The Universal Timekeepers

Atoms are unfathomably tiny. It takes fifteen million trillion of them to make up a single poppy seed-give or take a few billion. And there's hardly anything to them: atoms are more than 99.9999999999 percent empty space. Yet scientists have learned to count these slivers of near nothingness with precision and to peer into their internal states. In looking so closely, we have learned that atoms, because of their inimitable signatures and imperturbable internal clocks, are little archives holding the secrets of the past.