By precisely calculating odd shifts in the orbit of Mercury, Einstein gained important support for his theory of general relativity, which posits that gravity arises because mass bends spacetime. By studying the orbits of two manmade satellites around Earth, scientists now say they have confirmed a much smaller effect predicted by the theory: namely, that mass drags spacetime with it as it rotates.
Astrophysicists think this “frame dragging” is a big deal for rapidly rotating neutron stars and black holes, but that near Earth it is extraordinarily small. In April, NASA launched the Gravity Probe B satellite, which carries high-precision gyroscopes designed to measure the effect with about 1 percent accuracy. Researchers writing today in Nature analyzed position measurements of two earlier satellites over an 11-year period, and determined that the 40-million-meter-long orbits of the satellites shifted by about two meters per year.
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