Quote:
Originally posted by Nisses
stingc:
if an accelerated object emits a gravitational radiation, and for a planet to revolve around the sun, it takes a constant accelaration towards the sun, does that mean that the earth produces gravity in 2 kinds? as in: acceleration gravity and mass-based gravity?
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That's right. The 'acceleration gravity' is immeasurably small in the case of the earth though. It takes a much more extreme situation for the effect to become significant.
There are systems with two very small (but massive) stars orbiting each other once every few hours (!) where the acceleration is enough to cause an observable energy loss. Hulse and Taylor were given a Nobel prize a few years ago for showing that this happened in exactly the amount predicted.
denim,
The speed of gravity is yet another thing that is hard to define. You measure how fast something is going by finding how far it goes in a given time. Since gravity is itself changing the nature of space and time as it goes along, its hard to think of what it would mean to define a speed for it.
It only makes sense if we ask what is the speed of a very weak gravitational wave, so geometry remains almost constant as the wave passes. In that case, relativity predicts that the speed of gravity has components travelling at the speed of light and all slower speeds in general. In the solar system, the components moving at less than the speed of light would be negligible, but they're important in more extreme situations.