Editing General relativity (section)

=== Gravitational time dilation and frequency shift ===
{{Main|Gravitational time dilation}}
[[File:Gravitational red-shifting.png|thumb|Schematic representation of the gravitational redshift of a light wave escaping from the surface of a massive body]]
Assuming that the equivalence principle holds,<ref>{{Harvnb|Rindler|2001|pp=24–26 vs. pp. 236–237}} and {{Harvnb|Ohanian|Ruffini|1994|pp=164–172}}. Einstein derived these effects using the equivalence principle as early as 1907, cf. {{Harvnb|Einstein|1907}} and the description in {{Harvnb|Pais|1982|pp=196–198}}</ref> gravity influences the passage of time. Light sent down into a [[gravity well]] is [[blueshift]]ed, whereas light sent in the opposite direction (i.e., climbing out of the gravity well) is [[redshift]]ed; 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.<ref>{{Harvnb|Rindler|2001|pp=24–26}}; {{Harvnb|Misner|Thorne|Wheeler|1973 |loc=§&nbsp;38.5}}</ref>

Gravitational redshift has been measured in the laboratory<ref>[[Pound–Rebka experiment]], see {{Harvnb|Pound|Rebka|1959}}, {{Harvnb|Pound|Rebka|1960}}; {{Harvnb|Pound|Snider|1964}}; a list of further experiments is given in {{Harvnb|Ohanian|Ruffini|1994|loc=table 4.1 on p. 186}}</ref> and using astronomical observations.<ref>{{Harvnb|Greenstein|Oke|Shipman|1971}}; the most recent and most accurate Sirius B measurements are published in {{Harvnb|Barstow, Bond et al.|2005}}.</ref> Gravitational time dilation in the Earth's gravitational field has been measured numerous times using [[atomic clocks]],<ref>Starting with the [[Hafele–Keating experiment]], {{Harvnb|Hafele|Keating|1972a}} and {{Harvnb|Hafele|Keating|1972b}}, and culminating in the [[Gravity Probe A]] experiment; an overview of experiments can be found in {{Harvnb|Ohanian|Ruffini|1994|loc=table 4.1 on p. 186}}</ref> while ongoing validation is provided as a side effect of the operation of the [[Global Positioning System]] (GPS).<ref>GPS is continually tested by comparing atomic clocks on the ground and aboard orbiting satellites; for an account of relativistic effects, see {{Harvnb|Ashby|2002}} and {{Harvnb|Ashby|2003}}</ref> Tests in stronger gravitational fields are provided by the observation of [[binary pulsar]]s.<ref>{{Harvnb|Stairs|2003}} and {{Harvnb|Kramer|2004}}</ref> All results are in agreement with general relativity.<ref>General overviews can be found in section 2.1. of Will 2006; Will 2003, pp. 32–36; {{Harvnb|Ohanian|Ruffini|1994|loc=sec. 4.2}}</ref> However, at the current level of accuracy, these observations cannot distinguish between general relativity and other theories in which the equivalence principle is valid.<ref>{{Harvnb|Ohanian|Ruffini|1994|pp=164–172}}</ref>