Assuming that the equivalence principle holds,
[61] gravity influences the passage of time. Light sent down into a
gravity well is
blueshifted, whereas light sent in the opposite direction (i.e., climbing out of the gravity well) is
redshifted; collectively, these two effects are known as the gravitational frequency shift. More generally, processes close to a massive body run more slowly when compared with processes taking place farther away; this effect is known as gravitational time dilation.
[62] Gravitational redshift has been measured in the laboratory
[63] and using astronomical observations.
[64] Gravitational time dilation in the Earth's gravitational field has been measured numerous times using
atomic clocks,
[65] while ongoing validation is provided as a side effect of the operation of the
Global Positioning System (GPS).
[66] Tests in stronger gravitational fields are provided by the observation of
binary pulsars.
[67] All results are in agreement with general relativity.
[68] However, at the current level of accuracy, these observations cannot distinguish between general relativity and other theories in which the equivalence principle is valid.
[69]