Chen-Ning Yang and Tsung-Dao Lee, Chinese-born physicists who had emigrated to the United States and were working at Princeton and Columbia respectively, propose in 1956 an idea that contradicts an assumption fundamental to physics and until then considered unquestionable: that the laws of nature are symmetric with respect to their mirror image (parity), so that an experiment and its mirror-reflected version should give physically equivalent results. Yang and Lee theoretically predict that this symmetry, valid for electromagnetism and the strong nuclear force, might be violated in the weak nuclear interaction, responsible for beta radioactive decay. Chien-Shiung Wu, also a Chinese experimental physicist, designs at Columbia University an extraordinarily delicate experiment with cobalt-60 nuclei cooled to near absolute zero and aligned by a magnetic field, observing that the electrons emitted in radioactive decay preferentially come out in one direction relative to the nuclear spin — an asymmetry that would violate parity if nature respected mirror symmetry. The result, confirmed in January 1957, shocks the theoretical physics community and forces a revision of fundamental assumptions about the structure of physical laws.