In 1949 physicist Chien-Shiung Wu devised an experiment that documented evidence of entanglement. Her findings have been hidden in plain sight for more than 70 years
I n November 1949 Chien-Shiung Wu and her graduate student, Irving Shaknov, descended to a laboratory below Columbia University's Pupin Hall. They needed antimatter for a new experiment, so they made their own, using a machine called a cyclotron. The machine's multiton magnet was so gigantic that, according to university folklore, a decade earlier administrators had to blast a hole in an exterior wall and recruit the football team to maneuver the block of iron into the building.
The magnetic field produced by a cyclotron accelerates particles to dizzying speeds. In the lab, Wu and Shaknov used it to bombard a sheet of copper with deuterons, generating an unstable isotope, Cu 64, as a source of positrons—the antimatter. When a positron and an electron collide, they annihilate each other, releasing two photons that fly apart in opposite directions. A few years earlier physicist John Wheeler had predicted that when matter and antimatter met, the resulting photons would be orthogonally polarized. Wu and Shaknov were looking for conclusive proof of Wheeler's so-called pair theory.
They weren't the first. An earlier team of experimentalists had a high margin of error, so their results were not sufficiently reliable. A second team came back with results that were too low to match Wheeler's predictions. But Wu was known for her extreme precision and strategic experimental design. The prior year she had proved Enrico Fermi's theory of beta decay after more than a decade of attempts by others.