Talk:General relativity/Archive 7

"Symmetrical Relativity"

There's a new article called Symmetrical Relativity, which I assert is original research and hence unsuitable for Wikipedia. If you have a background in the subject your review and opinions would be welcome on the talk page. Thanks! --Craig Stuntz 17:17, 3 August 2006 (UTC)

Dark energy

I am puzzled by this paragraph:

Dark energy: An unobserved energy that is spread throughout the universe. Recent observations of distant supernovae indicate that the expansion of the universe is accelerating. The Einstein field equations can support this type of universe only if 70% of its stress-energy is in the form of this dark energy.

First, the energy is observed indirectly, as implied by the next sentence. Second, the accelerating expansion contradicts general relativity. Second, the field equations do not support this type of universe unless they are modified. Not too much need to be said, as there is more info on other pages anyway. Roger 18:14, 10 August 2006 (UTC)

This section may need some tweaking, as your comments about the first sentence is reasonable. However, no change to the EFE is needed to deal with dark energy, as it can be modelled as a locally constant stress-energy field throughout spacetime. (OTOH, if you do want to change the EFE, then the best way is to bring in Einstein's cosmological constant term. The effect is the same.) --EMS | Talk 18:35, 10 August 2006 (UTC)

Ten years ago everyone said that general relativity requires that the expansion of the universe is slowing down. Yes, the equations can be fudged to approximate dark energy, and maybe that will gain acceptance someday, but it is a deviation from what has long been understood as general relativity. Do you want to suggest something brief? There is more detail on other pages. Roger 20:53, 10 August 2006 (UTC)

In a Robertson-Walker spacetime, the size of the universe (dictated by $a(t)$ in the equations in the Wikipedia article) can either be expanding or contracting, and that change of size can either be accelerating or decelerating. When the contents of the universe is mostly a "dust" of superclusters of galaxies and the leftover CMB radiation, these equations predict an expanding but decelerating universe. So there was an assumption being made as to the contents of the universe which in turn drove the prediction of how it was expected to behave. As you correctly note, the finding that there is an accelerating expansion of the universe was not expected. However, as more and more studies came to support it, the issue quickly arose of whether it could be modelled in GR. The answer was a resounding "yes"! However, there is a catch: To do so, it has to be assumed that the contents of the universe are 4% durectly observed matter and energy, 22% dark matter, and 74% dark energy! (These numbers come from this Sky and Telescope article on the most recent WMAP results.)

So first things first: The dark matter/energy results do not contradict GR, but instead are obtained from it. In essense that is how GR is reconciled with the current observations. So no change to GR is needed to expain dark matter & energy. Instead, change would be needed to explain the current observations without it. BTW - Observations of galaxies seem to support the view that 80% of the matter in the universe is not accounted for in existing observations. OTOH, those same observations do not provide any independent evidence for the existance of dark energy.

As for any changes to the article, the paragraph that you cite should be reviewed and revised a ways to make the situation somewhat clearer. Also, I can support wording in the "current status" section indicating that the dark matter/energy results do strain credulity and raise the issue of whether GR may be in need of revision. However, wording that GR is "contradicted" by the accelerating universe observations will not be tolerated. --EMS | Talk 22:25, 10 August 2006 (UTC)

You have partially agreed with me, and yet someone else has reverted my changes without comment. Perhaps there is a change that will make everyone happy. Suggestions? Roger 22:44, 10 August 2006 (UTC)

That "someone else" was me. Just see where my sig links to. I said that I would not tolerate wording that GR is contradicted by the accelerating expansion of the universe, and I meant it. The only reason that I did not revert the change earlier was because I did not notice it earlier. I have just done a rewrite of that piece to deal with your concern about calling it "unobserved" and to make it clear that dark energy is a prediction of GR needed to explain the observed accelerating expansion of the universe.

If you want to do more with the article, I am willing to work with you if I can. I would rather use the contributions of others than discard them, but I will not accept edits which I know for a fact to be in error. I made the case against your edits above. If you have futher questions about why I am discarding them, feel free to ask. --EMS | Talk 02:21, 11 August 2006 (UTC)

I am confused. Someone edited it again, and now it refers to solutions that "may require the reintroduction of the cosmological constant". Is the cosmological constant needed or not? —Preceding unsigned comment added by User:Schlafly (talk • contribs) 05:56 UTC, August 11, 2006

"May" means "maybe"; the cosmological constant's probably the most popular solution, but there are competing ideas, such as quintessence. --Michael C. Price ^talk 10:58, 11 August 2006 (UTC)

Roger - If you get confused this easily, you should not be here.  :-( This is Wikipedia, after all. What is going on is that you have openned a can of worms, and the text will be in flux until a new consensus is reached. --EMS | Talk 15:46, 11 August 2006 (UTC)

Being Wikipedia doesn't justify nonsensical explanations. It seems to me that general relativity does not predict or explain dark energy. If there is more than one way to modify general relativity to make it consistent with what is known about dark energy, then just say so. A consensus on what dark energy is (or how to model it) might take ten years or more. Roger 17:29, 11 August 2006 (UTC)

I don't see what is nonsensical here. GR is not easy to understand, but we are not dealing that the math itself in this thread. So let me try again:

1. GR does predict dark energy!!!
2. Dark energy is that means by which GR is reconciled with the observations of an accelerating expansion for the universe.
3. GR is contradicted only if dark energy does not exists.
4. An alternate way of explaining the accelerating universe is to use a cosmological constant term. This does fall under the umbrella of GR, but is a change to the theory.

You can keep saying that "GR does not predict dark energy" all that you like. That is simply false. GR indicates that if universe has a sufficient energy density that there will be an accelerating expansion of the universe. The accelerating expansion is observed. The needed energy density is unaccoundeted for. Therefore there must be a "dark energy" in the universe. That is the logic. That is how GR predicts dark energy. --EMS | Talk 21:25, 11 August 2006 (UTC)

As to the edit I just made, the energy of "unknown composition" has been observed. For example, measuring the velocity of clouds of hydrogren gas as you move to larger and larger radii ($r$) out a galaxy. One would expect these velocities to fall off like $r^{-1/2}$ since the mass appears to get less dense as you move out. However, instead the velocities are proportional to $r$, which means that there must be some gravitational effects (mass/energy) that we cannot directly observe. There may be some errors in that example, as I was working from memory and its been a while since I've worked with actual applications of the theory. Archmagusrm 14:57, 11 August 2006 (UTC)

I have reverted your edit. The evidence that you cite is good for the galaxy, but not for the universe. It is evidence for dark energy (which is also being called for) but not necessarily dark energy. There is an ongoing debate about dark energy, as noted above by MichaelCPrice. That needs to reflected here. --EMS | Talk 15:46, 11 August 2006 (UTC)

"Homeopathy relativity"

Readers are invited to have a look at the recent article "Homeopathy relativity" which attempts to apply General Relativity to Homeopathy. Reads like nonsense to me - other views are welcomed. Camillus (talk) 23:55, 21 August 2006 (UTC)

Recent rv regarding "effective field theories".

Effective field theories are hardly Original Research. They have been around for quite a while. There is even a Wikipedia article about them (one which I have not created or edited). The reference I cited is a pedagogical one from arxiv published in a conference notes.

A search on arxiv for "effective field theory" yields over 300 hits.

I don't want to start a "lame edit war", but I firmly feel that the recent removal of my edits was misguided. Therfore I am attempting to open a discussion. The views presented in the current article, that theories must be renormalizable, were current at one time, but are now quite dated. If there is a creditable source that even suggests that effective field theories are even mildly controversial, I would certainly be willing to include such a source in the interests of NPOV. I am not aware of any creditable source making such a suggestion, howver.

Pervect 06:05, 28 August 2006 (UTC)

You are claiming that there has been a recent paradigm shift in this regard. If that is the case, then I am happy to learn about it, and happy to let this article reflect it. However, as a general rule-of-thumb here, something new and unexpected that comes at you "out of the blue" is original research. I think I need to start a discussion over at the effective field theory article, and also ask the others here who have advanced degrees in the field about this. That way I can get a sense of what effective field theory is, how highly it is thought of, and whether and how it should be discussed here. EMS | Talk 14:14, 28 August 2006 (UTC)

I know a little bit about effective field theory. I agree with the spirit of the changes by User:Pervect but perhaps not with the tone. Statements such as "Nonrenormalizable theories used to be considered problematic" and "General relativity [...] at ordinary energies appears to be a good quantum theory" sweep a lot of subtleties under the rug. It is true that through the semiclassical approximation, and work on gravity as an effective field theory, general relativity is now seen as something that you can calculate quantum effects with at low energies. But I don't know if that makes it a "good quantum theory", even at low energies, or whether it means that nonrenormalizable theories are unproblematic. Certainly they are statements that a lot of physicists would agree with – tons of people work in the framework of EFTs these days – but they're not uncontroversial. I do think something about EFTs should be included, though. –Joke 15:23, 28 August 2006 (UTC)

Joke - Thank you much for this. I knew those edits to the POV as soon as I saw them. Also, while I suspected that the topic was OR it just did not read like real genuine original research. So it looks like some mention that GR works with (or seems to work with) EFT at low energies may be useful. However, some work on the tone is needed. I also would like to see a few more opinions on this issue. This is a change of perspective on the issue of GR and QM, and I want to be sure that we get it right. --EMS | Talk 15:59, 28 August 2006 (UTC)

What's this, User:Pervect and EMS are arguing over effective field theory? That is not a happy development, since I worked for almost a year with Ed on the (now aborted) WikiProject GTR and Pervect has generally been doing a fine job (as far as I can see) over at his native habitat, Physics Forums.

I probably won't have time to comment much on any ensuing discussion (having abandoned content to the cranks and all that), but FWIW, effective field theory per se is certainly not OR. (Just looked for a nice review paper I have seen on the idea that gtr should be an effective field theory for some quantum theory of gravitation, but coudn't find it, darn.) The quotes cited by Joke do sound very odd to me, however. I haven't heard about any recent paradigm shift, but then, unless this should be evident from classical gravition eprints at the arXiv, I wouldn't have done. Sorry I couldn't be of more help off the top of my stack :-/ Ed, maybe you can email to Steve Carlip? ---CH 04:58, 29 August 2006 (UTC)

I was caught fairly off-guard by this. I guess that I need to look over a few of the ArXiv references and decide about this. Chris - If you think that User:Pervect's edits are appropriate I will happily restore them. This is a shift in the status-quo, as I was hearing nothing about this six years ago when I took GR courses. --EMS | Talk 13:31, 29 August 2006 (UTC)

I very vaguely remember some papers on this. Testing my Google skills, I came up with this article in Living Reviews:

• Quantum Gravity in Everyday Life: General Relativity as an Effective Field Theory by Cliff P. Burgess

Pjacobi 13:49, 29 August 2006 (UTC)

That was a good reference. Looking at it and looking back at Pervect's edits I am now satisfied that what Pervect wrote is OK. The article has now been reverted to use Pervect's last version. OTOH, I have no regrets about challenging these edits. It is all too often that unexpected new data turns out to be WP:OR, and nice to see something withstand being challenged. --EMS | Talk 15:22, 29 August 2006 (UTC)

Ed, no-one cited links to the edits in question, so my remarks above were neccessarily very general. And Peter, thanks, you found the very article I was looking for! And good, it seems Ed and Pervect are on the same page now.---CH 19:53, 29 August 2006 (UTC)

References

I'm on a minicrusade to try and incorporate reliable references to statements made in general relativity articles. I've made a start at Einstein field equations, but it's probably better to start here at GR because there are a lot of statements here that need references. It's ok to give general references such as textbooks, but for statements about experimental results etc., it may be better to give solid research paper references (for example). Compare articles such as psychosis (former featured article) for the sort of thing I am trying to aim for. MP (talk) 18:15, 28 August 2006 (UTC)

That sounds like a really good idea. Where are you getting the research papers from? Archmagusrm 04:56, 29 August 2006 (UTC)

M, this might be a bit OT but I hope you'll read "What is the Einstein Field Equation?" from this page as well as The Meaning of Einstein's Equation, by John Baez and Ted Bunn. ---CH 05:02, 29 August 2006 (UTC)

Hi CH (long time no see!) and Archmagusrm. CH, I read those exposes by Baez and Bunn - been a while, but I'll have another look. Responding to your question Archmagusrm, for papers on experimental results, journals such as CQG (Classical and Quantum Gravity), GRG (General Relativity and Gravitation) and some others would be a good place to start (I'm not that interested in such papers, so other people may be able to help out here). As for other types of references, Einstein's original papers would be a good start, as well as various review monographs. The idea is to get a good selection of reliable references from journals like CQG and GRG to strongly back up many claims that to relativists are 'obvious', 'well-known' and 'well-documented' so that nonspecialists can't complain. It also gives us ammunition to counter any dodgy/POV/cranky edits by setting an example of including solid references (especially from reliable journals). MP (talk) 18:54, 29 August 2006 (UTC)

Hi, M, the first citation above is a post by me, and the second citation above is the well known essay by Baez and Bunn. I'd add Living Reviews to the list of superb on-line sources of good information. These review papers are often among the very best I've seen anywhere.---CH 19:56, 29 August 2006 (UTC)

M - This is a good project. If you have not already done so, do look at Wikipedia:Footnotes. This page describes a style of footnoting and supporting mechanisms that has been used to good effect in other large articles. This article could become a feature article. (It already is being called "A-class", which is a definite compliment.) Robust referencing and cutting down on the use of math are two keys to obtaining the honor of being a featured article. --EMS | Talk 20:01, 29 August 2006 (UTC)

The pages on specific aspects of GR contain more detailed references. Perhaps the 'flagship references' can be used in the GR article. For example, I've just included Hubble's 1929 paper - which I found quickly by googling - but was already in the article Metric expansion of space.

Alternatives to GR

Mollwollfumble has created an article (on the 24th of August) called Alternatives to general relativity which discusses various theories of gravitation and gravity - maybe it's just my imagination, but don't we already have an article that discusses other gravitation theories ? MP (talk) 17:56, 30 August 2006 (UTC)

The answer to my question above is YES, we have classical theories of gravitation - this makes things a little awkward, as there is too much overlap between this article and alternatives to general relativity. Looks like a merger or deletion should resolve this. MP (talk) 18:21, 30 August 2006 (UTC)

classical theories of gravitation is almost a stub, anyway. I've only glanced at it, but the new article looks MUCH better Pervect 20:17, 30 August 2006 (UTC)

Although the 'alternative theories' section is a nice little summary, the section is too long for my liking; there is a main article link to Alternatives to general relativity which does a comprehensive enough job (or will do) of describing the alternatives to GR. I think that the list of alternatives to GR should be scrapped from here and a few paragraphs be written about the types of alternatives that exist. MP (talk) 16:02, 6 September 2006 (UTC)

EFE publication date

See Priority_disputes_about_Einstein_and_the_relativity_theories#General_relativity. Actually, I gave the submission date in my revert instead of the publication date. The latter was 12/2/1915. Note that this is still 1915. 1916 is when Einstein published his landmark review article on general relativity in Annalen der Physik. --EMS | Talk 03:25, 4 September 2006 (UTC)

How about fixing this up? Submitting it for publiciation in 1915 is not publishing the paper in 1916. I thought my prose was pretty clear, I really don't understand why you reverted it. Come to think of it, I think I'll just revert it back, because I'm miffed. Please do fix it up to make the point clear if you think something in my prose was unclear or clunky or whatever - please don't just revert it mindlessly again. I believe that the reason this section was pasted with a {{Fact}} tag was because of the inconsistency with the very first reference in the article, the date of publication of the theory. People need to know that the theory was written down by Einstein in December of 1915 and published in 1916. If someone wants to address the Hilbert dispute or add a link to it, that's great, I think that the article could use expansion. I don't know enough of the details to add a lengthly explanation, I just want users to understand the 1915 and 1916 dates. Pervect 04:21, 4 September 2006 (UTC) Pervect 04:24, 4 September 2006 (UTC)

When a manuscript was really written down in its final form is usually hard to establish, for publications commonly only the publication date is taken into account. And if my English isn't completely off (and I'm not completely drunk), then EMS stated here above that the first publication date was 12/2/1915 (is that American for 2/12/1915?). Of course, that must be backed up by a reference to the 1915 publication. Harald88 21:44, 7 September 2006 (UTC)

The first publication of the EFE occurred in a printed journal dated December 2, 1915. That is my point. --EMS | Talk 02:43, 8 September 2006 (UTC)

This doesn't match up with the web sources I've seen. Consider, for instance http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/General_relativity.html

This states that the correct field equations were submitted on December 25 in 1915, and so could not have been published in a printed journal on Dec 2. Note that there were several previous papers by Einstein that were about general relativity but did not have the correct field equations. There were also 2 papers in quick succession with the correct field equations - the first paper, and an expanded version.

I've been trying to dig around more to confirm my information, but I've been a bit busy on other projects, and also I have more technical books than history books. But I think this area needs to be looked at in more detail. Right now I don't have the references to support my view firmly in hand, but I'm still quite suspicious of the 1915 date for publication. Britannica also puts it in 1916 http://www.britannica.com/eb/article-9109465. Pervect 19:08, 9 September 2006 (UTC)

I just looked at the first page you referenced, and the date given was November 25, 1915. Besides, who would have been around accept a submission on Christmas day? As for the Britannica article, you need to realize that November 25, 1915 is the end of the initial development of general relativity. At this point, the EFE are in place, and except for the (later regreted) cosmological constant no further changes will be made to the field equations of general relativity. However, as of 12/2/1915, GR has no coherent presentation in a single article. Instead, it is scattered over numerous articles which give its final 1915 form. In 1916, Einstein published a comprehensive overview of this achievement in Annalen der Physik which described in one place what he had achieved in late 1915. That classic article is often cited as being the publication of GR as it is the first time that the whole is presented in a single place. However, GR in its modern form first came out in 1915. --EMS | Talk 20:27, 9 September 2006 (UTC)

Note that the date given by my on-line reference was submsission of the paper for publication on December 25, 1915, not publication of the paper in 1915.

A few more references. http://www.bun.kyoto-u.ac.jp/~suchii/gen.GR5.html talks about Einstein's 1915 papers on relativity. These papers were about general relativity but did not have the correct form of the field equations, and seem to match your reference. http://www.bun.kyoto-u.ac.jp/~suchii/gen.GR7.html gives a reference to a single paper in 1916 that seems to match the Mpatel reference, which I believe to be the correct reference for the first publication of the correct field equations. I am not a historian, however this is what I found online. I'd like to drag Mpatel into this debate, because I'm not sure where he got his information. While my first online reference suggested two 1916 papers, my second only quotes one, the same one quoted by Mpatel. It does appear from the information available that the 1915 paper cited in the Wikipedia is an early article which, while written by Einstein, did not yet have the correct field equations.

Pervect 21:36, 9 September 2006 (UTC)

Ok, I'll get involved in this debate. I got my 1916 source from here: http://www.alberteinstein.info/gallery/gtext3.html (the current reference number 2 in the article). There it states that "This article was the first systematic exposé of Einstein's general theory of relativity" and also "Einstein completed the general theory of relativity in 1915 and published this first exposé the following year." Also, http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/General_relativity.html says that Einstein submitted a paper (The field equations of gravitation) on Nov. 25 1915 with the correct field equations (but Hilbert had done the same 5 days earlier).

From what I can gather, all the correct bits and pieces of GR were in place and sent for publication in 1915 (and published in 1915 ?). The first published overview of the theory was in the 1916 Annalen der Physik paper. A reference to Hilbert and Einstein's November 1915 papers should be given for the correct field equations, if those papers were actually published in 1915 - the Einstein one appears to have been published on Dec 2 1915, but I'm not sure about Hilbert's.

About the publication date ? Well, the theory was really present(-ed) and completed in 1915, meaning that all the bits and pieces were published in 1915. The first proper overview of the theory was published in 1916. Therefore, I agree with EMS and say that the theory was first published in 1915. I think that means I have to remove my reference (the second one) in the first line.

Since the other reference in that line appears to refer to a paper which appears (from my secondary sources) to have incorrect version of the EFE, I'd feel obligated to tag it with a "verify source", as a doubtful statement that really isn't very harmful to the article. A really thorough investigation would involve getting a hold of the original paper (in German) and verifying whether it did (or did not) have the correct field equations, I suppose.

One peaceful but rather uninspired, solution is to remove all conflict by removing both references. Then at least we won't have anything that's actually wrong, it'll just be rather vague.

Of course, someone will then put in a "citation requested", and we'll be back where we started :-) Pervect 20:06, 10 September 2006 (UTC)

It sounds like I somehow referenced the wrong article. Einstein first submitted field equations which were $R=T$ early in November, 1915. After it was realized that the only solution to these field equations is $T=constant$ (meaning that the universe should be of a constant density everywhere), Einstein figured out that $G=T$ are the real field equations. Those were published on 12/2/1915. As both articles had the same name, that could be where the confusion arose.

I advise against removing the 1916 reference. That was the first comprehensive overview, and so is in some ways a better reference that the 1915 article.

As for Hilbert's paper, you need to see relativity priority dispute for the full story. In a nutshell, it does not appear that Hilbert submitted the correct field equations initially. However, the by the time that article came out in March, 1916, it did contain the correct fields equations. it is notable that Einstein and Hilbert settled that fledgling priority dispute by agreeing the Einstein discovered the field equations, while Hilbert was responsible for the action. I strongly suggest that the wishes to there two great men be respected in this matter. --EMS | Talk 21:45, 10 September 2006 (UTC)

I found the problem: The correct reference was called "Ein1915a" in relativity priority dispute, but the way the articles are cited in the text I assumed that the corrent entry had to be "Ein1915d". The correct acticle is now being cited. --EMS | Talk 21:58, 10 September 2006 (UTC)

They aren't in the modern form, but the field equations in this paper, available online at the following link http://nausikaa2.mpiwg-berlin.mpg.de/cgi-bin/toc/toc.x.cgi?dir=6E3MAXK4&step=thumb sure look like the correct version to me (for instance, eq 6, pg 2) R_uv = -x (T_uv - g_uv T), and the paper is clearly dated as November 25, 1915. So I think we've got the right reference, this time around, at least as far as I can tell. I'm a bit surprised at the amount of seemingly contradictory information on the WWW about this topic. I'll probably add the above html link to the reference if I can figure out how to do it. Pervect 21:47, 12 September 2006 (UTC)

Ooops, I just noted that there is still a date conflict, now it's the day that's the conflict, not the year. Sigh. I'm going to change it to what I think is right based on the above online reference from Echo, which I think is a creditable reference Pervect 22:05, 12 September 2006 (UTC). I should add - to read the date information, click on the thumbnail of page 1, and enlarge it to about 2x. To read the field equation, click on the thumbnail of page 2, and enarge it to about 2x. Pervect 22:09, 12 September 2006 (UTC)

The "date conflict" is because the journal article header lists the submittal date (November 25), while my original citation listed the publication date (December 2). BTW - The publication date appears at the bottom of page 4 of the thumbnails in your Project Echo reference. In any case, I thank you for that link, and leave it up to you as to which date will be listed. My advice is to use the December date, but I am not going to start an edit war over two dates that are separated by only a week. --EMS | Talk 02:37, 13 September 2006 (UTC)

I'd really like to do whatever is standard. http://adsabs.harvard.edu/abs/1915SPAW.......844E for instance only gives the year. Any third part comments, or is everyone else as tired of the issue as I am? The important thing is that the user be able to identify the referenced work, and I think we've met that criterion. Pervect 20:43, 13 September 2006 (UTC)