
Originally Posted by
Leszek Luchowski

Originally Posted by
Janus
You seem to be thinking that there might be some type of "aberration of gravity", where the felt force of gravity comes from where the source was, not where it is. This will not happen. The short reason for this is that effects from General Relativity cancel out any tendency for this to occur.
I find this strange, because it would mean - in my opinion - that gravity transmits information instantenously, making a body at point A "feel" the presence (or absence) of another body at point B at the very moment when that other body is (or is not) there.
I thought such instantenous transmission of information was impossible, precisely because of the Theory of Relativity.
As I don't understand TR I am probably wrong, but I am curious to know where I err.
What happens is that certain GR effects due to velocity "cancel out" the effect of aberration.
This causes the effect of gravity to come from the where the source "is". The thing is this only works as long as the velocities are uniform. It's kind of like leading a target. You can hit a moving target, not because you aim at where it is, but where you know it will be when the bullet gets there, so I don't need my bullet to travel at an infinite speed to hit a moving target (this is a
really loose analogy).
Of course, if your target makes a sudden change of velocity after you pull the trigger, you will miss.
The same holds for cancellation of gravity aberration. It works if the velocity between the objects is uniform. If the relative velocity of the source suddenly changes, there will be a propagation delay before you would notice.
With objects in orbit, the relative velocity, on average, remains constant and you get no net aberration.
If this were not the case, no orbits would be stable. Consider the Earth. If the above mentioned effects did not cancel out aberration, the Earth would hit the Sun's gravity at an angle. There would be a cumulative forward component that would accelerate the Earth forward in its orbit, causing it move out from the Sun at a fairly good rate.