Speed of gravity found to be same as speed of light

Sunday, January 12, 2003

SEATTLE -- Einstein was right. The speed of gravity matches the speed of light, according to astronomers who took advantage of a rare planetary alignment to measure one of the fundamental forces of nature.

Edward B. Fomalout of the National Radio Astronomy Observatory and Sergei Kopeikin of the University of Missouri measured the amount that light from a distant star was deflected by the gravity of Jupiter as the planet passed in front of the star.

Albert Einstein, who formulated basic theories about space, time and relativity, had assumed that gravity moved with the speed of light, about 186,000 miles per second, "but until now, no one had measured it," said Kopeikin.

"Einstein was right, of course," said Fomalout.

Much not understood

The measurement is one of the last fundamental constants in physics to be established and Fomalout admitted, "gravity is not well understood."

The researchers used 10 radio telescopes scattered across the Earth from Hawaii to Germany to precisely measure how light from a distant quasar, a type of star, was bent as it passed by Jupiter on its way to the Earth. Jupiter is in the precise position for such a measurement only once a decade.

To make the measurement, the instruments had to detect a minute deflection of the light. Fomalout compared the required precision to being able to measure the size of a silver dollar sitting on the moon's surface, or measuring the width of a human hair from 250 miles away.

Craig Hogan, a University of Washington astronomer, said the achievement "is an important advance for physics," but he predicted that new techniques will be developed that will measure gravity's speed even more accurately.

"You can expect a series of experiments now," he said.

Fomalout and Kopeikin said their results are accurate within about 20 percent.

Knowing the precise speed of gravity is important to physicists testing such modern ideas as the superstring, which holds that fundamental particles in the universe are made up of small vibrating loops or strings. It also affects some basic space-time theories.

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