A spinning nucleus ejects an electron while decaying, as does its reflection in a mirror. The electron is ejected in the direction of the nuclear spin (represented by the vertical arrow) in the real world, but opposite to the direction of spin in the mirror, violating mirror symmetry. Steven Weinberg writes, ‘In 1957 experiments showed convincingly that, while the electromagnetic and strong nuclear forces do obey mirror symmetry, the weak nuclear force does not. Experiments showed, for example, that it was possible to distinguish a cobalt nucleus in the process of decaying—as a result of the weak nuclear force—from its mirror image, spinning in the opposite direction.’ Adapted from an illustration in A. Zee, Fearful Symmetry: The Search for Beauty in Modern Physics (Princeton University Press, 2007).