When Newton proposed his original mechanics, he had point particles exerting forces at each other instantaneously over great distances. Many people, including him, were bothered by this. The introduction of a gravitational field provides a medium of sorts for broadcasting that information, but changes in positions of objects still instantly affects the field infinitely far away. In general relativity, the currently accepted theory, the gravitational field is equated with the curvature of space-time, and propagations (including [gravity wave]?s) can be shown to travel at the speed of light. A faster speed would violate causality. It is well known that the speed of gravity can't be appreciably less than the speed of light, since that could be detected by changes in the angular momentum of the planets.
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