Gravity Probe A
Encyclopedia
Gravity Probe A was a space-based experiment to test the theory of general relativity
, performed jointly by the Smithsonian Astrophysical Observatory
and the National Aeronautics and Space Administration. It sent a hydrogen maser
, a highly accurate frequency standard, into space to measure the rate change of a clock in lower gravity with high precision.
The probe was launched on June 18, 1976 on top of a Scout rocket
and remained in space for 1 hour and 55 minutes, as intended. It then crashed into the Atlantic Ocean.
The experiment was thus able to test the equivalence principle
. Gravity Probe A confirmed the prediction that gravity slows the flow of time, and the observed effects matched the predicted effects to an accuracy of about 70 parts per million.
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, performed jointly by the Smithsonian Astrophysical Observatory
Smithsonian Astrophysical Observatory
The Smithsonian Astrophysical Observatory is a research institute of the Smithsonian Institution headquartered in Cambridge, Massachusetts, where it is joined with the Harvard College Observatory to form the Harvard-Smithsonian Center for Astrophysics .-History:The SAO was founded in 1890 by...
and the National Aeronautics and Space Administration. It sent a hydrogen maser
Hydrogen maser
A Hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference....
, a highly accurate frequency standard, into space to measure the rate change of a clock in lower gravity with high precision.
The probe was launched on June 18, 1976 on top of a Scout rocket
Scout (rocket)
The Scout family of rockets were launch vehicles designed to place small satellites into orbit around the Earth. The Scout multistage rocket was the first orbital launch vehicle to be entirely composed of solid fuel stages....
and remained in space for 1 hour and 55 minutes, as intended. It then crashed into the Atlantic Ocean.
Experimental setup
The 100 kg Gravity Probe A spacecraft housed the hydrogen maser system that ran throughout the mission, and a microwave repeater to measure the Doppler shift of the maser signal. The satellite was launched nearly vertically upward to cause a large change in the local gravity seen by the maser, reaching a height of 10000 km (6,213.7 mi). At this height, general relativity predicted a clock should run 4.5 parts in 1010 faster than one on the Earth.Results
The clock rate was measured from the ground by comparing the microwave signal from the clock to a maser on the ground and subtracting a signal from the spacecraft that measured the Doppler shift. The clock rate was measured for most of the duration of the flight and compared to theoretical predictions. The stability of the maser permitted measurement of changes in the rate of the maser of 1 part in 1014 for a 100-sec measurement.The experiment was thus able to test the equivalence principle
Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
. Gravity Probe A confirmed the prediction that gravity slows the flow of time, and the observed effects matched the predicted effects to an accuracy of about 70 parts per million.
See also
- Timeline of gravitational physicsTimeline of gravitational physics and relativityTimeline of gravitational physics and general relativity* 3rd century BC - Aristarchus of Samos proposes heliocentric model, measures the distance to the moon and its size...
- Gravity Probe BGravity Probe BGravity Probe B is a satellite-based mission which launched on 20 April 2004 on a Delta II rocket. The spaceflight phase lasted until 2005; its aim was to measure spacetime curvature near Earth, and thereby the stress–energy tensor in and near Earth...