Physicists at Last Measure Speed of Gravity

From Times Staff and Wire Reports

Using a rare alignment of Jupiter against a far-off group of quasars, researchers have finally succeeded in measuring the speed of gravity, a fundamental constant of physics described by Albert Einstein in his general theory of relativity.

The new number, announced Tuesday, will curb some of the more exotic notions of theorists working to formulate a "theory of everything" that unifies concepts of particle physics with scientists' understanding of gravity. While Einstein theorized that gravity exerts its effects at the speed of light, others have speculated that its effects are instantaneous.

At a meeting of the American Astronomical Society in Seattle, Sergei Kopeikin of the University of Missouri-Columbia and his colleagues reported that the force of gravity propagates outward from a source at 1.06 times the speed of light (186,000 miles per second in a vacuum), with a 20% margin for error.

The team took advantage of the once-in-a-decade alignment of Jupiter against a grouping of quasars billions of light-years away. The researchers measured how much light from the quasar grouping was "bent" by Jupiter's gravitational field, and from that calculated gravity's speed.

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