Talk:Gravitational wave/Archive 1

Spooky action
Does "spooky action at a distance" have anything to do with gravitation radiation? I though gravity waves travelled at (or near) c? Isn't spooky action to do with quantum entanglement? miterdale 14:13, 25 Jun 2004 (UTC)

Gravitation radiation has very little to do with quantum entanglement. Also a lot of the assertions in the article were factually wrong.

Roadrunner 16:34, 25 Jun 2004 (UTC)


 * (William M. Connolley 17:39, 25 Jun 2004 (UTC)) Not at all sure I believe this dipole/quadrupole stuff, especially since someone can't spell quadrupole.

Note: I rewrote larger parts of it in July 2004. I hope I deleted every such stuff. Gravitaional radiation has in fact little to do with entanglement or spooky action on a distance. Simon A. 15:49, 16 Oct 2004 (UTC)

What about an orbital cosmic ray gravitic wave detector? lysdexia 11:37, 16 Oct 2004 (UTC)

Yes, that is planned and called the LISA project. I think it is mentioned in the article, albeit only briefly. Simon A. 15:49, 16 Oct 2004 (UTC)

Where has it: "been proposed that certain conductors, especially superconductors, could be made to emit gravitational waves in the laboratory"? If we're talking about this: kook's theories then this mention needs to be removed post haste, because then it's complete pseudoscience. --Deglr6328 04:42, 6 Dec 2004 (UTC)


 * No reply->removed.--Deglr6328 19:44, 12 Dec 2004 (UTC)


 * Although the idea is far-fetched, some astronomers already dream of [&hellip;] gravity phones to communicate via gravity waves across p-branes.

To be sure, astronomers probably have all sorts of exciting dreams. Still, I would like to propose against recording them in Wikipedia in general, and this article in particular. &mdash;Herbee 13:51, 2005 Apr 12 (UTC)

Expert
Ok so I guess I am kind of an expert here. I am a graduate student in large gravity (theory) group at a large university. But I have nothing to do with any actual measurement experiments. My advisor is very well known in gravity.

So looking over the article it seems ok. There are a few factual errors. There are also some places with big words that could use some better explaination for the casual reader. I think gravity waves should be accessable to the casual reader on up to the advanced reader.

I am going to clean over this and make minor corrections. Then I will add a section deriving the gravitational wave from the flat metric and explain what it is and what it means. And I will probably add a paragraph comparing it to E&M radiation and talk more about the Weyl tensor.(CHF 08:40, 27 October 2005 (UTC))


 * Ok I gave the article a good cleaning. It took more work than I thought. Next I think some derivations might be valuable to physics students. (CHF 11:24, 27 October 2005 (UTC))
 * Nice job! And long over due. A true encyclopedia treatment and very accesible. DV8 2XL 12:19, 27 October 2005 (UTC)
 * Thanks. I think I am pretty much done, at least for the parts that I am knowledgable in.(CHF 13:16, 27 October 2005 (UTC))


 * Hi, CHF, good to see someone taking an interest in this. I had a buncha stuff in mind when I added the flag, which I should have listed in a todo list.  I wanted to see a much more discussion at a higher level later in the article, as part of WikiProject GTR, a project which is still in its incipient stages.  One thing which has not yet been addressed is the distinction between the linearized theory of the generation of gravitational radiation (with links to multipoles, far-fields, weak-fields) and exact solutions such as pp-waves which are or will be extensively discussed in Category:Exact solutions in general relativity.

Content
Hi all. The articles linearized gravity and linearised Einstein field equations may be worth a quick perusal to decide exactly where certain derivations should be. The derivation given here (or part of it, at least) is in the linearised EFE article; I planned to go into that in more detail, but server problems put an end to that idea. As there is no point in having duplicate derivations, can we come to an agreement over which article should have the derivation ? Thanks. ---Mpatel (talk) 09:22, 29 October 2005 (UTC)


 * I don't know. I am torn. But I do think that the linearized gravity would need other derivations as well. There is more to linearized gravity than this one derivation. And this article still needs the derivations of the monopole, dipole, quadropole stuff. Personally I see no problem with duplication. (CHF)

"Also, because gravitational waves are so weak (and thus difficult to detect), objects opaque to light are often transparent to gravitational radiation." Objects are transparent to gravitational radiation because the waves are weak? --24.94.189.89 14:13, 2 November 2005 (UTC)

Sources of Gravitational Waves
Someone changed the paragraph to read

In gravity, an object's quadropole moment must be changing in time for it to radiate. This means that a single moving mass (monopole) will not radiate unless it "has bumps" and is spinning. A pair of orbiting masses is kind of like a very bumpy, spinning mass, so they may radiate. A changing monopole moment will not radiate due to the conservation of translational energy-momentum. The changing dipole moment will not radiate due to the conservation of angular momentum. But the changing quadropole moment will radiate gravitational waves. The system will lose energy due to this process. Much as a speeding charge decelerates as it radiates, the gravitational system will spin down as it radiates.


 * It was right, but this is completely wrong. "Bumps" don't do anything and neither does spinning. Changing quadropole moment - learn it, understand it.


 * Please don't edit stuff you aren't sure about. Pick up a book and be sure.(CHF 10:24, 8 November 2005 (UTC))


 * Good grief. In the past I've often tried to explain this in newsgroup posts, and have frequently been dismayed to see how badly people lacking the standard math/physics training often misunderstand what I (or others) wrote.  Ineed, the textbooks by Carroll and by Schutz both offer excellent presentations of quadrupole radiation, but both suffer from notational quirks which can and have misled some well-intentioned but not very well-prepared readers.  (To be fair, both authors are of course writing for well-prepared students, and quite properly so.)
 * Sigh... this is an important and often misunderstood topic. I've been making noises for some time about writing one or more articles on multipole moments in gtr, e.g. distinguishing between "Weyl multipoles", which bear an obvious relation to multipoles in Newtonian gravitation, and the various relativistic multipole moments (known to be equivalent under favorable circumstances).  But right now I'm rather discouraged about the future of WP (see the sad history of Albert Einstein).---CH  (talk) 22:38, 9 November 2005 (UTC)


 * I'd like to appologize for that. (CHF 14:36, 14 February 2006 (UTC))

Last two sections
Removed "trivia" since these topics are in fact extremely important. Alas, these two sections are seriously misleading in various ways. This entire section is so misleading it should be removed entirely:

''Gravitational waves differ markedly from electromagnetic waves in that electromagnetic waves can be derived exactly from Maxwell's equations. However gravitational waves, as a linear, spin-2 wave, as they are often thought of are only perturbations to certain space-time geometries. In other words, classically there are always linear, spin-1 E&M waves, but there are never linear, spin-2 gravitational waves. There are still wave-like fluctuations, but in general things are nonlinear, as is always the case in General Relativity. This is one of the reasons there may be no graviton.

The thing that is analogous to electromagnetic radiation is the Weyl curvature, not the linear, spin-2 wave.''

Thinking of the graviton as something which might "exist" is not, I think, fruitful. Better to think of it as a theoretical concept useful in some ways, but having serious limitations. Much like spacetime is undeniably useful, even essential for formulating metric theories of spacetime, but according to some approaches to a possible quantum theory of gravity, ultimately this is a seriously misleading notion. But I hesitate to say that the concepts which are put forth to replace it are things which might "exist"; rather, they might be conceptually useful under more extreme conditions where the spacetime concept is no longer useful.

I think the author of the last sentence might have been trying to compare Ricci (or Einstein) and Weyl curvature. Ricci curvature is directly proportional to the immediate presence of energy-momentum; Weyl curvature can propagate as a wave, and all that. But of course, in gtr, exact EM waves exist (see monochromatic electromagnetic plane wave for an example) and the immediate presence of EM field energy at various events "causes" Ricci curvature there.

As for "always non-linear", I would avoid using that language. Sometimes general relativity does admit linear superposition. In particular, two pp-waves traveling in the same direction can be linear superimposed.

In static Weyl vacuums, there is a simple formula for superimposing an arbitrary Weyl vacuum and a line mass. It is possible to write down a general formula, but this is not very useful. The fact that this special case is so simple reflects the symmetry group of the class of Weyl vacuums (transformations taking a Weyl vacuum to a new Weyl vacuum, or even the same one but in a different chart). CH

Deleted nonsense
To whoever wrote this:

''Our solar system can not radiate very much gravitational radiation because the planets are mostly on the same plane of orbit with spins aligned to that plane. And even if this were not the case, the gravitational forces in our solar system are very weak with the exception of Mercury and the Sun.''

FYI, spins aligned with the orbital plane or not has nothing to do with the quadrupole moment formula giving the most elementary estimate of the power (energy per unit time) radiating by a binary system such as Mercury, Sun.---CH 00:04, 20 December 2005 (UTC)


 * Spin-spin interactions can make the orbits more stable (decay slower) if they have a certain alignment (both up or one up and one down (but pointing in a little in both cases)). But, yeah it won't totally stop the radiation. (CHF 14:41, 14 February 2006 (UTC))

To whoever wrote this:

The thing that is analogous to electromagnetic radiation is the Weyl curvature, not the linear, spin-2 wave.

This doesn't even make sense. Maybe you were confusing the linearized gtr approximate treatment of weak gravitational radiation with the real McCoy? See the textbooks by Carrol, Stephani, Schutz, and MTW for some excellent discussion of analogies of weak-field gravitational radiation with EM radiation. Analogies with strong-field radiation also exist but are harder to explain, so let's get the simple stuff right first. ---CH 00:17, 20 December 2005 (UTC)


 * (LOL) Now I recall that I wrote the second paragraph I quoted!  That is, I wrote it in on this talk page--- and then someone else added that sentence to the article without the context!    This illustrates the point that context is everything: if you yank meaningful sentences from another users writings and insert them randomly into an article, you are likely to turn sense into nonsense, in addition to disrupting the flow of ideas in the article.---CH 13:30, 21 December 2005 (UTC)

Detection
The section Detection in the current version is very badly written. I started to try to improve it, but threw up my hands. Surely the WP community can do better than this! See the expository papers listed at Relativity on the World Wide Web for some excellent sources you can use. ---CH

Derivation
This section needs a new introduction clarifying that this "derivation" involves the linearized approximation. Indeed, this is a linearized perturbation of Minkowski vacuum. ---CH 00:20, 20 December 2005 (UTC)

One part in 1021, not 10&minus 21
Hi, 24.61.41.56, thanks for noticing the wrong sign. Someone other than myself keeps putting in the incorrect minus sign, and I don't know why that is attributed to me (I don't even use that syntax!). Whoever you are, do you see that one part in ten expresses the ratio one:ten? So one part in 1021 is what we want here.---CH 10:29, 23 December 2005 (UTC)

FWIW, apparently this happened when I tried to revert to a previous version, and didn't notice that version contained the mistaken sign.---CH 10:52, 23 December 2005 (UTC)

Production of gravitational radiation
139.184.30.17, you wrote  A gravity wave arises due to density changes in a fluid. But this claim is not only ambiguous, it is incorrect under either interpretation: Please do not edit this article if you do not have your facts straight, OK? TIA---CH 10:37, 23 December 2005 (UTC)
 * density perturbations do not always produce gravitational radiation (consider a spherically pulsating star)
 * gravitational radiation will be produced by any source having a time varying quadrupole moment tensor (or time-varying higher multipoles), regardless of whether the source contains a fluid (consider a bar rotating about an axis orthgonal to its symmetry axis).



Ooops, I get it, you said "gravity wave", not "gravitational wave". However, since this article is about gravitational waves, I'll try to replace that explanation with a link to the appropriate article. Also, my reversion fixed another problem in which someone replaced "distant events" (correct) with "massive events (nonsensical). Sorry for falsely accusing you!---CH 10:43, 23 December 2005 (UTC)

On second thought, I think the link to Gravity wave should suffice. ---CH 10:48, 23 December 2005 (UTC)

Possibly inappropriate links
This paper is apparenlty an unpublished comparison of gravitational radiation in various theories, which I haven't read, but the fact that it is unpublished may be a warning sign:
 * Gravitational Radiation by Warren F. Davis

—Preceding unsigned comment added by Hillman (talk • contribs) 13:39, 24 December 2005

(Sorry all, I did indeed forget to sign my own comment, so I just signed after the fact.---CH 08:53, 6 May 2006 (UTC))


 * Before gratuitously flagging a "warning sign" about this paper, I would suggest that the writer take the trouble to read it or, :at the least, exert himself sufficiently to learn something of the paper's origin. It is, in fact, a slightly edited version of :my MIT Ph.D. thesis.  It passed muster at MIT; it ought to pass muster here!


 * --- Warren F. Davis, Ph.D.


 * —Preceding unsigned comment added by Wfdavis (talk • contribs) 21:12, 23 March 2006

Please take no offense, Warren, but if your thesis was published shouldn't you also have sent it to the arXiv? If you had so (and it's probably not too late, if this is a recent thesis), I would almost certainly have seen it and at least skimmed it. With experience (if you choose to hang around WP for very long), I think you will soon learn why we old timers react with suspicion to links to personal webpages! (Now I'm curious--- why wouldn't you put up your Ph.D. thesis at your academic website? Oh well, send it to the arXiv; I look forward to reading it.  Well, more likely skimming it...)

BTW, you can sign and datestamp your comments simply by typing ~ (four tildes) at the end of your future comments.---CH 08:53, 6 May 2006 (UTC)

More linkspam and POV pushing
Anon 130.184.88.44 (University of Arkansas at Fayetteville) added several links within the body of the article to a cranky website promoting alleged proprietary "technology",. These links were embedded in text promoting claims about high frequency gravitational radiation which must be regarded as fringe at best. Such edits are somewhere between linkspam and POV pushing and are deprecated. ---CH 19:18, 3 March 2006 (UTC)

Improvement urgently needed
Wrt User:Miserlou's addition of flag, gravitational waves do carry energy in general relativity, and this has been been proven long ago, but the current version is not quite right in what it says about this and many other matters. Caveat emptor.---CH 22:38, 17 April 2006 (UTC)

Someone apparently queried this oddly phrased statement: ''Just as an electromagnetic wave has electric and magnetic components, so too does a gravitational wave have gravitoelectric and gravitomagnetic components. A "+" wave has an "X" gravitomagnetic component, and vice versa.'' I certainly wouldn't put it like that, and I haven't seen anything in the literature phrased like that. Possibly whoever wrote it was thinking of the case of vacuum pp waves, wherein for a plus polarized wave the electrogravitic and magnetogravitic tensors, taken wrt a family of Rosen observers (certain inertial observers with purely transverse expansion) with spatial basis vectors "aligned" with the wave, look like
 * $$E[\vec{X}]_{\hat{a}\hat{b}} = \left[ \begin{matrix}0 & 0 & 0 \\ 0 & \alpha & 0 \\ 0 & 0 & -\alpha \end{matrix} \right], \; \; B[\vec{X}]_{\hat{a}\hat{b}} = \left[ \begin{matrix} 0 & 0 & 0 \\ 0 & 0 & \alpha \\ 0& \alpha & 0 \end{matrix} \right]$$

whereas for a cross polarized wave, these look like
 * $$E[\vec{X}]_{\hat{a}\hat{b}} = \left[ \begin{matrix}0 & 0 & 0 \\ 0 & 0 & \beta \\ 0 & \beta & 0 \end{matrix} \right], \; \; B[\vec{X}]_{\hat{a}\hat{b}} = \left[ \begin{matrix} 0 & 0 & 0 \\ 0 & \beta & 0 \\ 0 & 0 & -\beta \end{matrix} \right]$$

where $$\alpha,\; \beta$$ are appropriate functions. But I have never seen these called gravitomagnetic components and gravitoelectric components.

Every time I look at this article, it seems to be in worse shape, which tends to discourage any plans to try to improve it, unfortunately. ---CH 08:26, 6 May 2006 (UTC)

My recent rework
I've made some drastic edits recently to arrange things in a way that non-experts (like myself) will find more accessible than it was before. There were a lot of parenthetical remarks that didn't seem to contribute much, and a lot of digressions that, while interesting, destroyed the flow of the article. If anyone wants to reinstate material I deleted, please consider re-adding it rather than reverting my changes. --Doradus 17:30, 9 May 2006 (UTC)


 * Thanks. I was just about to say its still a "sprawling mess" :-/ but at least you are trying.  Someone asked for citations to pseudotensors and gravitational radiation: recently there have been a ton of arXiv eprints on pseudotensors.  Some years ago, Bel superenergy tensor was also popular for gravitational plane waves. ---CH 03:34, 12 June 2006 (UTC)


 * Hmm... lemme try to make a more helpful critique :-/ The basic reason this is a sprawling mess is that WP desperately needs good articles on various topics, or already has articles, on which this article should draw rather than reinventing the wheel:
 * multipoles (separate articles on conventional multipoles in Newtonian gravitation, linked to electrostatics &c., and on relativistic multipoles, linked to related notions)
 * weak fields in gtr
 * far fields in gtr
 * LIGO
 * I'd like to write these myself, but arguing with idgits over work I've already done is taking all my time.---CH 03:38, 12 June 2006 (UTC)

Todo list
As a courtesy, I have removed the "expert" items from the todo list. I am leaving WP and doubt anyone else will know how to implement the suggested improvements since this was mostly a note to myself.

While I never extensively edited this article, and mostly hadn't the heart to monitor it, since I feel it is rather awful and concerns a subject dear to my heart, from time to time I make more or less specific suggestions in this talk page. Sadly, I am now abandoning this article to its fate.

Good luck to all students in your search for information, regardless!---CH 00:09, 1 July 2006 (UTC)