Talk:Einstein field equations/Archive 1

Archive 1

Notation change?

I propose that the notation should be changed in the article to reflect more standard usage in the field. In particular, spacetime indices should be greek letters, and the Einstein tensor should have the letter G, instead of E

Comments? Lethe 22:13, Jun 16, 2004 (UTC)

I think that many people nowadays use Latin letters for spacetime indices (although a lot of people still use Greek). I think that for consistency's sake, as well as keeping up to date with most research papers, it's better to use Latin letters for spacetime indices.

Totally agree with the comment about the Einstein tensor (I made the new change).

I think that the article is inappropiately using Wald's abstract index notation, where tensors are denoted by G_{ab} (with subindices a,b,c,d...) , and the subindices do not indicate the component of the tensor but rather the rank of the rank of the tensor. G_{ab} represents the tensor itself, and not its components. Tensor components are still indicated by Greek letters, G_{\mu\nu}. Many research papers use Wald notation. So I propose:

• Either to use the abstract index notation adequately (and explaining it),

• Or to work in components, as traditional, and thus change the indices from ab to \mu\nu (or ij).

--Daniel Arteaga 12:31, 28 Mar 2005 (UTC)

That last comment has clarified a few things. I would prefer to use Wald's abstract index notation. Let's try this approach with proper explanation.

Cleaning up needed ?

I think this article could do with some cleaning up.

(1) The notation change has been mentioned.

(2) Is the section on 'tensor geometry' needed ? (should it be in another article ?).

(3) The section on solutions of the field equations should surely come after the section on the field equations themselves.

(4) Perhaps a qualitative discussion of the initial value problem should be included.

That's all I can think of just now.

edits made

moved discussion of exact solutions to page on exact solutions. Mpatel 17:32, 6 Jun 2005 (UTC)

Exact solutions discussion

Lethe -

In dicussing the solutions of the EFE, the new text is much more correct than the older one you reverted to. Also, the exact solutions are of secondary importance here. Mpatel has moved them to a new page, with an updated explanation of what an exact solution is. I for one approve of this organization, but just as importantly I approve of what he is saying in the editted pages.

At the least, if you think that the exact solutions belong in the EFE page then kindly make your case for that here first. Then if we decide that it is proper one of us will merge the pages back together. In the meantime, I repeat that I highly approve of the totality of Mpatel's edits: What is written about EFE solution, both in general and of the exact kind, is a wast improvement over the previous text. At the least, that older text should be kept in the history were it belongs.

--EMS | Talk 18:44, 6 Jun 2005 (UTC)

Apologies for not fully explaining (here) why I moved that chunk of text on exact solutions. I was doing a lot of chopping and changing and must have forgotten to explain it here. I was also worried that some1 might revert my edits, but I'm glad that at least 1 person approves of them. Mpatel 11:11, 8 Jun 2005 (UTC)

Firstly, let me say that I mostly reverted because I thought I saw a fairly large deletion, without edit summary, and just assumed it was a mistake. If I had known that it was intentional, I would not have reverted. So I apologize for that misunderstanding.

But now that I bring my attention to it, I don't really agree with the change. Here are my reasons:

1. exact solutions of einsteins equations are probably the most important results of the theory, and at least some of them (and why not a list of all of them?) should be here
2. the text in question is actually quite short. it's just a list with a very short summary and separate link to each solution. if the list of solutions is nothing more than a list, why should it warrant its own article?
3. the new article has a wrong name. exact solutions?? That's far too specific! Do you envision eventually including exact solutions to Navier-Stokes, Yang-Mills, the Klein-Gordon equation, and Maxwell's equations, all in one place? What about the rest of the diff eqs? I suppose this objection could be easily laid to rest by renaming the article exact solutions to the Einstein field equations or some such.
4. the link is not prominently displayed enough. as I mentioned earlier, this is a very important part of the theory. It should have its own section, and if there is so much to write on the subject, then a subpage should be made, and linked prominently from the top of the summary in the main page, as is always done in featured articles.

Basically, I think I could be happy with the way things are, if we still had the list of exact solutions, with a 5 word commentary on each solution, along with two sentences about the general principles of exact solutions, and a link to a (properly renamed) subarticle. the subarticle would then have detailed information about the methods of finding exact solutions. maybe a synopsis of the solution for each guy in the list. well... I'm dreaming.

Anyway. I think the move could be good in the long run, might not be so good now, but whatever, as long as it wasn't a weird deletion, I don't care. The text is still somewhere, and is linked to from this article. I didn't realize that at the time, and that is reason enough to revert my undeletion. -lethe ^talk 12:53, Jun 8, 2005 (UTC)

I agree with you that the "exact solutions" article should be renamed. I can also agree that some coherent discussion of solutions of the EFE should be in this page. Indeed, solving the EFE in general needs to be discussed.

My big reason for being vehement on keeping the changes is that exact solutions of the EFE are not "physically realizable" solutions as the old text claimed. Instead they are a subset of the EFE whereby a metric tensor can be formulated with an Einstein tensor that is equal to the stress-energy of the spacetime in question. Some exact solutions (such as the Alcubierre metric) may not be physically realizable. (The Alcubierre metric uses negative energy, whetever that is and if it even is.) At the same time, other cases lacking exact solutions (such as two bodies orbiting each other) are quite obviously not just physically realizable but physically realized.

So overall you have some good thoughts here. All I ask is that their implementation be done in a way that moves the article forwards.

--EMS | Talk 14:56, 8 Jun 2005 (UTC)

In making the edits, I was thinking long term. The articles exact solutions (ES) and Einstein's field equation are quite short at the moment and they will no doubt be expanded in the future (for example, in ES, I was thinking of perhaps including descriptions of various techniques used in searching for exact solutions of EFE - Petrov types, Segre types etc.).

I agree that some of the exact solutions could be mentioned in the EFE article, but they should definitely be mentioned in ES - the problem/question then becomes: 'is it OK to have that same list on both pages ?' Certainly, a handful of the important ones could be mentioned in passing on the EFE page but a more complete list should be in ES. I don't think there is much point in listing all the known exact solutions (Kramer, Stephani et al have done that).

Agree about the name change for ES.

Mpatel 16:51, 8 Jun 2005 (UTC)

Mpatel - I think that you are on the right track with these thoughts. "'[I]s it OK to have the same list on [multiple] pages?'" To me it depends on the size of the list and its importance to the article(s). However, in general the answer would be "No". Some overlap is needed to relate the pages, but their covering the same ground to a large extent often is troublesome. (For example, before you fixed it the EFE and math-of-GR pages disagreed as to what an "exact solution" is. [I was about to fix that myself, but you beat me to it.]) So I approve of your idea of listing some important metrics here, while keeping the full list in the exact solutions page.

We may also want to consider having the subarticle be on solutions instead of just exact solutions. However, it should be noted that exact solutions are preferred since anything odd in a non-exact solution begs the question of "is this real, or is it an artifact of the computation that would vanish if a (more) exact solution was known?".

--EMS | Talk 02:18, 9 Jun 2005 (UTC)

teleparallelism

Considering the fact that Einstein himself wrote a paper on the teleparallel formulation of gravity, I think it should stay in the article. No? -lethe ^talk 22:35, Jun 10, 2005 (UTC)

I have been looking at this, and it seems to be a failed attempt to create a unified field theory incorporating causitive mechanisms for both gravitation and electromagnetism. I will first direct you to the comment at the end of this report of Einstein's. It seems that he was having trouble generating correct equations of motion. This is only one of the reports I found on this teleparalellism web site. Overall it seems to be an effort that got dropped after 1930, and that probably says a lot about it. I have also found a 2002 newsgroup posting by Chris Hillman on this matter which leads to me believe that the theories are distinguishable. There is also an implication that the predictions become identical in the special case of no torsion, in which case the theories have become identical!

I don't know anything about teleparallelism being used to unify field theory and gravity, but it doesn't surprise me that people would try that. I mean, they tried it with GR (cf. Kaluza-Klein), and teleparallelism looks superficially more similar to field theory than GR does. For a more conservative introduction to teleparallelism (i.e. without any unification), see gr-qc/0011087. Equation 21 of that paper is the Einstein field equation in the teleparallelism formalism. My recollection is that the two theories are identity on topologically trivial spacetimes, but not necessarily so on nontrivial spacetimes. Of course, no one knows the topology of our spacetime yet, so there's no experimental reason to prefer one over the other. -lethe ^talk 02:36, Jun 11, 2005 (UTC)

Unification seems to have been Einstein's goal with this. I gather he at some point decided that he was barking up the wrong tree, and dropped it.

I also agree that there is no known experimental reason to prefer GR to teleparallelism. However, I will point out three things:

• I myself am working on a modification of GR to remove the black hole from theory. It also agrees with extant experimental results within the margins of error for the observations. However, I will not present it in Wikipedia due to its being truly original research, and its lacking any support in the field at this time.
• There is this little thing called Occam's Razor which says that when two theories give results which are in accord with observation, preference should be given to the simpler one. GR seems to be a special case of teleparallelism, and so is the simpler theory.
• Unless I am mistaken, noone has seen if teleparallelism works in the medium strength gravitational fields of the binary pulsars. This is a serious issue as the binary pulsar observations blew Rosen's bimetric theory out of the water, and have set a lower bound on the parameter ω for the Brans-Dicke scalar-tensor theory which is so large that the difference between it and GR is at best miniscule.

Einstein embraced teleparallelism as a chance to extend GR. It seems to have failed to to live up to that promise. If one cannot distinguish between the theories, then teleparallelism is useless unless you can show that it is a better fomulation of GR. On the other hand, if there is a difference, then the statement that they give the same results is false and this is an alternate theory. Either way, it does not belong here. --EMS | Talk 05:21, 11 Jun 2005 (UTC)

My impression is that this is an alternate theory, and one that is followed by a small minority of physicists and possibly not any of any standing. If you like, we can ask Chris for his opinion. (I don't want to bother him unless it is needed and will be a good use of his talents, but this issue appears to qualify on both counts.) However, mine is that it does not belong here. If it deserves mentioning then it should be done in the GR article itself in the alternate theories section or in a subarticle on alternate theories. Indeed, the Einstein-Cartan formalism (which this seems to be related to) also should not be here as I see it, but I am not yet sure of what to do with it.

Yes, teleparallelism is an alternate theory, but a viable one. It deserves a mention on wikipedia. If you argue that it should go in the GR article and not this one, I'm willing to listen to that argument (though I think it could well go in both). Maybe I agree with your argument, and then we can move it, but at the time, it looked like you were deleting, not moving, and that's why I reverted (as with MPatel's edit earlier.) -lethe ^talk 02:36, Jun 11, 2005 (UTC)

I was deleting it. I felt that it does not belong here, and still do. Yes. I think it should be moved to the GR article and listed as an alternate theory. Either that or the reference should be dropped completely. Once again, this article is about the EFE. If an alternate theory supports the EFE, it is redundent here. If it does not support the EFE, then it is irrelevant here. Either way, discussions of alternate theories belong elsewhere. So I won't touch that reference for now. I make no promises for later, or for anyone else.

BTW - I couldn't care less about whether teleparallelism is viable, and neither does Wikipedia according to the Wikipedia NPOV policy. In that policy it is stated that if only a very small part of a community supports a certain viepoint, its being covered in Wikipedia is inappropriate even if their viewpoint is correct. So don't show me the teleparallelism is viable. Instead show me that it has support in the community of physicists! --EMS | Talk 05:21, 11 Jun 2005 (UTC)

So for the moment I will leave the teleparallelism reference alone. It's having a defender is reason enough to do so for now. However, I ask you consider that for reasons of scope that it does not belong here, and that for reasons of its being followed by a very small minority of physicists may not even be appropriate to mention in Wikipedia as an alternate theory (but it may be worthy of mentioning in the development of general relativity article). --EMS | Talk 01:48, 11 Jun 2005 (UTC)

I would have to agree with EMS's view that teleparallelism is a theory and does not belong in the EFE article. I think that even a passing mention of teleparallelism's field equations are inappropriate in the EFE article, as 'EFE' is almost always taken to mean 'the field equations that Einstein wrote down in formulating GR', no mention of teleparallelism being mentioned - as EMS wrote above, teleparallelism was taken to be an extension of GR. --- Mpatel 10:30, 11 Jun 2005 (UTC)

I moved teleparallelism and  Einstein-Cartan theory to general relativity.  -lethe talk 06:57, Jun 13, 2005 (UTC)

nonlinearity of EFE

Mentioned a few things about EFE being nonlinear in the metric. I think it's ok to mention differences between EFE and other dynamical equations like Maxwell's equations (ME) and Schrodinger's equation (SE) (yes, I know there is more than 1 type of SE), as it's a pretty important mathematical difference with physical implications - of which I have yet to explore properly. ---- Mpatel 14:20, 11 Jun 2005 (UTC)

vacuum solutions

Vacuum solutions incorporated into EFE, as the vacuum field equation page seemed more like a definition rather than an article. --- Mpatel 15:45, 13 Jun 2005 (UTC)

I also had the same idea. Thanks for doing this. ---CH

Tetrad section

That 'tetrad formalism' section has been bugging me for a while now, as it didn't quite seem to fit in to the general discussion of the Einstein field equations (on the face of it, it's really got nothing to do with the field equations). Therefore I have moved it to the exact solutions page where it fits in a lot more naturally (many techniques used to obtain exact solutions, for example, employ the tetrad formalism). Hope this is ok. ---Mpatel 28 June 2005 14:01 (UTC)

Franklin Felber?

Shouldn't this claim http://www.physorg.com/news10789.html be mentioned somewhere in the article? --Gene s 07:48, 12 February 2006 (UTC)

Hold your horses :). Felber hasn't actually presented the 'solution' to the EFE. Once it's presented, we'll be better able to assess the claim that it is actually an exact solution of the field equations of general relativity. There have been claims by people of discovering an alleged solution, but which are, in fact, not (for example, the Alcubierre drive). The whole business of anti-gravity (especially antigravity in GR) is a little wild. So let's just wait and see exactly what Felber is claiming and once all the maths has been ploughed through and interpretations have been clarified, then we'll all be in a better position to assess what significance Felber's research has. However, I do agree that it is worthy of mention (whatever the outcome), but it should be mentioned in the exact solutions page, not here. MP (talk) 08:53, 12 February 2006 (UTC)

Reasons for tag

There are two problems here that need resolving:

• This article is misnamed. It should be the Einstein field equations, not Einstein's field equations Einstein's field equation.
• The introduction is in error. The field equations are a tensor expression representing up to 10 differential equations. This is mentioned in the bodu of the article, but the inconsistency between the intro and the text is not good.

The big problem with resolving the name problem is that the current redirect at Einstein field equations has to be removed before this page can be moved to that title. --EMS | Talk 05:54, 17 February 2006 (UTC)

Are you saying you think the name should be singular (equation) or plural (equations)? The singular fits better with my mathematical sensibilities, but the plural sounds better to my ears. -- Fropuff 17:33, 17 February 2006 (UTC)

I agree with EMS' first suggestion: the article should be called the Einstein field equations (most popular designation). How do we get rid of the redirect - is it simply a matter of deleting the redirect (once consensus has been reached) ? MP (talk) 18:17, 17 February 2006 (UTC)

Yes, any admin can delete the redirect and move the page if there is a consensus to do so. -- Fropuff 18:37, 17 February 2006 (UTC)

1. I have corrected my statement above. (darm those pestky "s"-es.)
2. The redirect can be removed and the rename done by using WP:RfD WP:RM.

If one of you would like to initiate the RfD move request, I would appreciate it. Otherwise I will get around it over the long weekend. --EMS | Talk 22:10, 17 February 2006 (UTC)

The following discussion is an archived debate of the proposal. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.

The result of the debate was move. —Nightstallion (?) 21:59, 23 February 2006 (UTC)

Requested move

Einstein's field equation → Einstein field equations – This article is misnamed. Einstein field equations is the correct name, but this is currently a redirect to the current name.

Add *Support or *Oppose followed by an optional one-sentence explanation, then sign your vote with ~~~~

(Note that this section has been created per the WP:RM instructions, as is to permit the administrators to judge if there is a consensus in favor of the move.)

Discussion

• Support - "Einstein field equations" is the correct name as described above. Due to its using tensors, this is a set of equations, not just one. Also the use of the possessive 's is uncommon in scientific literature. Usually the attachement of a proper noun is enough to state whose equation it is. --EMS | Talk 05:22, 18 February 2006 (UTC)

I would hardly say that one name is correct and the other wrong; its just a matter of how you look at it. You can consider it a single tensor equation, or a set of ten component equations. If you think of a tensor has an object in its own right then setting one tensor equal to another is a single equation. (I'm not voting as I'm ambivalent.) -- Fropuff 06:36, 18 February 2006 (UTC)

I think that the real issue is what the standard terminology is. I am used to this being called the "Einstein field equations", but as I do a textbook search I am finding an amazing lack of consistency in this regard. What I am seeing is:

• Einstein's equation - Wald, Robert M. (1984). General Relativity. Chicago: University of Chicago Press. ISBN 0-226-87033-2.
• Einstein's field equation - Ohanian, Hans C. & Ruffini, Remo (1994). Gravitation and Spacetime (2nd ed.). New York: W. W. Norton. ISBN 0-393-96501-5.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Einstein field equations - 2 books:
  • Bowler, M. G. (1976). Gravitation and Relativity. Oxford: Pergamon Press. ISBN 0-08-020408-2.
  • Earman, John (1995). Bangs, Chrunches, Whimpers, and Shrieks : Singualtities and acausalities in relativitistic spacetimes. New York: Oxford University Press. ISBN 0-19-509591-X.
• Einstein equation - O'Niell, Barrett (1995). The Geometry of Kerr Black Holes. Wellesley, MA: A. K. Peters. ISBN 1-56881-019-1.
• Einstein gravitational field equations - Ellis, George F. R. and Williams, Ruth M. (1988). Flat and Curved Space-Times. Oxford: Clarendon Press. ISBN 0-19-851169-8.{{cite book}}: CS1 maint: multiple names: authors list (link)

Let me put it this way: The plurality at least is for "Einstein field equations". --EMS | Talk 17:20, 18 February 2006 (UTC)

• Comment I much prefer "Einstein equation" or "Einstein field equation", because I think the possessive is unnecessary and infrequent in today's usage. I like the singular because it is almost always written as one (tensor) equation, but I could live with Einstein field equations. –Joke 17:53, 21 February 2006 (UTC)

  On the possessive part, agreed! As for singular vs. plural, I realize that the EFE is a single tensor equation, but it contains multiple "scalar" equations. --EMS | Talk 20:34, 21 February 2006 (UTC)

The above discussion is preserved as an archive of the debate. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

More general intro ?

Perhaps the introduction could be made slightly more intuitive. I can't help thinking that general readers will be turned away by the words, 'stress-energy tensor' and 'Einstein tensor'. I think what I'm saying is that there is too much technicality for the opening sentence. The article should start by generally telling the reader what the EFE describes. Also, all the definitions should be at the start. Begin quite generally, then we can get specific. I think the opening sentence should be fairly short, snappy, and intuitive with minimum technicalities.

Perhaps something like the following:

'The Einstein Field Equations (EFE) are a set of ten equations in Einstein's theory of general relativity describing the fundamental force of gravitation as a curved spacetime caused by matter and energy'. The EFE are sometimes called Einstein's equation or Einstein's equations.

'The EFE collectively form a tensor equation and equate the energy-momentum tensor (representing the sources of curvature) with the Einstein tensor (representing spacetime curvature)'.

Then keep the second paragraph of the present version. MP (talk) 12:15, 3 March 2006 (UTC)

I think that it is a good start, but you cannot use the term "energy-momentum tensor" because

1. the proper name is "stress-energy tensor", and
2. that link redirects to something called "stress tensor" which is much less then helpful.

Also, do be aware that momentum is also a source term for gravitation. So it is really four-momentum as expressed in the stress-energy tensor that is the real source of gravitation. --EMS | Talk 16:24, 3 March 2006 (UTC)

I've tried out an amalgam of our approaches to writing the introduction. Is it ok ? MP (talk) 18:06, 17 March 2006 (UTC)

Solutions of the EFE

There really needs to be an article on Solutions of the Einstein field equations, as non-exact solutions need to be discussed too. This future article could discuss solutions in general, then have a subsection on exact solutions (with a 'main article' thingy). Some techniques for finding non-exact solutions can be briefly discussed. Oh dear, that link redirects to exact solutions. We should get rid of that redirect. MP (talk) 15:52, 28 March 2006 (UTC)

I think it's a good idea to have such an article. Now we just need to find someone to write it. If someone does write it, then it makes sense to remove the redirect, but until then, it should stay. -lethe ^{talk +} 16:03, 28 March 2006 (UTC)

I'll second these views. --EMS | Talk 06:09, 29 March 2006 (UTC)

Created the new stubby article as above. MP (talk) 11:12, 16 April 2006 (UTC)

Other uses of the term ?

Should there perhaps be a mention somewhere in the article of other uses of the term EFE? - I'm just referring to the fact that in higher-dimensional theories, Einstein's field equations appear and they are still called by the same name. MP (talk) 15:43, 18 June 2006 (UTC)

I don't think there is anything in the article that is particular to 4 dimensions. -lethe ^{talk +} 16:27, 18 June 2006 (UTC)

Actually, the article mentions Einstein's theory of general relativity and that is 4-dimensional. MP (talk) 21:38, 18 June 2006 (UTC)

edits by Allen McC.

This editor is making two sets of changes. The first is changing the indices from a and b to μ and ν. I have little issue with this.

The other change is in the sign of the right hand side of the EFE. This editor claims that a negative sign is more common (and equivalent), but I rarely see that, and it was not presented that way in my GR courses at the University of Maryland, College Park. (This editor is also claiming that the two forms are equivalent, which could be the case but for which I would want documentation by an authoritative source.) I wonder if this editor is not being confused by the trace of the EFE being $R=-T$. --EMS | Talk 20:51, 19 December 2006 (UTC)

Ever since I came to Wikipedia, I have been trying to use the abstract index notation in most relativity articles, so I do have an issue with the first set of changes by Allen McC. As for the second set of changes (the minus sign), I must confess that I don't recall seeing the -ve sign on the RHS of the EFE too often (I think I may have seen it somewhere, but it was an old book). Usually, the RHS has a plus sign. In the university where I learned GR (in Britain), we used a plus sign. MP (talk) 22:02, 19 December 2006 (UTC)

We seem to be on the same page about the RHS sign. If this is an equivalent form, then it should be noted, but like you I have rarely seen a minus sign there. I will support you on the indexing. --EMS | Talk 22:33, 19 December 2006 (UTC)

Do a newtonian approximation of the metric $g_{\mu \nu }=\eta _{\mu \nu }+h_{\mu \nu }$ where h is a disturbance in the flat metric eta=(-1,1,1,1). You end up with: $g_{00}=-\left(1+{\frac {2\phi }{c^{2}}}\right)$ where phi is the gravitational potential. Rewriting yields $\phi =-c^{2}{\frac {g_{00}+1}{2}}$. Then you look at Poisson's equation $\nabla ^{2}\phi =4\pi G\rho$. The stress energy tensor holds $T_{00}=\rho c^{2}$. Combining all that you get $\nabla ^{2}g_{00}=-{\frac {8\pi G}{c^{4}}}T_{00}=-\kappa T_{00}$. That's where the form comes from, you see? I seems that eta=(-1,1,1,1) is commonly used. Weinberg uses it, Dirac uses it, some lecture pdf's use it, Misner-Thorne-Wheeler use it too. When I first derived the field equations I used $\eta =(1,-1,-1,-1)$ (i think adler-basin-schiffer uses this one) and got $g_{00}=1+{\frac {2\phi }{c^{2}}}$ for the approximation. But when I tried to determine the constant, I just looked at the potential phi=-GM/r which goes with a minus by definition(!) and I found just another way to get the minus there. So no matter what eta you use, you end up with the minus which comes in because of the convention to write the grav. potential with a minus. I've looked though all the books and papers now and I've not yet seen another notation so it must be correct - or at least the convention that is commonly used. Hence I suggest that we use the same notation here - same applies for the indices! It is common to write $\mu ,\nu$ when you have 4 dimensions - in three dimensions it's common to write i,j. We should use the same notations here as those that are used in the textbooks because there is no good reason to make the equations look different; it only confuses it think. Even Einstein used $\mu$ and $\nu$ in his original papers; I just found a "-k" in his paper too and now I think I figured it out: the minus sign is part of the konstant, but it is common to write "-k" and make the constant positive, surely you can write "k" and say the minus is in there. But when you write 8piG/c^4 you should not forget the minus; I guess that's where this error comes from. --Allen McC. 23:35, 19 December 2006 (UTC)

There isn't any error in the RHS being positive. nor do I see anything in your math other than a bunch of handwaving. I am more interested in your sources as they are what matter here, but I need more explicit refereces rather than a bunch of name dropping. From what I am seeing, Ohanian uses a negative RHS while Wald and most web sites use a positive RHS. I don't see that value that you are adding to this article. --EMS | Talk 06:30, 20 December 2006 (UTC)

P.S. Rather than firghting over the sign, we need to obtain some better sense why the sign is arbitrary, or if it is not what considersations control it. That is information that should be in the article. --EMS | Talk 17:56, 20 December 2006 (UTC)

I'm backing up EMS on this one. I believe I've seen "-k", but I've never seen -8piT. The positive sign is conventional in textbooks, research literature, and on Wikipedia. Introducing confusion over the sign will not be helpful to the reader. Allen, you'll have to be much more persuasive if you want to make a substantial change like this.

P.S. I could care less about the alphabet of the indices. It was just easier to revert that way. --MOBle 22:31, 20 December 2006 (UTC)

Thanks for the support. Now I can continue to revert as needed without worrying about WP:3RR. --EMS | Talk 22:37, 20 December 2006 (UTC)

I notice that Allen is fairly new to Wikipedia, so let's try to stay cool. Allen, I hope you don't feel ganged-up-on (and please do take note of WP:3RR). The way to resolve this is to use outside sources, develop consensus here, then make any changes. For my part, all outside sources I've ever used have the positive sign, and that's how I think we should keep it.

Again, I have no issue with "ab" versus "\mu\nu", so anyone who wants to can feel free to change that part back, if you ask me. --MOBle 22:56, 20 December 2006 (UTC)

Someone was so kind to send me this link: Sign_convention, so it looks like that it's conventional - just as I said. I've had a conversation with several profs at my university and they all agreed that the minus sign is more commonly used (at least by them). Now this link tells us a lot and it seems to be an old issue. Still, the textbooks use the minus sign - I've looked though about 7 of em. Of course, the sign doesn't change the physics. So my idea was to change the formulation (indices, sign) to the formulation that is more commonly used, so that it doesn't conflict with that. It should be mentioned though, or maybe we should write something like $\pm k$ together with a little comment on what that's all about; or we should mention the article about the sign convention. Not loosing a word on this is definately not an option. I'm gonna change it back, save for the minus. The other changes are ok I think.--Allen McC. 01:01, 21 December 2006 (UTC)

Excellent! That edit explains what I wanted explained. I took a look at Wald and Ohanian again. Sure enough, Wald is using the -+++ signature with a positive RHS, while Ohanian uses a +--- signature and has a negative RHS! I won't quibble over the tensor index notation myself, but MP does prefer the use of Wald's abstract index notation and its use of latin indices for pure tensor equations (such as the EFE). --EMS | Talk 02:22, 21 December 2006 (UTC)

I enthusiastically agree that the equation could be right if you changed the sign of the metric. The definition of the Riemann tensor would be unchanged. The Ricci tensor is one contraction of Riemann with the metric, so you would get a relative minus sign there. You get two more with the Ricci scalar times the metric. In this case, $R_{\mu \nu }-{\frac {1}{2}}Rg_{\mu \nu }$ gets a relative minus sign. I also agree that the signature of the metric is entirely arbitrary.

However, I have never seen a source in General Relativity which uses the +--- signature. Particle physicists use +--- all the time, but they do Special Relativity. I've only ever read Wald and MTW as textbooks, but all the GR literature I've ever read (which is a lot) uses the -+++ convention. Also, +--- appears in no lecture notes I've taken at Caltech or the University of Chicago.

The point is: Saying +--- is "widely used in the standard textbooks" suggests that all those textbooks use that convention. I don't believe that this is true (though I don't have my books with me at the moment), and it's unnecessary at best. I'm just deleting that one sentence. --MOBle 06:10, 21 December 2006 (UTC)

I've looked though Einstein's original paper and he used $\eta _{\mu \nu }=diag(-1,-1,-1,1)$ where the time coordinate is $x^{4}$. I've seen it in many books but I'm not sure anymore if there's a "common signature" - I think both are in use and that's it.--Allen McC. 14:23, 21 December 2006 (UTC)

-+++ is definitely the most common signature used in GR, to the point that some people claim it is a de-facto standard. +--- is used with spinors however, and I prefer it myself as it makes the invarient length squared of timelike path and the associated mass-energy sqaured positive. However, I feel that it is incumbent on us to reflect the current wisdom (or habits) in the field. Only in the lack of direction from the literature should our personal preferences be used. --EMS | Talk 15:53, 21 December 2006 (UTC)

Oh, yeah; I always forget about spinors when I'm arguing that -+++ is the way to go. +--- is okay by me (and most general relativists, it seems) only when using spinors. Consistency is something we should strive for on Wikipedia. --MOBle 15:59, 21 December 2006 (UTC)

Alright, but when you use -+++ you end up with the minus sign :) Then -k should also be more common (?) --Allen McC. 16:55, 21 December 2006 (UTC)

I'm not sure what you just said. To clarify, using $g_{\mu \nu }\rightarrow \sim \mathrm {diag} [-1,1,1,1]$ results in the field equations

$R^{\mu \nu }-{\frac {1}{2}}Rg^{\mu \nu }={\frac {8\pi G_{N}}{c^{4}}}T^{\mu \nu }\ ,$

and you're mathematically free to swap the sign of the metric if you swap the sign on one side of the equation above. The definition and usage of "k" isn't something I'll take issue with. --MOBle 19:14, 21 December 2006 (UTC)

Vandalism

There has been a high rate of vandalism recently, specifically from Keller Dude. He should either be banned or cautioned. His behaviour has been unacceptable. —The preceding unsigned comment was added by PepperySugar (talk • contribs) 01:16, 9 February 2007 (UTC).

He was banned by Wikipedia admins a short time ago, after ignoring a number of warnings. —Krellis 01:17, 9 February 2007 (UTC)

what were the basic assumptions?

I think there were six. Obviously, one was relativity. I think it is important to give readers an idea of how the six assumptions became the EFE.CorvetteZ51 01:03, 15 February 2007 (UTC)

There are six fundamental principles listed in General_relativity#Fundamental_principles. Perhaps that is what you are thinking of. However, only a subset of these contribute to the EFE. --EMS | Talk 01:27, 15 February 2007 (UTC)

some of those six are results,not inputs. From memory, one of the other assumptions was 'Poisson's equation'. I can't find, on the internet,what I once read, that was a reasonable discussion of where the EFE came from, what I don't like about the current text is that it reads like a college textbook.CorvetteZ51 08:15, 15 February 2007 (UTC)

There are some things to be said both for and against its reading like a textbook. As for Poisson's equation, that is a starting point for the linear approximation, but is only indeirectly relevant to the EFE. Relativity requires that gravitational interacts be described with a 10-component rank-two (or symmetric) tensor. Once you toss in the requirement that gravitation be described with curvature, the stress-energy of the spacetime being a determining factor in the amount of the curvature, and the need for general covariance, you quickly arrive at a tensor expression where stress-energy is associated with a rank-two curvature expression. That is what the EFE are. --EMS | Talk 17:56, 15 February 2007 (UTC)

Matter models

Shal we add other matter models than Einstein-Maxwell equations, like elasticity, fluid, null dust, Enstein-Yang/Mills? Temur 22:18, 24 October 2007 (UTC)

Th Einstein-Maxwell equations are very common, which is why I included them here (I want to expand the section a little). I suppose that other models could at least deserve a brief mention. MP ^{(talk•contribs)} 19:53, 21 November 2007 (UTC)

Derivation of Newton's law

I've included a derivation of Newton's law of gravity in a show/hide box. I have to check the minus signs ! MP ^{(talk•contribs)} 19:48, 21 November 2007 (UTC)

The idea is good. But it's not just a minus sign. The whole section needs fixing signs, factors of two, etc. Could someone have a look at a couple of textbooks and fix it? —Preceding unsigned comment added by 83.243.113.85 (talk) 00:01, 1 April 2008 (UTC)

Sign conventions

I am not so sure, but it seems to me that in mostly plus conventions, as used in this article, the sign of the cosmological term is other then written here. i think the sign of the term is wrong in the first equation of "the cosmological constant", under "vacuum field equations" however it is correct again. (when written on the right hand side, the cosmological term should have a minus sign in mostly plus conventions, no?) could someone check this? —Preceding unsigned comment added by 140.247.123.225 (talk) 00:07, 10 March 2008 (UTC)

History

I think that a History section would be appropriate for this article, perhaps showing Einstein's route to the field equations (probably in a show/hide box). Comments ? MP ^{(talk•contribs)} 17:58, 27 February 2008 (UTC)

Why is there a sign difference on the right hand side?

At Einstein field equations#Mathematical form, the article says "The above form of the EFE is for the −+++ metric sign convention, which is commonly used in general relativity, and which is used by convention here. Using the +--- metric sign convention leads to an alternate form of the EFE which is" where the two versions shown differ by the sign in the coefficient of the stress-energy tensor. This is wrong. The signature of the metric has no effect upon any of the signs in the EFEs. Just imagine replacing $g_{\alpha \beta }$ by its negative. The sign cancels out in the Christoffel symbols. Therefor it has has no effect upon the curvature tensors (which do not use the metric directly, but only through the Christoffel symbols). The scalar curvature changes sign, because it contracts Ricci with the reciprocal of the metric, but then it is multiplied by the metric again and so regains the same sign as before. The time-time component of the contravariant stress-energy tensor should always be defined to be positive by anyone, and lowering both indices cancels out the effect of the sign change in the metric.

The sign might be affected by the definition of the Ricci curvature in terms of the Riemann curvature tensor. I do not know of any other possible reason. JRSpriggs (talk) 14:20, 28 June 2008 (UTC)

I fixed this problem since no one disagreed here. JRSpriggs (talk) 21:08, 7 August 2008 (UTC)

Trace-free version of EFE

If we calculate the trace of EFE

$R_{\mu \nu }-{1 \over 2}g_{\mu \nu }\,R+g_{\mu \nu }\Lambda ={8\pi G \over c^{4}}T_{\mu \nu }$

we get

$-R+4\Lambda ={8\pi G \over c^{4}}T\,.$

If we add $-{1 \over 4}g_{\mu \nu }$ times this to EFE, we get

$R_{\mu \nu }-{1 \over 4}g_{\mu \nu }\,R={8\pi G \over c^{4}}(T_{\mu \nu }-{1 \over 4}g_{\mu \nu }\,T)$

which is a trace-free equation. At first glance, this appears to contain less information than EFE because it has only nine independent equations rather than ten. However, we can recover the trace from this as follows. Take the covariant divergence of the contravariant form of this equation to get

$(R^{\mu \nu }-{1 \over 4}g^{\mu \nu }\,R)_{;\nu }={8\pi G \over c^{4}}(T^{\mu \nu }-{1 \over 4}g^{\mu \nu }\,T)_{;\nu }\,.$

Using the facts that

$(R^{\mu \nu }-{1 \over 2}g^{\mu \nu }\,R)_{;\nu }=0\,$ and

${T^{\mu \nu }}_{;\nu }=0\,$

we get

$({1 \over 4}g^{\mu \nu }\,R)_{;\nu }={8\pi G \over c^{4}}(-{1 \over 4}g^{\mu \nu }\,T)_{;\nu }\,.$

Since the covariant derivative of g is zero, this is the same as saying

$0=(R+{8\pi G \over c^{4}}\,T)_{;\nu }=(R+{8\pi G \over c^{4}}\,T)_{,\nu }\,$

which implies that

$R+{8\pi G \over c^{4}}\,T=\operatorname {constant} \,$

which is equivalent to the trace equation above for some value of Λ. So the trace-free version of EFE is equivalent to the full EFE with cosmological constant. JRSpriggs (talk) 21:08, 7 August 2008 (UTC)

Small Discrepancy in the article.

I believe there is a small discrepancy in the article. The equation in the “Equivalent formulation” section is derived not from the standard equation (the first one), but from the second one, which is misleading. Regards, Boris Spasov, bspasov@yahoo.com —Preceding unsigned comment added by 72.87.242.181 (talk) 18:36, 4 September 2008 (UTC)

Oops, my bad. The equation is OK. Boris. —Preceding unsigned comment added by 72.87.242.181 (talk) 20:16, 4 September 2008 (UTC)

A Question about EFE Deduction?

Dear Editors, I read the deduction of Einstein Field Equation. However, I cannot understand one part of the deduction. In the deduction, it says that T00=rho*c^4 based on the "low speed and static field assumptions". I don't know why these assumption can get T00=rho*c^4. Can you provide more detail deductions? Thank you very much. —Preceding unsigned comment added by Wanchung Hu (talk • contribs) 07:22, 10 October 2008 (UTC)

In section Einstein field equations#The correspondence principle, I was not deducing EFE from Newtonian gravitation. I was showing that they are the same in a certain limit which represents the experience which most of us have of gravity (and incidentally determining the value of the constant factor in EFE). The Solar System is close to that limit.

It is essential to understand that in this article as in Stress-energy tensor, I am assuming that the Minkowski metric has the form

$\eta _{\alpha \beta }={\begin{pmatrix}-c^{2}&0&0&0\\0&1&0&0\\0&0&1&0\\0&0&0&1\end{pmatrix}}\,.$

From the article on the stress-energy tensor, we get

$T^{00}=\rho \,.$

Then in the approximation where

$g_{\alpha \beta }\approx \eta _{\alpha \beta }\,,$

we get

$T_{00}=g_{0\mu }T^{\mu \nu }g_{\nu 0}\approx g_{00}T^{00}g_{00}\approx (-c^{2})(\rho )(-c^{2})=\rho c^{4}\,.$

Is that clear? JRSpriggs (talk) 08:34, 10 October 2008 (UTC)

Hi, dear editor, thank you very much, sincerely,Wanchung —Preceding unsigned comment added by Wanchung Hu (talk • contribs) 03:39, 5 January 2009 (UTC)

Potential

The potential applies globally, not just locally. See Maxwell's_equations_in_curved_spacetime.Ferrylodge (talk) 03:48, 3 January 2009 (UTC)

Any chance we could have some discussion here, instead of simply reverting each other? It is very common to use a potential formulation of the generally covariant Maxwell Equations.Ferrylodge (talk) 04:16, 3 January 2009 (UTC)

Pending a response, I will restore the potential formaulation, with a new footnote to Brown, Harvey. Physical Relativity, page 164 (Oxford University Press 2005).Ferrylodge (talk) 04:25, 3 January 2009 (UTC)

It's pretty well-known that there may not exist a global field potential outside of certain special solutions. Even on a priori grounds, there is no justification at all for believing that the local electromagnetic field potentials must patch together globally (over all of spacetime) to give a coboundary, since the field itself is the physical datum rather than the potential. In general, of course, there are topological obstructions to the existence of a vector potential. It would be nice to find a definitive source that treats in detail the difference between these cases, and although sources such as arXiv:gr-qc/9305022 (by Robert Wald) allude to local-versus-global issues, they seldom seem to play a major role. Some sources asserting a "global" electromagnetic potential, such as arXiv:gr-qc/0608108, assume that the space is a small perturbation from Minkowski space (and thus in particular is contractable), in addition to imposing suitable decay at infinity. At the other end of the balance, although volume 1 of Penrose and Rindler states the electromagnetic field equation first in terms of a vector potential, it then cautions its reader against doing this in a curved spacetime. Annoyingly, it does not give detailed reasons for this beyond lack of physical meaning of the vector potential itself. Misner, Thorne, and Wheeler, despite being referenced in the article that you cite Maxwell's_equations_in_curved_spacetime, as well as your reference to Brown, only mentions the formulation via vector potentials after stating that that the version given in my own edit forms "the basic equations of electrodynamics in the presence of gravity. From them follows everything else" (p. 568, which seems to be a fairly definitive endorsement of my own version). Other references include:

• Ward, J. P. (1979), "Interpretation of global integrability conditions for stationary Einstein-Maxwell equations", General Relativity and Gravitation, 10 {{citation}}: Unknown parameter |Number= ignored (|number= suggested) (help). Here Einstein-Maxwell equations refers specifically to the ones I prefer.
• Ward, Richard Samuel; Wells, Raymond O'Neil (1991), Twistor geometry and field theory, Cambridge University Press, ISBN 052142268X {{citation}}: Text "Twistor Geometry and Field Theory" ignored (help). Here is a quote directly supporting my assertion that the potential is only local, amid a more general discussion of Maxwell's equations in a curved spacetime (p. 254): "Here we restrict a priori to field configurations $F_{ab}$ satisfying $\nabla _{[a}F_{bc]}=0$. Thus $F_{ab}$ is a closed two-form, and therefore exact: there exists a one-form $A_{a}$, called the potential, such that
    $F_{ab}=2\nabla _{[a}A_{b]}.$
  We are assuming for the time being that the region Ω we are working in is topologically trivial, so there is no obstruction to the existence of $A_{a}$." (Bold emphasis is my own)

At any rate, Maxwell fields on a non-trivial spacetime may lack a globally-defined potential. This is quite well-known to people who know some differential geometry, and the quotations and references provided above provide reasonable prima facie evidence that this is the case. If you require more convincing, the necessary details depend on the de Rham cohomology (specifically H²) of the spacetime under consideration. siℓℓy rabbit (talk) 07:29, 3 January 2009 (UTC)

There is no evidence that the universe is not topologically flat, i.e. contractible to a point. Consequently, there can be no evidence (in classical physics) as to whether it must be possible to integrate the electromagnetic field to get an electromagnetic potential in the presence of a non-trivial topology, i.e. π₁ ≠ 0. Theory alone cannot answer this question, only experiment.

However, it appears that quantum mechanics requires the existence of a potential field (please correct me, if I am wrong here). JRSpriggs (talk) 15:24, 3 January 2009 (UTC)

In partial response to your latter question, Yang-Mills instantons require the opposite: that the field should not have a global potential. I don't know if the potential is used elsewhere in quantum mechanics in a way that breaks local gauge invariance, but it seems unlikely to me at any rate. siℓℓy rabbit (talk) 15:58, 3 January 2009 (UTC)

(undent) I've rearranged the equations so that a manifestly covariant form is provided first. Hopefully, no one will have a problem with that. I hope everyone agrees that:

$F_{\alpha \beta ;\gamma }+F_{\gamma \alpha ;\beta }+F_{\beta \gamma ;\alpha }=0\!$

And consequently, according to the definition of a covariant derivative, I hope everyone agrees that:

$F_{\alpha \beta ,\gamma }+F_{\gamma \alpha ,\beta }+F_{\beta \gamma ,\alpha }=0\!$

And just as in Special Relativity this implies the existence of a four-potential.Ferrylodge (talk) 17:52, 3 January 2009 (UTC)

To Ferrylodge: Although I am inclined to agree with you that there is a potential field, I am afraid that this last argument of yours fails. The equations are correct, but the conclusion that they can be integrated is based on the fact that you are using a coordinate patch whose domain is simply connected. Consider the Klein bottle × R². Being non-orientable, it requires at least two separate such patches to cover it. If one makes the potential fields agree on one part of the overlap, it will disagree on the other for some choices of the force field. (If my geometric intuition is correct.) Of course, this does not prove that potentials do not exist, it merely points to a possible flaw in the argument.

To Silly rabbit: But instantons are not known to exist, are they? So that proves nothing. JRSpriggs (talk) 19:17, 3 January 2009 (UTC)

Thanks for the comment. While it is true that a single coordinate system may not exist that covers the entire universe (and hence the need for coordinate patches), I do not think that was known when Einstein formulated the Einstein-Maxwell Equations. In any event, can we all live with the section as it stands now? I hope so.  :-) The big problem was that the section did not even give the covariant Maxwell Equations until yesterday.Ferrylodge (talk) 19:21, 3 January 2009 (UTC)

To JRSpriggs:
(1) Yang-Mills instantons, and even more specifically Maxwell instantons, most definitely do "exist" in the sense that such solutions can categorically be established (here I have put my mathematician hat on). See, for instance, Trautman, Andrzej (1977), "Solutions of the Maxwell and Yang-Mills equations associated with hopf fibrings", International Journal of Theoretical Physics, 16 (9). Whether this article also describes a solution of Einstein-Maxwell is unclear to me. The spacetime is vacuum with cosmological constant, so the answer is likely in the negative, although I am not prepared to wager large sums of money either way.
(2) In regards to your earlier query regarding the existence of a potential in quantum mechanics: I believe this is true, but the potential is only supposed globally to exist as a connection on a suitable principal bundle (in the sense of gauge theory). This is totally consistent with the usual formulation of Maxwell's equation dF=d*F=0, since Poincare implies the existence of a local gauge for the connection. siℓℓy rabbit (talk) 22:22, 3 January 2009 (UTC)

I've gone ahead and restored the more general version of the equations. Absent any unequivocal source, the references provided above are more than sufficient to establish that the definition of the Einstein-Maxwell equation does not presuppose topological triviality. In fact, there is an active area of research into global solutions of this equation (and related PDEs), which lack global potentials. No sources have been presented which contest this point of view, whereas I have given some sources that maintain a degree of agnosticism on this point. The cited Harvey source correctly takes the potential as the variation, but quite incorrectly also as a primitive part of the theory, which we know quite well is not physical. More properly the Maxwell equations arise from compactly supported variations within the cohomology class of the field strength. siℓℓy rabbit (talk) 02:09, 11 January 2009 (UTC)

First of all, why must this section state that brackets denote skew-symmetry? Skew-symmetry is synonymous with the more common term "anti-symmetry". See the Wikipedia article "Antisymmetric". Additionally, the brackets absolutely do not indicate skew-symmetry or anti-symmetry in this instance. They indicate permutation of indices. If you want to keep saying that the brackets in this instance denote anti-symmetrization, then please include a reference that uses that odd and (in my view) erroneous phraseology. And why use brackets at all, when it's perfectly easy and normal to write out what we mean?

$F_{\alpha \beta ;\gamma }+F_{\gamma \alpha ;\beta }+F_{\beta \gamma ;\alpha }=0\!$Ferrylodge (talk) 02:17, 11 January 2009 (UTC)

Secondly, you have inserted the statement that, in a local coordinate system, F[βγ;α] = 0 can be rewritten in terms of partial differentiation, denoted by a comma, rather than covariant differentiation. I have several questions for you. (1) Do you mean by a "local coordinate system" a local inertial coordinate system in the neighborhood of a point-event at which the derivatives of the metric tensor have been transformed away? If that's what you mean by local coordinates, then your statement is much too narrow, since the semicolon can be replaced with a comma in general coordinates too. (2) Would you please tell me, yes or no, whether you agree that the covariant derivative of a scalar equals its partial derivative, not just in local coordinates, but in general coordinates? (3) Do you agree that the following equation holds true not just in local coordinates, but in general coordinates?

$F_{\alpha \beta ;\lambda }{\stackrel {\mathrm {def} }{=}}F_{\alpha \beta ,\lambda }-\Gamma _{\alpha \lambda }^{\mu }F_{\mu \beta }-\Gamma _{\beta \lambda }^{\mu }F_{\mu \alpha }\,$

(4) Do you also agree that this next equation holds true not just in local coordinates but in general coordinates?

$F_{\alpha \beta ;\gamma }+F_{\gamma \alpha ;\beta }+F_{\beta \gamma ;\alpha }=0\!$

If you agree that these two equations are correct, look what happens when we plug the first equation into the second. (5)We get F[βγ,α] = 0 correct? Thanks in advance for hopefully answering these questions.Ferrylodge (talk) 02:30, 11 January 2009 (UTC)

One question at a time, please. First, in relativity theory, skew-symmetrization is almost universally denoted by square brackets, and I would challenge you to come up with references to the contrary. Please consult any basic textbook on relativity theory (e.g., Misner, Thorne, and Wheeler, p. 126; Hawking and Ellis, p. 20). As for whether you prefer "anti-symmetrization" or "skew-symmetrization", I believe that the latter is more common (although I may have a certain geographical bias there). The term "alternation" is sometimes used as well. The article antisymmetric is largely irrelevant to this discussion. Perhaps you should refer instead to the article exterior algebra, and more specifically the section on the alternating tensor algebra. Finally, you continue to labor under the misapprehension that, for a skew-symmetric two-tensor F, the following two equations are somehow different:

$F_{\alpha \beta ;\gamma }+F_{\beta \gamma ;\alpha }+F_{\gamma \alpha ;\beta }=0$

and

$F_{\alpha \beta ;\gamma }+F_{\beta \gamma ;\alpha }+F_{\gamma \alpha ;\beta }-F_{\beta \alpha ;\gamma }-F_{\gamma \beta ;\alpha }-F_{\alpha \gamma ;\beta }=0.$

But these are exactly the same (for an antisymmetric two-tensor F), unless you are doing relativity theory over a field of characteristic two, which I believe is rarely done. siℓℓy rabbit (talk) 02:44, 11 January 2009 (UTC)

I agree that skew-symmetrization is almost universally denoted by square brackets. What I'm saying is that many people understand the word "skew-symmetrization" to mean simply change of sign upon exchange of two indices. It's true that the word "skew-symmetrization" is sometimes given a broader meaning, e.g. to antisymmetrize, one adds even permutations and subtratcts odd permutations. But we have to say what we mean, and the simplest way to do that would be to simply say that

$F_{\alpha \beta ;\gamma }+F_{\gamma \alpha ;\beta }+F_{\beta \gamma ;\alpha }=0\!$.

We're trying to be as understandable as possible here, so why not explain what we mean? At least we should explain what we mean by the term "skew-symmetric", but I would much prefer to simply write out the whole equation without the brackets.Ferrylodge (talk) 02:59, 11 January 2009 (UTC)

Secondly, what do you mean by "general coordinates"? If by general coordinates you mean a mapping into R⁴ that holds throughout the space-time, then I fail to see the relevance of the question. Many physically realistic solutions of the field equations lack such a coordinate system globally. Rather, for maximum generality, spacetime is instead conceived of as a differentiable manifold consisting of a variety of coordinate charts (local coordinates). Personally, I fail to see the need to introduce coordinates at all. But at your own insistence of using something called "partial differentiation" (whatever that is in a generally-covariant theory), local coordinates need to be invoked explicitly at some point. I would of course be fine with getting rid of this needless reference to coordinates, as long as you are equally willing to agree that the reference to "partial derivatives" should also be removed. siℓℓy rabbit (talk) 02:44, 11 January 2009 (UTC)

Look, you put the term "local coordinate system" into this article, and I'm simply asking you what you mean. I didn't put the term "general coordinates" into this article, but I'd be glad to tell you what I mean by it: coordinates with respect to which the field equations are generally covariant, rather than merely local inertial coordinates in which space-time is locally flat.

I numbered my questions above so that you could respond clearly and easily. Are you not going to answer question #2: "Would you please tell me, yes or no, whether you agree that the covariant derivative of a scalar equals its partial derivative, not just in local coordinates, but in general coordinates?" It's a very simple question, and would help me understand what our disagreement is about.Ferrylodge (talk) 03:09, 11 January 2009 (UTC)

Please enlighten me: What is a "general coordinate"? I think I have been reasonably clear as to the meaning of a local coordinate (at least in our discussion here). If it is unclear, perhaps atlas (topology) or more likely differentiable manifold will help. At any rate, if you want to make sense of a partial derivative, it is at least implicit that you have a background coordinate system. On the other hand, as is common in relativity theory, covariant notions can be interpreted through abstract indices, and so do not in principle rely on the presence of coordinate systems. If by "general coordinates" you mean what physicists sometimes refer to as "non-holonomic coordinates", then all bets are off, and the formulas are generally false in such "coordinates". siℓℓy rabbit (talk) 03:18, 11 January 2009 (UTC)

Did you not see what I wrote above? "I didn't put the term 'general coordinates' into this article, but I'd be glad to tell you what I mean by it: coordinates with respect to which the field equations are generally covariant, rather than merely local inertial coordinates in which space-time is locally flat" (i.e. in which the partial derivatives of the metric tensor vanish). Why are you asking me to repeat myself?

And why will you not answer any of the numbered straightforward questions that I posed for you?

(1) Do you mean by a "local coordinate system" a local inertial coordinate system in the neighborhood of a point-event at which the derivatives of the metric tensor have been transformed away?

(2) Would you please tell me, yes or no, whether you agree that the covariant derivative of a scalar equals its partial derivative, not just in local coordinates, but in general coordinates?

(3) Do you agree that the following equation holds true not just in local coordinates, but in general coordinates?

$F_{\alpha \beta ;\lambda }{\stackrel {\mathrm {def} }{=}}F_{\alpha \beta ,\lambda }-\Gamma _{\alpha \lambda }^{\mu }F_{\mu \beta }-\Gamma _{\beta \lambda }^{\mu }F_{\mu \alpha }\,$

(4) Do you also agree that this next equation holds true not just in local coordinates but in general coordinates?

$F_{\alpha \beta ;\gamma }+F_{\gamma \alpha ;\beta }+F_{\beta \gamma ;\alpha }=0\!$

If you agree that these two equations are correct, look what happens when we plug the first equation into the second.

(5)We get F[βγ,α] = 0 correct?

You said above, “I would of course be fine with getting rid of this needless reference to coordinates, as long as you are equally willing to agree that the reference to ‘partial derivatives' should also be removed.” Absolutely not. It would be very unwise to remove the concept of a "partial derivative" from Wikipedia's articles about General Relativity. A covariant derivative is defined by using partial derivatives. The affine connection is defined in terms of the metric tensor by using partial derivatives. The curvature tensor is defined in terms of the affine connection by using partial derivatives. I can scarcely imagine a more central concept that should not be removed from Wikipedia’s discussion of General Relativity than the concept of a partial derivative. Ferrylodge (talk) 03:23, 11 January 2009 (UTC)

(1) Well, because local coordinates are not generally considered to be "locally flat" coordinates. Indeed, such things typically do not even exist. Here the term "local coordinates" refers to four smooth functions real-valued x⁰, x¹, x², x³, x⁴ from an open set U in the spacetime manifold M such that

$dx^{0}\wedge dx^{1}\wedge dx^{2}\wedge dx^{3}=0$

and that the R⁴-valued function (x⁰, x¹, x², x³) maps U homeomorphically onto an open subset of R'⁴. That is to say, local coordinates of a spacetime are (for a mathematician) the coordinate charts which give that spacetime the structure of a differentiable manifold. Please see an elementary relativity textbook, such as Hawking and Ellis p. 12, for further details about the terminology. siℓℓy rabbit (talk) 03:36, 11 January 2009 (UTC)

(2) Ordinarily, I would say "yes", but since you don't seem to understand coordinates at all, please write down a specific equation, explaining exactly what things like an index "α" means in various contexts, as well as exactly what you mean by a "partial derivative", and then I may give my provisional assent, assuming that I am satisfied.

"Yes" is exactly correct.Ferrylodge (talk) 04:08, 11 January 2009 (UTC)

(3) Again, what are general coordinates? This term seems to have no clear meaning whatsoever. But if it means what physicists sometimes call "non-holonomic coordinates", then no, your equation is wrong. If it means "actual coordinates which are defined on all of spacetime", then your equation is correct but possibly vacuous, since there are important solutions of the field equations which lack such a coordinate system (for instance, conformally compactified Minkowski space).

(4) See (3).

(5) Covariant differentiation does not, in fact, rely on partial differentiation as a matter of definition. There are purely geometrical ways in which to define it. Many modern differential geometry textbooks take this point of view. At any rate, if you are going to go around defining things in terms of local coordinates, then you had better say that you are using coordinates (rather than, e.g., abstract indices). siℓℓy rabbit (talk) 03:47, 11 January 2009 (UTC)

Something I have noticed we agree on

We at least agree on the mathematically trivial fact that in a (holonomic) coordinate system with a (torsion-free) connection,

$F_{[\alpha \beta ;\gamma ]}=F_{[\alpha \beta ,\gamma ]}$

$={\frac {1}{3}}\left(F_{\alpha \beta ;\gamma }+F_{\beta \gamma ;\alpha }+F_{\gamma \alpha ;\beta }\right)$

$={\frac {1}{3}}\left(F_{\alpha \beta ,\gamma }+F_{\beta \gamma ,\alpha }+F_{\gamma \alpha ,\beta }\right)$

$={\frac {1}{6}}\left(F_{\alpha \beta ;\gamma }+F_{\beta \gamma ;\alpha }+F_{\gamma \alpha ;\beta }-F_{\beta \alpha ;\gamma }-F_{\gamma \beta ;\alpha }-F_{\alpha \gamma ;\beta }\right)$

May I suggest that we remove this particular point of putative contention from the above discussion? siℓℓy rabbit (talk) 04:01, 11 January 2009 (UTC)

I can't see what you've written due to an apparent formatting error that you've made.

For physicists (or at least many of them), the word "local" refers to a locally inertial freely falling coordinate system where the laws of Special Relativity approximately hold true. For example, according to one textbook author, “local inertial frames are identical with the frames in which Maxwell’s Equations are valid locally....The local inertial coordinates are approximately realisable by freely falling, nonrotating small laboratories…” "Relativity, Astrophysics and Cosmology" by W. Israel. Therefore, I will clarify this in the article, which still uses the word "locally".Ferrylodge (talk) 04:08, 11 January 2009 (UTC)

Okay, now it's fixed, and I see what you're saying. I'm glad we agree about that, but can we just leave the conversation as-is without deleting? Thanks.

So, let's suppose we're using a coordinate chart in which:

$0={\frac {1}{3}}\left(F_{\alpha \beta ,\gamma }+F_{\beta \gamma ,\alpha }+F_{\gamma \alpha ,\beta }\right)$

Doesn't it follow that the four-potential exists within that coordinate chart? Why do we have to start talking about a "star-shaped region" in the article? I guarantee you that there are lots of physics books that introduce readers to the mathematics of general relativity without talking about "star-shaped regions".Ferrylodge (talk) 04:21, 11 January 2009 (UTC)

The four-potential exists locally within a coordinate chart, it is true. But it may not exist globally, because there may be a gauge transformation from one coordinate system to the next. See the cited paper by Trautman for an explicit example of this. This presents a global solution of the Einstein-Maxwell equation without a global potential (called an instanton solution). siℓℓy rabbit (talk) 04:29, 11 January 2009 (UTC)

A couple responses. First, you've cited the following in this Wikipedia article: Trautman, Andrzej (1977), "Solutions of the Maxwell and Yang-Mills equations associated with hopf fibrings", International Journal of Theoretical Physics 16 (9): 561-565.

This does not seem to be available online. It is far far better to cite online material, so that other Wikipedia editors can easily check it, and so that Wikipedia readers can more easily access it. Surely, with all of the articles and texbooks now available online, one of them should discuss the point in the Trautman article, right? We're supposed to be providing only the briefest summary of General Relativity here, so it seems like online sources ought to fully support what we write. If this were a Wikipedia article about a more obscure subject, then it might be more appropriate to cite offline sources.

Second, if the four potential exists within a coordinate chart, then why do we have to further specify that it only exists in a star-shaped region of that coordinate chart? Not one reader in a million is going to have the slightest idea of what significance the term "star-shaped region" has here. I dare say Einstein would not have had the slightest clue what it means. Can't we at least confine it to a footnote?Ferrylodge (talk) 04:35, 11 January 2009 (UTC)

(undent)And would you please explain what your objection is to the following equation?

$F_{[\alpha \beta ;\gamma ]}{\stackrel {\mathrm {def} }{=}}{\frac {1}{3}}\left(F_{\alpha \beta ;\gamma }+F_{\beta \gamma ;\alpha }+F_{\gamma \alpha /;\beta }\right)=0.\!$

Thanks.Ferrylodge (talk) 04:44, 11 January 2009 (UTC)

Anyway, the section looks pretty much okay to me now. Cheers.Ferrylodge (talk) 04:54, 11 January 2009 (UTC)

Potential (part 2)

To Silly rabbit: If by "non-holonomic coordinates" you mean something like a multi-sheeted function (i.e. a relation rather than a function) from a domain within spacetime to R⁴, then neither I nor Ferrylodge nor any sensible person would ever consider using such a thing. Every event in spacetime which falls within the domain of the coordinate system should be mapped to one and only one point in R⁴. JRSpriggs (talk) 01:47, 12 January 2009 (UTC)

By "non-holonomic coordinates" is sometimes meant a non-coordinate system whose "infinitesimal coordinate axes" are a system of vector fields which do not commute. The precise meaning of such a situation (and why it should be called a coordinate system) are always left somewhat more to the imagination than I am comfortable with. Nonetheless, it appeared to me at the time that by "general coordinates" Ferrylodge meant just such a thing. (As primarily a differential geometer, "local coordinates" always means a "coordinate system in a local neighborhood". What, then, is the meaning of "general coordinates"? It was not explained.) siℓℓy rabbit (talk) 02:35, 12 January 2009 (UTC)

I should add that, while I do not believe in "non-holonomic coordinates", I am also aware that there are some physicists who believe in such things. Now, amid various edit conflicts in the above discussion, and being pressed to answer five questions at once, I managed to miss Ferrylodge's assertion of what, in his/her view, a local coordinate system is. (Namely it is a local coordinate system in which the metric is trivial.) Now, you know what a local coordinate system is in the context of differentiable manifolds: this is established terminology, and is quite independent of any metrical notions. In a polemic, having this contrasted with a "general coordinate system" produces an immediate cognitive dissonance which allows the imagination to run wild as to what the author intends by the latter phrase. Presumably a "general coordinate system" must be something which generalizes in some way a "local coordinate system." siℓℓy rabbit (talk) 02:57, 12 January 2009 (UTC)

(undent)Pardon me for interrupting, but (as far as I can tell) I never said that a local coordinate system is "a local coordinate system in which the metric is trivial". If you want to see how physicists normally define the term, see local reference frame. Or see the textbook I quoted from and linked to: “local inertial frames are identical with the frames in which Maxwell’s Equations are valid locally....The local inertial coordinates are approximately realisable by freely falling, nonrotating small laboratories…” "Relativity, Astrophysics and Cosmology" by W. Israel.

I do not believe that the metric is trivial in a local freely falling coordinate system. Only its first partial derivatives are trivial.Ferrylodge (talk) 03:14, 12 January 2009 (UTC)

I see. I misunderstood your original post (the original wording was "rather than merely local inertial coordinates in which space-time is locally flat" (i.e. in which the partial derivatives of the metric tensor vanish)") to indicate that the metric is locally flat: that is that all of its derivatives have been completely transformed away in a neighborhood, not just at a single point. Anyway, it is a moot point, since this is not what a "local coordinate system" usually refers to when talking about a differential manifold (in relativity theory or otherwise). siℓℓy rabbit (talk) 14:33, 12 January 2009 (UTC)

To Silly rabbit: I do not understand what you mean by "a system of vector fields which do not commute". Vectors (as I understand them) are not operators. Thus commutation is meaningless (unless you want to replace points in spacetime with matrices as in matrix mechanics?). Perhaps you are talking about frame fields? Or are you referring to something to do with Lie derivatives? JRSpriggs (talk) 03:55, 12 January 2009 (UTC)

Vector fields are indeed operators: they are derivations on the algebra of functions. No matter, see Lie bracket of vector fields. In my little part of mathematics, it is usually understood that "commuting vector fields" means "vanishing Lie brackets". siℓℓy rabbit (talk) 14:29, 12 January 2009 (UTC)

(undent) Just FYI, here's an interesting factoid....Suppose we have an arbitrary 4x4 symmetric metric tensor, in a coordinate system with respect to which the metric tensor's first and second partial derivatives are all smooth and continuous. It's well-known that at a point-event "P" it's possible to perform a coordinate transformation that transforms the first partial derivatives of the metric tensor to zero. Less well-known is that it is possible for that coordinate transformation to also transform the second partial derivatives so that certain combinations of the second partial derivatives also transform to zero. Eddington discussed this fact, here.Ferrylodge (talk) 16:10, 12 January 2009 (UTC)

Indeed, this is well-known at a point (see normal coordinates). It is generally not possible locally, that is, in an open set, contrary to my interpretation of your above explanation: "rather than merely local inertial coordinates in which space-time is 'locally flat' (i.e. in which the partial derivatives of the metric tensor vanish)." Local flatness means that the metric can be reduced to a flat metric by a change of coordinates in an open subset of the spacetime manifold, not just at a single point. Anyway, I suggest that we drop the whole thing, since it apparently boils down to some confusion over the meaning of the word "local": in mathematics, local means "in an open set", never just "to first order at a point". Apparently the word is used with both meanings, depending on the context, in certain areas of physics. siℓℓy rabbit (talk) 17:27, 12 January 2009 (UTC)

The article on normal coordinates only seems to talk about transforming away the Christoffel symbols at a point, but does not seem to discuss transforming away any term involving partial derivatives of the Christoffel symbols. Eddington, however, did discuss that.Ferrylodge (talk) 17:41, 12 January 2009 (UTC)

Actually, normal coordinates are a natural coordinate system in a neighborhood of a point. Higher order derivatives are all specific polynomials in the curvature tensor (and its covariant derivatives), so one actually has control in principle over infinitely many derivatives at a point. I believe that the relation of Eddington's that you are intent on pointing out to me is already satisfied for normal coordinates, and essentially reduces to the Jacobi identity for the curvature tensor. I have not, however, done this calculation, since the point hardly seems to matter. siℓℓy rabbit (talk) 22:42, 12 January 2009 (UTC)

As far as practical applications are concerned, the entire theory of General Relativity hardly seems to matter, but there are those who are fascinated by it nonetheless. As to your suggestion that we "drop the whole thing", I agree. Cheers.Ferrylodge (talk) 23:17, 12 January 2009 (UTC)

Regarding your first sentence, that really isn't what I meant at all. Rather the thing is this: according to Eddington, some specific linear combination of the second partials of the metric (at a point) is zero, and that the remaining components of the 2-jet of the metric are entirely due to curvature. My point is that this is just as well true of a normal coordinate system. While it seems likely that the very same identity holds (specifically Eddington's "preferred" linear combination), even if not, the charge that this implies that Eddington's coordinates give more control over the second derivatives of the metric than normal coordinates is erroneous. In fact, in principle normal coordinates give the entire Taylor series of the metric in terms of the curvature tensor and its covariant derivatives, not just the first two terms. siℓℓy rabbit (talk) 00:01, 13 January 2009 (UTC)

Silly Wabbit, there was never any "charge that this implies that Eddington's coordinates give more control over the second derivatives of the metric than normal coordinates." On the contrary, it appears that Eddington was describing a particular type of normal coordinate system, rather than describing something different from normal coordinates. Incidentally, one interesting attribute of those coordinates described by Eddington is that, at a point P, not only the acceleration of a freely-falling neutral particle equals zero, but also the first derivative of the acceleration vanishes as well at that point.Ferrylodge (talk) 00:10, 13 January 2009 (UTC)

(unindent) Anyway, I have checked that both of Eddington's requirements are satisfied by normal coordinates. siℓℓy rabbit (talk) 00:46, 13 January 2009 (UTC).

Mix up

Sorry for the mix up on 24 May 2009. I had no intention of vandalising the article. I was trying to correct a version that had been. Xxanthippe (talk) 12:32, 24 May 2009 (UTC).

I don"t believe a second of the bullshit> I am close to explaining that time time has no meaning. Which means a fews things. Hold onto your onself> —Preceding unsigned comment added by 41.247.125.121 (talk) 23:31, 18 October 2009 (UTC)

Archive 1

Invariant formulation

Meanwhile there is an invariant (basis free) formulation of curvature structures (by Singer and Thorpe) and hence an invariant formulation of the field equations. In this formulation the Lorentzgroup easily is chased around the structures. The index notation here is disgusting and not very powerful! — Preceding unsigned comment added by 130.133.134.44 (talk) 18:12, 29 June 2012 (UTC)

EFE in spinor formalism

I think, it's useful to have a section about EFE in spinor formalism. Anyone has a comment?Earthandmoon (talk) 08:26, 22 December 2013 (UTC)

I would be interested in that, if you know enough about it to write a section on it. JRSpriggs (talk) 06:41, 23 December 2013 (UTC)

Oh, i'm not familiar with spinor. :( Earthandmoon (talk) 15:53, 23 December 2013 (UTC)

Should the equations be named after Einstein, Hilbert, or both?

IP user 12.72.186.xx has twice changed the name of the equations from "Einstein field equations (EFE)" to "Einstein-Hilbert field equations (EHFE)". His edit summaries explain this by saying "Einstein field equations should correctly read Einstein-Hilbert field equations with Hilbert having them before Einstein in the form of Hamilton's principle" and "Hilbert had the same equations before Einstein in the form of Hamilton's principle with the Ricci curvature scalar as the Lagrange density". As I said when I reverted him the first time, "whether or not Hilbert deserves credit, it is called EFE, not HFE or EHFE".

To aid the users' understanding and searches for terms, we are required to use the most common names for things (in reliable sources) regardless of the origin or technical accuracy of the name. See WP:AT for the application of this to article names. It is not appropriate to re-argue here the issues discussed at Relativity priority dispute#General relativity and History of general relativity#Einstein and Hilbert.

If you wish to change the name, please provide evidence that most reliable sources refer to the equations by a different name than that which has historically been used in this article (i.e. EFE). JRSpriggs (talk) 01:17, 31 March 2014 (UTC)

Proper definition of trace

It should be clarified that the trace is the trace defined according to the metric not the banal (Euclidean) trace of a matrix.TonyMath (talk) 14:22, 14 April 2015 (UTC)

Calling it a trace at all is a misnomer, IMO, the result of a degree of sloppiness in terminology. A trace is really only defined on a tensor over a pair of indices of opposite variance, and the term "trace with respect to the metric" is probably an attempt to find a brief description that sort of works. I agree with the sentiment, and have made the suggested change, but would like to see a more precise description, perhaps a term involving "contraction" rather than "trace". —Quondum 20:33, 14 April 2015 (UTC)

Agreed but I am nonetheless happier with this result. At least the hyperlink is going to the right place.TonyMath (talk) 04:52, 18 April 2015 (UTC)

10 equations?

The introduction says there are 10 equations, but nowhere in the article does it identify which are the 10 equations. — Preceding unsigned comment added by 23.242.49.114 (talk) 00:31, 24 March 2016 (UTC)

The ten equations are:

$R_{00}-{\tfrac {1}{2}}R\,g_{00}+\Lambda g_{00}={\frac {8\pi G}{c^{4}}}T_{00}$

$R_{01}-{\tfrac {1}{2}}R\,g_{01}+\Lambda g_{01}={\frac {8\pi G}{c^{4}}}T_{01}$

$R_{02}-{\tfrac {1}{2}}R\,g_{02}+\Lambda g_{02}={\frac {8\pi G}{c^{4}}}T_{02}$

$R_{03}-{\tfrac {1}{2}}R\,g_{03}+\Lambda g_{03}={\frac {8\pi G}{c^{4}}}T_{03}$

$R_{11}-{\tfrac {1}{2}}R\,g_{11}+\Lambda g_{11}={\frac {8\pi G}{c^{4}}}T_{11}$

$R_{12}-{\tfrac {1}{2}}R\,g_{12}+\Lambda g_{12}={\frac {8\pi G}{c^{4}}}T_{12}$

$R_{13}-{\tfrac {1}{2}}R\,g_{13}+\Lambda g_{13}={\frac {8\pi G}{c^{4}}}T_{13}$

$R_{22}-{\tfrac {1}{2}}R\,g_{22}+\Lambda g_{22}={\frac {8\pi G}{c^{4}}}T_{22}$

$R_{23}-{\tfrac {1}{2}}R\,g_{23}+\Lambda g_{23}={\frac {8\pi G}{c^{4}}}T_{23}$

$R_{33}-{\tfrac {1}{2}}R\,g_{33}+\Lambda g_{33}={\frac {8\pi G}{c^{4}}}T_{33}$

Since all these tensors are symmetric, the six equations where the first index is larger than the second are redundant, and thus not counted. JRSpriggs (talk) 16:48, 24 March 2016 (UTC)

Einstein field equations

The typical reasoning about space-time curvature is since light travels in a curve around massive objects, space time must be curved.  In fact, the effective refractive index of space time around the massive object is increased because spacetime is compressed around these objects.  The only curving involved is when light moves into a medium of different refractive index. Len loker (talk) 06:30, 26 March 2016 (UTC)

Is gravity due to: curvature of space-time, a force proportional to mass, or a change of refractive index? I think that it is more useful to think of these as several different, but equivalent ways of thinking about what is happening. JRSpriggs (talk) 01:01, 28 March 2016 (UTC)

Why are those equations called "Einstein" field equations here ?

Those equations were introduced as "Hilbert field equations" at my university and many - maybe most - scientific papers also call them Hilbert field equations. I wonder, why they are called "Einstein field equations" here ? This doesn't seem to reflect the scientific consensus. --Lambda C (talk) 01:42, 10 July 2016 (UTC)

Perhaps you should consider attending another university. YohanN7 (talk) 14:02, 11 July 2016 (UTC)

The integral form of the equations?

It is unclear whether there is an integrated form of these equations? For example, see here. — Preceding unsigned comment added by 178.120.182.18 (talk) 12:22, 2 January 2017 (UTC)