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General relativity

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This peer review discussion has been closed.
Through the help of a number of contributors, this article has evolved to the point where I feel we are up to FA standard. It provides a good overview of the theory, touches upon all important subjects, and all its statements are referenced. In preparation for FA candidacy, I'm putting the article up for peer review. Any corrections and/or suggestions for improvement would be most welcome.

Thanks, Markus Poessel (talk) 17:53, 6 April 2008 (UTC)[reply]

Comments by Mike Peel

[edit]

Some initial comments, from a quick scan through the article:

  • The article is incredibly long: 149kb of wikicode! Can the different elements be summarized more concisely? Related to this (and I'm amazed that I have to say this), you have too many references. Can some of the secondary references be moved into the articles that are being summarized, leaving only the main references?
  • Going by readable prose, which is the criterion for WP:SIZE, it's 59kB, and thus less than the threshold for articles that "Probably should be divided". It's taken a lot of work to get to this point, and in the run-up to this peer review, we've done a lot of streamlining already. If you can suggest a section that can be tightened up, please do. As for moving references elsewhere, most of the current references were carefully selected so as to be most useful: original articles whenever I found them, up-to-date reviews, but also reviews that themselves have become classical. I am reluctant to change that, especially given the fact that WP:SIZE explicitly doesn't count references when it comes to size restrictions. Markus Poessel (talk) 19:12, 6 April 2008 (UTC)[reply]
  • The image from WMAP should probably be updated to the 2008 version (I think the current one is from 2005). Having it on a white background would also be good.
I've swapped it over to the 2008 version that was already on Commons. Mike Peel (talk) 20:40, 6 April 2008 (UTC)[reply]

I'll add more comments when I have more time... Mike Peel (talk) 18:39, 6 April 2008 (UTC)[reply]

Some more comments:

  • Introduction: "multiple images of one and the same astronomical object " doesn't need "one and" in the middle of it.
  • Introduction: Have gravitational waves been "measured indirectly", i.e. quantified, or just "observed indirectly"?
  • Cosmology: " seconds" needs a number at the top of it.
  • There are a number of very long sentences within the article. Examples: the first sentences of the second and third paragraphs, as well as the first sentence of the "The geometry of Newtonian gravity" section. These should be broken up where possible.
  • Some of the page ranges in the references are using the wrong sort of dash, e.g. Trautman 2006 uses - instead of –
  • Should the "history and current status" be moved to just below the introduction? That would seem to be the natural place for it to me, but then I maybe rank history too highly.
  • The main reason for leaving it where it is is that it makes casual mention of black holes, the cosmological constant, quantum gravity, and other concepts that have been explained in the preceding text. Once you know the physics, you can give a brief overview of the history; if you start with the physics, you would need to include explanation that would more properly belong in the physics sections. Also, I think that having the current status as a wrap-up is nice. Markus Poessel (talk) 15:43, 8 April 2008 (UTC)[reply]
  • It would probably be worth mentioning in the introduction that General Relativity isn't the only theory of gravity around.
  • The introduction says that "Although general relativity is not the only relativistic theory of gravity, it is the simplest such theory that is consistent with the experimental data." – I would have thought this sentence implies that GR is not the only theory of gravity around. Which further information do you think is needed? Markus Poessel (talk) 01:19, 9 April 2008 (UTC)[reply]
  • The geometry of Newtonian gravity: "Conversely, it would seem that inertial motions ... can be used to define the geometry of space and a time coordinate." You may want to clarify this: are you meaning "the geometry of three dimensional space, as well as a time coordinate"?
  • Relativistic generalization: "As intriguing as geometric Newtonian gravity may be, its basis, classical mechanics, is merely a limiting case of (special) relativistic mechanics;[8] in the language of symmetry: where gravity can be neglected, physics is Lorentz invariant, not Galilei invariant – and the differences between the two become significant when we are dealing with fast motions or high-energy phenomena." has a total of 5 commas, a semi-colon, a full colon and a dash in it. Please can it be broken up a little with some full stops?
  • Definition and basic applications: "As a result of the derivation sketched in the previous section..." The previous section was more of an outline than a derivation; a derivation implies a mathematical proof.
  • There is a derivation (all the mathematics included), but it was only sketched (that is, not described completely - the details are in the references). I've made it "As a result of the derivation outlined in the previous section" now; if that is still misleading, please advise. Markus Poessel (talk) 01:24, 9 April 2008 (UTC)[reply]
  • Definition and basic properties: "Its core are Einstein's equations" would be better as "At its core are ..."
  • Model-building: matter which is not particularly compact and moves quickly (but slowly compared with the speed of light)"
  • Light deflection and gravitational time delay: "In both cases the case of the planets and the probes"
  • I'm a bit wary of Image:Psr1913+16-weisberg en.png for two reasons: first, it was done in Excel; second, it has no error bars. Can this plot be improved at all?
    Difficult. I'm not aware that the actual numbers are published anywhere, so it's not easy to re-create the plot from scratch. On the plus side, if you look at figure 7 here, the error bars are mostly so small as to fall inside each dot of data. As a compromise, I've put a reference to that figure into the caption. Markus Poessel (talk) 14:59, 9 April 2008 (UTC)[reply]
  • Orbital decay: "using the binary pulsar PSR1913+16 that they had discovered in 1974"
  • Gravitational lensing: "This and similar effects are known as gravitational lensing[80] and, depending on the configuration, scale, and mass distribution, it can result in two or more images, a bright ring known as an Einstein ring, or partial rings called arcs."
  • Cosmology: "the universe has evolved over the past 14 billion years from a hot, early Big bang phase": Big Bang has both words capitalized.
  • Cosmology: It would be worth mentioning the influence that gravitational waves are expected to have on the polarization of the cosmic microwave background, provided that the universe has a suitably high value of the scalar-to-tensor ratio that we can detect it (see Cosmic Microwave Background#Polarization and the academic literature).

Mike Peel (talk) 21:55, 7 April 2008 (UTC)[reply]

Comments by Chris Lintott

[edit]
  • Will add comments as I go; not sure how much time I'm going to have this week. First note : I'm not sure how useful it is to use 'mass-energy' in the introduction and then link to mass and energy. The links won't help anyone unfamiliar with the concept. How about 'directly related to the mass (or more strictly the mass-energy)' Chrislintott (talk) 14:44, 7 April 2008 (UTC)[reply]
Hi Chris, first of all: many thanks for your comments. I've just deleted "energy" and put the other properties into parentheses in a phrase that is, hopefully, more accessible. Markus Poessel (talk) 16:11, 7 April 2008 (UTC)[reply]
  • I'm not too happy about saying that GR predicts the presence of black holes as the end state of massive stars; that's a prediction of stellar evolution models and not all black holes (especially those in AGN) may come from massive stars. Why not say 'It allows for the existence of black holes -— regions of space in which space and time are distorted in such a way that nothing, not even light, can escape. There is evidence...' and leave it at that? Chrislintott (talk) 14:48, 7 April 2008 (UTC)[reply]
I would think that "allowing for the existence" is too weak, but I agree that the additional input (models of stellar evolution) should be in there. How about something along the lines of "Applied to current models of stellar evolution, it predicts the existence of black holes"? Or a less awkward version of that sentence? As for AGN, I had hoped that was taken care of as the text goes on, talking about such [i.e. stellar] black holes and their more massive kin. Markus Poessel (talk) 16:18, 7 April 2008 (UTC)[reply]
I think my problem is that I'm not happy with the suggestion that models of stellar evolution are general relativistic in nature...you do need GR to handle the collapse to a black hole, but that's a bit different from what I'd normally mean by a model of stellar evolution. How about turning it around 'It predicts the existence of black holes, which current models of stellar evolution suggest are the final end of massive stars' (or something like that)? Chrislintott (talk) 12:13, 8 April 2008 (UTC)[reply]
Returning to that sentence, it now reads "the final stages of gravitational collapse". That should make it clearer that we are not talking about what falls into the realm of stellar models. Markus Poessel (talk) 21:36, 2 June 2008 (UTC)[reply]
  • A general reader is unlikely to be able to follow the point of the sentence 'however, as can be shown using simple thought experiments, the Newtonian connection is not integrable – space-time is curved.'. Can you either give a reference to such simple thought experiments, or just simplify the statement. Chrislintott (talk) 15:40, 7 April 2008 (UTC)[reply]
The reference is, well, in the reference. Currently, it's reference [6]. It would certainly be possible to be more explicit, but the article is rather long as it is. Also, some of the more elementary versions of thought experiments alluded to in the text are in Introduction to general relativity. Markus Poessel (talk) 16:18, 7 April 2008 (UTC)[reply]

There doesn't seem to be a reference tied directly to that sentence. I agree with you that adding more detail here wouldn't be useful. I would still rather just say 'space-time is curved' than nod to thought experiments we don't have time to describe (and where there isn't an obvious link to click if you want to know what they are). I hope to get to the rest of the article later today - thanks for the hard work. Chrislintott (talk) 12:13, 8 April 2008 (UTC)[reply]

I did try to be very thorough with the references, but not every single sentence has one. I've changed the sentence in questions in any case, and I think it should be more easily understandable now. Markus Poessel (talk) 21:36, 2 June 2008 (UTC)[reply]

Comments by RJHall

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  • I am wondering who is your target audience? Some of the language requires that the reader already be familiar with advanced calculus and phyics. "Lorentz invariant", "reference frames", "inertial frames", "Minkowski metric", "symmetry", "metric tensor", "partial differential equations", &c. That seems self-defeating. Also please see Wikipedia:Explain jargon.
Hi RJHall, thanks for your comments. Target audience is mixed, both the general reader who wants to learn more (beyond the existing Introduction to general relativity) and the more advanced reader, for whom this article should serve to tie all the other, more specialized general relativity-related articles together. Markus Poessel (talk) 18:33, 7 April 2008 (UTC)[reply]
  • The second sentence of the lead is much too long and it doesn't flow well. For example:
    It unifies special relativity and Newton's law of universal gravitation through the Einstein field equations, a system of partial differential equations. The result is a theory in which gravity is a property of the geometry of space and time; in particular, the curvature of space-time is directly related to the mass (and further properties such as the momentum) of whatever matter is present.
  • Per the MoS, there shouldn't be spaces around em-dashes. Spaced en-dashes are okay.
I tried to be consistent with en-dashes, but apparently two em-dashes slipped in. I've replaced them now. Markus Poessel (talk) 18:59, 7 April 2008 (UTC)[reply]
  • Per the MoS, the title of the article should not appear in the section titles.
  • I've changed it for "General relativity: Definition and basic applications". I don't see a good solution for "From classical mechanics to general relativity". All substitutions for "general relativity" I could come up with sounded rather awkward. Markus Poessel (talk) 18:37, 7 April 2008 (UTC)[reply]

—RJH (talk) 17:58, 7 April 2008 (UTC)[reply]

As far as I can see, in the one sentence that has it, the "it would seem" is not a weasel word. It's not there to introduce a vague statement; it's there to introduce a statement that might be thought to be true, given the preceding statements, but in fact isn't, as stated directly afterwards ("However..."). Would "one might think" be better? Markus Poessel (talk) 18:33, 7 April 2008 (UTC)[reply]

Comments by RelHistBuff

[edit]

Overall it’s a great article. Some suggestions that you might want to consider:

  • It does seem quite long as one commenter mentioned. Could a summary style help? For example, the quantum theory/quantum gravity section could be in a separate article. A lot of tests/measurements of general relativity are given in this article, but there is already a Tests of general relativity article which seems a bit redundant.
  • I'm still not quite convinced it is too long. As far as I can see, it's about the same length (by WP standards, counting only the main text) as the FA Evolution. And there is the need to be comprehensive, if only in summary style. I will try to shorten the text snippets about the various measurements and tests, but I think leaving them out altogether would create the wrong impression. I will try and see whether the quantum gravity section can be summarized more than it is already. Markus Poessel (talk) 16:51, 20 April 2008 (UTC)[reply]
  • I also have to agree with the previous commenter that the history ought to be at the beginning. The history could be written without the need of explaining difficult concepts. Some of the complex math articles were able to do this. The final two paragraphs (the "current status") however should remain at the end.
  • Why are the acronyms GR and GTR defined when they are never used ?
  • For "fast motions", do you mean “speeds approaching the speed of light”? If not, then could you explain what you mean?
  • Why are some solutions of "great theoretical interest"? Maybe add a sentence or two to explain.
  • Not so important, but another time example would be better rather than the "Minute Waltz" since "minute" was supposed to mean "small" and not 60 seconds.

I hope this helps. --RelHistBuff (talk) 10:29, 15 April 2008 (UTC)[reply]

Many thanks! Markus Poessel (talk) 17:10, 16 April 2008 (UTC)[reply]

Comments by Timothy Rias

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Overall impression: pretty good. I'm in a bit of a hurry so some of these comments can seem a bit staccato. This is not to be rude, so please do not take offense.

  • Overall. As I noted on the talk page decide between spacetime or space-time the article should at least choose a preferred spelling and stick to it. (you know my preference.)
  • Overall note. The prose seems to over use semi-colons. Readability can be improved by replacing many of these by fullstops.
  • General: it might be worth to consider using inline harvard citations instead of footnotes. Having harvard style citations in footnotes and having the footnotes section directly follewed by the general references sort of defeats the purpose of harvard style referencing. (i.e. not having to move the page to see what the reference is.) I would suggest either this of full references in the footnotes. The later obviously would be a lot of work, the former is a simple search and replace.
  • I did give this some thought, but I don't think it will work. There are too many footnotes with comments, which aim to bring the quotations into some kind of order; these would become much less readable if all sources were given in full, and would make the text unacceptably unreadable if included in the main text. With this solution, at least two clicks bring you to the full reference. And it's not uncommon; I've come across this style in a number of books. Markus Poessel (talk) 03:10, 13 May 2008 (UTC)[reply]
Harvard style referencing is indeed quite common, but in most case it is either inline or as a footnote. I haven't seen a book using harvard style referencing in endnotes, which is what the article currently is doing. But I see your point. And at any rate, inline style harvard referencing seems very unpopular on wikipedia, which alone might be a reason not do that.(TimothyRias (talk) 08:13, 13 May 2008 (UTC))[reply]
My goal is to have the resources presented in the way that is most helpful to the reader. They're there to be used, and they are most effectively used, I think, if the reader knows: X is an original article, Y is a review covering that particular aspect, Z is more pedagogical, but less rigorous. If I give the complete sources, this kind of source overview becomes very much less legible. Given how the automatic links work, the proper reference is, as I said, a mere two clicks away. Also, a number of references are used more than once in the text, so having Harvard style referencing avoids "ibid" or similar complications. I do respect wikipedia traditions, since they're very often based on what works best for the articles, but in this case, I think this is the best solution. Markus Poessel (talk) 21:43, 2 June 2008 (UTC)[reply]
  • In the sentence Already in 1916, Schwarzschild found a solution to the Einstein field equations that is nowadays known under his name link solution to Schwarzschild metric.
  • The second part of the sentence stretches the truth somewhat. The Schwarzschild solution by it self says very little about gravitational collapse. It think you could probably better just not that it was the first example of a black hole solution. (you might need to do some research to find when this was first realized (maybe mid 1920's ?).
  • I don't think it stretches the truth. It only says the Schwarzschild solution laid the groundwork for the description of gravitational collapse. Oppenheimer and Snyder found an interior spherically-symmetric matter solution and matched that to exterior Schwarzschild - in fact, their 1939 article starts with exterior Schwarzschild and proceeds from there. I'm not saying Schwarzschild describes gravitational collapse by itself but, well, it laid the groundwork. Markus Poessel (talk) 00:26, 30 April 2008 (UTC)[reply]
  • In the sentence the Newtonian connection is not integrable integrable links to an disamb page. Needs a direct link. Maybe better to not mention integrability at all and just paraphrase. (even most physicists would not known what the current sentence means exactly.
  • dealing with speeds approaching that of light -> dealing with velocities approaching the speed of light The speed of light is more than just the speed at which light travels. It is a fundamental parameter of nature.
  • Lorentz symmetry introduces an additional structure, in mathematical terms: a conformal structure. Shouldn't this say casual structure. I'm not sure it is enough to just specify light cones to uniquely determine the conformal class of a (pseudo)Riemannian manifold.
  • I see the problem – what is meant to be a further description of the light-cones in fact can be read as if I meant to describe the conformal structure as a causal structure. I've re-structures those sentences, being more explicit about the various structures involved. Markus Poessel (talk) 02:24, 5 June 2008 (UTC)[reply]
  • ...show that free-falling frames in which... needs a verb
  • As in the Newtonian case, this is suggestive of a more general geometry: where all reference frames in free fall are equivalent, and approximately Minkowskian, we are dealing with a curved generalization of Minkowski space: instead of Minkowskian, assume the metric tensor to be, more generally, semi-Riemannian. Is unreadable. Better to split in parts. Moreover it is pseudo-Riemannian.
  • next sentence a Levi-Civita connection->the Levi-Civita connection. Moreover, this might need some more explanation on why this choice is natural. (Why no torsion?)
  • The Einstein's equations section may need some discussion about what to choose as the stress-energy tensor.

I'll stop here cause I'm out of time for today. (TimothyRias (talk) 14:25, 29 April 2008 (UTC))[reply]

I'll continue:

  • At several places in the article semi-Riemannian manifolds/metrics are mentioned. I'm pretty sure this should be pseudo-Riemannian.
You're right, both terms are used, which to use is a matter of taste pseudo-riemannian seems to be the fashionable choice nowadays but semi is fine also. You might to change the wikilink from Riemannian to semi-riemannian (which then redirects to pseudo-Riemannian this will help readers that get confused because they are used to other terminology. (TimothyRias (talk) 07:38, 16 May 2008 (UTC))[reply]
  • blue-shifted / redshifted; needs a consistent choice of spelling. Either it is blueshift and redshift of it is blue-shift and red-shift. There seems to be no wikipedia consensus on this topic. Redshift has found its way into dictionaries while blueshift apperently has not. Still when using both words in the same article they should be spelled according to the same scheme.
  • General remark: The prose in the "definition and basic applications" section seems to be of lesser quality than the rest of the article. This might be the result of forcebly trying to provide a "definition" for GR. This not a big point, but just something I noticed.
  • Astronomically, the most important property of compact objects is that they provide a superbly efficient mechanism for converting gravitational into radiation energy. might be better in the form ... converting gravitational energy into radiation. The current sentence looks funny.
  • In On the basis of isotropic and homogeneous solutions of these enhanced equations isotropic and homogeneous need relevant wikilinks.
  • The section cosmic partitions: horizons could probably just be called Horizons. Partitions still suggest more something like a split where there is no way to get from either side to the other. (instead of the one-way traffic meant here.)
"Cosmic one-way streets?" - probably not sufficiently encyclopedic, so I'll go with your suggestion. Markus Poessel (talk) 20:38, 28 May 2008 (UTC)[reply]
  • FRLW metrics can also have future singularities in the form of a Big Crunch.
  • Numerical situations are known that violate cosmic censorship, making the last line of the relevant section inaccurate. I'll try to find a reference when I have time.
  • However, starting with the usual quantum field theories used in elementary particle physics to describe interactions, while leading to an acceptable effective (quantum) field theory of gravity at low energies,[177] results in models devoid of all predictive power at very high energies.[178] is a one sentence paragraph. It should probably be slightly extended and split into two or more sentences. You might also want to mention the relevant term; non-renormalizable. It tends to be used a lot in other treatments and it is nice for a reader to know that you are talking about the same thing.

And that's it. On the whole, pretty good job, some of the nitpicking above aside. (TimothyRias (talk) 12:30, 6 May 2008 (UTC))[reply]

Thanks, great! I'll address the comments one by one. Markus Poessel (talk) some time on 6 May 2008 (UTC)

Comments from Ealdgyth (talk · contribs)

[edit]
You said you wanted to know about FAC, so I looked at the sources for the article.
  • The Futamase, T. & Itoh, Y. ref has some weird formatting error in it. Very minor, but needs fixing.
  • The Kahn, Bob Gravity Probe B Website ref needs a publisher and last access date.
  • Same for the next Kahn ref
  • The Kochanek, C. S.; Falco, E. E., etc. CASTLES Survey Website needs a publisher
All the sources looked fine to me, at least in so far as they are published by reputable journals or publishing houses. I did NOT read the article, so no claims on proofreading are made. 16:26, 18 May 2008 (UTC)
Many thanks for your comments! Markus Poessel (talk) 19:22, 18 May 2008 (UTC)[reply]

Note to self: Heckmann-Schuecking reference appears to have gone missing. Markus Poessel (talk) 20:44, 30 May 2008 (UTC)[reply]

As far as I can see, I have now addressed all reviewer's comments. Unless new comments come up, or reviewers ask for clarification of my responses, I would like to take this to FA candidacy in a few days. Markus Poessel (talk) 01:31, 7 June 2008 (UTC)[reply]

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