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Last week I ran across an article about mathematician Emmy Noether, decided to make a diary out of it, and dedicate it to Women’s History Month. I didn’t intend to start a series, but, this being Women’s History Month, there turn out to be a lot of articles about the achievements of prominent women in science, and this week, I stumbled onto an article about Chien-Shiung Wu, an experimental physicist of whom many thought should have won the Nobel Prize, though she never did.
She was born in 1912, in Liuhe, Taicang, in Jiangsu province in China. Her father, Zhongyi Wu, was an engineer and a social progressive, determined that his daughter receive a thorough education. She eventually earned a degree in physics from National Central University in 1934. On advice from her mentor in China, she decided to come to the U. S. to pursue graduate study at the University of Michigan. Tragically, after coming to the U. S., she never saw her family again. First, there was World War II, then the revolution in China, and then the decades where China was closed to all visitors. By the time Nixon reopened the relationship with China in 1972, her parents and siblings were dead.
Once she arrived in the U. S., due to negative experiences while visiting the University of Michigan, and positive ones at the University of California at Berkeley, she chose to study at Berkeley. While Earnest O. Lawrence, the head of the Radiation Lab at Berkeley, was her nominal dissertation director, she also worked closely with the Italian physicist Emilio Sergrè. Her dissertation committee was rounded out by J. Robert Oppenheimer. She completed her Ph. D. in 1940; her dissertation was on bremsstrahlung (radiation caused when an electron is deflected by an atomic nucleus) caused by the xenon nucleus, and on production of radioactive xenon as a product of nuclear fission of uranium. After her dissertation defense, Oppenheimer was convinced that Wu knew everything there was to know about neutron absorption cross sections.
After graduation, Wu experienced frustration in her search for a faculty position at a research university. However, in 1944, she was hired by the Manhattan Project where she worked on the separation of uranium isotopes. In 1945, after World War II ended, she joined the faculty at Columbia University where she would spend the rest of her career. She was the first woman to be tenured by the Physics Department at Columbia.
Wu’s research focused primarily on beta decay, where an atomic nucleus either emits an electron or positron, or captures an electron in the atom’s inner shell, so that the atom is transformed into the element to the right or the left of the original one in the periodic table, depending on which beta decay process takes place. Enrico Fermi had created a theory of beta decay in 1934, but the theory remained unconfirmed by experiment a decade later. In 1949, Wu published her confirmation of the theory.
Parity violation was demonstrated in an experiment led by Chien-Shiung Wu. She and her colleagues measured the emission of beta-decay electrons from cobalt-60 nuclei polarized in an applied magnetic field. If the weak interaction conserved parity, then the direction of electron emission should be independent of the direction of the nuclear spin. The observation of a dependence on the polarization direction of the nuclei confirmed the theory that weak interactions do not obey mirror symmetry.
Wu’s most groundbreaking experiments were on parity in the weak nuclear force (the source of beta decay), and on entanglement. Of these two, her experiment on parity in the weak nuclear force has the highest profile. The principle of parity in physics states that if you were to run an experiment in a mirror world, the outcome would be the same as it was in this world; in other words, the universe shows no preference for handedness (right or left).* This is certainly true for the electromagnetic force and for gravity, as well as the strong nuclear force. Up to the 1950s, parity was assumed to apply to all forces. However, two particles were discovered with identical properties in all respects except for how they decayed: one decayed into two particles, while the other decayed into three particles. These decays were beta decays, which are caused by the weak nuclear force. Theorists struggled to find a possible explanation for what what these two particles were, and why they had different decay modes. This was called the tau-theta problem, where tau and theta were the names assigned to these particles. In 1956, two theoretical physicists, Chen-Ning Yang of the Institute for Advanced Study at Princeton, and Tsung-Dao Lee of Columbia University, found that they could explain the two decays if they recognized that tau and theta were in fact the same particle (now called the kaon), and that the weak force violated the parity condition. Wu immediately set out to design an experiment to determine if beta decay did, in fact, violate parity. She placed a sample of radioactive cobalt-60 atoms in a magnetic field so that the nuclei of the atoms would align their spins with the magnetic field, all pointing in the same direction. Cobalt-60 emits an electron when it decays. If the weak force obeyed the parity condition, there should be as many electrons emitted at the north pole end of the sample as at the south pole. What she found was that more electrons emerged from the south pole than the north pole. Thus, the weak force violates the principle of parity, which was a mind-blowing conclusion. The universe is left-handed. Wu completed this experiment in early 1957. The theorists Yang and Lee were awarded the Nobel Prize in Physics in 1957, but not Wu. (This is the inverse of what usually happens; Usually it’s the experimentalist who confirms the theory that gets the Nobel, while the theorist gets overlooked.)
The other groundbreaking experiment initiated by Wu had to do with entanglement. Whenever two or more identical quantum particles are found in a single system and interact with each other, they are said to be entangled. In a paper written by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935 (abbreviate as EPR), the concept of entanglement was used to demonstrate that quantum mechanics predicts that entangled particles separated by long distances must still communicate with each other, and they do so at a speed faster than light. (I’ve written before about quantum entanglement here.) Wu performed one of the first experiments after the EPR paper was published looking at the correlations between entangled particles. When matter-antimatter pairs, an electron and a positron, annihillate each other, the event produces two correlated gamma ray photons propagating in opposite directions. In 1947, two theoretical groups had made predictions regarding the correlations between these two gamma photons. Wu and her student Irving Shaknov published a letter in 1950 on the correlations they measured on this type of correlated gamma photons, confirming the theorists’ predictions. While it wasn’t the first effort to perform the measurement, Wu’s was much more precise. Later, in 1975, after John Bell published his recipe for experimentalists to test quantum versus classical correlation between correlated particles (the Bell inequality), Wu and her students attempted Bell’s test, again using correlated gamma ray photons originating from the annihilation of an electron and a positron, the result suggesting that the quantum result was correct, as has been since confirmed many times. As Wu passed away in 1997, she was not eligible when the Nobel Committee finally got around to awarding the Physics Prize to researchers tackling the entanglement problem in 2022. (Nobel Prizes are not awarded posthumously.)
Lars Brink, a former member of the Nobel Committee for Physics, admitted privately that Wu’s had performed work worthy of a prize, though he did so in 2022. I think we can safely chalk up Wu’s lack of a Nobel Prize to sexism.
Comments are below the fold.
*More accurately, parity is inversion symmetry, but a “mirror world” is easier to imagine and works for the purposes of describing the phenomenon.
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