A clock built from thorium-229 has crossed an important line, from a long-discussed concept to a working device. The shift matters because this clock does more than keep time. It can also watch for tiny changes in the forces that shape the universe. The new system uses a nuclear transition in thorium-229 rather than the electron-shell transitions used in conventional atomic clocks. That distinction has made the isotope one of physics’ most closely watched candidates for next-generation timekeeping. The transition sits at 148 nanometers, making it accessible to lasers, and its unusual origin gives it an added role as a probe of fundamental physics. For years, thorium-229 was valued for its potential. Now the device has been demonstrated as a stand-alone nuclear clock, with a continuous-wave laser stabilized directly to the isotope’s nuclear transition inside a calcium fluoride crystal at room temperature. A fluorite crystal containing atoms of the radioactive element thorium-229. It was used to precisely measure the absorption spectrum of atomic nuclei at the National Metrology Institute of Germany (PTB). (CREDIT: Weizmann Wonder …