Albert Einstein' general Theory of Relativity, published in 1915/16, unified special relativity and Newton's law of universal gravitation with the insight that gravitation is not due to a force but rather is a manifestation of curved space and time,
with this curvature being produced by the mass-energy and momentum content of the space-time.
Gravity Probe B (GP-B) is a satellite developed by NASA and Stanford University (California), launched on April 20, 2004, from Vandenberg Air Force Base in California. It orbited at about 400 miles, traveling over the North and South Poles. However, GP-B was first started in 1959 by Stanford University professors and funded by NASA beginning with 1964. Consequently, it is considered the longest lasting physics-research programs at Stanford and NASA.
It was designed to measure the stress-energy tensor (the distribution, and especially the motion, of matter) in and near Earth, thus testing Einstein's general theory of relativity and our precise models of space.
Einstein did not see, for example, the solar system's planets being pulled in by the Sun. Instead, he saw them following the curvedness of space-time due to the distorting effects of the Sun's mass.
One measurement made by GP-B is the geodetic effect, which is the amount by which the mass of the Earth warps the local space-time in which it resides. Space-time is a math model that brings together the three dimensions of space and the one dimension of time into one single entity called the space-time continuum. The other effect, called frame-dragging, is the amount by which the rotating Earth drags local space-time around with it.
According to Einstein's theory, over the course of one year, the geodetic warping of Earth's local space-time causes the spin axes of each gyroscope to shift from its initial alignment by a minute angle of 0.0018 degrees (6.606 arc-seconds ) in the plane of the spacecraft's orbit. Likewise, the twisting of Earth's local space-time causes the spin axis to shift by an even smaller angle of 0.000011 degrees (0.039 arc-seconds) in the plane of the Earth's equator.
Although the distortion effects of space-time have been considered true for years, this experiment proves these two previously unverified predictions from Einstein's General Theory of Relativity. Such knowledge means an enormous amount with respect to the structure of the universe.
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