Speed of Gravity: Experts' Opinion

[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.

Thanx in advance

 

[KaneOris]

I don't get speed of gravity, i thought f=ma, so unless your talking about the speed of an actual gluon, i don't understand

 

[KaneOris]

I just realized, i have no idea why i just said f=ma, what i was meant to say is that, doesn't the amount of gravitational pull depend on the mass of something?

 

[bayan]

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

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

 

[pervect]

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.

 

[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.

 

[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.

 

[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 wouldn`t know about it for another few minutes. (the sun is 8.5 light minutes away from earth)
When I say we wouldn`t 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.

 

[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.

 

[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

 

[pervect]

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.

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 ...

 

[Nereid]

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 wouldn`t know about it for another few minutes. (the sun is 8.5 light minutes away from earth)
When I say we wouldn`t 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.

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.

 

[marlon]

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.

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

 

[Nereid]

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.

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'?

 

[Nereid]

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.

"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.[/color]" 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?

 

[pervect]

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.

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?

 

[Nereid]

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

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

 

[pervect]

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 wouldn`t know about it for another few minutes. (the sun is 8.5 light minutes away from earth)
When I say we wouldn`t 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.

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.

 

[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 canceled 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

 

[Roberth]

Physical effect faster than light

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

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

 

[Roberth]

Aspect experiment

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

Please, see my answer below.

 

[Roberth]

see below

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?

I thought that somebody would pick on the fact that I used the term massless object and did not mention light. I am aware that light is massless. However, the point that I wanted to make is that if and only if something is massless it may be able to travel faster than light.

As far as the more important part of the argument goes, see my posting below.

Roberth

 

[marlon]

My point is this: Quantum entanglement shows non-locality, hence there is something that is not bound by the speed of light.

I agree on the non-locality of entanglement, yet faster then light communication is not possible because a measurement needs to be performed in order to get "something" usefull out of this info. isn't this what QM and more in particular quantum info technology is all about?(at least to a certain extent?, for example look at the massive quantum-parallelism in Deutsch's problem)

I am sure you know the examples of Alice and Bob and their supposed faster then light communication because of entanglement. You know the flaw in this one : the denstity matrix for the A and B atom is the same, meaning an ad random distribution of the outcomes. To put it more clearly, if for example Alice were to perform some measurement on her atom she would get a 50/50 chance to get one of the two possible outcomes (you know, spin up or down along some axis). Basically she knows "nothing" by measuring...the info ain't lost, it just ain't accessible...

marlon

 

[Roberth]

I agree on the non-locality of entanglement, yet faster then light communication is not possible because a measurement needs to be performed in order to get "something" usefull out of this info. isn't this what QM and more in particular quantum info technology is all about?(at least to a certain extent?, for example look at the massive quantum-parallelism in Deutsch's problem)

I am sure you know the examples of Alice and Bob and their supposed faster then light communication because of entanglement. You know the flaw in this one : the denstity matrix for the A and B atom is the same, meaning an ad random distribution of the outcomes. To put it more clearly, if for example Alice were to perform some measurement on her atom she would get a 50/50 chance to get one of the two possible outcomes (you know, spin up or down along some axis). Basically she knows "nothing" by measuring...the info ain't lost, it just ain't accessible...

marlon

Hi Marlon,

My statement was essentially about my belief that it is time to try to create a "new physics" in which our 4 dimensional Minkowsky space is a subspace of a higher dimensional space or 'order'. In this upper space there acts a 'control mechanism' that controls the entanglement via some symmetry rules, for example. If the EPR experiments are correct in proving that there cannot be a LOCAL hidden variable theory (and I think that they do via the Bell theory) then we have to search for a NON-Local hidden variable theory. However, non-local means to me 'outside' Minkowsky space or 'cuts' through Minkowsky space (like the wormholes of Stephen Hawking).
A higher dimensional Group theory may be a good candidate since symmetries are often well described by Groups. Entanglement is a non-local symmetry effect.

However, a Group theoretical approach is again epistemological and not ontological, ie. Group theory is like QM which can be considered an advanced spreadsheet (I know, QM is a rather difficult one), only even more so. It is not an explanation of nature, only a description of possible mathematical connections.

Anyway, all this is a bit too metaphysical at the moment and I will come back to this topic if and when I can talk about an, at least conceptual, ansatz.

Your explanations from your angle of approach seem sound and I have not really anything to add at the moment. I will, however, think about this whole issue again and in more depth as soon as I have time (I am pretty busy over the next weeks).

Cheers for now and thanks for your interest.

Roberth

 

[Nenad]

I don't get speed of gravity, i thought f=ma, so unless your talking about the speed of an actual gluon, i don't understand

I think you mean graviton.

 

[bayan]

why do you guys believe in enistiens theory and totally ignory isac Newtons theory?

 

[selfAdjoint]

Newton's theory is a limiting case of Einstein's GR. GR has made many predictions that Newton's gravity has different answers for, and the experiments and observations support Einstein, not Newton. We have to go with the evidence.

 

[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.

What about this?

If Gravity acts like magnet, it means it will be attracted or repaled by the Gravity force of another planet.

wouldn't the coulision between two planets happen?

 

[Nereid]

What about this?

If Gravity acts like magnet, it means it will be attracted or repaled by the Gravity force of another planet.

wouldn't the coulision between two planets happen?

The trouble with this idea is that there is, AFAIK, no experimental or observational result which even hints that gravity is anything but an attractive force.

As you've probably gathered by now in your reading of posts on PF, speculation is a wonderful thing, and some lucky folk can come up with twenty-five new ideas even before they've had their morning coffee. However, within minutes to hours, a little careful thought and some checking nearly always shows each bright sparkling speculation to have at least one fatal flaw ... either inconsistent with well-established experimental or observational results (e.g. your 'gravity acts like a magnet' idea), or internally inconsistent, or wildly inconsistent with well established theories (e.g. GR, QM).


Could I ask that you please do some serious thinking before posting a speculation here?

 

[NoTime]

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 canceled 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

Ok, that seems to be a GR take on things. As stated elsewhere in this thread gravity is not a force in GR.

My problem is with the spin 2 graviton.

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

There seems to be a requirement that gravitons travel >> than the speed of light, being force carriers.

What am I missing here and how is this resolved?
Are gravitons optional in quantum gravity theory?

 

[Chronos]

Ok, that seems to be a GR take on things.

Did you have a non-GR take in mind?

As stated elsewhere in this thread gravity is not a force in GR.

Agreed, gravity is not a force in the usual sense under GR, so, what is your point?

My problem is with the spin 2 graviton.

There seems to be a requirement that gravitons travel >> than the speed of light, being force carriers.

There is? By what?

What am I missing here and how is this resolved? Are gravitons optional in quantum gravity theory?

Hard to say, it is not clear what you are assuming.

 

[pervect]

There seems to be a requirement that gravitons travel >> than the speed of light, being force carriers.

What on Earth makes you think that?

As far as spin 2 gravitons go, if you quantize gravity, that's what you'd get. I'd strongly suggest to anyone and everyone that getting a basic handle on non-quantized gravity is a Very Good Idea before one gets too worked up over quantized gravity and gravitons.

 

[bayan]

Gravity does act like a magnet, there is no doubt about that!

It is just that it acts like a magnet to mass, and not to force.

Why do we not get pulled towards the Earth in space? there are many factors one is the inverse square law, the other one is the fact that you are also being pulled by the gravitational fields of other planets and stars.

 

[Nereid]

Gravity does act like a magnet, there is no doubt about that!

It is just that it acts like a magnet to mass, and not to force.

Being as respectful of your opinion as I can, it's extremely easy to demonstrate that gravity doesn't 'act like a magnet' at all - there's no 'repulsive' counterpart.

Why do we not get pulled towards the Earth in space? there are many factors one is the inverse square law, the other one is the fact that you are also being pulled by the gravitational fields of other planets and stars.

Are you joking? The motions of solar system bodies are predictable, to a very high degree of accuracy, under Newtonian physics, and if you add General Relativity, the only apparent inconsistencies are the motions of some comets (which are now well understood in terms of the 'rocket' effect of the jets).

 

[bayan]

hah!

lets get sirous now.

i was kidding about the second part, but with the first part wouldn't they be attracted to each other? when i say "act like magnet" i do not mean that they have "+" & "-" ends. Rather what i meant was that they will get "pulled towars each other". If there is an explenation why it is not like that please post.

 

[NoTime]

Did you have a non-GR take in mind?

Yes QFT, LQG.

Agreed, gravity is not a force in the usual sense under GR, so, what is your point?

That I think I have some idea of how GR does its thing?

What on Earth makes you think that?

I don't. It's actually something I have read.
With a light speed graviton the force vector points in the wrong direction.

Also, it seems to be the reason for the question "What's the speed of gravity?" in the first place.

Does this question of the speed of gravity arise from some other source?
Am I supposed to accept that gravitons are not limited to the speed of light?
Is there some way, retaining the speed of light, to handle this?
Are these quantum gravity theories just plain wrong?

 

[Nereid]

hah!

lets get sirous now.

i was kidding about the second part, but with the first part wouldn't they be attracted to each other? when i say "act like magnet" i do not mean that they have "+" & "-" ends. Rather what i meant was that they will get "pulled towars each other". If there is an explenation why it is not like that please post.

As there are no magnetic monopoles (at least, no experimental evidence of such), one cannot ignore the 'dipole' nature of magnets and magnetism 'just because'. Further, since there are no monopoles, it's very easy to demonstrate that magnetic fields don't follow the inverse square law (gravity does). Finally, there are an awful lot of 'pulled towards each other' things out in the universe - gravity, electrostatic, magnetic, springs, surface tension, ... - one needs to go at least a tiny step further before sharing 'ideas before morning coffee' with others, especially in PF.

bayan 0, Einstein+Newton 2