# Speed of Gravity

1. Sep 23, 2004

### bayan

Hi, all

I have done some researches about the speed of Gravity. in some sources they state that the speed of Gravity is equal to speed of light even though it is not experimented with the right technology, and in some recources they state it is at least 2^10 faster than C, again not experimented with right technology.

I wanted to know what you guys think and what is the reason in that.

2. Sep 23, 2004

### KaneOris

I dont get speed of gravity, i thought f=ma, so unless your talking about the speed of an actual gluon, i dont understand

3. Sep 23, 2004

### KaneOris

I just realised, i have no idea why i just said f=ma, what i was meant to say is that, doesnt the amount of gravitational pull depend on the mass of something?

4. Sep 24, 2004

### bayan

i do not mean the amount of force that is applied.

What i mean is the speed at which the force is acting

5. Sep 24, 2004

### pervect

Staff Emeritus
The speed of gravity according to general relativity is expected to equal to the speed of light, using the following definition

If you change the state of motion of a mass, a distant mass will experience no change in force until some later time, equal to it's distance from the mass divided by the speed of light.

The speed of gravity by this definition is also the speed of gravitational radiation. Unfortunately we don't currently have the technology to detect gravitational radiation, though experiments are almost underway (LIGO) that have a good chance of doing so.

6. Sep 24, 2004

### flexten

There was a measurement of gravitational wave velocity a year or two ago .. can't remember reference but it was in this solar system. And it found approx c.

7. Sep 24, 2004

### Chronos

Think frame dragging. An invariant speed of light works perfectly in that model. It violates causality to have a speed of gravity that exceeds the speed of light.

8. Sep 24, 2004

### primal schemer

So, from (my understanding of) general relativity, say a star mysteriously vapourises for some reason, then any other objects nearby (planets/other stars etc) will only feel its effect as they 'see' the star vapourising.

For example, our sun could have vapourised a few minutes ago, and we wouldnt know about it for another few minutes. (the sun is 8.5 light minutes away from earth)
When I say we wouldnt know about it, I mean that earth would still remain orbiting the sun (that isn`t there anymore), as its gravitational effect would remain. After the 8.5 minutes are up, and the last bit of light from the, now non-existant, sun hits us, we would fly off in any direction we happened to be facing.

9. Sep 24, 2004

### bayan

If it was to be C. When a planet get formed it would be attracted towards the closer planet than the farther planet, meaning that it would start accelerating towards the closer planet due to unbalanced forces, some time latter the force from the farther planed would balance the forces and make it equal, therefor no acceleration. But the muzzle velocity would still remain the same until both planets colide causing destruction.

If it was to travel at least 2^10 faster than C. the forces would be balanced at same time meaning no velocity.

I think it sounds more acceptable to go with the second tought.

10. Sep 24, 2004

### Roberth

Gravity and speed of light

Hello,

I do not believe that gravitons (if they exist at all) do only travel with the speed of light.

It is about time that physicists get away from the speed of light barrier. As EPR experiments show (in particular, Aspect et al, 1982), information can be transmitted faster than the speed of light. The light barrier is only for bodies with masses.

I also do not think that normal quantum physics will be able to shed light (pun intended) on the subject. We have to start thinking about physics where masses in normal space-time follow the Lorentz transformation, but 'information carriers' do not.

Most likely, there is a higher dimensional (5-dimensional or higher) space above Minkowsky space where symmetry considerations are observed and they act instantly. Minkowsky space is then a sub-space of this higher dimensional 'order'.

Another example of this higher space is the Aharanov-Bohm effect which shows that a magnetic potential can influence the path of an electron although there is no magnetic field where the electron travels. Although this effect is predicted by 'conventional' quantum mechanics, QM does not (as usual) explain why it is happening. It gives a lovely mathematical explanation based on Stokes Theorem, but ends up with this "spooky action at a distance", to cite the great Maestro of Physics. Like entanglement this effect has been experimentally proven.

I believe that gravity acts out of this 'higher' space.

Roberth

11. Sep 24, 2004

### pervect

Staff Emeritus
I don't see how we can measure the speeds of gravitational waves until we can detect them. And I'm pretty sure we haven't detected them, it would be big news.

I think the best prospects for a direct measurement of the speed of gravitational waves would involve comparing the light, neutrino, and gravity wave signatures from some nearby supernova. All we need now is the gravity wave detector ....

12. Sep 24, 2004

### Nereid

Staff Emeritus
Welcome to Physics Forums, primal schemer!

[nitpick]Let's rephrase 'our sun could have vapourised' shall we? After all, the Sun is, right now, nothing but vapour (well, plasma)! How about 'the mass of the Sun ceased to exist'?[/nitpick]

Yes indeed, that's just what would happen (we'd know about the fate of the Sun, gravitationally as well as via neutrinos and EM, all at the 'same time'), at least that's what GR predicts.

13. Sep 24, 2004

### marlon

Hi,

may i ask where you get this kind of info on faster then light-communication ??? It does not exist to my knowledge and the light barrier is NOT only for bodies with masses...

marlon

14. Sep 24, 2004

### Nereid

Staff Emeritus
But the devil is in the details!

"When a planet get formed" ... we could ask how this happens; we could ask in one's wildest dreams, 'how *could* this happen?' No matter how you answer these questions (as long as you stick to known physics), you will see that there's no 'acceleration contradiction' of the kind you speculate about.

But that's just me, maybe you can tell us how a planet could form in such a way to lead to an 'acceleration contradiction'?

15. Sep 24, 2004

### Nereid

Staff Emeritus
"On September 8, 2002, the planet Jupiter passed almost directly in front of the radio waves coming from a quasar, a star-like object in the center of a galaxy billions of light-years away. When this happened, Jupiter's gravity bent the quasar’s radio waves, causing a slight delay in their arrival on Earth. Kopeikin believed the length of time that the radio waves would be delayed would depend upon the speed at which gravity propagates from Jupiter." Fomalont did the observations (using the VLA), Kopeikin did the analysis, and found that the speed of gravity was c, +/-20%.

However, Stuart Samuel re-did the analysis, and claimed Kopeikin missed a few important aspects. According to Samuel's analysis, there indeed would be an effect, but that it would be too small to have been measured using Fomalont's set-up.

Maybe one of you will re-do the experiment, in 20 years time or so, with sufficient sensitivity to measure the effect?

16. Sep 24, 2004

### pervect

Staff Emeritus
The Aspect experiment most definitely does not show that information can be transmitted faster than the speed of light.

The argument that massless objects can travel faster than light is a bit disingenuous, considering that light is the prime example of a massless particle.

Does light travel faster than light?

17. Sep 24, 2004

### Nereid

Staff Emeritus
I think you may have misunderstood the Aspect (and later) experiments, testing the Bell inequality. Perhaps you could read some of the threads on this topic, in the Quantum Physics section of PF. In particular, this thread

18. Sep 24, 2004

### pervect

Staff Emeritus
Yep. That's the right idea. Since mass can't actually disappear, though, the more precise thought experiment is to imagine the sun being separated into two (or more) bodies, moving away from each other at a high velocity. The effects of such a catastrophic event would not be seen here on earth until 8 minutes later.

19. Sep 24, 2004

### Chronos

Putting aside the Kopeikin result for the time being, the indirect evidence that gravity speed is 'c', is still pretty overwhelming. Most of the confusion centers around the effect of propogation delay on orbital bodies. As it turns out, the apparent orbital instability this would cause is cancelled out because the total angular momentum of the system must be conserved. There is, however, a tiny amount of momentum that is radiated away via gravitational waves: which has been observationally confirmed.
http://arxiv.org/abs/astro-ph/0408457
So, eventually the earth will spiral in and collide with the sun. No cause for alarm though. The earth will be incinerated long before that can happen when the sun goes red giant on us. See here for further explanation why gravity speed is 'c'.
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

20. Sep 24, 2004

### Roberth

Physical effect faster than light

Hello,

I am talking about the experiment by Aspect et al which was carried out to test whether or not the Bell inequalities are observed, ie whether or not quantum entanglement exists. In their experiment, they made sure that the two detectors which measured the correlation of the two entangled particles were far enough apart that only a signal faster than light would be able to inform particle 1 what was measured at particle 2. Similar experiments in Geneva, 1997 showed the same result.

My point is this: Quantum entanglement shows non-locality, hence there is something that is not bound by the speed of light. This 'something' causes entanglement. I believe that our known "Physics" is only a subpart of a larger structure and entanglement or the Aharanov-Bohm effect are evidence of that structure.

The fact that you cannot transmit information via entanglement is always used to 'save' locality (ie. the speed of light barrier). However, it does not matter whether you can use it to send meaningful information. The fact remains that there is an action that has a physical effect which acts faster than the speed of light.

Roberth