Einstein proved right on gravity

It all adds up: Albert Einstein would be pleased

By Dr David Whitehouse BBC News Online science editor

The speed of gravity has been measured for the first time, revealing that it does indeed travel at the speed of light.

It means that Einstein's General Theory of Relativity has passed yet another test with flying colours.

The measurement was made by Ed Fomalont of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, and Sergei Kopeikin of the University of Missouri, in Columbia, both US.

Writing in New Scientist magazine, they say: "We became the first two people to know the speed of gravity, one of the fundamental constants of nature."

Higher dimensions

Isaac Newton believed the influence of gravity was instantaneous. Later, Albert Einstein assumed it travelled at the speed of light and built his 1915 General Theory of Relativity around that assumption.

If gravity travelled at the speed of light it would mean that if the Sun suddenly vanished from the Solar System, the Earth would remain in orbit for about eight minutes - the time taken for light to travel from the star to our planet. Then, in the absence of gravity, Earth would move off in a straight line.

Modern researchers say that knowing the speed of gravity is important in the study of branches of cosmology where the Universe has more spatial dimensions than the usual three.

Some of those theories suggest that gravity could take a short cut through higher dimensions and so appear to travel faster than the speed of light.

Jupiter's help

To measure gravity's velocity, Kopeikin determined that it could be determined with the help of the planet Jupiter, if its mass and velocity were known.

The perfect opportunity arose in September 2002, when Jupiter passed in front of a quasar - a distant, very active galaxy - that emits radio waves.

Fomalont and Kopeikin combined observations from a series of radio telescopes to measure the apparent change in the quasar's position as the gravitational field of Jupiter bent the passing radio waves.

From the observations the researchers determined that that gravity does indeed move at the same speed as light.

The results of the study have been presented to this weeks meeting of the American Astronomical Society (AAS) in Seattle.