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Author Topic: At what speed does gravity propogate?  (Read 7469 times)

Offline realmswalker

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At what speed does gravity propogate?
« on: 02/06/2005 04:46:45 »
I donno? can anyone give an insight please (ive heard speed of light or instantaneous)


 

Offline gsmollin

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Re: At what speed does gravity propogate?
« Reply #1 on: 02/06/2005 14:04:28 »
quote:
Originally posted by realmswalker

I donno? can anyone give an insight please (ive heard speed of light or instantaneous)



The answer depends upon what you mean by "propagation". If you are refering to gravity waves, i.e. gravitons, they propagate at c. However, gravitational radiation requires a mass quadrupole moment to radiate. So there are negligible gravitons coming from the sun, since there is neglible quadrupole moment in the solar system. The earth orbits the sun about its true location, not the location we observe when we look at it (using solar filters, of course). We see photons, which radiate from the sun and take 8 minutes to get here, so we see the sun as it was 8 minutes ago. The earth orbits about the instantaneous location of the sun. No gravitons needed, the spacial curvature necessary to orbit the sun is already in place, much like a road for the earth to follow.
 

Offline DoctorBeaver

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Re: At what speed does gravity propogate?
« Reply #2 on: 02/06/2005 14:43:00 »
This ties in with something I've been waffling about in another thread.
If you take 2 plates that intersect at a right angle and apply a force to 1 of them causing it to move, the entire other plate will also move. That will also be the case if you have a plate in each of x, y & z axes of 3D space. Now imagine a plate existing in a further dimension that intersects the other 3. Any movement of that plate will affect all 3 others simultaneously.
So, imagine that the 4th plate is gravity. it doesn't matter where on that plate a movement occurs, it will affect all the others simultaneoulsy. C is not violated.
(I know what I mean but I'm having difficulty explaining it)
 

Offline gsmollin

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Re: At what speed does gravity propogate?
« Reply #3 on: 03/06/2005 13:57:58 »
You have to be careful with the "simultaneous" word. If you have a material, and you move it, the movement cannot propagate through the material faster than the speed of sound in that material. In the case of gravity, the movement of a mass *may* produce a quadrupole moment, and gravity waves would be required to transmit the change in space curvature. That change would propagate at c.

So for instance, in the hypothetical situation where the sun disappears, the earth orbits about the old position of the sun for 8 minutes, then reacts to the change in gravitation. There would have been massive gravity waves caused by the disappearance of the sun.

Something similar is being sought by gravity wave researchers as they seek to find gravity waves with their gravity wave telescopes.
 

Offline DoctorBeaver

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Re: At what speed does gravity propogate?
« Reply #4 on: 03/06/2005 14:14:49 »
Maybe my analogy wasn't very good. I wasn't talking about physical plates (that's just the way I was visualising it), nor do I mean propogation. If gravity exists in a curled-up dimension that is subatomic in size, it could still touch every point in the 3- or 4-dimensional universe we can percieve. As such, a tiny movement in that dimension would affect a much larger part of our visible universe. A Planck-sized movement could affect, say, a cubic parsec apparently instantaneously but C would not have been violated.
 

Offline gsmollin

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Re: At what speed does gravity propogate?
« Reply #5 on: 04/06/2005 21:40:36 »
I don't know what the properties of curled-up dimensions are. I had enough problems with the 4 that did expand. As far as I know, gravity is included in the 4 large dimensions. I also know that quantum gravity researchers use 11 dimensional mathematics to describe their theories. What nobody is too sure of is if those dimensions really exist. Just because the mathematics works, doesn't mean nature used it, and we don't even know if the math works. So to recap, I don't understand what you are proposing.
« Last Edit: 04/06/2005 21:42:40 by gsmollin »
 

Offline DoctorBeaver

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Re: At what speed does gravity propogate?
« Reply #6 on: 05/06/2005 14:01:51 »
quote:
So to recap, I don't understand what you are proposing.

Neither do I! [V]
It's just something that's nagging at the back of my brain.
I saw a science program on TV a while back where they tried to explain how an infinitessimally small extra dimension could touch every point of the 3D universe we see. It just occurred to me that if that were the case, a movement in just a tiny part of that dimension could have immense effects in the 3 dimensions we see.
Imagine a cone with the large end in contact with a rubber sheet. By applying a force to the point of the cone a much larger area of the rubber sheet is affected than the area on the cone to which the force was applied.
Oh, I'm confused
 

Offline gsmollin

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Re: At what speed does gravity propogate?
« Reply #7 on: 05/06/2005 16:15:10 »
Of course you are. Who can say for sure what the properties of 7 "curled up" dimensions would be? I would expect that to mean they are all independent of each other, and have no effect on each other, let alone the 4 big ones. Somebody is saying the opposite, that they are all connected, and move together. What I'm not seeing is the way they are connected. But maybe its still true, and maybe it expalins QM absurdities like entanglement, and "spooky action at a distance". Maybe we're all clutching at straws.
 

Offline DoctorBeaver

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Re: At what speed does gravity propogate?
« Reply #8 on: 05/06/2005 17:27:48 »
Maybe everyone else is as stupid as me!
 

Offline chimera

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Re: At what speed does gravity propogate?
« Reply #9 on: 05/06/2005 20:27:53 »
Only differently. :)

Maybe those extra dimensions are to provide 'leverage' for the action in the other 4 dimension where movement IS allowed. So they would maybe not be 'fixed' by nature, but only 'locked'.

The living are the dead on holiday.  -- Maurice de Maeterlinck (1862-1949)
 

Offline DoctorBeaver

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Re: At what speed does gravity propogate?
« Reply #10 on: 05/06/2005 21:27:31 »
" I vill a little tink haf". heh
 

Offline chimera

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Re: At what speed does gravity propogate?
« Reply #11 on: 05/06/2005 21:55:04 »
And probably another Stella... :)

The living are the dead on holiday.  -- Maurice de Maeterlinck (1862-1949)
 

Offline DoctorBeaver

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Re: At what speed does gravity propogate?
« Reply #12 on: 05/06/2005 22:05:58 »
Indeed! :D
 

Offline Sandwalker

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Re: At what speed does gravity propogate?
« Reply #13 on: 16/06/2005 14:03:01 »
Is not the 4 dimensions we experience, curled up for the photon and similar wavicles (from there existence/experience (If they do))?!

Its all matter of perception!
 

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Re: At what speed does gravity propogate?
« Reply #14 on: 17/06/2005 11:46:41 »
Sandwalker - In effect that may well be the case. Travelling at C means the entire life of the universe will pass in zero time from the perspective of a photon. It follows, then, that motion means nothing to a photon as motion involves travelling a certain distance in a finite time. So, if a photon were sentient, would it percieve the entire universe as a point of zero size or itself as infinite in size? Would it ever realise what it was missing having never gone and got completely blathered then woken up the following morning next to a real minger? :D
 

Offline qpan

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Re: At what speed does gravity propogate?
« Reply #15 on: 28/06/2005 16:42:35 »
quote:
Originally posted by gsmollin

quote:
Originally posted by realmswalker

I donno? can anyone give an insight please (ive heard speed of light or instantaneous)



The answer depends upon what you mean by "propagation". If you are refering to gravity waves, i.e. gravitons, they propagate at c. However, gravitational radiation requires a mass quadrupole moment to radiate. So there are negligible gravitons coming from the sun, since there is neglible quadrupole moment in the solar system. The earth orbits the sun about its true location, not the location we observe when we look at it (using solar filters, of course). We see photons, which radiate from the sun and take 8 minutes to get here, so we see the sun as it was 8 minutes ago. The earth orbits about the instantaneous location of the sun. No gravitons needed, the spacial curvature necessary to orbit the sun is already in place, much like a road for the earth to follow.



Gsmollin - surely the earth is rotating about the sun at its position 8 minutes ago and not its instantaneous position? Surely the speed of speed limit of c also applies to the speed that space can warp?
If you suggest otherwise then surely you are violating one of the fundamental laws that infomation cannot be transmittted faster than the speed of light? I'd count the position of all the molecules of the Sun to be a lot of information!


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Offline gsmollin

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Re: At what speed does gravity propogate?
« Reply #16 on: 28/06/2005 18:34:56 »
I'm reasonably certain this is true. Gravitons propagate at c, but they are only radiated from a gravitational quadrupole. A rotating sun contains no such quadrupole, so there is no gravitational radiation. There is also no information involved. Newtonian gravity was "action at a distance", and implicitly propagated at an infinite speed. Relativistic gravity is a spacetime curvature. The space around the earth is already curved by the sun's mass, and the space has been curved for billions of years. The earth is just following the shortest path through this space, which is an ellipse.

If the sun were to "vanish", the earth would continue to orbit its old position for 8 minutes, until the gravitational radiation arrived that would flatten the space around the earth. Then the earth would travel off at a tangent to its old orbit. If the sun vanished, a huge quadrupole would be formed. Such an event would be measurable by gravitational detectors for light years in every direction. However, I don't expect that to happen.
« Last Edit: 28/06/2005 18:48:07 by gsmollin »
 

Offline qpan

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Re: At what speed does gravity propogate?
« Reply #17 on: 28/06/2005 19:40:44 »
Bearing in mind that the sun is moving in space, the curvature in spacetime must also move, so the space around the earth has been curved for billions of years, but the curve is constantly moving in space. The sun is affected by various gravitational fields in the galaxy and beyond, and so the curvature would also vary with time. I find it incredibly hard to believe that change in spacetime curvature can be instantaneous - and in any such case, isn't it the dissapearence of space time curvature which would cause the earth to fly off at a tangent 8 minutes after the sun dissappeared? Why would the change in curvature of spacetime due to the sun disappearing propagate at C when you say it propagates instantaneously under normal conditions?

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Offline qpan

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Re: At what speed does gravity propogate?
« Reply #18 on: 28/06/2005 19:55:51 »
Having thought about the problem further, i think that you are definately mistaken. Information can almost certainly be interchanged by measuring variations in space-time curvature, so there is no possible way that variations in space time curvature can occur faster than the speed of light. Where the sun is in relation to the earth is most certainly information and i can see no possible reason how the earth would know the instataneous position of the sun?

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Offline ukmicky

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Re: At what speed does gravity propogate?
« Reply #19 on: 28/06/2005 21:19:23 »
has there been an experiment,a direct test  which has shown that gravity moves at the speed of light.
or is it just an assumption
« Last Edit: 28/06/2005 21:20:20 by ukmicky »
 

Offline gsmollin

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Re: At what speed does gravity propogate?
« Reply #20 on: 29/06/2005 03:43:13 »
qpan, conservation of angular momentum requires the force of gravity be transmitted on a line through the centers of mass of the two orbiting bodies. If this does not happen, the orbit will become unstable, and in the case of the earth-sun system, the 8 minute delay will lead to an increase in the earth's angular momentum. I submit a quotation from an internet source, and another reference below. Feel free to research this yourself until you are satisfied. Your confusion on this subject is not unfounded. Understanding the way that gravity actually works has consumed some of the brightest minds in physics.

"Consider the conceptual model of Newtonian gravitation. In this system the Sun basically "pulls on the Earth" using an unknown mechanism, the same mechanism causing the Earth to pull on the Sun. Given any particular snapshot in time, the force between the Earth and Sun lies along the line between them. If this were not the case, for instance, if the Sun pulled slightly to one side, then there would be a leftover torque and the angular momentum of the Earth would change over time. This effect is not seen, in fact it is the conservation of angular momentum that leads to the original statements of celestial mechanics, Kepler's laws of planetary motion.

There is a "twist" that needs to be considered. From the Sun's perspective the Earth is moving about it. In modern physics all forces are transmitted at, or below, the speed of light. Given this wrinkle a problem arises: if the Sun pulls on the Earth then the direction of the force when it arrives 8 minutes later will be in the wrong direction -- it will be where the Sun was and not where it is now. This leads to the problems noted above, it is exactly the same as if the force is not directly along the line between the two bodies. For this reason, Newton felt that the speed of gravity must be instantaneous, although he was at a loss to provide a model for this interaction.

In the 19th century a new conceptual model evolved, one that indirectly addressed these problems. Under these so-called field theories, gravity does not work as a force directly between two bodies, but on the surrounding space itself. In the case of the Sun-Earth system, the Sun does not pull on the Earth, but instead creates a potential field in space, which the Earth interacts with by following the path to the lowest potential, toward the Sun. The implications of this change in model are profound. In the same "heliocentric" view of the Sun-Earth system, the force the Earth feels is always directed towards the Sun because it is interacting with the spherically symmetric field set up by the Sun. In a moving case the potential field that is set up is "sloped" in the direction of the Sun's motion, and once again the Earth moves towards the Sun. Field theories have since become the standard model for most of physics. Similar models are used to explain electromagnetism for instance.

Under a field theory the introduction of a maximum speed of propagation is of no serious concern. There are effects if the source is accelerating, but the magnitude of the effect is directly related to the speed of propagation relative to the acceleration. When relativity suggested a finite speed for all things, this presented no particular problem, notably given the fairly high speed of light. Astronomical evidence of the era had no counterexamples to demonstrate a problem. Note that similar effects occur in the electromagnetic force as well, but, for a variety of reasons, the resulting effects are much different, we call them magnetism.

General Relativity, the currently accepted model of gravitation, works on a field basis. However it proposes a different nature to the field, which many interpret in a geometric fashion; the field is actually the "bending" of space as opposed to some sort of potential. The Earth moves around the Sun because that it the shortest path between today and tomorrow, not because it is being pulled on."

http://en.wikipedia.org/wiki/Tom_Van_Flandern
see also
http://en.wikipedia.org/wiki/Speed_of_gravity

 

Offline qpan

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Re: At what speed does gravity propogate?
« Reply #21 on: 29/06/2005 09:22:18 »
Hi gsmollin-hmmm i'm still quite confused. The second article says that the field is delayed (as i said in my post above) but that this equates to a force in the direction of the instantaneuos position of the sun...Unfortunately i on't really understand how this can happen!
"By viewing gravity as being transmitted by a field rather than a force, it is possible for gravity to be transmitted at a finite speed without running into the problems that Newton sees. If gravity is transmitted by a field, a moving object will cause the field potentials to be non-circular. Hence by using a delayed field rather than a delayed force, one can show that the force will point to where an object is currently rather than were it was in the past. Gravity is still traveling at a finite speed because a sudden change in the direction of an object will not be noticed by the object it is pulling without a delay."
Surely a delayed field produces a delayed force? I understand your argument about conservation of angular momentum, but cannot understand why this mechanism cannot be used to transmit information at speeds faster then C?

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Offline gsmollin

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Re: At what speed does gravity propogate?
« Reply #22 on: 30/06/2005 00:16:15 »
Gravity is a heavy subject.[8D]

You should read some of the papers by Tom Van Flandern. He's quite sure gravity is transmitted at superluminal speed. I don't agree with all of what he says, and most other scientists don't agree, but remember he's a Yaley, and if he's got problems, then we shouldn't feel bad.

I think the geometric model makes the most sense, and shows why two bodies can orbit about a common center of mass. More complicated than that, I don't understand either. I suppose, if I studied GR for long enough, I might get an epipheny, but that's not my field:D.

Puns intended.

P.S. Here is one of those papers. I don't agree with everything in here BTW, but it shows some of the (mis)understanding that dogs gravitational physics.
http://www.ldolphin.org/vanFlandern/gravityspeed.html
« Last Edit: 30/06/2005 19:56:05 by gsmollin »
 

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Re: At what speed does gravity propogate?
« Reply #22 on: 30/06/2005 00:16:15 »

 

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