scientists have at last measured the speed at which the force of gravity moves. A landmark experiment shows its speed is similar to that of light (300,000 kilometres per second). The find supports Einstein's theory of relativity. Assuming that gravity moves at the speed of light, Einstein evolved the theory of relativity, according to which speed of no substance in the Universe can exceed the speed of light.
The speed of gravity and light being the same implies that if the Sun were to suddenly disappear from the centre of the solar system, the Earth would remain in the orbit for about 8.3 minutes - the time it takes light to travel from the Sun to the Earth. Then, suddenly feeling no gravity, the Earth would shoot off into space in a straight line.
But how can you measure the speed of gravity? One way is to detect gravitational waves that are transmitted by accelerating masses. However, no one has managed to do this till date. Ed Fomalont of the Virginia-based National Radio Astronomy Observatory and Sergei Kopeikin from the Colum-bia-based University of Missouri found another way. They reworked the equations of the theory of relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. In other words, if you could measure the gravitational field of a planet while knowing its mass and velocity, you could work out the speed of gravity.
The opportunity to do so arose on September 8, 2002, when Jupiter passed in front of a quasar (an object that emits bright radio waves). The planet's gravity shifted the quasar's radio signals. This resulted in a small change in the quasar's apparent position in the sky. The change (equivalent to the width of five human hairs) depended on the speed at which gravity propagated from Jupiter. Fomalont said there could be a 20 per cent uncertainty in the measurement due to distortions caused by clouds and air turbulence. For most cosmologists, it is a relief to have a fundamental assumption of physics corroborated by science.