International atomic clock comparison achieving $5\times 10^{-18}$ uncertainty
A. Tofful, M. Filzinger, T. Schmidt, A. Amy-Klein, E. Benkler, E. Cantin et al. · arXiv: 2610.29980v1 · Summary page
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Clocks precise enough to hear gravity
Independently developed optical clocks in the United Kingdom and Germany have agreed in a comparison conducted over a long optical fiber link. Both clocks use the same transition in a trapped ytterbium ion. Their comparison reached an uncertainty that the authors describe as the lowest yet achieved internationally, marking progress toward a future optical definition of the second.
At this precision, knowing the clocks is not enough: one must also know the gravitational potentials where they sit, because gravity changes the rate at which clocks tick. The correction for that relativistic effect, including changes in potential over time, dominated the uncertainty. The researchers also used the separation between the clocks to search for violations of Lorentz invariance, a foundation of relativity. They found no statistically significant modulation and tightened existing constraints—a reminder that metrology infrastructure can double as an experiment on fundamental physics.
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