Speed of Gravity

[jeheron]

Does gravity travel at the speed of light? (Meaning is there a lag between when an object moves and another object 1000km’s away is affected by a different level of gravitational pull)

 

or…

 

Is gravity instantaneous? If so, doesn’t this defy Einstein’s Theory of Special Relativity?

[cyeokpeng]

From what I know from Einstein's GR, there is no speed in gravity. Gravitational force is one of the hardest forces for scientists to try to unify under a single unified theory, just as electic field and magnetic field can be classified under electromagnetism. Its nature is due to the distribution of mass in the space-time fabric, and the effect of gravitational force is instantaneous.

 

Gravitational force are not physical objects or particles. So the ultimate speed c does not apply here.

[Severian]

Yes, gravity 'travels' at the speed of light (as your first option).

[maverick88]

Gravitational force are not physical objects or particles. So the ultimate speed c does not apply here.

 

What about gravitons?

[[Tycho?]]

From what I know from Einstein's GR' date=' there is no speed in gravity. Gravitational force is one of the hardest forces for scientists to try to unify under a single unified theory, just as electic field and magnetic field can be classified under electromagnetism. Its nature is due to the distribution of mass in the space-time fabric, and the effect of gravitational force is instantaneous.

 

Gravitational force are not physical objects or particles. So the ultimate speed c does not apply here.[/quote']

 

I'm pretty sure it does apply. Hasn't the speed of gravity been measured to be ~c?

[Martin]

'']I'm pretty sure it does apply. Hasn't the speed of gravity been measured to be ~c?

 

there is always to distinguish between theory and experiment

 

so far the only direct measurement of the speed of propagation of gravity AFAIK was not too well received and people quarreled with it

not because they dont believe!

 

they mostly all take for granted (like Severian just now) that gravity waves and all that DOES travel at c

 

they just didnt respect the way the guy measured it

 

However even tho it hasnt been reliably measured, to everybody's satisfaction, people still assume gravity goes at c because that is part of standard vanilla General Rel, which is a beautiful coherent theory and predicts several things with exquisite accuracy!

 

so one assumes its right, because it would mangle the theory if it werent so.

 

nevertheless as loyal followers of the Venerable Western Tradition of Empiricism we also have to say "does it really?" and we have to want for someone to

actually directly measure, like they do with light or radio signals

[Severian]

Well, I would say it is stronger than that. Irrespective of what the final quantum theory of gravity is, it will have a particle which at low(ish) energies transmits the force - we may as well call that a 'graviton' (although recognising it may not be fundamental in that form). In other words we must have an effective theory containing a particle which mediates gravity (even if the effective theory is non-renormalisable like Fermi's theory).

 

OK, then this particle must be massless, since gravity is long range - if the graviton had a mass, gravity would become short range, just like the weak nuclear force. Of course, this is really only a limit on the mass since we only have to have gravity having a range of > 16 billion light years, but the mass must be very very small if it is there. (This would be stronger than the bounds on the photon mass for example.)

 

Now if a particle is massless, then it travels at c, and thus gravity propagates at c.

 

In other words it is more certain that the graviton travels at c than that the photon does!

[Gilded]

Doesn't the graviton exchange theory presume that gravitons have negative mass? Or then I confuse it with the tachyon theory.

[Severian]

That's a tachyon.

[Gilded]

Alrighty then. So generally it's thought that if graviton exchange exists, a graviton is a 0 mass particle (thereby travelling at c)?

[swansont]

nevertheless as loyal followers of the Venerable Western Tradition of Empiricism we also have to say "does it really?" and we have to want for someone to

actually directly measure' date=' like they do with light or radio signals[/quote']

 

Measurements of a binary star orbit decay is consistent with gravity waves propagating at c, to a reasonable degree.

 

We've discussed this before

[ydoaPs]

That's a tachyon.

 

grrrrrrrrrrrr, their mass squared is negative, not the mass. the mass of a tachyon, if they exist, would be imaginary.

[Severian]

grrrrrrrrrrrr, their mass squared is negative, not the mass. the mass of a tachyon, if they exist, would be imaginary.

 

I am sure that is what he meant. A bosonic particle can never have negative mass; in the Lagrangian the mass term is proportional to mass squared, so if it were negative the Lagrangian would be identical to if it were positive and you would just redefine it to be positive.

[gpdone]

For the posters here who believe that gravitational force acts at c. I would like to know how you account for the fact that the instanious position of target bodies are required for any sucessful orbital insertion?

 

gpdone

[Severian]

NASA don't use GR for that - Newtonian gravity is good enough on a planetary scale.

[Les Sleeth]

Does gravity travel at the speed of light? (Meaning is there a lag between when an object moves and another object 1000km’s away is affected by a different level of gravitational pull)

 

or…

 

Is gravity instantaneous? If so' date=' doesn’t this defy Einstein’s Theory of Special Relativity?[/quote']

 

This was a pretty big media event about a year ago. You can read about it at: http://www.nrao.edu/pr/2003/gravity/

 

Excerpts:

 

"We have determined that gravity's propagation speed is equal to the speed of light within an accuracy of 20 percent," said Ed Fomalont, an astronomer at the National Radio Astronomy Observatory (NRAO) in Charlottesville, VA. The scientists used the National Science Foundation's Very Long Baseline Array (VLBA), a continent-wide radio-telescope system, along with the 100-meter radio telescope in Effelsberg, Germany, to make an extremely precise observation when the planet Jupiter passed nearly in front of a bright quasar on September 8, 2002.

 

The observation recorded a very slight "bending" of the radio waves coming from the background quasar by the gravitational effect of Jupiter. The bending resulted in a small change in the quasar's apparent position in the sky.

 

"Because Jupiter is moving around the Sun, the precise amount of the bending depends slightly on the speed at which gravity propagates from Jupiter," Kopeikin said.

 

"Our main goal was to rule out an infinite speed for gravity, and we did even better. We now know that the speed of gravity is probably equal to the speed of light, and we can confidently exclude any speed for gravity that is over twice that of light," Fomalont said.

[gpdone]

The following is an excellent example of the other side of the debate. It hinges on the difference between gravitational fields and gravitational force.

http://www.metaresearch.org/media%20and%20links/press/SOG-Kopeikin.asp

 

gpdone

[Les Sleeth]

For the posters here who believe that gravitational force acts at c. I would like to know how you account for the fact that the instanious position of target bodies are required for any sucessful orbital insertion?

 

Good point. After reading the article you referenced, I think there are problems with Kopeikin's results. Thanks.

[gpdone]

I also found Kopeikins conclusions to be 'not all they were quacked up to be.'

[JaKiri]

For the posters here who believe that gravitational force acts at c. I would like to know how you account for the fact that the instanious position of target bodies are required for any sucessful orbital insertion?

 

Because the difference between instantaneous and c-delayed is negligable?

[swansont]

The following is an excellent example of the other side of the debate. It hinges on the difference between gravitational fields and gravitational force.

http://www.metaresearch.org/media%20and%20links/press/SOG-Kopeikin.asp

 

gpdone

 

The problem here is that Van Flandern is a crackpot.

[Gilded]

"The problem here is that Van Flandern is a crackpot."

 

Lol. :> You're not a big fan of the "meta model" theory, now are you? )

[RawThinkTank]

So what are the other methods to find out speed of G. Did anyone of U searched the Net yet ?

 

Do U have any idea of what GravityScope is ?

[swansont]

So what are the other methods to find out speed of G. Did anyone of U searched the Net yet ?

 

Do U have any idea of what GravityScope is ?

 

Well, I did one better. I went to a talk by Kopeikin on Friday - he did the measurement with a quasar almost being occulted by Jupiter, and the experiment was part of the talk.

 

I don't have the expertise in GR to evaluate or give details, but he did present his explanation why the bending of the quasar light would be different if gravity were instantaneous vs. c. However, he presented it in terms of the angle of deflection, and not as speed of propagation, as reported in some of the popular-press articles.

[5614]

read this:

 

http://curious.astro.cornell.edu/question.php?number=573

 

(i know this is an old thread but thought it'd be worth posting this)