Atomic clocks that excite the nucleus of thorium-229 embedded in a transparent crystal when hit by a laser beam could yield the most accurate measurements ever of time and gravity, and even rewrite some of the fundamental laws of physics. Thorium-229-doped crystals are both scarce and radioactive. A thin film using a dry precursor of thorium-229 shows the same nuclear excitation as the crystal, but its low cost and radioactivity, and smaller size mean its production could be more easily scaled up to make smaller, less expensive, more portable atomic clocks.
