Is gravity instant?

by zsweyd
Tags: gravity, instant
Thanks
P: 1,948
 Quote by MikeGomez I’m not understanding this “moving at a constant velocity” concept as it applies to the motion of planets. As they have elliptical orbits, are they not undergoing constant acceleration? Wouldn’t that indicate the constant velocity argument in not valid in the case of celestial bodies? If gravitation travels at the same speed as light, it makes sense to me that the effect of gravitation would be exactly in line with our visual interpretation, meaning we should see the sun and feel it’s gravitational effect both at the same time, and at its retarded position. It seems to me that to say otherwise is to indicate a separation from the rays of light from the sun with the rays of gravitation (whatever form that may take).
Yes, Naively one would think that. But turns out both electromagnetic waves and gravitational waves seem to emanate from the retarded position but the electric field of a moving charge and the gravitational field of a moving mass point to the actual position of the object if the object is moving at constant speed. If the object accelerates than just pretend it doesn't and use the speed at the retarded location to extrapolate where the object would've been had it not been accelerating and that's where the fields point to. To understand that somewhat surprising fact one must thoroughly study the Liénard–Wiechert potentials and the fields derived from them.
P: 4,079
 Quote by MikeGomez I’m not understanding this “moving at a constant velocity” concept as it applies to the motion of planets. As they have elliptical orbits, are they not undergoing constant acceleration?
The key for planet orbits is how the Sun moves. The changes in direction of the planets are relevant for their moon orbits.

 Quote by MikeGomez Wouldn’t that indicate the constant velocity argument in not valid in the case of celestial bodies?
It's an approximation, based on the assumption that the velocity of the source doesn't change much during the propagation duration.

 Quote by MikeGomez If gravitation travels at the same speed as light,
Changes in gravity propagate at the same speed as changes in the EM-filed. Gravity itself doesn't really "propagate", just like a the E-filed doesn't "propagate".

 Quote by MikeGomez it makes sense to me that the effect of gravitation would be exactly in line with our visual interpretation, meaning we should see the sun and feel it’s gravitational effect both at the same time, and at its retarded position.
But that's not the case. If the Sun was charged, the E-force would be towards the current position, and so is gravity.

 Quote by MikeGomez It seems to me that to say otherwise is to indicate a separation from the rays of light from the sun with the rays of gravitation (whatever form that may take).
Yes, for EM there is this separation: If the Sun was charged, the E-force would be towards a different direction that the visual image of the Sun. Similarly for "gravitational pull" and gravitational waves.
 P: 211 Thanks duato and A.T. Also, after reading the reference by Stevendaryl (and Wiki), my understanding of gravitational waves is that they are the result of the third time derivative of position of a massive body (jerk). Is that correct?
 C. Spirit Sci Advisor Thanks P: 5,639 No. In the linearized approximation, the power radiated is related to the third time derivative of the mass quadrupole moment so it isn't that simple. This is akin to electromagnetic dipole radiation wherein the power radiated is related to the second time derivative of the electric dipole moment. Gravitational waves themselves just arise from linear perturbations of a background curved or flat space-time through the resulting wave equation.
P: 490
 Quote by jtbell Why "therefore"?
Not quite sure , if it flies like a duck etc, but as duato points out the speed of light isn't an electromagnetic quality?