Consider the ghostly neutrino. This elementary, subatomic particle carries with it not only an uncanny reminder of a time eons ago when symmetries were perfect, but also a clue as to how they came to be broken. For every neutrino that now spins to the left, there was once one that spun to the right: these parallel twins were destroyed in the "Big Bang," that cosmic apocalypse that, most scientists now agree, created the universe. And this decay of...