A Chinese-American physicist whose name many people have never heard will soon share a rare honor typically bestowed on the field’s mononymous greats: Einstein, Fermi, Feynman. On 11 February, the International Day of Women and Girls in Science, the U.S. Postal Service (USPS) will issue a stamp commemorating Chien-Shiung Wu, the service announced this week. In 1956, Wu proved, essentially, that the universe knows its right hand from its left.
Wu, who died in 1997 at age 84, never received a Nobel Prize for her demonstration of the effect called parity violation. Instead, she numbers among the women many scientists think were unfairly overlooked by the Nobel Committee. “It was an incredibly important experiment and she was an amazing scientist,” says Melissa Franklin, a particle physicist at Harvard University.
The universe can be thought of as a huge assemblage of fundamental particles interacting through four forces: gravity, electromagnetism, the strong force that binds the atomic nucleus, and the weak force that produces a type of nuclear decay called beta decay. Physicists once assumed that if you inverted all the particles’ positions—swapping left and right, up and down, forward and back—and reverse all their momenta, the universe should work just the same. If you performed such a “parity” transformation on a clock, for example, the weird mirror-image clock that would result would tick just like the original one.
By the 1950s, however, physicists were producing exotic, fleeting subatomic particles by firing high-energy protons into targets, and certain particle decays seemed to defy parity symmetry. In June 1956, theorists Tsung-Dao Lee of Columbia University and Chen Ning Yang of Brookhaven National Laboratory suggested parity might not hold in weak interactions. Their paper proposed an experiment to find out—although for that part, they consulted Wu.
The trick was to study a nucleus that both spins and undergoes beta decay, in which it spits out an electron and a nearly undetectable neutrino. If parity holds, the electron should emerge with equal probability in all directions. If parity is violated, electrons would be more likely to emerge in one direction relative to the nucleus’ spin than in the other.
That’s just what Wu and colleagues observed in experiments at the National Bureau of Standards (now the National Institute of Standards and Technology) in December 1956. They placed a sample of radioactive cobalt-60 in a strong magnetic field and cooled it to nearly absolute zero to make most of the nuclei spin in the same direction. They found a strong correlation between the spin and the directions of emitted electrons, proving the weak interaction has a handedness: Curl the fingers of your left hand in the sense the nuclei are spinning and the electrons emerge in the direction of your thumb.
Lee and Yang shared the 1957 Nobel Prize in Physics for the discovery of parity violation, but Wu did not. Some scholars have argued the Nobel Committee left her out because it wasn’t clear that her group deserved priority for the experimental observation. Wu and colleagues were almost scooped by rivals led by Leon Lederman, also at Columbia, who measured the same effect in the decay of a particle called the muon. Lederman’s team started later but had a manuscript ready first and it held off on submitting it for publication at Lee’s request, recounts Magdolna Hargittai, a chemist and historian of science at the Budapest University of Technology and Economics in a 2012 essay.
However, Lee and Yang had already sharpened their theoretical ideas in discussions with Wu, notes David Kaiser, a physicist and historian of science at the Massachusetts Institute of Technology. And she suggested a cryogenic cobalt-60 source would be the best way to achieve a spin-polarized beta source. In addition, Kaiser notes, the Nobel Committee could have given a second prize for experimental observations in a later year. “I think we can make a very strong case that she was overlooked or at least eminently deserving of such an honor,” he says.
The Nobel controversy aside, the discovery of parity violation had a tremendous impact on particle theory. Particle physicists’ whole standard model of elementary particles and forces derives from various abstract symmetries and how interactions among the particles sometimes obscure or break those symmetries. The discovery of parity violation set the conceptual stage for that development, Kaiser says.
A physicist has a much better chance of winning a Nobel Prize than appearing on a postage stamp. USPS officials say they haven’t tracked exactly how many physicists have been on stamps, but the list is short. In addition to Wu, Einstein, Fermi, and Feynman, others include rocket scientist Robert Goddard, experimentalist Robert Millikan, aerodynamicist Theodore von Kármán, condensed matter theorist John Bardeen, and nuclear theorist Maria Goeppert Mayer. Regarding Wu, Franklin says, “I don’t think it matters about the Nobel Prize, the stamp is much more important.”
Wu’s name went through the standard selection process for commemorative stamps, says William Gicker, director of stamp services at USPS. Anyone can nominate a person for a stamp, Gicker says, and USPS receives roughly 30,000 nominations per year. Those that conform to the requirements then go to the Citizens’ Stamp Advisory Committee, which makes recommendations to the postmaster general. USPS issues just a few dozen commemorative stamps each year.
The committee was looking to issue more stamps involving the sciences and to diversify the figures on them, Gicker says. It’s OK if Wu is not a household name, he says. “We hope to engage a viewer through a little 1-inch-by-1-inch work of art to ask, ‘Who is this and what did they do?’” He adds, “You should be on the lookout for other prominent women in the sciences in future years.”