# Consensus about Van Flandern's ideas on speed of gravity?

1. Jan 12, 2007

### hkyriazi

Is there any consensus among experimental and/or theoretical physicists about Tom Van Flandern's ideas about gravity propagating (and having to propagate) much faster than the speed of light?

2. Jan 12, 2007

### ZapperZ

Staff Emeritus
How could there be ANY consensus when there are no definitive experimental evidence yet? Let's wait till LIGO starts producing results, why don't we?

Zz.

3. Jan 12, 2007

### Staff: Mentor

4. Jan 12, 2007

### hkyriazi

Thanks, JTBell. From those archived discussions, I found a 1999 paper by Steve Carlip, addressing Van Flandern's Physics Letters A article (vol 250, p.1, 1998). (Carlip's article is available at: http://xxx.lanl.gov/abs/gr-qc/9909087.)

It seems to have everything I need. (My interest was in knowing a) whether gravity in general requires multi-light speed of propagation (it does only for Newtonian gravity, and, I guess, any mechanical form of gravity, such as "push gravity"), and b) how general relativity got around this (by the use of velocity-dependent terms).

Now I just need to read and be able to make sense out of Carlip's article, and understand what is meant by "the quadrupole nature of gravitational radiation."

5. Jan 12, 2007

### Daverz

You really don't have to wait for a consensus, because you don't have to be an expert to evaluate Flandern's credibility. See

http://math.ucr.edu/home/baez/RelWWW/wrong.html#speed [Broken]

Last edited by a moderator: May 2, 2017
6. Jan 12, 2007

### Daverz

See https://www.physicsforums.com/search.php?searchid=568693 [Broken]

Last edited by a moderator: May 2, 2017
7. Jan 13, 2007

### robphy

Why is this thread in this forum?

8. Jan 13, 2007

### pervect

Staff Emeritus
If the intent is to ask what General relativity has to say about the speed of gravity, the answer is fairly clear - it travels at 'c'. This is a prediction of GR, not yet an experimentally established fact.

If the question is more general, it may belong in some other forum, as robphy has perhaps suggested - though I'm not sure exactly which one.

I belive that Van Flandern has actually been published, so that while his ideas may be full of errors and far away from the mainstream, they are probably "fair game" according to PF guidelines (to discuss only published papers, not personal theories).

9. Jan 13, 2007

### MeJennifer

Well obviously if the speed of gravity would not be c then GR would be incorrect. So I would say that GR implies it instead of that it predicts it.

But perhaps I am misunderstood, how does GR predict that the speed of gravity is c?

10. Jan 13, 2007

### tehno

Good question Jennifer!
In Maxwell's theory of EM waves these propagate in vacuum at "speed" determined by two constants,vacuum permeability and permitivity namely.
Why the speed of propagating of a gravity wave,which at first glance has nothing to do with electrical charges,has to be linked with these two electrical constants, in the same manner?

11. Jan 13, 2007

### pervect

Staff Emeritus
I should probably add that the speed of gravity is predicted to be equal to 'c' only for weak fields in a vacuum.

When you linearize the Einstein field equations around a vacuum solution, you come up with a set of linear differential equations. The solution to these linearized equations is a plane wave, just as it is in the case of Maxwell's equations. This plane wave travels at a speed of 1 in geometric units, i.e. it travels at the speed of light, since c=1 in geometric units.

12. Jan 14, 2007

### yogi

Pervect - can you clarify - are you saying that a plane wave arises from the sudden destruction of matter (conversion to another form such as photons) which is other than gravitational radiation.

13. Jan 14, 2007

### tehno

Only becouse STR is taken as guide in linearization where c=1.
But this fact about EM was found both experimentally,and theoretically.
The claim that gravity wave propagate with c,isn't found experimentally or theoretically...

14. Jan 14, 2007

### pervect

Staff Emeritus
This is incorrect. As I explained earlier, it's a standard textbook exercise to derive the theoretical prediction of the speed of propagation of gravitational waves in GR.

As I re-read your remark, I suppose I should add that GR does presuppose SR is true - it doesn't make any sense to postulate GR without also including SR as a special case.

The result is that according to GR, the speed is equal to 'c'. Just about any GR textbook will go into this, see for instance chapter 18 of MTW's "Gravitation". But if you don't happen to have that textbook handy, pick ANY textbook that covers the topic of gravitational waves.

Certain other assumptions are made to make this derivation. One assumes that one has a metric n_uv which satisfies Einstein's field equations. G_uv can be written as a compiclated second-order non-linear differential equation of n_uv. It is simplest and usually assumed that n_uv is a Minkowski metric, so that the background space-time is not only a vacuum solution, but it is flat. This is for ease of computation (and ease of interpretation) though, it's perfectly possible (though trickier) to talk about the speed of gravity in a a Schwarzschild vacuum as well as a Minkowski vacuum.

One then assumes a pertubation metric g_uv = n_uv + h_uv, where h_uv is "small". One then linearizes Einstein's field equations, getting LINEAR second order differential equations in terms of the pertubations to the metric h_uv. These equations are found to be the wave equations, and represent gravitational waves. These waves travel at 'c', the speed of light.

While it is incorrect, as I have attempted to explain at length, to say that there is no theoretical foundation for the speed of gravity being c -according to GR (which is I might add, the title of this forum, i.e. this is a GR forum), it is basically correct to say that we currently have no experimental measurements of the speed of gravity. Here we have a few authors such as Kopeikin arguing that they have performed experiments which indirectly measure the speed of gravity and other authors arguing that since one needs to assume some theory other than GR even to talk about the speed of gravity not being c, that the above measurements which assume GR is true in order to interpret the results as a 'speed' have assumed their conclusion rather than actually measuring the speed. I agree with Carlip on this point, and while Kopekin continues to defend his position I don't think he currently has a lot of support (this is a judgement call).

(see for instance http://www.arxiv.org/abs/astro-ph/0302462, http://arxiv.org/abs/gr-qc/0403060, http://arxiv.org/abs/gr-qc/0510048) [Broken].

Last edited by a moderator: May 2, 2017
15. Jan 14, 2007

### pervect

Staff Emeritus
Analyzing the source of gravitational waves is actually a bit different from the simpler task of determining how fast they move. Conversion of matter to energy is not really the central issue behind creating gravitational waves. A spinning assymetrical bar or plate will, for instance, generate gravitational waves without any such conversion. What's important turns out to be the third time derivative of the quadropole moment of the matter distribution. I'm sorry if that's too technical, I'm not sure how to describe it more simply and still be exact.

But it's basically true that the in order to measure the speed of gravity by accepted defitnions, one wants to create a disturbance "here" and then detect the effects "there", and then compute the propagation speed. So, for instance, while the decay of the orbits of the spinning pulsars (Taylor & Hulse) has provided us with indirect evidence that gravitational waves exist (for which they won the Nobel prize), this smooth decay process doesn't really offer us any "handles" on a way to measure the actual speed of gravitational radiation.

One of the ways that I envision the speed of gravity being measured at some point in the future is for us to observe an binary inspiral or other catastrophic event which emits gravity waves both visually and with gravitational wave detectors such as Ligo, assuming they come on-line and work as expected. This is the sort of experiment that will give us the best information about the "speed of gravity" IMO.

It doesn't appear to be technologically possible in the forseable future to create artiically a gravitational wave disturbance that we can detect, therfore we will have to wait for a catastrophic astrophysical event to occur and measure the waves from it.

Currently, though we've built gravitational wave receivers, they aren't very sensitive, and they have yet to detect any signal at all, much less provide timing information about how fast the signal travels. The former issue (detection of signals) is still expected to change as we improve the sensitivity of the receivers - the lack of detection is not considered to be alarming considering the expected frequency and magnitude of natural sources of gravitational radiation.

Last edited: Jan 14, 2007
16. Jan 14, 2007

### tehno

So there you go...And what constant,if not "electromagnetic" c ,is fixed in a Minkowski metric?:tongue:

17. Jan 14, 2007

### pervect

Staff Emeritus
And your point is - what, exactly?

It sounds like we might actually agree if you would restrain what appears to be some anti-relativity sentiment. At least that's the way it's coming across to me.

18. Jan 15, 2007

### yogi

Pervect - so the plane wave you were referring to in post 11 is the quadrapole gravitational radiation. Thanks

One more question with regard to your post 15 - if we assume for example a catastrophic event - say electrons combining with positrons to extinquish matter and release photons (a visual event).. is not the total energy of the original particles accounted for in the radiating photon flux - and if so - where is the energy that is conveyed by the gravitational radiation come from?

19. Jan 15, 2007

### tehno

Impression from the books that "electromagnetic" c sets the "gravitational" c.
Quite comfortably,I would rather say that it's the other way round .
However,I don't think this could be the correct standpoint either.
c must be the universal constant,not exclusively reserved for electromagnetism or gravity.
Beside the fact that it doesn't deal with the gravity,Maxwell's theory cannot be considered as the complete theory.
Covariance:Maxwel's eqs. for empty space stay unchanged if we apply to space-time coordinates linear tranformations->Lorentz transforms.Covariance holds for a transformation composed of more such transformations.Mathematically that's the property of a Lorentz group.Accordingly,from Maxwell's eqs. arise the Lorentz group,but Maxwell's eqs. from the Lorentz group don't arise .The group can be defined independently of these eqs. as the group of linear transforms with c=1 kept constant.
In GR things are even more interesting ,nonlinear transformations must be applied,and Lorentz group aren't generally valid .
But the point is :in electromagnetism where charges oscillates,we find c. In the gravity,where masses oscillate,we will probably verify one day the same velocity c of the field disturbance propagation.
Also the curiosity :A propagating EM wave induces a gravitational field,but a propagating gravitational wave does not induce a magnetic field.

Last edited: Jan 15, 2007
20. Jan 15, 2007

### pervect

Staff Emeritus
I'm still not following you - and I have to run.

Basically, though, the point is that one doesn't know what the speed of gravity (I should perhaps say the speed of gravitational radiation) is just by inspecting the Minkowski line element. One actually have to solve Einstein's field equations. When one does so, using the method I sketched earlier, one finds that the speed of gravitational radiation in a vacuum is 'c'. This is a mathematical result, very similar to the way that Maxwell's equations show that the speed of light is equal to 'c' in a vacuum.

Someone has emailed me that I should be more precise on this point, and I will attempt to do so. When I say that the speed of gravitational radiation is 'c', I don't mean the coordinate speed of gravitational radiation is equal to 'c'. That would be rather silly, for the coordinate speed of light is not always equal to c in GR as GR allows arbitrary coordinate systems. What I mean is that the local speed of gravitational radiation, like the local speed of light is equal to 'c'.

Last edited: Jan 15, 2007
21. Jan 15, 2007

### rbj

but these constants are manifestations only of our anthropometric units used to measure them. they are not fundamental properties of the universe. but, even though the numerical value of the speed of E&M propagation is still an anthropocentric number (unless we were to use natural units like Planck units), the quantity of such a speed is fundamental and believed to be universal (and if everyone, including us humans and the aliens on the planet Zog, agree to use Planck units, this speed is always 1 and is the speed reference to measure and describe all speeds against).

That speed is the speed of the disturbance of an E&M field that would happen to a test charge some distance away from a "transmitting" charge that would be accelerated. you disturb a charge at point A and the charge at point B reflects such a disturbance at a time |B-A|/c later as viewed by a distant observer on a line that perpedicularly bisects B-A. now Coulombs law for electrostatic attraction is modeled as instantaneous also, that a change in A would cause an instantaneous change in B, but that doesn't happen. the speed of this ostensibly instantaneous reaction is some universal property we call the "speed of light".

but why would it be reasonable if the speed of propagation of the ostensibly instantaneous electromagnetic action first described as Coulombs law be this finite c and, on the other hand, the speed of propagation of the ostensibly instantaneous gravitational action first described as Newtons law be infinite? i see no good reason to assume this and neither did Einstein.

now, it's just an approximation (for low gravitational fields or nearly flat space-time) but you can imagine a thought experiment where you have two identical infinite and parallel lines of charge moving together along in the direction of the lines at some speed, v relative to some observer. it turns out that, for this observer that the lines are charge are whizzing by that they repel each other more slowly than they appear to repel each other for an observer traveling alongside the moving lines of charge (due to time-dilation). this reduction in repulsive force is identical in direction and quantity to the magnetic force that you would arrive at in classical physics. so the effect of the magnetic field can be thought of as the same as if only the electrostatic field existed, but we took into account the consequences of special relativity.

now apply that same thought experiment to two infinite lines of uncharged mass. they will attract each other, but for the "stationary" observer their rate of attraction will be reduced due to the same time dilation with the same c in the time dilation formula. this reduction of attraction can be thought of as a gravito-magnetic effect and, for the "classical" model formula very similar to Maxwell's equations (called the GEM equations) can be constructed with mass replacing charge, mass density replacing charge density, $-G$ replacing $1/(4 \pi \epsilon_0)$ but the same c !!! if it were a different speed of propagation for gravity, then the time-dilation formula for this second thought experiement, would need a different c to go into it. so different formulae for time-dilation depending on what it is that is moving past an observer??? why?

those two electrical constants are anthropometric crap. it's the speed of propagation of these ostensibly "instantaneous" effects that is fundamental and is the same for all things instantaneous. then, given your set of units you choose to use, you measure $G$ or $\epsilon_0$ to come out to be whatever numbers they do.

neither is true.

Sergei Kopeikin and Edward Fomalont have experimental data that the speed of gravity is within +/- 20% of the speed of light: http://arxiv.org/abs/astro-ph/0302294 . also, there is good theoretical reason to expect the same speed for both, which, i think GR is supposed to nail. if the Gravity-Probe B ends up consistent with the predictions of GR, i think that's another nail in the coffin. these frame-dragging or gravito-magnetic effects would have a different magnitude if the speed of gravity was not the same c.

Last edited: Jan 16, 2007
22. Jan 16, 2007

### yogi

Just to clarify what Van Flanderen is saying - gravitational radiation, like electromagnetic radiation, propagates at velocity c. But the field(s) that couple the forces between separated objects (electrical, magnetic and gravitation) is not determined by the same factors. In other words, the force is not coupled by a wave as such, but by some other mechanism. Van Flanderen mistakenly concludes that G acts intantaneously because the earth is pulled toward the Sun at its Now position rather than its retarded position...Carlip shows this conclusion is unjustified (this has been known for many years - the subject being discussed by Feynman in Vol I published in 1963). So all that can really be said is that Van Flanderen's proof is poof - but the issue of how forces are communicated and whether they differ from c is still uncertain.

23. Jan 16, 2007

### pervect

Staff Emeritus
They way I'd describe Van Flandren's results is that he offers a novel defintion of "speed" that is not compatible with the usual defintion.

Furthermore, by Van Flandren's defintion of "speed", the speed of electromagnetism is greater than 'c'. I gather that Van Flandren has actually admitted as much, but this hasn't deterred him from arguing his point.

The standard defintion of the speed of electromagnetic radiation or of electromagnetic forces insists that you move or wiggle something "over here", and get some sort of physical reaction (a reading on a dial, the movement of a charge) "over there". You can then take the distance divided by the time, and compute the speed as a number. (At least you can once you are able to synchronize your clocks properly, and if you use small distances and small times so you don't get into the local vs global issues I mentioned earlier).

We can sidestep some of these issues about "how to measure speed" and syncrhonize clocks and the entire "local vs global" issue by simply saying that weak-field gravitational radiation travels at the same speed light does.

This still leaves the issue of what strong field gravity does, or the issue of what travelling through media does. I'm pretty sure I recall that the result is that media and strong fields can only slow the propagation speed down, not speed it up. Unfortunately I don't have time to look the issue up further in my textbooks at the moment to confirm this. The place I would start looking is Wald's section on gravity as "a well-posed initial value problem" in his book "General Relativity".

24. Jan 16, 2007

### tehno

I don't know why do you feel to write at lenght about set/system of units?
I don't rise that question up becouse I don't find it an issue at all.
I didn't say permeability,permitivity are fundamental units of the universe.
Matter of fact,I expressed my opinion about $c$ being universal constant not reserved exclusively for electromagnetism or gravity.
Hmm...

In the paper it's only interpretation of the experiment that speed of gravity is close to the speed of light .But that is far from being the measurment of speed of gravity (gravity waves namely).
What is Gravity-Probe B?I don't find it in the paper.

25. Jan 16, 2007