Gravity Probe B proves Einstein right

Science : Gravity Probe B, launched in 2004, has confirmed two predictions derived from Einstein’s theory of general relativity. The satellite, which circled Earth from pole to pole for 17 months, used gyroscopes to measure the geodetic effect caused by Earth’s mass creating a dimple in spacetime; the circumference of a circle around Earth should be slightly shorter than 2 π times the circle’s radius. Gravity Probe B measured the 2.8-centimeter decrement to 0.25% precision. The satellite’s measurement of the frame-dragging effect, in which Earth twists the surrounding spacetime as it rotates, fell short of expectations and achieved 19% precision. The gyroscopes were geometrically the roundest objects ever manufactured—but trapped charges in their niobium coating made them far less precisely round electrically. Those imperfections combined with ones in the gyroscope’s housing to create uncertainties that took five years to correct. In 2004 Ignazio Ciufolini (University of Salento in Lecce, Italy) and colleagues used data from less expensive satellites, LAGEOS and LAGEOS II, to measure the frame dragging effect to 10% precision. Whether Gravity Probe B merely confirms those results, or has additional importance, is a matter of perspective.