AMO
In the literature: open
Intercontinental optical clock comparison at 1e-18
In plain words
Clocks on different continents must be compared to check that they agree, but satellite links are not yet good enough. Height differences also matter, since a clock 1 cm higher runs faster by about one part in $10^{18}$.
Precise statement
Achieve a comparison of optical clocks separated by more than 5000 km with total uncertainty below 1e-18, including link noise and the gravitational redshift (1.1e-18 per cm height), which requires geopotential differences known to $1e3\,\mathrm{cm}^2\,\mathrm{s}^{-2}$.
What would settle it
An intercontinental clock comparison via free-space optical satellite link, transportable clocks or fiber, with an uncertainty budget below 1e-18 and agreement with independent geodesy.
Related problems
- Special case of Can optical clocks meet the redefinition criteria by 2029?