Talk:Gravitoelectromagnetism

The "4" factor and speed of gravity?

I am very disturbed by the "4" factor in GEM equations. Taking curl of (curl E) equation in vacuum (rho=j=0), we get: laplacian E = (2/c)^2 * d^2E/dE^2 suggesting c/2 speed of wave propagation? Doesn't gravity propagate with speed of light? — Preceding unsigned comment added by 46.205.198.40 (talk) 13:32, 9 August 2014 (UTC)

This article has been through a few revisions of some constants (2) being moved about, and I'm not sure about the consistency – different authors place them differently, effectively changing the definition/scaling of the gravitoelectric and gravitomagnetic fields. Gravitoelectromagnetism#Scaling of fields was written to address this, but was not updated with the last change, and is thus inconsistent with the current formulae.

I would still expect to arrive at a wave equation that predicts propagation at the velocity of light, though who knows what has been lost due to the assumptions for simplifying the equations. —Quondum 00:53, 10 August 2014 (UTC)

Not only must gravity waves propagate at the speed of light, but also the divergence GEM equation that is the counterpart to Gauss's law in EM, that must have an ε₀ that is consistent with -1/(4 π G) or the static strength of gravity, indicated in the GEM equations, will be wrong.

In my opinion, the GEM equations should look exactly like Maxwell's equations with μ₀ eliminated (replaced by 1/(ε₀ c²)) and ε₀ replaced with -1/(4 π G). then in the magnetic term of the Lorentz force equation, they can toss in whatever factor of 2 or 4 they need to make it consistent with the Einstein equation. But the static part of the Lorentz equation should be the same. This is the way it was in the earlier life of this article. Someone who's real knowledgeable about the physics (GR) should look into this and fix it. Otherwise, if I fix it, it will be only to revert it to a previous state that was considered sufficiently accurate at the time. 207.136.213.243 (talk) 16:15, 11 August 2014 (UTC)

The factor of 4 only appears when GEM is derived from Einstein's theory. O Jeffimenko does not use it, nor does Th. de Mees. de Mees is able to reproduce the exact procession of Mercury, the mass-velocity distribution of galaxies etc, without recourse to it. If the value is open to debate, it ought be set as a variable, and the different variations noted by theory. GEM is older than Einstein's GRT, and the main objections seem to come from variance with GRT rather than any inate analysis of GEM. Wendy.krieger (talk) 12:20, 19 July 2016 (UTC)

The claim of de Mees is 'original research' and would need to be checked before being given any credence, I believe his approach is to treat the galaxy as a single mass and apply an average rotation velocity rather than integrating over the stellar mass distribution using the velocity relative to the Solar System. It would perhaps add to the article if other relevant tests of GEM were added, for example being based on Newtonian gravity plus GM effects, it fails some basic tests as the Shapiro delay so should not be seen as a viable alternative to GR. George Dishman (talk) 13:16, 7 February 2018 (UTC)

A deep-seated problem, here, is that gravitomagnetism cannot coexist with the equations of special relativity.

There are a number of mainstream texts that point out that if the path of a forcibly-accelerated mass is broken down into an arbitrarily-great number of constant-velocity-stages, which are then described using SR, individually and in bulk, that we can prove that the accelerated mass produces no deformation of spacetime, other than the deformations associated with the relative velocities (which under SR are zero). Since the general principle requires the accelerated mass to be associated with an intrinsic deformation of spacetime, under a general theory the constant-velocity motion of matter, logically, cannot be correctly described using SR: Accelerative GM is only geometrically possible if there is also a velocity-dependent GM effect, and velocity-dependent GM breaks special relativity. Since SR's equations of motion have to apply identically to strong-gravity and weak-gravity objects, we cannot create a weak-gravity "safe haven" for SR where the 1905 equations apply.

So a universe can either support the SR equations and show no gravitomagnetic effects ("SR disproof of GM"), or it must support full gravitomagnetism, but not use SR ("GM disproof of SR"). Geometrically, we aren't allowed both.

Einstein's general theory tries to "have its cake and eat it", by nominally supporting both the SR equations and gravitomagnetism ... which is impossible.

So any attempt by us to try to make sense of how this subject works under Einstein's general theory is pretty much doomed to failure. We can't create a legitimate WP narrative of how GM works under GR1916 that makes sense, because under GR1916, the thing doesn't make sense. ErkDemon (talk) 17:37, 25 June 2023 (UTC)

There should be a separate section where the gravitoelectromagnetism as presented in the article is compared to General Relativity. 2001:14BB:678:E599:85EF:1C5F:59EE:5C69 (talk) 12:19, 9 December 2023 (UTC)

There's no problem here. GEM and SR are two different simplifications (limiting cases) of GR. So that they contradict each other should be no surprise. SR works for vanishingly small spacetime curvature but relativistic velocities; GEM works for moderate spacetime curvature but nonrelativistic velocities. So one has to start from GR to write up a consistent narrative. One possible derivation from GR > GEM can be found in [1]. Slovborg (talk) 22:26, 3 November 2024 (UTC)

A mistake?

According to references 8 and 9, the third equation of GEM should be in another form and has 1/4:

$~\nabla \cdot \mathbf {E_{g}} =-4\pi G\rho _{g},$

$~\nabla \cdot \mathbf {B_{g}} =0,$

$~\nabla \times \mathbf {E_{g}} =-{\frac {1}{4}}{\frac {\partial \mathbf {B_{g}} }{\partial t}},$

$~\nabla \times \mathbf {B_{g}} ={\frac {4}{c^{2}}}\left(-4\pi G\mathbf {J_{g}} +{\frac {\partial \mathbf {E_{g}} }{\partial t}}\right),$

Only in this case the force has the form as in the article

$~\mathbf {F} _{m}=m\mathbf {E_{g}} +m\mathbf {v} _{m}\times \mathbf {B_{g}} .$

On the other hand, if in ref. 9 (S.J. Clark, R.W. Tucker (2000). "Gauge symmetry and gravito-electromagnetism". Classical and Quantum Gravity17 (19): 4125–4157. arXiv:gr-qc/0003115.) we multiply Equations 7.12 by 4 we come to equations of GEM in the form as in the article

$~\nabla \cdot \mathbf {E_{g}} =-4\pi G\rho _{g},$

$~\nabla \cdot \mathbf {B_{g}} =0,$

$~\nabla \times \mathbf {E_{g}} =-{\frac {\partial \mathbf {B_{g}} }{\partial t}},$

$~\nabla \times \mathbf {B_{g}} ={\frac {1}{c^{2}}}\left(-4\pi G\mathbf {J_{g}} +{\frac {\partial \mathbf {E_{g}} }{\partial t}}\right),$

But in the case the force in Equation 5.14 will have the form

$~\mathbf {F} _{m}=m\mathbf {E_{g}} +4m\mathbf {v} _{m}\times \mathbf {B_{g}} .$

The same is from ref. 8 (B. Mashhoon, F. Gronwald, H.I.M. Lichtenegger (1999). "Gravitomagnetism and the Clock Effect". Lect.Notes Phys.562: 83–108. arXiv:gr-qc/9912027). In both cases gravitomagnetic force more 4 times then gravitoelectric force. May be it better to exclude 4 in forth equation GEM and add 4 to the force? . Also why Lorentz factor is in the force? it is absent in the ref. 8 and 9. — Preceding unsigned comment added by 95.31.126.196 (talk • contribs) 2014-08-26T11:37:43‎

Effects Section

To make it easier to understand gravitomagnetism, I propose an effects section (which some dick jerk simply deleted without even informing me or creating a talk page section). Here's the text:

Gravitomagnetism has two primary effects: [2]

• Inducing an angular momentum in nearby objects of the same rotational moment. This effect causes nearby objects to feel a rotational acceleration in the same direction as the spinning body, which would cause previously non-rotating masses to start rotating in the same plane as the spinning body.
• Changing the acceleration felt by two rotating bodies along moment of rotation. This effect causes two rotating mass to be more attracted to each other when they have the same direction of spin along the plane between them, and to be less attracted to a spinning body when they have opposite directions of spin along that plane. This is analogous to how two magnets are attracted or repelled from eachother.

It has a source, but I may be misinterpreting and I've read conflicting things about whether gravitomagnetism affects non-spinning bodies, and how spinning bodies interact. I'd appreciate anyone's thoughts, corrections, or additional sources. Fresheneesz (talk) 20:05, 27 August 2014 (UTC)

For the record here is the revert in question.

If you seriously consider a website like this a source, then it isn't reliable. Any website can claim whatever, you need a real paper published in peer-reviewed journals, or books/monographs published by academic authority. The "relativity book" in question looks self-published, hence original research. We have already had this issue before with another editor years ago. The burden is on you to write the section with reliable sources, not for you to write then expect others to correct and provide such sources for you.

I admit I don't know much about gravitoelectromagnetism. Not sure about the first point. But the second point could be wrong - don't the magnets have to be antiparallel to be more attracted, since the magnetic moment of one magnet is aligned with the B field of the other? If the magnets are antiparallel, doesn't the N pole of one moment attract to the S pole of the other more easily? So perhaps the analogy would be the opposite way round to ordinary magnetism. M∧Ŝc²ħεИ_τlk 21:51, 27 August 2014 (UTC)

And why is there a need for another "effects" section anyway?? There is a "Higher-order effects" section, which even includes the analogies or partial anaologies with classical electromagnetism. M∧Ŝc²ħεИ_τlk 22:51, 27 August 2014 (UTC)

Maschen, I think you'll find that you can use the EM analogy quite well; just remember that the tension and compression in the fields is opposite: longitudinal compression, lateral tension. —Quondum 23:27, 27 August 2014 (UTC)

The analogies are one thing, but what I'd like an effects section to do is make it clear very concisely what the full list of effects are conceptually, while further explanation can be done elsewhere in the article.

The background doesn't really explain the effects. The equation section describes it mathematically but not conceptually, same with the Gravitomagnetic fields of astronomical objects section. The higher-order effects section gives examples of some higher order effects, but by no means makes it clear if there are other higher order effects it's leaving out and doesn't talk about basic effects (ie non-higher-order effects). The images describing the analogy to electromagnetism are somewhat helpful, but still don't make it clear what the effects really are. The intro certainly doesn't enumerate the effects.

What I'd like to see is something that enumerates all the effects of gravitomagnetism. My understanding are that there are essentially two:

• A moving mass X will put a force (distinct from gravity) on a mass Y in the same direction as the direction of X's motion
• A moving mass X will put a force on a mass B in the direction perpendicular to X's trajectory that passes through the mass Y.

This is the kind of thing I'd like to see, sourced and corrected of course where wrong or incomplete.

Fresheneesz (talk) 22:41, 29 August 2014 (UTC)

Okay, let's try to step back a bit. I'll give my rationale for the original deletion, though I don't expect it to generate any less reaction than the simple deletion did. I had tried first to fix the passage, then I scrapped that and tried critiquing it on the talk page, but found that this just resulted in demolishing the whole passage anyway and I judged that it would have a negative impact to little avail. So I ended up simply deleting it with an understated comment as what I felt was the most constructive route, with the implication that it was OR. I actually spent quite a bit of time in this process with various drafts, much more than I'm spending on this now, before I decided that the simplest approach would be the least stressful all around. My critique:

• The section attempts to give an interpretable overview to create an intuitive picture. The introduction (calling these points "primary effects") is overstated, though it would be easy enough to fix this; we'd just call any listed points examples of predicted effects. Eventually the list of examples could in principle include enough to be illustrative in the way intended.
• The first bullet point is stated in a way that makes it difficult to even interpret what was intended, and potentially gives an incorrect impression: that angular momentum might be directly transferred somehow. There are many effective forces, such as moments acting on each other (which depend of the relative angles, just as with magnets), as well as velocity-related forces and torques (e.g. a mass moving towards a rotational pole of another would feel a torque in one direction, but in the other direction if the velocity is reversed, and zero if the relative velocity is zero. Similarly a mass moving on the equatorial plane of a rotating body will feel a deflection to one side when falling directly towards a rotating body, and experiencing a deflection in the opposite direction if travelling in the opposite direction. What should not happen is angular momentum being transferred from one rotating body to another when their relative positions are kept fixed in a simple configuration (pole to pole or equator to equator). that I really don't think that this is even remotely captured by what was there, and replacing it with something adequately descriptive would be a challenge. It would be simpler to say that the gravitomagnetism of rotating bodies, from these equations, should behave rather similarly to normal magnets, with certain tweaks such as that like poles attract and "like" masses attract. There is little to do here but remove the bullet, unless one can find a properly researched source for proper descriptions, though. What one will find in sources is the effects on precession and the like, but these are probably adequately described already.
• The second point does mention a particular manifestation that has validity. The wording is difficult, but a guess that it is trying to describe two bodies with the same equatorial plane of rotation. As with side-by-side magnets, one would expect a lateral force that varies I guess with the cube of the separation, attractive for parallel, repulsive for antiparallel, and a torque if they are at right angles (each being opposite to the same effect with magnets). Yet since, as Maschen has pointed out, this is already covered under § Higher order effects where it describes coaxial rotation, which I saw at the time. Since the only part with some merit was is already covered and better-described, there seemed no reason to keep it.
• Pretty much everything I'm saying here is OR, and I have no intention of including it in the article. I could not, in fairness, even correct what was there, as my changes would be OR.

I was pretty confident that others would concur with my feeling that the addition did not improve the article, and was too underdeveloped to fix without extensive effort and research: it added more confusion than insight, partly because of the wording, and partly because it wasn't saying anything sensible that could be discerned. Finally, I feel that the deletion was well within WP's criteria. We all initially react with anger when something of ours is reverted, but soon learn that most people have a reason and that generally we can assume good faith. —Quondum 23:46, 29 August 2014 (UTC)

I'll have to get back to this after looking into reasonably well-established sources, like S. Carroll, and I. Ciufolini and J.A. Wheeler. M∧Ŝc²ħεИ_τlk 00:39, 1 September 2014 (UTC)

So I apologize for my angry response. I certainly should have assumed good faith, but you have certainly put in a lot of thought into the deletion. For what its worth, I would have received even a small note on the talk page much more gracefully (since doing that indicates a willingness to work together). In any case, I'm sorry.

As far as your points, I disagree that most of your descriptions above ^ are OR. Your definition of OR would seem to include almost any text on wikipedia that isn't a direct quote. I think as long we can can reasonably interpret information that is already in the article, or information in a source, it need not be considered OR. If you're saying that the already-cited information in the article isn't sufficient to build such an effects section, that's another problem entirely that should be fixed.

I suppose what I want to do is expand the "Higher-order effects" section to include effects both "high-order" and more basic. For example, how does a non-rotating point-mass in motion interact with another one. What effects could be talked about that *are* left out of the higher-order effects section?

Fresheneesz (talk) 06:19, 3 September 2014 (UTC)

I concede that I could have been more thoughtful and informative about my deletion.

I do not mean by OR that it must be nearly a direct quote, but rather that it must be fairly directly substantiated by a believable source (and not involve much interpretation by one individual: this is liable to introduce actual errors). Expanding the higher-order effects would be fair, and if you want we can analyze specific sources for effects to mention. As I have pointed out, I expect many of these to be qualitatively identical to the behaviour of electromagnetism, but it may help to describe them nevertheless. —Quondum 06:49, 3 September 2014 (UTC)

My Thesis

Feel free to study my own derivation of Maxwell's type Euations of Gravitoelectromagnetism, as I publish from my graduation thesis: [3] — Preceding unsigned comment added by 85.240.131.23 (talk • contribs) 2014-12-20T00:42:01

Compendium of original sources and derivations

These proceedings, presented as a book and published in 2000, contain many of the original sources and derivations for the whole content of this article.

Reference Frames and Gravitomagnetism: Proceedings of the XXIII Spanish Relativity Meeting

extreme examples

I very much like the actual values given in the comparison between the gravitomagnetic field on earth and that of the pulsar. A similar comparison could be made for the proton's gravitomagnetic field on an orbital electron in the hydrogen atom versus that field at its "surface".

A similar provision for the approximation due to relativistic effects, as made for the pulsar, would be appropriate. Since it is unlikely that a reference could be found for any of the relativistic effects, calculation would have to be limited to results of the equation. Aqm2241 (talk) 19:35, 30 June 2022 (UTC)

Specific reference needed for the ‘Gravitomagnetic fields of astronomical objects’ section

This article is confidently written and makes a very good impression. A specific reference is badly needed for the formula for the gravitomagnetic field Bg near a rotating body as derived from the GEM equations (given as exactly half of the Lense-Thirring precession rate). I am not even sure that it is true.

I suspect that providing a specific reference is more easily asked for than provided – the original provision of this formula may be lost in the mists of time. I have consulted References 11, 12 and 13 and they are either silent on the subject, very complicated (and expressed in raw GR form), or both.

Biemond (Biemond, J., "The gravitomagnetic field of a sphere, Gravity Probe B and the LAGEOS satellites." arXiv:physics/0802.3346v2 [physics.gen-ph], 14 Jan 2012) examines the GM field of a spinning sphere without, however, being able to give a closed-form answer ‘near’ the sphere. DavidMMIX (talk) 15:39, 18 September 2022 (UTC)

Choice of scaling for this article

My suggestion is to choose the scaling in this article so that the quantities are directly meaningful/understandable to the reader. My suggestion is:

• ρ_g, J_g, F_g and S_g have their standard intepretations
• E_g and B_g are defined via the force equation F_g = m(E_g + v × B_g).

The effect is that B_g gets a multiplying factor in the analogous Maxwell equations. —Quondum 00:38, 18 November 2024 (UTC)