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US Astronomers Measure Speed of Gravity for First Time - 2003-01-08

U.S. astronomers have measured the speed of gravity for the first time. To do so, they took advantage of a rare alignment of Earth, Jupiter, and a far off quasar, a celestial object resembling a star.

A falling apple is said to have caused the 17th-century English scientist Isaac Newton to think seriously for the first time about gravity, the attraction between objects. Newton said that the bigger an object, the more gravitational pull it exerts on another object. He also believed gravity to be an instantaneous force, taking no time to travel between objects.

But Albert Einstein thought otherwise in his 1916 treatise on relativity. He suggested that gravity travels at the speed of light, about 300,000 kilometers per second.

Now, astronomers from the University of Missouri and the National Radio Astronomy Observatory in Virginia say Einstein was right.

To determine this, they observed Jupiter passing in front of an extremely bright heavenly body called a quasar in September. Quasars are the most brilliant known objects in the heavens. They are thought to be the release of energy by matter as it is sucked into black holes at the center of galaxies.

The instruments the researchers used to observe the rare Jupiter-quasar alignment were 11 radio telescopes scattered from Hawaii to Germany all linked together.

The scientists told the American Astronomical Society conference in Seattle that they measured radio waves emitted by the quasar as Jupiter moved in front of it. They noticed that Jupiter's massive gravitational force bent the waves very slightly during its passage. The deflection caused the radio telescopes to sense the quasar's position to be slightly different when Jupiter moved in front of it than it actually was.

This bending allowed them to calculate the speed of gravity as virtually the same as the speed of light, plus or minus 20 percent. The researchers say if the speed of gravity were instantaneous, the amount of bending would have been different.

The researchers said the precision involved in this observation was like measuring the size of a large coin on the moon's surface, or the width of a human hair from a distance of 400 kilometers.