[Southack]

"But how can you measure the speed of gravity? One way would be to detect gravitational waves, little ripples in space-time that propagate out from accelerating masses. But no one has yet managed to do this.

Measuring the speed of gravity Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity. The opportunity to do this arose in September 2002, when Jupiter passed in front of a quasar that emits bright radio waves. Fomalont and Kopeikin combined observations from a series of radio telescopes across the Earth to measure the apparent change in the quasar's position as the gravitational field of Jupiter bent the passing radio waves."

Then again, perhaps all that they really measured was the speed of the radio waves bending around Jupiter...

[Semper911]

perhaps all that they really measured was the speed of the radio waves...

Yes, you are on the right track, since radio waves travel at the speed of light.

Isn't that a co-inky-dink.

[Dan Day]

Then again, perhaps all that they really measured was the speed of the radio waves bending around Jupiter...

No, they took that into account (of course).

But the point is that the incoming radio waves would bend around Jupiter in slightly different ways if Jupiter's gravitational field "instantly" followed Jupiter around as Jupiter moved in its orbit, versus whether Jupiter's gravitational field lagged a bit behind it.

If instantaneous, Jupiter's "gravitational lens" (which was bending the incoming waves from the distant star) would be spherical, whereas if gravitation travelled at the speed of light, it would be subtly "conical", like the sonic boom coming off of a supersonic jet. The two different "lens" shapes would result in different kinds of "warping" of the incoming waves from the distant star, thus allowing the experimenters to answer the "is it or isn't it" question they were posing.

Pretty clever.

[Southack]

Post #6 is a blast from the past!