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"Imagine the Earth as if it were immersed in honey. As the planet rotated on its axis and orbited the Sun, the honey around it would warp and swirl, and it's the same with space and time." This is how Francis Everitt, physicist at Stanford University (California) and principal investigator of the Gravity Probe B satellite describes the behaviour of space around our planet.
The Gravity Probe B mission, which started in 2004 and ended last December, was conceived to test two of the main predictions of Einstein's theory of General Relativity, as described by Everitt; that is, a deformation of spacetime around the planet and a dragging of spacetime caused by its rotation.
The main problem in measuring these phenomena is their tiny magnitude. The satellite measured the precession of 4 precisely engineered gyroscopes by using the star IM Pegasi as a point of reference while in its polar orbit.
If Einstein were wrong, the gyroscopes would have remained pointed in the same direction with respect to the star for the entire mission, but if the theory was correct the terrestrial gravitational field would have caused minute variations in the rotational axis of the gyroscopes. This is indeed what was measured.
Apart from providing another confirmation of Einstein's theory of General Relativity, the technology developed for this satellite has led to innovations in every-day applications such as GPS. Gravity Probe B technology was also applied to another NASA mission which measured the cosmic microwave background and led to the award of the Nobel Prize to the physicist John Mather.
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