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Is gravity instant?

by zsweyd
Tags: gravity, instant
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dauto
#19
May26-14, 07:44 PM
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Quote Quote by MikeGomez View Post
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.
A.T.
#20
May27-14, 03:39 AM
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Quote Quote by MikeGomez View Post
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 Quote by MikeGomez View Post
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 Quote by MikeGomez View Post
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 Quote by MikeGomez View Post
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 Quote by MikeGomez View Post
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.
MikeGomez
#21
May27-14, 05:52 AM
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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?
WannabeNewton
#22
May27-14, 07:42 AM
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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.
Buckleymanor
#23
May28-14, 04:15 AM
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Quote Quote by jtbell View Post
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?
stevendaryl
#24
May28-14, 05:31 AM
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Quote Quote by Buckleymanor View Post
According to Newton the gravitational effects would be instant.Einstein predictions would have gravity propergate at the speed of light.
To do that the mass-less particle, the graviton, would have to exist as only these along with other particles that don't have any rest mass travell at the speed of light.
For all that to happen you would have to have a correct theory of quantum gravity, which at present
we don't.
So at present it's all a bit speculative.
You don't need a quantum theory of gravity to predict that it propagates at the speed of light. General Relativity predicts that.


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