How fast is Gravity?

25 May 2008

Question

If the light we see from the sun is eight minutes old what about the gravitational field? Does gravity travel at the same speed as light?

Answer

There was a recent experiment to look at just this sort of thing, which looked at how light was bent by the gravity of Jupiter. Our recent observations are yes, we think the gravity travels at exactly the same speed. If you plucked the sun from out of its orbit, just removed it completely - we would still orbit around the sun for eight minutes before suddenly the Earth realised that it wasn't there anymore and disappeared off into space. A wonderful statistic I heard from Brian Fulton, who was professor of Astrophysics at York, is that the light emerging from the sun (because the sun's so big and massive and has so much gravity) the light is already something like a million years old. Even if the sun went out tomorrow we'd still have a million years' more light.

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