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]


The objective of the Gravity Probe A experiment was to test the validity of the equivalence principle. The equivalence principle was a key component of Albert Einstein's theory of general relativity, and states that the laws of physics are the same regardless of whether you consider a uniformly accelerating reference frame or a reference frame that that is acted upon by uniform gravitational field.

Equivalence Principle[edit]

The equivalence principle can be understood by picturing a rocket ship in two scenarios. First, imagine a rocket ship that is at rest on the Earth's surface; objects in the rocket ship are being accelerated downward at 9.81 m/s². Now, imagine a rocket ship that has escaped Earth's gravitational field and is accelerating upwards at a constant 9.81 m/s² due to thrust from its rockets; objects in the rocket ship that are dropped will fall to the floor with an acceleration of 9.81 m/s². This example shows that a uniformly accelerating reference frame is indistinguishable from a gravitational reference frame.

Further, the equivalence principal guarantees that phenomena that are caused by inertial effects will also be present due to gravitational effects. Imagine, for example, a beam of light that is shined horizontally across a rocket ship that is accelerating uniformly upwards. According to an observer outside the rocket ship, the floor of the rocket ship accelerates up towards the light beam. The light beam does not seem to travel on a horizontal path according to the outside observer, rather the light seems to bend down toward the floor (because the floor is accelerating uniformly upward). This is an example of an inertial effect that causes light to bend. The equivalence principle states that this inertial phenomenon will also occur in a gravitational reference frame as well. Indeed, the phenomenon of gravitational lensing states that matter can bend light, and this phenomenon has been observed by the Hubble Telescope.

Time Dilation[edit]

Time Dilation refers to the expansion or contraction in the rate of which time passes, and was the subject of the Gravity Probe A experiment. Under Einstein's theory of general relativity, matter bends spacetime similar to the way a bowling ball would bend a sheet of fabric if it were dropped in the middle of the sheet. This analogy shows how space would bend in the presence of matter, and since space and time are unified, time also distorts under the presence of matter. A far away observer from a massive object would measure time slowing down in the region near that massive object. This is due to the fact that the speed of light is constant in all reference frames. The far away observer in this example sees bent space while the reference frame on the large mass sees flat space. Speed is defined as a change in distance divided by a change in time, and the speed of light in both reference frames is constant. A curved path is by necessity longer than a flat path, so the far away observer must measure a longer time interval than the reference frame on the mass. This is what is meant by time slowing down, the reference frame on the massive object would measure a lower amount of time than the reference frame far away from the massive object.

Time dilation is caused by inertial effects as well, in that objects that travel at speeds near the speed of light experience a slowing down of the rate of time compared to a stationary observer. The Gravity Probe A experiment was meant to verify that the time dilation effect due to inertia translates into a time dilation effect due to gravity. In this way, it was a test of the equivalence principle, as the equivalence principle states that inertial reference frames are indistinguishable from gravitational reference frames.

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]