A laser beam as faint as a whisper of light has stirred a thorium-229 nucleus into a higher energy state. Most nuclei demand a blast of energy, but thorium-229 has such a tiny energy gap that even this gentle nudge works. Instead of waiting for the nucleus to emit light, scientists used absorption spectroscopy — measuring the fraction of the whisper absorbed, giving a fast, clean signal. They placed the thorium in a calcium fluoride crystal, in a spot so quiet that the crystal's hum was almost inaudible.

A clock based on this could be so steady it would miss less than a second over the age of the universe. It might reveal whether the standard model, the rulebook of fundamental forces, ever drifts. And the laser's power? A firefly's brief glow outshines it by far.