Editing Gravity Probe A (section)

{{short description|Space-based experiment to test the theory of general relativity}}
{{redirect|GP-A|other uses|GPA (disambiguation)}}
{{Infobox spaceflight
| name              = Gravity Probe A
| image             = Benjamin Crowell, General Relativity (2009) p17 GPA.gif
| image_caption     = Diagram of the ''Gravity Probe A'' experiment<ref>{{cite book | author=Benjamin Crowell | title=General Relativity | location=Fullerton/CA | url=https://commons.wikimedia.org/wiki/File:Benjamin_Crowell,_General_Relativity_(2009).pdf| year=2009 }}</ref>{{rp|17}}
| mission_type      = Astrophysics
| operator          = [[NASA]]
| mission_duration  = 1 h 51 m
| spacecraft        = GP-A
| spacecraft_bus    = 
| manufacturer      = 
| dry_mass          = <!-- {{convert| | | |abbr=on}} -->
| launch_mass       = {{cvt|60|kg}}
| dimensions        = <!-- {{convert| |x| | | |abbr=on}} -->
| power             = {{val|22|u=W}}
| launch_date       = {{start-date|18 June 1976}}
| launch_rocket     = [[Scout (rocket family)|Scout]]<ref name="NSSDCA">{{cite web |title=NASA - NSSDCA - Spacecraft - Telemetry Details |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=GRAVR-A |website=nssdc.gsfc.nasa.gov |access-date=13 May 2020}}</ref>
| launch_site       = [[Wallops Flight Facility]]<ref name="nssdc">{{cite web |title=NASA - NSSDCA - Spacecraft - Details |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=GRAVR-A |website=nssdc.gsfc.nasa.gov |access-date=13 May 2020}}</ref>
| launch_contractor = 
| disposal_type     = Decommissioned
| deactivated       = {{start-date|18 June 1976}}
| suborbital_range  = 
| suborbital_apogee = {{cvt|10224|km}}
| COSPAR_ID         = GRAVR-A
}}

'''Gravity Probe A''' ('''GP-A''') was a space-based experiment to test the [[equivalence principle]], a feature of Einstein's theory of relativity. It was performed jointly by the [[Smithsonian Astrophysical Observatory]] and the [[National Aeronautics and Space Administration]]. The experiment sent a [[hydrogen maser]]{{mdash}}a highly accurate [[frequency standard]]{{mdash}}into space to measure with high precision the rate at which time passes in a weaker [[gravity|gravitational field]]. Masses cause distortions in [[spacetime]], which leads to the effects of [[length contraction]] and [[time dilation]], both predicted results of Albert Einstein's theory of [[general relativity]]. Because of the bending of spacetime, an observer on Earth (in a lower gravitational potential) should measure a slower rate at which time passes than an observer that is higher in altitude (at higher gravitational potential). This effect is known as [[gravitational time dilation]].

The experiment was a test of a major consequence of Einstein's general relativity, the equivalence principle. The equivalence principle states that a reference frame in a uniform gravitational field is indistinguishable from a reference frame that is under uniform acceleration. Further, the equivalence principle predicts that the phenomenon of different time flow rates, present in a uniformly accelerating reference frame, will also be present in a stationary reference frame that is in a uniform gravitational field.

The probe was launched on June 18, 1976 from the NASA-Wallops Flight Center in Wallops Island, Virginia. The probe was carried via a [[Scout (rocket)|Scout rocket]], and attained a height of {{cvt|10000|km}}, while remaining in space for 1 hour and 55 minutes, as intended. It returned to Earth by splashing down into the Atlantic Ocean.<ref>{{cite web|url=http://funphysics.jpl.nasa.gov/technical/grp/grav-probea.html|title=Fundamental Physics of Space - Technical Details - Gravity Probe A|publisher=[[NASA]] [[Jet Propulsion Laboratory|JPL]]|date=May 2, 2009|access-date=May 5, 2013|archive-url=https://web.archive.org/web/20110918231727/http://funphysics.jpl.nasa.gov/technical/grp/grav-probea.html|archive-date=September 18, 2011|url-status=dead}}</ref>