Gravity and light move at the same speed

8 January 2003

Scientists have succeeded in measuring the speed of gravity for the first time. Sergei Kopeiken of the University of Missouri-Columbia and Ed Fomalont of the National Radio Astronomy Observatory in the US used a rare cosmic alignment to check that gravity and light travel at the same speed -- as predicted by Einstein. The astronomers presented their findings today at the American Astronomical Society meeting in Seattle.

On September 8 last year Jupiter passed almost directly between the Earth and the quasar J0842+1835. Kopeikin and Fomalont used the Very Long Baseline Array of radio telescopes in the US and a 100-metre radio telescope in Effelsberg, Germany, to measure how radio waves from the quasar were deflected by Jupiter. Previously they had shown that the size of the deflection depends on the speed at which gravity propagates from Jupiter. From their measurements Kopeikin and Fomalont calculated the speed of gravity to be 95% of the speed of light, with an error margin of plus or minus 25%.

Prior to this work, physicists had assumed that the only way to measure the speed of gravity was to detect gravitational waves. Kopeikin believes that this new result is the first of many observations of gravitation that will shed new light on the general theory of relativity.