Gravity Probe A

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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. It sent a hydrogen maser, a highly accurate frequency standard, into space to measure with high precision the changed rate of a clock that is high up in a gravitational field.

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.[1]

Experimental setup[edit]

The 100 kg Gravity Probe A spacecraft housed the atomic hydrogen maser system that ran throughout the mission, and a microwave repeater to measure the Doppler shift of the maser signal. The probe was launched nearly vertically upward to cause a large change in the gravitational potential seen by the maser, reaching a height of 10,000 km (6,200 mi). At this height, relativity predicted a clock should run 4.5 parts in 1010 faster than one on the Earth.

According to the 1976 press release by Joyce B. Milliner: "The interaction of the electron and proton in the hydrogen atom generates a microwave signal (1.42 billion cycles per second) stable to one part in a quadrillion (1 x 10−15), or the equivalent of a clock that loses less than two seconds every 100 million years."[2]


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 10−14 for a 100-sec measurement.

The experiment was thus able to test the equivalence principle. Gravity Probe A confirmed the prediction that gravity slows the flow of time,[3] and the observed effects matched the predicted effects to an accuracy of about 70 parts per million.

See also[edit]


  1. ^ "Fundamental Physics of Space - Technical Details - Gravity Probe A". Nasa JPL. May 2, 2009. Retrieved May 2013. 
  2. ^ Milliner, Joyce B. (June 10, 1976). "Space Probe to Test Einstein's "Space-Time Warp" Theory". Retrieved May 2013. 
  3. ^ Than, Ker (May 5, 2011). "Einstein Theories Confirmed by NASA Gravity Probe". National Geographic Society. Retrieved May 2013. 

Additional reading[edit]