The first nuclear clocks have begun ticking

Two teams, in Vienna and Beijing, have built working clocks that use a transition inside the nucleus of thorium-229 to set a laser’s rhythm. Today’s atomic clocks do something similar with an atom’s outer electrons; moving the reference into the much smaller nucleus could make a clock less vulnerable to outside disturbances. That is why physicists think nuclear clocks could eventually exceed the best atomic clocks and become unusually sensitive tools for detecting tiny changes in the laws of physics. These first devices are still early prototypes, not replacements for the timekeepers behind navigation and communications, but they turn a decades-old idea into an operating technology.