Time passes slightly faster higher above Earth. The difference is tiny, but modern optical clocks can measure it.
Gravity changes the rate
General relativity connects the flow of time with gravitational potential. Two sufficiently precise clocks at different elevations accumulate a measurable difference.
An optical clock uses an extremely stable atomic transition as its reference. Lasers probe that transition while a frequency comb connects the optical signal to countable time.
Turning time into height
Researchers compare clocks through fiber links or transportable systems. With careful calibration, the frequency difference can be translated into a difference in gravitational potential.
A complement to existing maps
Clock measurements will not make leveling surveys or satellite geodesy obsolete. They provide another kind of observation that may be useful where mass changes or difficult terrain complicate conventional methods.
