Atomic clock performance beyond the geodetic limit

The passage of time is tracked by counting oscillations of a frequency reference, such as Earth's revolutions or swings of a pendulum. By referencing atomic transitions, frequency (and thus time) can be measured more precisely than any other physical quantity, with the current generation of optical atomic clocks reporting fractional performance below the 10^{-17}level. However, the theory of relativity prescribes that the passage of time is...