Another "speed of gravity" question.

[LivingInThePast]

@astro

The Speed of Gravity - What the Experiments Say

Gravity and light are somewhat different animals, and while it can be said that gravity “radiates” in a sense, it does not propagate in the same way that light does, so talking about the “speed” of gravity is a somewhat different concept than the radiant “speed” of light or other EM phenomena if you’re trying to apply a metric to the comparison.

@Ring

Something, someone, somewhere, may find interesting is that the gravitational force vector points to the actual position of the Sun rather than its retarded position. (Where the sun was 8 minutes ago). If this were not true stable orbits would not be possible. This is what forced Newton to postulate that gravity propagated instantaneously. (Action at a distance)

I guess I would like someone better versed than I to address the issue of whether orbits would be stable if gravity propagated non-instantaneously…

[LSLGuy]

@Ring

Something, someone, somewhere, may find interesting is that the gravitational force vector points to the actual position of the Sun rather than its retarded position. (Where the sun was 8 minutes ago). If this were not true stable orbits would not be possible. This is what forced Newton to postulate that gravity propagated instantaneously. (Action at a distance)

I am FAR from an expert in this area, but are you sure? That implies the Sun’s entire gravity well, out to infinity, moves instantly as the Sun moves. Any cite (net or book) for my further education would be appreciated.

[Optihut]

@Q.E.D.

How about this, from New Scientist:

Again I say “Wow”! That was enlightening; thanks for sharing!

[Ring]

Let me see if I can be a little clearer. Gravity does not propagate instantaneously. But the vector does point to the present position of the Sun. Not its retarded position. This forced Newton to postulate an “action at a distance” even though he knew this was ridiculous.

The same sort of phenomenon occurs with electromagnetism and without it there would be no such thing as an electromagnetic wave.

A lot of cranks attempt to use this fact to claim that gravity propagates greater than c, and a lot of legitimate scientists have shown that even though the vector points to the instantaneous position there is no violation of faster than light information transfer.

In effect what is happening is that, to first order, in E&M the actual position is extrapolated from the velocity of the source of the field, and in GR it is extrapolated from both the velocity and acceleration.

For E&M the easiest way to see this is to examine the change in the shape of the field envelope as the velocity increases and SR length contraction sets in.

[MrFantsyPants]

@Ring

But the vector does point to the present position of the Sun. Not its retarded position.

So, if the sun suddenly moved 20 feet to the left, and got 3 times bigger, objects around the sun would react to the change in position immediately, but not the change in mass?

I don’t see how this doesn’t violate causality. Can you explain please? (not a challenge, I’m just confused)

[Q.E.D]

@MrFantsyPants

I don’t see how this doesn’t violate causality. Can you explain please? (not a challenge, I’m just confused)

You misunderstand him. Ring is stating that under Newtonian physics, the gravitational vector has no correction for propagation velocity–it doesnt’ need one. If one tries to incorporate it, then calculated orbits become unstable. However, when we look at General Relativity, we see that the effect of propagation delay is actually almost exactly cancelled out, so that under most conditions, Newtonian gravity and GR gravity agree to a very close approximation. From here:

In general relativity, on the other hand, gravity propagates at the speed of light; that is, the motion of a massive object creates a distortion in the curvature of spacetime that moves outward at light speed. This might seem to contradict the Solar System observations described above, but remember that general relativity is conceptually very different from Newtonian gravity, so a direct comparison is not so simple. Strictly speaking, gravity is not a “force” in general relativity, and a description in terms of speed and direction can be tricky. For weak fields, though, one can describe the theory in a sort of Newtonian language. In that case, one finds that the “force” in GR is not quite central–it does not point directly towards the source of the gravitational field–and that it depends on velocity as well as position. The net result is that the effect of propagation delay is almost exactly cancelled, and general relativity very nearly reproduces the Newtonian result.

[Chronos]

And the speed of gravity is only meaningful if the mass and hence curvature is changing over time. Which generally doesn’t happen.

Not necessarily. You can’t change the total mass at all, but you can change the mass distribution, which will cause a detectable (in principle) change in the gravitational field, which would propagate outwards at c. These propagating disturbances are gravitational waves, and are analogous to the electromagnetic waves produced by changing charge distributions. We currently have several ground-based gravitational detectors, which are hoped to detect gravitational waves within a few generations of upgrades, and there are plans for space-based detectors which should detect them as soon as they’re turned on. Once we can detect gravitational waves from objects we can also detect electromagnetically, it will become very simple to measure the speed of gravity, since we’ll be able to just compare the arrival times of the gravitational and electromagnetic waves. Presumably, these two speeds will be the same. If not, somebody’s got some 'splaing to do.

[mangeorge]

A related question. I trying to comprehend “gravity”;
If I burn a log in my fireplace I wind up with ash, unburned gasses, water vapor, heat and light, and a lot less gravity. Where did it go? I’ve seen experiments (back in high school) where something was burned in a closed vessel and the vessel weighed less afterward. I know light and heat, but where did the gravity go to?

[Chronos]

If you burn your log in a sealed container, then it will weigh the same before, during, and after burning. For that matter, if you do anything at all (including set off nuclear bombs and annihilate matter and antimatter) in a truly well-sealed container, it’ll still always weigh the same. If your high school experiments indicated otherwise, then you had leaks in your container, or some other flaw in the experiment.

[mangeorge]

@Chronos

If you burn your log in a sealed container, then it will weigh the same before, during, and after burning. For that matter, if you do anything at all (including set off nuclear bombs and annihilate matter and antimatter) in a truly well-sealed container, it’ll still always weigh the same. If your high school experiments indicated otherwise, then you had leaks in your container, or some other flaw in the experiment.

Now that I think about it, maybe that is exactly what we were proving. HS was a long time ago.
Nevermind

[barbitu8]

@astro

The Speed of Gravity - What the Experiments Say

Gravity and light are somewhat different animals, and while it can be said that gravity “radiates” in a sense, it does not propagate in the same way that light does, so talking about the “speed” of gravity is a somewhat different concept than the radiant “speed” of light or other EM phenomena if you’re trying to apply a metric to the comparison.

That link also says that the speed of gravity is c [2x10] to the 10th power.

[RM_Mentock]

@barbitu8

That link also says that the speed of gravity is c [2x10] to the 10th power.

It says “Conclusion: The speed of gravity is >/= 2x10^10 c”

[Squink]

@RM Mentock

It says “Conclusion: The speed of gravity is >/= 2x10^10 c”

If it’s that fast, LISA will pick up a signal from Betelgeuse going supernova 520 years before the explosion lights up our skies.

[EllisDee]

@Squink

If it’s that fast, LISA will pick up a signal from Betelgeuse going supernova 520 years before the explosion lights up our skies.

Wouldn’t all the matter (and thus all the gravity) still be in the same position (relative to LISA) after the supernova as before?

[Squink]

@Ellis Dee

Wouldn’t all the matter (and thus all the gravity) still be in the same position (relative to LISA) after the supernova as before?

The explosion spreads the mass out, so that instead of one deep gravity dimple you have a bunch of smaller ones. That change is supposed to produce gravity waves:

Gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe, for example by the collision of two black holes or by the cores of supernova explosions. Gravitational waves are emitted by accelerating masses much as electromagnetic waves are produced by accelerating charges. These ripples in the space-time fabric travel to Earth, bringing with them information about their violent origins and about the nature of gravity.

(LIGO)

[mangeorge]

Been doing a little outside reading, and what’s on this thread. There’s a lot we don’t know about gravity, isn’t there.