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Aracanth
2008-Jan-19, 06:24 PM
Hi Guys,
Another theoretical question I am afraid :-)

I was asked yesterday: "If the Sun were to vanish (dunno why this would happen, lets assume Aliens steal it), we would notice light go after 7 minutes (approx). BUT, how long would the effect of gravity take to go?" I guess the question simply asks, "At what speed does the effect of gravity travel"?? Anyone got any ideas? Is it c??

What would happen to us? Does the Earth now travel straight on, or would we start to orbit around the next large mass, maybe Jupiter? Or is that not enough mass for us........

Cheers Again,

Ara

Tim Thompson
2008-Jan-19, 07:10 PM
The speed of gravity has been discussed numerous times in the BAUT forums. Here is one example: Speed of Gravity (http://www.bautforum.com/general-science/16377-speed-gravity.html), but there are more no doubt.

Theory and observation are consistent with each other. General relativity holds that the speed of gravity is the speed of light. Observation of the change in period of binary pulsars is consistent with the interpretation of energy loss through the generation of gravitational waves, which in turn depends on the finite speed of gravity.

If the sun just "goes away", it is most likely that the planets would simply fly off into space in whatever direction they are going. It is unlikely that Earth would begin orbiting anything else.

alainprice
2008-Jan-19, 10:19 PM
That is one gravitational wave that we'd be sure to detect.

Ken G
2008-Jan-20, 02:48 AM
The problem with making the Sun "go away" is that it disobeys the very laws of physics you are asking about when you discuss gravity. Gravity comes from energy, and the energy of the Sun is not allowed to just "go away", that would not conserve energy. So you'd actually have to explode the Sun, or some such thing, but note you could never do that faster than the speed of light anyway. You could get the Sun to wobble and spit out gravitational waves-- those should travel at c also.

Arcane
2008-Jan-20, 08:37 AM
There you go again Ken ruining a perfectly reasonable question.

Ok, let's agree that the sun cannot just disapear into nothing, but for the sake of argument a worm hole appeared next to the sun and something sucked it in and now it is in a nother place in the universe ok? Or do you want to argue that worm holes don't exist and that is simply impossible?

SHEESH!

Haha, I'm just kidding man, but you get the point.

BTW, you don't play the flute by any chance do you?

astromark
2008-Jan-20, 09:58 AM
NB.;) I am in full agreement with Ken; He has not spoiled a good question he has simply pointed out the very laws of this universe dictate that matter can not simply vanish... It can be changed or even transformed into energy. Tim Thompson has show en you that this question was thrashed around not to long ago else where in this forum.., and finally I am suggesting that yes. Gravity is the end result of Mass Which is or can be energy, and that yes c seems to be the most obvious speed for this force to travel time over distance...:)

Ken G
2008-Jan-20, 04:26 PM
Haha, I'm just kidding man, but you get the point.
I and astromark are just saying that when a physics question is posed about some hypothetical situation, the situation has to be possible within that physics or the answer will make no sense. The very gravity you are asking about makes it impossible for a white hole to "just appear". You see, Einstein's gravity is a theory in which there is no action at a distance-- what happens to spacetime is all connected, so you can't just rip it open to pull out the Sun-- you can't arbitrarily suspend the physics in one part of the question and still get a valid answer. Indeed, elimination of action at a distance is one of the great triumphs of general relativity. To do that some other way, you'd need a quantum view, with gravitons and whatnot, but I'm unclear on the status of theories like that.

alainprice
2008-Jan-20, 05:41 PM
Ok, if the moon suddenly exploded into dust(and expanded throughout the solar system), how long would tidal waves continued to be generated by the moon? Is it 1 or 0 seconds?

I'm with the camp that says 1 second.

Ken G
2008-Jan-20, 05:51 PM
When you ask a question like "what happens if I explode the Moon", the only way to answer such a question scientifically is to do the experiment. If you can't or don't want to do the experiment, you can ask a slightly different question: "what does such-and-such theory, which we have every right to expect to apply, predict would happen if I explode the Moon". The latter is really what you are asking, but then you actually have to apply that theory to get the answer. If you exploded the Moon, you would have to track what that exploding mass was doing to determine what would happen to the tides. You'd have to solve the dynamical equations of general relativity. It would certainly take far longer than 1 second for the tides to change-- it would take far longer than that to disperse the Moon over the scale of the Earth-Moon distance. Don't get me wrong, it is certainly valid to say "the signals of gravity move at speed c, not instantaneously", but you can't verify that by doing thought experiments that violate the very laws you are trying to explain.

John Mendenhall
2008-Jan-24, 05:57 PM
Ken G is right. When doing thought experiments, or attempting to answer questions about someone else's thought experiment, the first thing to do is to make sure that the experiment could actually be done. For example, spaceships traveling at near c are allowable; traveling at c is not. Instantaneous changes in velocity (jumping between meeting spaceships) are not allowable. And there are more subtle things, as Ken G calls attention to above. The sun cannot disappear, fun though it is to think about it. (I think this is an eight minute case, since it lies outside GR. Oh, Lordy, I've gone and tried to answer a bad question!)

Point is, in a good question the conditions need to be possible within the laws of physics. That way, the posts can thrash out a good answer eventually.

Good question = good answer.

dgavin
2008-Jan-26, 05:48 PM
Well there is a kink in all this. Even if Gravity is driven by a Particle/Wave function, the warpage of space itself due to gravities influance is not constrained to any speed limit. GR and QM both allow for space/time to be warped, bent, twisted, etc...

Where binary stars inteacting are a good indication of the speed limit of gravitational generated waves (which are -not- gravity) and frame dragging, it doesn't really apply to the effects of gravity on space.

QM and Quantum Tunneling and entanglement are also a good indication that there are properties and effects of matter end energy that can trasmit at extreeme speeds, faster the light.

If you hold to GR, then gravity of an object is not trasmited to another object. It's trasmitted to space/time which gets warped in all four dimensions in response. Objects in that warp will follow it's curvature.

It's all a bit convoluted still, and it will be a long time before science can really answer what the true speed of gravity transmision is verses the true speed that space warpage from gravity is.

publius
2008-Jan-26, 07:10 PM
Well there is a kink in all this. Even if Gravity is driven by a Particle/Wave function, the warpage of space itself due to gravities influance is not constrained to any speed limit. GR and QM both allow for space/time to be warped, bent, twisted, etc...

Where binary stars inteacting are a good indication of the speed limit of gravitational generated waves (which are -not- gravity) and frame dragging, it doesn't really apply to the effects of gravity on space.



Actually it does. Changes in the curvature of space-time propagate at the same speed light does -- they follow null paths through the space-time.

A gravitational wave is one type of propagating change in space-time, in the same way EM radiation is a type of propagating change in the EM field. When you wiggle a mass around, the changes in space-time due to that wiggling propagate out, at 'c', in the same manner that wiggling a charge around propagates changes in the electric field.

When you wiggle a charge, you get a very small radiation field, by the definition of radiation field. However, the basic Coloumb field of that charge wiggles around too, it's just not a radiation field.

And the same with GR gravity, it's just the equations that describe the wiggling are much more complex for gravity than EM.

For example, suppose I accelerate away from a big mass, say a star, along a Rindler hyperbola. There is a point beyond which light from the star will never catch me -- it will get asymptotically close according to rest frame of the star, but in the accelerating frame, one never sees it.

Now, suppose the star blows up and scatters its mass to the four winds. That gravity well just spreads out and dissipates to nothing. But in my accelerating Rindler frame, I never see the change in space-time because it will never catch me.

-Richard

publius
2008-Jan-26, 07:19 PM
And BTW, here is a paper by Steve Carlip which I think should be the standard reference for the speed of gravity questions that pop up here with some regularity:

http://arxiv.org/abs/gr-qc/9909087

The main point of that paper is to show how GR gravity pulls a clever little trick in the weak field, low velocity limit of making it look like instantaneously updating Newtonian gravity to high order. And then he goes on to show that it really isn't some strange trick, because EM itself does it, but just not as good as GR gravity. That difference is in the dipole vs quadrapole order of radiation.

-Richard