Talk:General relativity

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Contents


[edit] Plans for cutting down on maths

Recalling EMS' suggestion of cutting down on the maths in the article, I was thinking in particular of the 'Relationship to other physical theories' section. For example, in the electromagnetism subsection, do those equations really need to be written down (they're in the main article Maxwell's equations in curved spacetime) and what really is our aim in that little subsection ? The equations themselves illustrate the partial/covariant derivative dichotomy, and that may help more advanced readers understand the relationship between the theories, but I think something wordy can be used to replace the maths. For instance, some mention of electromagnetism's relationship to GR (e.g. e/m laws get modified in a covariant fashion - but no need to mention the maths here).

EMS' revolutionary rewrite some time ago served the purpose of having an accurate and more or less complete overview of GR. I believe that the maths can be reduced by giving qualitative descriptions of the equations instead. MP (talk) 20:58, 5 September 2006 (UTC)


I would imagine that this part of the article is to show the analogy between gravity and electromagnetism. After all, if I recall correctly, Einstein initially set out to form a theory of electromagnetism, not gravity. Although, I could be dead wrong on that point. Regardless, there are distinct correlations between the two in the tensor formulation and the covariant derivatives.
As for the scope of the whole article, I think we can get rid of all the actual math shown except for perhaps the field equations. I feel that those are important enough to keep, if just to demonstrate the simplicity of the general concept in its geometric notation. After all, we do have the mathematics of general relativity article. Archmagusrm 23:20, 5 September 2006 (UTC)
I just made a few edits to the math. I think certainly the math in the "inertia" and "gravitation" subsections is not too hard to be intimidating to anyone who would bother reading this article. I'm not sure about the E&M or "conservation of energy-momentum" sections, though. In any event, I think cleaning up those easier sections would be better than just getting rid of everything, especially because the mathematics of general relativity page is substantially harder than the math appearing on this page. That's my vote, anyway. I don't mean my edits as an attempt to preempt discussion, so have at it. MOBle 02:29, 6 September 2006 (UTC)


I disagree on the case of the "inertia" section. The "gravitation" section is certainly okay, as most people would be familiar with math of that level. But I'm not so sure in the case of "inertia" due to the existence of calculus and has some hefty mathematical concepts in there. I don't think the gain from having these equations in the article outweighs the heftiness of the equations. Archmagusrm 03:13, 6 September 2006 (UTC)
I'd like to see more pictures, in at least one area. This would be the classic illustration of the geodesic deviation equation via two great circles on a sphere. One of the questions I see a lot is "how does curvature cause gravity"? I think a picture of this sort could help a lot.
I'm not sure what sort of free software would be needed to draw these sorts of pictures. (Any suggestions?).
I was also thinking that pictures could be used to illustrate the egg-crate like structures used for differential forms, and that E&M in curved spacetime could be presented pictorally using this technique. Of course this would probably make more sense in a different article.
Anyway, one of the big obstacles is drawing tools (at least from my POV). Perhaps there are some suggestions out there. Pervect 18:43, 6 September 2006 (UTC)

[edit] Maths increasing ?

It looks like the maths in the article is actually increasing - not quite what I had in mind when I wrote my original comment at the start of this section ! The summation sign in the geodesic equation is not needed as it is made clear in the article that this convention is adopted. One of the beautiful features of GR is that the maths of it can be expressed so succinctly. Example: I have never seen - and will never write - the conservation law Tab;b = 0 as

Tab;b = 0
b

. MP (talk) 20:44, 7 September 2006 (UTC)

I think this particular sum sign appears in Wald despite the fact that he's certainly not afraid of implied summations. (I don't have a copy of Wald on me to confirm this.) The reason is that this is best interpreted as a coordinate equation, rather than a tensor equation like \nabla_b {T_a}^b=0.
Where in this article is the summation convention mentioned? I think the sum sign you mention is both more clear than the implied summation and more accurate. I reiterate my opinion that the conservation of stress-energy and E&M sections do contain more math than is necessary here. As the inertia section stands, I think it's a nice little sample of the math that has physical relevance, but is much easier than the math in the mathematics of GR page. Again, just my two cents... MOBle 21:44, 7 September 2006 (UTC)

[edit] relativity of simultaneity

The relativity of simultaneity article is in very bad need of attention. It is currently poorly written, and the last half is an anti-Einstein diatribe which belongs in relativity priority disputes if it belongs anywhere in Wikipedia. I made an attempt to reduce this article to a stub, but other editors objected and reverted it back.

This is an important sub-topic of relativity, and it is a disgrace to have this important article in the hideous shape that it is now in. I am hoping that others can help me to correct it. Otherwise, I will attempt to have it deleted. --EMS | Talk 02:38, 8 September 2006 (UTC)

[edit] Penzias and Wilson refs.

I've just created this section to dump this link, which may or may not be useful in the article (it may be better somewhere else). I'm still trying to get a hold of the original paper(s) by Penzias and Wilson which describes their findings. MP (talk) 19:21, 16 September 2006 (UTC)

NASA ADS, Penzias and Wilson to get PDF, click the link for GIF, then "other document options" then "get PDF". –Joke 19:34, 16 September 2006 (UTC)
I found it by a little googling, but thanks anyway for responding quickly. Now the reference (currently number 9 in the article) comes out strangely. MP (talk) 20:06, 16 September 2006 (UTC)
This problem has been bugging me for a while now - how come the Arno and Penzias ref. (currently ref. number 15 in the article) doesn't appear correctly in the references section - the link is fine, but the url is highlighted (and not the name of the paper) ? Also, both the url and paper name seem to be in a single set of square brackets - argh. Help ! MP (talk) 14:44, 27 September 2006 (UTC)

[edit] Bell's spaceship paradox

Just ran across this article, and I must say I think it's a horrible article, pushing a minority and rather silly POV. As others have commented on the talk page, hyperbolic motion is not any sort of "mystery" with "two different resolutions".

At the current time, I'm not sure what to do about it, though - in the meantime I stuck an NPOV tag on it, to at least warn readers to look at the talk page. Pervect 20:24, 17 September 2006 (UTC)

That page is in need of a thoughtful rewrite. It needs more math (but of the algebra/into. calculus type) and a short but thoughtful explanation of why the counter-arguments are wrong. You should look at the talk page archives (if you have not done so already), as you can gleam some sense of what we have been through with it from those pages. Do be advised that as soon as you start any major modification that Rod Ball will come out of the woodwork to challenge you, but me feeling is that we can cross that bridge when we come to it. --EMS | Talk 02:14, 19 September 2006 (UTC)

[edit] Update on references

I've included some important references regarding experimental results and theoretical predictions. I also found this site which could be useful (for other articles too): http://www2.corepower.com:8080/~relfaq/experiments.html. MP (talk) 20:21, 18 September 2006 (UTC)

[edit] Another test of GR....

... is reported here - the Shapiro effect. MP (talk) 16:35, 20 September 2006 (UTC)

[edit] Alternatives to GR section: reduction proposal

I'd like some feedback on my previous suggestion in the 'Alternatives to GR' (above) regarding cutting down significantly the material in the section. Thanks. MP (talk) 16:29, 6 October 2006 (UTC)

Pls just go ahead. IMHO explicitely mentioning Nordström, Brans-Dicke and Einstein-Cartan, and a nearly unchanged paragraph about testing using PPN would be fine.
Classical theories of gravitation looks somewhat redundant, but perhaps can be recycled as a non-technical introduction section for Alternatives to general relativity.
Pjacobi 16:50, 6 October 2006 (UTC)
And the Will paper should better referenced to LRR, http://relativity.livingreviews.org/Articles/lrr-2006-3/. I'll do that later. --Pjacobi 17:02, 6 October 2006 (UTC)
I think the section could use trimming too. Pervect 22:32, 6 October 2006 (UTC)
http://en.wikipedia.org/wiki/Brans-Dicke_theory at the end of the article has a nice table of alternative theories in wiki-link form, which seems to me to be somewhat misplaced. I'll splice a copy here on the talk page for now, maybe somebody can find it a new home. There might be some debate as to how to or whether or not to include things like "Le Sage" gravity, and totally pseudophysical theories (currently labelled with a P, currently only one entry. On reading the web-page for dynamic gravity, it might be a candidate for deletion, as it is a "theory" that has apparently never actually been published.
edit
Theories of gravitation
Gravity Alternatives to GR Unified field theories Other

(S) = Stub only
(P) = Pseudophysical

—The preceding unsigned comment was added by Pervect (talkcontribs) 17:26, October 30, 2006.

This is an in-line version of the template {{theories of gravitation}}, which is also included at the bottom of this article. I do see your point about Tesla's Dynamic theory of gravity. It usually is a bad sign when the theory is never published. Perhaps it should be replaced with Einstein-aether theory. --EMS | Talk 03:07, 31 October 2006 (UTC)
See relevant meta-discussion at Wikipedia:Requests_for_arbitration/Pseudoscience/Workshop#Inclusion of non-mainstream scientific theories in articles about mainstream topics. --Pjacobi 12:46, 31 October 2006 (UTC)
Indeed, and my advice in that discussion may be immediately applicable here: why not give that table a separate page, and simply link to it from the main article in one simple sentence? That's an effective way to remove that clutter form the topic( which happens to be GRT and not alternative theories), without denying fair mention of alternatives. Thus I propose trimming it back to one sentence in this article without doing the information any harm. Harald88 11:00, 4 November 2006 (UTC)

[edit] Relationship to other physical theories

Is it just me, or is the article missing something glaringly obvious, such as discussions of relativistic hydrodynamics and relativistic thermodynamics ? The latter isn't even restricted to the study of black holes. MP (talk) 00:37, 7 October 2006 (UTC)

[edit] Scrapping some maths in the article

I've boldly scrapped the equations in the electromagnetism subsection. Reason: The 'Relationship to other physical theories' seems to be about comparing some of the GR equations with their flat and non-relativistic counterparts; the first subsection - on inertia - does a good enough job in this regard . The subsections on conservation of energy and momentum and electromagnetism therefore seem redundant in terms of discussing the equations (I suppose that means I better scrap the equations in the conservation of energy-momentum subsection), as there are articles that discuss these issues in more detail. Is there an article on conservation of energy and momentum in general relativity (or just relativity) ?

In fact, now that I think about it, for the other subsections it's perhaps better to leave the GR version of the equations in and just state any comparisons. For the electromagnetism subsection, the first and third equations could be left (as the second one happens to be the same). MP (talk) 10:44, 10 October 2006 (UTC)

I'm sure that you are on the right track. I plopped the initial bunch of math into this article as something of mind dump when I was rewriting it. As time has passed, that math has more and more resolved itself into being the most dense and obtuse part of this article. It is good to see that mess being cleaned up--EMS | Talk 16:25, 10 October 2006 (UTC)

Well, it might look neat just to say that the semicolons go to commas for flat spacetime, but unfortunately Maxwell's equations are quite useful for things like dipole and higher multipole radiation in spherical coordinates. Sometimes one even wants cylindrical or other orthogonal coordinates. Unfortunately, divergence and curl have more complicated forms when you use curvilinear coordinates even in flat space. See for example the books on the mathematics of physics by Margenau and Murphy, or by Morse and Feshbach. Or (for the spherical case only) look at [1] and search for divergence or curl. The equations are complicated, but sometimes these forms are needed. Pdn 23:38, 10 October 2006 (UTC)

[edit] Black holes and NPOV

User:Carrionluggage edited the end of the black hole section to read:

A black hole exists at the center of our milky-way galaxy. This is proven by the concentration of mass too dense to be modeled (e.g.) by a cluster of neutron stars or any other candidate source of gravity. The movie from the Max Plank Institute for Astrophysics found at [2] shows actual observations of stellar orbits around the black hole. The shapes and sizes of the orbits are consistent with a black hole at the galactic center Sag A* and not with any other explanation yet proffered.

This is statement is not NPOV, and the edit history justification for it (that noone doubts that this is a black hole any longer) simply is not true. There is no doubt that a supermassive gravitationally collapsed object exists at the galactic center. However, there are still people who doubt that this is black hole. OTOH, I have retained the link to the site showing the orbits of stars around that object, as the central object certainly is a black hole if general relativity is 100% correct, and that site very much justifies the statement that a black hole is thought to be there. (Note that a journal article or two should also be cited.) --EMS | Talk 15:44, 17 October 2006 (UTC)

Well, I suppose a lot of people are NPOV on how many and what kind of dinosaurs were carried on Noah's Ark (I've seen that debate in Wikipedia), but the debate on what is at the center of Sag A* was closed by the paper: "Stellar Dynamics in the Central Arcsecond of Our Galaxy" by R. Schödel, T. Ott, R. Genzel, A. Ekhart, N. Mouawad, and T. Alexander, Astrophys. J. 596, 1015-1034 (2003) (3 yrs ago). To quote them, their observations "(leave) a central supermassive black hole as the only plausible explanation." You want to argue - come up with a theory that fits 3 million solar masses in 1/2 parsec (see their section 5) and that mass is not a black hole. So it is not an NPOV issue since Oct 20, 2003 unless somebody comes up with a refereed paper disputing the issue. Carrionluggage 21:04, 17 October 2006 (UTC)
Black holes have not been proven to exist and there is serious speculation among some scientists that they may not. So, writing that "A black hole exists in the center of our galaxy" is incorrect. That there is still speculation as to whether black holes exist or not is evident here [3]. User:Alfred Centauri 21:50, 17 October 2006 (UTC)
You have referred to a non-refereed news release. Chapline, has, however, contributed in his day some respectable physics (I believe he worked on the hydrogen bomb project); he published on shock waves in 1973. Nevertheless his paper in "Texas Conference on Relativistic Astrophysics" is unrefereed. I will try to see if there is a refereed version and citations to it. Anyway it seems to deal mainly with stellar mass "black holes" not supermassive ones. Carrionluggage 22:37, 17 October 2006 (UTC)
A search of the NASA ADS system for publications of G. Chapline did not yield any publications of the kind you implicitly refer to. This search engine finds not only publications but citations to them. It is free and accessible at: [4]. I assume that Dr. Chapline is being roundly ignored by all but the Livermore press office, but I sent them a query and will let you know if anything more emerges.

Here is what I got with my search:Query Results from the Astronomy Database Go to bottom of page Selected and retrieved 15 abstracts.

#       Bibcode         
        Score   Date    List of Links
        Authors         
        Title   
        Access Control Help
1        2006AIPC..822..160C    
        0.769   04/2006 A                      X              T      R  C                      U      
        Chapline, George; Mazur, Pawel  
        Tommy Gold Revisited: Why Doesn't the Universe Rotate?
2        1990fqm..meet...27C    
        0.769   08/1990 A                                                                          U      
        Chapline, G. F. 
        Information flow in quantum mechanics: The quantum Maxwell demon
3        1989nesa.conf..561C    
        0.769   00/1989 
        Chapline, G.    
        Cold Confusion
4        1982PhRvD..25..911C    
        0.769   02/1982         E                                      R  C                          H  
        Chapline, George F.     
        Binding of fractional charges to nuclei
5        1977PhRvD..16..450C    
        0.769   07/1977         E                                      R  C                      U  H  
        Chapline, George; Nauenberg, Michael    
        Asymptotic freedom and the baryon-quark phase transition
6        1977NYASA.302..191C    
        0.769   00/1977                                         T          C                      U      
        Chapline, George; Nauenberg, Michael    
        On the Possible Existence of Quark Stars
7        1976Natur.261..550C    
        0.769   06/1976 A                                                  C                      U  H  
        Chapline, G. F. 
        Quarks in the early Universe
8        1975Natur.253..251C    
        0.769   01/1975 A                                                  C                              
        Chapline, G. F. 
        Cosmological effects of primordial black holes
9        1974ApJ...192L..57W    
        0.769   09/1974 A          F  G                              R  C                              
        Weaver, T. A.; Chapline, G. F.  
        Dissipation in Supernova Shock Waves
10       1974PhRvD..10.1064C    
        0.769   08/1974 A      E                                      R                          U  H  
        Chapline, G. F.; Nuckolls, J.; Wood, L. L.      
        Gravitational-radiation production using nuclear explosions
11       1974BAAS....6Q.274W    
        0.769   03/1974                                                     C                              
        Weaver, Thomas; Chapline, George; Wood, Lowell; Silk, Joseph    
        The Structure of Supernova Shocks
12       1974BAAS....6..279D    
        0.769   03/1974 
        Dahlbacka, G. H.; Chapline, G. F.; Weaver, T. A.        
        Pion Gamma Ray Emission from Black Holes
13       1974Natur.250...36D    
        0.769   00/1974                                                     C                      U      
        Dahlbacka, G. H.; Chapline, G. F.; Weaver, T. A.        
        Gamma Rays from Black holes
14       1973ApJ...184.1041C    
        0.769   09/1973 A          F  G                              R  C                      U      
        Chapline, George, Jr.; Stevens, John    
        Effect of Compton Scattering on X-Ray Spectra
15       1973ApJ...179..277N    
        0.769   01/1973 A          F  G                              R  C                              
        Nauenberg, Michael; Chapline, George, Jr.       
        DETERMINAT10N of Properties of Cold Stars in General Relativity by a Variational Method . .

Carrionluggage 22:48, 17 October 2006 (UTC)

A couple of refereed articles on black hole alternatives: [5], [6]. Here [7] is a refereed article that argues that Sag A has an event horizon. The fact that this argument is being made in an article published in 2006 would seem to argue against your assertion that Sag A has been known to be a black hole since 2003. Alfred Centauri 23:52, 17 October 2006 (UTC)

OK - interesting! Carrionluggage 00:19, 18 October 2006 (UTC)

Hopefully this is settled. My concern was the absolutism of the statement more than anything else. If you look at my home page you will see that I do indeed have anti-black-hole views and an OR theory of my own. Normally I would not touch the Black Holes section because of that, but your statement was so over-the-top that I felt that I was on safe ground in making my revisions. (BTW - That section has always been somewhat tepid since I am the one who wrote it as part of the last major rewrite of this article. So it is good to see it finally being developed into something more than the very cut-and-dried thing that I felt both safe and honest in writing. I won't tolerate 100% certainly that black holes exist, but I assure you that I will be even quicker in reverting anything that says that black holes don't exist.) --EMS | Talk 03:02, 18 October 2006 (UTC)

[edit] Relativity WikiProject

I've suggested at the proposed wikiprojects page that a relativity wikiproject be created. If interested, you can add your name to the list and check out the plan for the project at WikiProject Relativity.

[edit] Abhas Mitra article deletion

I am trying to have the article on Abhas Mitra removed. Abhas Mitra is a contributor to the Magnetospheric eternally collapsing object or MECO theory, and apparently coined the term. In a nutshell, the MECO theory claims that radiation pressures prevent the collapse of an object into a black hole, and the you instead get an "eternally collapsing object" or ECO. If this ECO has a strong magntic field, that further prevents collapse, and makes the object a MECO. As this is being thoerized in the context of GR, I see this a being a theory which claims that GR is not GR. Several such theories are noted in the black hole article.

I won't contest that the MECO is encyclopedic, but I do not see that the notability extents to Dr. Mitra. Please see Wikipedia:Articles for deletion/Abhas Mitra. Thanks, EMS | Talk 03:37, 6 November 2006 (UTC)

[edit] Eddington's confirmation of GR

An anon edited the last paragraph of the "History" section to read:

Originally the only piece of observational evidence for which relativity provided a unique and correct answer was the perihelion precession of Mercury, which had been known as an anomaly for Newtonian mechanics for some time. Einstein himself proposed that one would be able to predict the effect of the Sun's gravity bending starlight if it could be carefully observed during a total solar eclipse. It was not until 1919 that a successful expedition to observe the bending of starlight was able to be undertaken by Arthur Eddington, who reported his observations as being consonant with Einstein's theory (and not Newtonian mechanics). Though this particular experiment had many difficulties — and has been cited in recent years as an example of the experimenter seeing what they wanted to see in the face of ambiguous data — it cemented Einstein's fame as an overthrower of Newton, though it would still be some years before general relativity was consided correct by the majority of physicists. Since then, many observations have confirmed the predictions of general relativity. These include studies of binary pulsars, observations of radio signals passing the limb of the Sun, and even the GPS system.

My objections to this edit are that a lot of data is being added mostly on one event (Eddington's 1919 expedition), the this data is fairly POV (by noting modern doubts about Eddington's results), and the this detail is unsourced. Another conideration is that this issue is dealt with in more and better detail in tests of general relativity, which is much more suited to this level of detail. --EMS | Talk 01:21, 20 November 2006 (UTC)

This brings up what may be an important point. Not too long ago, I read an article about the dispute over the eclipse expedition. I can't remember where it was -- maybe Physics Today or something. Anyways, it seemed like there was good evidence supporting the conclusion that the experimenters were indeed seeing what they wanted to see. I don't have the source, and I won't argue with your deletion of the unsourced comment. However, if this is the conclusion reached by contemporary science historians, I think it would be an excellent addition to any mention of the eclipse expedition, along with a quick reference to more modern confirmations of GR.
I don't know the source off the top of my head, but I'll look for it next chance I get. I thought maybe somebody would know what I'm talking about, though... --MOBle 02:11, 20 November 2006 (UTC)
I have also seen the same, most likely in Sky and Telescope. I'm less concerned with the veracity of the statement as with the level of detail it represents. That the data did not justify an unambiguous ratification of GR is a given. None-the-less, the announcement was a major turning point in the acceptance of GR. Of course, the subsequent confirmation of that announcement is equally important. (Had subsequent observations refuted Eddington's conclusion, GR would have ended up in the scrap heap and Eddington's claim would be just a piece of trivia. However, that is not what happened.) --EMS | Talk 04:46, 20 November 2006 (UTC)
It looks like a perfectly balanced piece of text to me. Readers should not be misled into thinking that Eddington's experiment was conclusive. What lacks at this point is a reference to a critical paper, preferably an overview article.
The text there can be reduced by referring to the experimental section or by putting part of it in a footnote. Harald88 08:05, 20 November 2006 (UTC)
I consider the text to be less than balanced because of its context. It places an empahsis on the ambiguities in Eddington's data, even though the announced conclusion has been confirmed. Please keep in mind that this is an overview article, and that voluminous detail is not appropriate to it. The issue is one of whether this item is important enough to rate mention here. I have editted the text to refer to "Eddington's announement" instead of saying the his observations did confirm GR. Beyond that, the idea of doing a footnote on this issue has also crossed my mind. --EMS | Talk 13:51, 20 November 2006 (UTC)

[edit] FA nomination

Because of the current FA nomination, I have expanded the lead to this article to be more complete and comprehensive. Others can help by adding in appropriate citations for what I have added, as I am a weak bibliographer. IMO, this fills the biggest hole in the article if it is to obtain FA status. Other possible issues are the size of the article and possibly the technical level of the section on comparisons with other theories. However, the comprehensive nature of the total material covered is most likely one of the reasons for the Fa nomination in the first place. In any case, let's see what others have to say about this article and adjust it as necessary. --EMS | Talk 21:50, 26 November 2006 (UTC)