The Clocks Feel the Tide

General relativity makes clocks at different gravity potentials tick at slightly different rates, an effect already used for static height comparisons. A European network of optical clocks and phase-stabilized fiber links has now measured the changing part: two distinct baselines of about 800 kilometers followed different combinations of daily and twice-daily tidal components, including one comparison that used a transportable clock as a remote sensor. The measured rate shifts resolved the predicted signal within one day and agreed with geophysical models. The demonstration tracks known tides rather than an unknown event, but it shows that future clock networks could monitor subtler mass redistribution or surface deformation if their stability and availability improve.