How Madame Wu Became First Lady of Physics (and why observational astro owes her legacy a great debt)

Madame Wu. Image from The National Science Foundation

Chien Shiung Wu (1912-1997) was a Chinese American physicist, known for her work on the Manhattan Project, but more specifically, her experiments on beta decay and the determination that double beta decay (the loss of 2 neutrons, or capture of 2 electrons) indicates a violation of parity – the idea that the processes underlying fundamental particles is invariant under mirror or reversal. Although Wu designed and implemented the experiments, the Nobel went exclusively to two of her colleagues, whom I will not mention here, because this is about Wu! And yet Wu, a proud Chinese immigrant and determined woman of science, she persevered, enduring sexism, racism and cultural discrimination to earn several prestigious awards, including the very first Wolf Prize, for her life’s work in physics. She was the first female instructor at Stanford, the first female President of the American Physical Society, and first female honoree of the National Medal of Science in Physics, which is the highest honor given to American scientists. The Nobel Prize that many think should include Wu was given to only two people. The Nobel is long contested for its denial of the team spirit of science. The denial of rewarding Wu and her colleagues is to this day considered one of the committee’s greatest failures.

Even though she was not an astronomer, Dr. Wu, aka the First Lady of Physics (or so called Madame Wu by her students), contributed to our fundamental understanding of particle parity and the weak interaction through her Cobalt-60 experiments and over her long career. The weak force, along with electromagnetism, gravitation, and the strong force, are known as the fundamental forces. The weak force describes the interactions of subatomic particles from the radioactive decay of an atomic nucleus. Parity describes the conservation of symmetry of a reaction under a mirror inversion; in this case, it’s radioactive beta decay, in which highly energized electron or a positron is emitted from an atom’s nucleus, leaving a more stable number of protons to neutrons. This weak interaction is involved in nuclear fission (like Wu’s work with the Manhattan Project); however, its influence is less than the diameter of a proton in range. In a nutshell, parity violation means that the beta particle emissions flowed in the opposite direction of the nuclear spin, meaning that the particles carrying the weak force do not behave at all like its cousins.

The weak force is essential to understanding its relationship to other non-contact forces, such as the elusive dark energy, dark matter, and the hypothetical fifth fundamental force.

In the field of physical cosmology, charge parity violation of electroweak force carrier particles is thought to be related to the parallel dominance of baryonic matter in our universe. The rest of it is composed of mysterious ‘dark’ energy, antimatter, or non-baryonic matter that behaves differently than ‘normal’ matter. During the electroweak epoch of the early universe, just after the Big Bang, rapid inflation expanded space-time and the strong force decoupled from the electric and weak forces; a third generation of particles emerged; the gravitational wave background appeared; then baryogenesis, the creation of baryons, or regular matter. It is by studying these fundamental interactions or forces that we may gain further insight into the astrophysical processes that we see in black holes and other powerful electromagnetic and gravitational wave sources.

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Moreover, by showing that some weakly-interacting particles violate charge parity and break symmetry, Wu blazed a path for modern astrophysics to study the intense sources that create the conditions for these violations to occur. Besides, even our most powerful particle accelerators can’t produce the energies required to recreate the conditions necessary to cause electroweak force carriers (electrons, W and Z bosons, and photons) to violate the charge-parity-time symmetry. For energies greater than 10¹⁵GeV, we must look to the stars – specifically, at black holes, or back in time, between 10^-32 and 10^-36 seconds after the Big Bang – where (and when) these extreme energies dominate instead of ordinary matter. As astronomy has progressed, there has been more focus on understanding high energy events that can only be observed in deep space. As our optics become more accurate, our science has become more precise and better able to measure the smallest fluctuations of radiation. Peering deep into space is also looking backwards in time; then it is the dream of the modern astronomer, as well as the physicist, to look as deep and as far as possible.

By placing a particle detector (much like NASA’s Alpha Magnetic Spectrometer) on a high-energy deep space observatory, we may be able to detect neutrinoless double beta decays. The observatory would study particles emitted from supernovae and black holes, and search for evidence of black hole decay, the data of which would help to answer questions around entropy and the relationship between the electroweak force and gravitation. The astrophysical observations can raise or lower the limits on properties of these particles, or of space-time itself. For instance, the fine structure constant, which quantifies the coupling charges of elementary particles to the electroweak force, is so named for the gap in the fine structure of the spectral lines of the hydrogen atom. Conversely, a particle physics approach to astrophysics that searches for muon neutrino sources can explain the appearances of exotic types of black holes, such as blazars, or how seemingly impossible black holes formed. All this, and more that we probably have yet to conceived of, thanks in great part to the contributions of Dr. Wu and her colleagues.