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Height differences also matter, since a clock 1 cm higher runs faster by about one part in $10^{18}$.","posed_since":"","precise":"Achieve a comparison of optical clocks separated by more than 5000 km with total uncertainty below $10^{-18}$, including link noise and the gravitational redshift ($1.1 \\times 10^{-18}$ per cm height), which requires geopotential differences known to $10^{3}\\,\\mathrm{cm}^2\\,\\mathrm{s}^{-2}$.","problem_ref":"d0ab5819f1c3b3ef3bd220bc093322312f68da8baed3eb76f4554eadf17aec60","references":"","settled_by":"An intercontinental clock comparison via free-space optical satellite link, transportable clocks or fiber, with an uncertainty budget below $10^{-18}$ and agreement with independent geodesy.","status_note":"","title":"Intercontinental optical clock comparison at 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