What is the Speed of Gravity?

<i>The Sun, as it is right now, won't have its gravity affect Earth for another 8+ minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8+ minutes ago! (Weird, isn't it?)</i><p>I'm almost positive this statement is incorrect.<p>Relativistic force laws tend to be forced to contain correction factors that ensure that constant velocity motion is "predicted" and the direction of the force is adjusted accordingly - as long as a body is not accelerating, a purely attractive or repulsive force will be pointing at its current position, <i>not</i> its position 8 mins ago.<p>To see why this must be the case for a repulsive force, at least, imagine two charges riding on frictionless rails that keep them at a constant, finite distance. Suppose they're both moving with some constant velocity (to start, at least) in the same direction - now the place that the force from the other charge <i>appears</i> to be coming from is behind the charge, so if there was no correction factor, each charge would be getting an extra push forwards. This would lead to a runaway "bootstrap" acceleration, and the particles would accelerate to the speed of light. That's a pretty clear violation of the conservation laws, so...<p>With attractive forces like gravity, there's still a conservation problem, since the charges would slow down to zero speed eventually, but it's always more convincing to cite the runaway solution as a violation of conservation laws than the run-down one, because while the energy could possibly leak out of the charges into the fields, there's nowhere to pull infinite energy from, so it's pretty clear there's a problem if we'd need to.<p>And this is somewhat different from the runaway self-action solutions that we grudgingly "accept" in E+M, because those tend to involve some limit to infinitesimal size, whereas this is a completely finite situation that we could theoretically set up in the real world with a couple of charged beads or something like that.

The point of the article is in<p>Clifford M. Will<p>"The Confrontation between General Relativity and Experiment" (2001)<p><a href="http://relativity.livingreviews.org/Articles/lrr-2001-4/" rel="nofollow">http://relativity.livingreviews.org/Articles/lrr-2001-4/</a><p>"Gravitational wave damping has been detected in an amount that agrees with general relativity to half a percent using the Hulse-Taylor binary pulsar,"<p>I also have something to say to the first question in comments there, to which the article author answered that he can't answer. The question was "Is the speed of gravity reduced by the medium through which it travels in a analogous manner to the slowing of the speed of light through various media?"<p>I was lucky enough to read Feynman Lectures recently, so as far as I understood him, the photons themselves don't really slow down in various media. That is, what we see as the result is the slowdown, but only because it's a new light (new photons) on the other side of the media. The photons on the other side are photons which were pushed out of the atoms of the media, that's why it appears that they come out slower. Inside, between the atoms of the media, photons still move at the speed of light.

I've always thought what we call 'speed of light' is the indirect observation of a structural property of (this) the universe. A property that has something to do with movement in general and not only the movement of light (the movement of space itself?).<p>Another interisting property is the 0º Kelvin (absolute zero) <a href="http://en.wikipedia.org/wiki/Absolute_zero" rel="nofollow">http://en.wikipedia.org/wiki/Absolute_zero</a>

Reminds me of a thought experiment that's always bugged me: if you had an incredibly long see-saw or lever, like the length of the galaxy, would movement at one end instantaneously be matched by movement at the other end? (I think the reason it bugs me is because the object itself is preposterous, but I still want to know)

I am not a physicist nor an astronomer. I wanted to ask however, when they explain gravity by the analogy of holding a blanket, first the blanket is on earth thus any object that is put on it is being pulled down by earth and the blanket is being held by two people, thus what is pulling the earth and sun down and who is holding the space blanket? Also, seeing some of the images on the site, it is clear that space is something and it is not nothing. Thus, what is space?<p>The second thing is that if you carry out the above experiment, the ball would eventually stop. That is because of friction. Why does the earth never stop? What makes it overcome friction?<p>Someone is probably going to shoot me down for being a complete idiot and knowing nothing, but I am genuinely interested to know the answer or to know if there is no answer.

It always seems rather low-class to take shots at Newton as if he was a fool for coming up with Newton's theory of gravity instead of Einstein's.

I hate that curved space graphic. Now you have some mysterious gravity pulling both the sun and earth down. I wish someone would come up with a better example to show gravity as curved space.

This is all well and good, but what the article is talking about is the speed of the propagation of gravity waves. I've never heard anyone who wasn't a crank argue that gravity waves travel at anything other than the speed of light. As the article confirms, observation of pulsars has confirmed the speed of gravity waves to very close to the speed of light.<p>However, I think the larger objection goes something like this:<p>Gravitational attraction contains information about the location of an object (say, for the purposes of argument, a singularity). Information may not travel faster than the speed of light, by relativity. Information may not escape the event horizon of a black hole, because to do so would require it to travel faster than the speed of light (or to use some funny quantum teleportation that Hawking describes as the mechanism behind Hawking radiation), but which is not described by relativity.<p>Therefore it follows that the information about the location of a singularity behind the event horizon of a black hole is somehow travelling "faster than light". This is of course impossible if relativity is correct, which leads to a big WTF?<p>My guess, and I am most emphatically not a physicist, is that you get some funny macroscopic quantum effects near a black hole, which allows Hawking radiation (and therefore also the encoded information about the objects that fell into the black hole) out, and also lets out the information about the location of the singularity itself, so that objects outside the event horizon can be attracted to it.<p>Perhaps studying black holes in sufficient depth (pardon the pun) will allow us to finally unify QM and relativity.

The article asks what would happen to gravity if you suddenly "removed" the sun. My question is- is that even possible? How could you instantaneously "disappear" a giant blob of mass/energy? Is this kind of like trying to divide by zero? I'm not being facetious, just wondering if the whole idea is predicated on an impossibility.

Personally I think it's easier to tell people to stop thinking of gravity as a particle or wave - it's not like a beam of light or radio wave.<p>Think of the space/time as the blanket the universe is wrapped in. If that blanket is stretched out like a trampoline, every object on it or moving around on it is causing flexing and ripples. That's gravity.<p>When a girl bounces off a trampoline, the specific instance of her gravity is gone but the ripples from that instance continue on for some time until everything "settles".<p>But apparently the goo that is space/time really does not tolerate anything going faster than the speed of light for some reason. Doesn't mean things aren't trying to go faster than that, they just cannot achieve it.

Something I wonder about gravity - While it is often mentioned that objects with a large gravitational force can 'bend' light (gravitational lens), can a sufficient amount of light 'pull' objects towards itself?<p>So I wonder if a star's gravity is mostly the mass of the star itself, but also, in small part, the huge amount of light that surrounds the star on all sides, being densest nearest the star itself.<p>There's some evidence that gravity may change a little during a solar eclipse <a href="http://en.wikipedia.org/wiki/Allais_effect" rel="nofollow">http://en.wikipedia.org/wiki/Allais_effect</a> and also the Pioneer anomaly, <a href="http://en.wikipedia.org/wiki/Pioneer_anomaly" rel="nofollow">http://en.wikipedia.org/wiki/Pioneer_anomaly</a><p>I am most definitely not a physicist!

My first thought was 'Find a way to create some matter and we'll measure it', but apparently they have a way. ;)

The speed of gravity is either infinity or millions of times faster than the speed of light. If the speed of gravity was the speed of light, then heavy objects passing near the earth would have more gravity than objects passing away. Less of the gravity beans would be arriving to be measured. Like the red shift in light or the doppler effect with sound.<p>I see gravity as more of a property of the universe, every atom in the universe is attached to each other atom in some way.