I have read this question:
where hawkeye says:
So what does that mean? It means that the "speed of gravity" is the speed of light … technically. Changes in the geometry of spacetime actually propagate at the speed of light, but the apparent effects of gravitation end up being instantaneous in all real-world dynamical systems, because things don't start or stop moving or gain or lose mass instantaneously for no reason. Once you factor in everything you need to in order to model a real system behaving in a realistic manner, you find that all the aberrations you might expect because of a finite speed of light end up canceling out, so gravity acts like it's instantaneous, even though the underlying phenomenon is most definitely not.
where Albert says:
Yes, in a sense, the field "instantly" moves together with it's source (if this source moves uniformly). That does not mean that the force propagates infinitely fast. The force on a test particle at any given instant is due to the electromagnetic field in the immediate vicinity of the particle at that instant.
where Ben Crowell says:
If something starts applying a force to the sun at a certain point in time, then there will be effects from this that will be detectable at the earth 8 minutes later.
So one says that when the source of the force field moves, the static field moves with it instantly. But this is not a violation of SR, since the field exists around the particle (that the field interacts with) already.
The other one says that the if something applies force at the Sun (the source of the gravitational field) then the static field's change will only be felt 8 minutes later on Earth.
This is a contradiction, because the static gravitational field already exist around the Earth too, so it is in the immediate vicinity of Earth, so when it moves instantly together with the source (Sun), we should feel it too on Earth.
Is this because EM static fields are different from gravitational, or are both fields working the same way in this context?
- Does the static gravitational field move with its source instantly?