Gravity Probe A

Gravity Probe A (GP-A) was a space-based experiment to test the <a href="/facts/Equivalence_principle/khkUXSLf">equivalence principle</a>, a feature of Einstein's theory of relativity. It was performed jointly by the <a href="/facts/Smithsonian_Astrophysical_Observatory/1Bz2ApPt">Smithsonian Astrophysical Observatory</a> and the <a href="/facts/National_Aeronautics_and_Space_Administration/TIMCV0yV">National Aeronautics and Space Administration</a>. The experiment sent a <a href="/facts/Hydrogen_maser/MKM884vQ">hydrogen maser</a>—a highly accurate <a href="/facts/Frequency_standard/VrTvqGam">frequency standard</a>—into space to measure with high precision the rate at which time passes in a weaker <a href="/facts/Gravity/bqAN7DdQ">gravitational field</a>. Masses cause distortions in <a href="/facts/Spacetime/uRIN1x4b">spacetime</a>, which leads to the effects of <a href="/facts/Length_contraction/4of503Dg">length contraction</a> and <a href="/facts/Time_dilation/fYj7FYbK">time dilation</a>, both predicted results of Albert Einstein's theory of <a href="/facts/General_relativity/jVsoIg8X">general relativity</a>. 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 <a href="/facts/Gravitational_time_dilation/z4XG7SYA">gravitational time dilation</a>.
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 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 <a href="/facts/Scout_(rocket)/y3J3BcVd">Scout rocket</a>, and attained a height of 10,000 km (6,200 mi), while remaining in space for 1 hour and 55 minutes, as intended. It returned to Earth by splashing down into the Atlantic Ocean.

Gravity Probe A open-in-new

Gravity Probe A