To measure time very precisely, scientists use the frequency of radiation as a stopwatch. But the current standard for atomic clocks, based on excited cesium atoms, is off by one second every 100 million years—an intolerable amount of sloppiness. In August, physicists at the National Institute of Standards and Technology debuted a new type of atomic clock that measures ytterbium atoms trapped in cages of light known as optical lattices. It’s more than 100 times as accurate, off by about one second over the lifetime of the universe—so precise that it redefines the second as a unit of time.