No one has ever tested this prediction, even though the assumption that gravity travels in waves or gravitons with a finite speed underpins much of theoretical physics. The difficulty is that if light and gravity travel at the same speed, how can you hope to see evidence of gravity's speed?The answer, says Sergei Kopeikin of the University of Missouri, Columbia, is by watching a distant quasar as the planet Jupiter moves in front of it and its gravity bends radio waves from the quasar. This event is due to happen over the weekend of September 7 and 8. "When I first gave a talk about the idea everybody got excited and said we have to do this," says Kopeikin.
Subtly warped
Together with Ed Formalont of the National Radio Astronomy Observatory, Kopeikin will get data on Jupiter's eclipse of the quasar from the NRAO's Very Long Baseline Array, a series of ten 25-metre radio telescopes strung between the Virgin Islands and Hawaii. The data will show the changes to the quasar's radio image to an accuracy of more than a millionth of one degree.
Astronomers usually assume that so-called "lensing events" convert a point-like source like the quasar into a ring, as the radio waves are bent round the edges of the lensing object - in this case Jupiter. But this assumes that the gravitational fields are static, while the light or radio waves move.
So in 2001 Kopeikin decided to develop a more realistic model. In this, as Jupiter moves, the changing gravitational field interacts with the radio waves coming from the quasar. The calculations show that if gravity has a finite speed, the ring-like image seen at Earth will be subtly warped compared to the shape expected if the gravitational changes propagate instantaneously.
It was a neat piece of work, but Kopeikin never expected to be able to test his idea so soon. "These lensing events only happen about once per decade," he says. The results from the experiment should be known within two months.