as well, the results should remain the same. This is called C Symmetry. Since if P symmetry is violated, C-symmetry has to be implicitly violated to conserve CP-symmetry. Since then, CP violation has also been identified in weak interactions.

Wolfgang Pauli and C. S. Wu in Berkeley © CERN

Wolfgang Pauli who had first theorised the existence of neutrino, and Wu’s close friend, was sceptical of the idea of P-Symmetry violation. The fall of a law was a shock to many physicists. In many letters between Wu and him he finally was convinced. He yielded to Wu in a letter in 1957 which said “I congratulate you” and “It is the courage to doubt what has long been believed and the incessant search for verification and proof that pushes the wheels of science forward”. Wu’s beautiful and definitive work on beta decay established the Fermi theory of weak interactions. Prof. Tsung-Dao Lee later went on to describe her as C. S. Wu was one of the giants of physics. In the field of beta-decay, she had no equal”.

Wu’s work however was far from over. In 1957 theorist Richard Feynman and Murray Gell-Mann proposed the CVC theory, which was a major step towards unification of two of the four fundamental forces, weak and electromagnetic. When the initial experiments failed to confirm CVC, they turned to Wu, pleading “How long did Yang and Lee pursue you to follow up on their work”. She conducted experiments and confirmed their theory, significantly changing the world of particle physics. She later went on to study double-beta decay, and also ventured into biochemistry, studying DNA in sickle-cell anaemia.

Wu began to speak out on social issues, especially on equality for women in science. Speaking about it at multiple avenues and advocating government funding for education and research. She eventually returned to China but never got to see her parents and brothers again before they died. She continued advise governments on science policy and promoting education for women. She retired from Columbia in 1981. Wu, while apolitical, campaigned against government crackdown on students demanding democracy and political reform, including the Tiananmen Square incident.

During her lifetime, she faced a lot of discrimination not just for being a woman, but for being a woman of colour. She persisted and thrived in a time when racism and sexism was out in the open and in policy, based on only her work and determination. The tiny woman, stood against the laws of physics and society alike, and they bowed down to her.

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