High-precision clocks with previously unobtainable sensitivities can be used to test the fundamental laws of the universe.

NPL is developing high-precision clocks that can be used for a future redefinition of the second and to investigate possible variation in a fundamental physical constant (the fine structure constant), which if found would violate Einstein's Equivalence Principle. The fine structure constant relates charge to electromagnetic radiation but some scientists doubt whether it remains a constant over space and time.