Talk:Classical theories of gravitation

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Contents

[edit] Order

Some users added to this page in chronological order and others in statement/response style, which messed uip some signatures and makes the discusion hard to follow. I am trying after the fact to put everything in order to add correct signatures.---CH (talk) 18:19, 21 August 2005 (UTC)

OK, I think I've finished reorganizing everything, so I have removed "inuse" template. Talk pages get very confusing very quickly if different contributors use different conventions, and particularly if they do not always sign all their contributions with their user name and date (four carets). I happen to prefer the indented/signed statement/response format, but after the fact, it would have been impossible to reformat the page in this style, especially since the signatures were all messed up, so please, on this particular talk page at least, henceforth let us put our comments in sections added in chronological order with our signature at the end of the section, OK? (If you want to comment on this, sheesh, dunno what to tell you about where to put that, but it's probably a more general issue than discussion of this particular article, so how about my user talk page?)

[edit] CH's critique of original version (reasons for adding "dispute" flag)

Hi, ErkDemon,

In honor of the World Year of Physics 2005, I am currently engaged in a long term attempt to greatly improve the content/organization/accuracy of the gtr related pages.

Currently, I believe we need

  • an in depth article comparing various (non-cranky) classical gravitation theories, including all those mentioned in the "Alternative theories" section of EMS's general relativity article,
  • a full length article on Mach's principle,
  • a full length on classical unified field theories, with coverage similar to the long review by Hubert Goenner.
  • a full length article on PPN formalism,
  • a section on experimental tests, including articles on the four classic solar system tests; as you probably know, many of these are not neccessarily specific to gtr.

I see you are (yes?) a new Wikipedian, so perhaps you might be interested in trying your hand at some of these projects.

I have inserted a "dubious" flag because you have said some things which I believe are misleading:

  • The term general theories of relativity is nonstandard (as is the concept as you have described it), and confusing (much too similar to gtr),
  • The claim about Mach's principle are controversial, if only because few authors bother to specify what they mean by "Mach's principle". It is true that gtr violates some formulations, but in their book Ciufolini and Wheeler argue that it is true to the spirit of MP in some sense (which I think they failed to clarify).
  • Your speculations about metric theories might better belong on your user page or your personal web page (see the Wikiproscription against "original research", which is a polite name for untested/unpublished/questionable ideas which probably do not belong in an encylcopedia, which focuses on matters of fact or establised bodies of theoretical knowledge). For example, it is legitimate to write in Wikipedia about loop quantum gravity in an NPOV way, since this is in fact a theory which is currently discussed in the research literature, but it would not be appropriate to write an article about some previously unpublished speculation in this area.

Accordingly, I'd like to merge some of this stuff with the existing general relativity article, or else to get you (and perhaps EMS, who also seems interested in alternative theories) to write an indepth but fair NPOV article on classical gravitation theories, including the ones mentioned in the existing article.

CH (talk) 23:57, 13 August 2005 (UTC)

[edit] Comments by MPatel on CH's critique

Hi ErkDemon, welcome to WP.

Regarding the article, I agree with CH that the term 'general theories of relativity' is nonstandard, but I can see that you want to mention alternative theories. The general relativity page has a section on 'relation to other theories' which contains some alternatives to Einstein's GR (we should really call it just 'GR' - that seems to be standard). I try to read through some of these articles to see what's there and to identify the terminology used, although sometimes I fall into the trap of using new phrases that are already there in a standard form. Also, we should take whatever is useful in this article (and isn't already elsewhere in WP), merge it into the GR page(s) and then delete this article.

Mpatel (talk) 10:19, August 14, 2005 (UTC)

[edit] CH describes wider context for proposal

Hi again, Erk, just to emphasis the wider context: I, MP, and EMS are trying to reorganize/reshape the gtr pages, as well as improve/elaborate existing articles and add many new ones. To some extent every editor here works according to his own plan, but clearly it is important to coordinate our activities as far as posssible, or we'll wind up with a mess.

I say something below about a forthcoming "Wikiproject GTR" page which will formalize our goals/methods/notation, and so forth, but first, there are some excellent guidelines which apply to all Wikiprojects and indeed to all edits, including The Five Pillars of Wikipedia, where you will find a link to the proscription I mentioned, No original research, and also to NPOV, to mention two pages which I think bear occasional rereading by all editors! Note that "guidelines" are just that, "policies" are enforced more strictly, but as I see it these are all tools to help Wikipedians work together to create something of value.

In our gr pages/revision project, we (myself, MP, EMS) have set ourselves a large task, and we'd like to recruit others, or at least try to help prevent editors of these pages from working at cross-purposes (e.g. creating articles with heavily duplicated material, conflicting notation).

CH (approx 17:30, August 14, 2005)

[edit] CH mentions "Wikiproject GTR" draft manifesto

Hi all, I can see a task which has been overdue for some weeks now: to create a formal project

"Wikiproject GTR" clearly explaining in the usual "broad to narrow" ("overview to detailed") wikistyle

  • the goals of the project
  • list of participants (where users can sign themselves up)
  • issues which have been identified and proposed responses
  • possible pitfalls to avoid
  • links to related projects

An example of an issue which I once again encountered yesterday: ideally, we should try to ensure that all the gtr articles use the "Landau/Lifschitz spacelike" sign convention (by far the most common and closest to being "standard") for metric signature, signs of Riemann tensor, etc., with occasional exceptions; e.g. most authors use -+++ and switch to +--- when using NP tetrads.

Today I am still trying to write the Robinson-Trautman spacetime hierarchy and to destubbify the closely related Calabi flow article, so I probably won't get to creating a draft project page until later today or tomorrow; interested users can watch my user page. My idea is to put up a draft in my own user space, invite comments, and when we have consensus on goals (so far my impression is that we already have consensus between myself, EMS, MP, and probably others too) to move the draft to create the formal project page.

CH | (talk) 17:30, 14 August 2005 (UTC)

[edit] Comments by EMS on CH's critique

I too am not too impressed by this article. While I have no objection to creating a full article devoted to GR atlernative which have been actively studied and considered academicly, this is not what I would be looking for in such an article. For one, I would recommend a title like "Alternative theories to general relativity". As I see it, this article needs to either be repaired or deleted. I remind people of Votes for Deletion (VfD) and even speedy delete. I will follow Chris' lead on this, but left to my own devices I would roll my eyes and VfD this.

EMS | Talk 00:28, 17 August 2005 (UTC)

[edit] ErkDemon's response to CH's critique

  • I agree that the term general theories of relativity is nonstandard, if you can think of a better term, please do suggest it. Since there is more than one theory that applies the principle of relativity generally, "general theories of relativity" seemed to me to be a valid "umbrella" term. There are general theories of relativity other than Einstein's. One might even suggest that Einstein's general theory came in two variants, depending on whether you choose the original version with the cosmological constant (pseudo-Euclidean universe) or the cleaned-up version without it (expanding hyperspherical universe).
  • I agree that Mach's principle is controversial, but it was also controversial back when Einstein used it in his development of GR, when he said that GR was an implementation of Mach's ideas, and when the community began to realise that Einstein's GR didn't seem to be fully Machian after all. I thought that a quick allusion to GR's perceived shortcomings was relevant to the question of why other similar theories exist, the issue of GR being considered by some to be "insufficiently Machian" was one of the reasons why Brans-Dicke theory was devised as an attempted alternative. I think that part of why people skirt around the subject of MP so much is because they don't want to be too rude about general relativity. I agree that I could have worded this section more tactfully, I shall try to phrase it more delicately. A more detailed breakdown of reasons why GR might be considered not to be quite optimal belongs on a different page, anyone working on more advanced theory ought to be able to trot off a list.
  • The part about metric theories is not speculation: Acoustic metrics are "metrics" (obviously), but theories built using acoustic metrics, which are technically "metric theories", do not fall under MTW's definition of a "metric theory", because they do not reduce to SR. Since Hawking decided that perhaps general relativity may need to be modified to allow fluctuating horizons (2004) work on metric theories that include this sort of "acoustic metric-type" behaviour has come under a lot more scrutiny as people look for routes to quantum gravity. Some of the guys working on the black hole information paradox have been playing with non-SR solutions for some time, now.

But the problem with trying to categorise this prototype QG work (by mainstream researchers) is that it doesn't fit into the usual classification schemes for GR-type theories. You can have a metric theory, which, according to MTW, isn't called a metric theory (even though it clearly is a metric theory).

I think that "non-metric metric theories" are a good example of one of the recurring problems that we have with cross-theory work and general relativity - some of the more enthusiastic GR proponents seem to have been busily subclassing and redefining new GR-specific variations of general terms to try to make them more GR-specific. It's a bit like Macdonalds teaching that a burger isn't a burger unless its a Macdonalds burger, and then when you then want to discuss the properties of non-Macdonalds burgers, Macdonalds-trained burgerologists patiently explaining that there can be no such such thing, by definition. Theory-specific redefinitions may be useful within a single theory, but they are a liability when one wants to compare different theories.

ErkDemon 23:53, 19 August 2005 (UTC)

[edit] ErkDemon responds to Mpatel's comments

Hello!

I suppose that it depends on whether we are taking the word "theory" in "general theory of relativity" to be singular or plural. If "general relativity" is to refer to a broad range of theories, then yes, merging would make sense, but some people argue quite strongly that Einstein's theory owns the name "GR" by default, which leaves us without an obvious name to refer to the larger set of general theories of relativity to which Einstein's GR belongs.

It seems a little odd to me to have a section in a page on "theory X" to be the default jumping off point for links to theories that are NOT theory X.

And there's also the issue of persistent linking: If you want to include a link in another page to something like "GR-like theories", then it would be handy to have a defined page to link to. I suppose that you could link to the appropriate subsection of the GR page, but then if the GR page gets rewritten and the section title changed, the link will break. If the list of non-EGR theories is on its own page, then if the page is ever moved, a redirect gets generated by default and the link still works.

Also, I think the GR page is a bit big. Large monolithic pages are nice when you find a good one, but the more sections you add to a single page, the more POV disputes and editing wars they seem to provoke. Beyond a certain point, snowballing everything up into a single page can start to defeat the object of using hypertext.

Another possible idea might be to merge this with the "alternative theories" section of the general relativity page and then extract the whole section to its own page. Again, if anyone can think of a better title for a listing page for theories similar to GR, then please suggest away. "Alternative theories of relativity"? No, not specific enough to gravitational theory. "Alternative gravitational theories" wouldn't specify that the principle of relativity should be involved. "Alternative relativistic theories of gravity" is a bit long-winded. "GR alternatives"? Maybe, but it would be nicer to have a list page that referred to a set of theories that included Einstein's GR.

ErkDemon 00:38, 20 August 2005 (UTC)

[edit] EMS comments on revised version

ErkDemon -

This page still needs a fair amount of work. I advise looking at the alternate theories section of the general relativity article. That is what this article should be improving and expanding on.

If you are willing to put this article into much better shape, I am happy to see the following occur:

  1. This article be listed as the "main article" associated with the alternate theories section of the GR article.
  2. Some of the material in the main GR article section be transfered into this article. (Chris did some work on that section, and now I worry that it is getting too big and detailed for an overview article. However, that material is valuable, and needs a new "home" if it is not to stay there.)
  3. This article added to the GR navigation template.
  4. This article become the feature article of a similarly named category.

Towards that end, I would like to see this article renamed something more appropriate. I suggest Alternatives to general relativity. (I thought about Metric theories of gravitation but Einstein-Cartan theory being an affine theory moots that idea.) That would remove the confusion between it and the general relativity article itself. It also would give it a scope that aligns better with the existing work, since GR itself is quite adequately documented in its own article. So what is needed is to expand on these theories and give some sense of how they relate.

In spite of my previous comments above I do agree that the subject of this article is encyclopedic. My concern is that the contents are quite weak in comparison with what already exists. GR seems to be something that people are happy to write about, but often without adequate knowledge. In your case you have picked a rat's nest of a subject, which demands some understanding of numerous theories many of which are more arcane than GR. Even so, there is no reason why a good job cannot be done with it. It just demands a lot of work. EMS | Talk 20:22, 20 August 2005 (UTC)

[edit] ErkDemon reponds to EMS's comments on revised version

Hi EMS! To be honest, when put the page up I had no intention of writing a major article or trying to "explain" anything, I just felt that there ought to be a nice useful basic listing page somewhere with maybe one line of comment on each theory, and a link to where someone could read about each one properly (if the page in question existed). If other people wanted to expand it and enlarge the list, great (that's why I added the stub). I figured that a listing page would be nice and simple and uncontroversial, all the "controversial" stuff could go into individual pages written by individual people who knew each theory well. I actually thought that the section in the GR page was quite useful (I borrowed a URL and "red links" from it), the only thing wrong with it was that it was tucked away at the bottom of a page on Einstein's GR, instead of being up in the daylight where everyone could find it and link to it.

It was just going to have two sections: metric and non-metric theories. Where the plan came unstuck was when I looked up the small print on the precise definitions being used in MTW, and found that since they'd mucked about with the definition of "metric theory" to make it SR-specific they'd effectively created an additional category with no name for theories specifically excluded from the MTW "metric theories" label. So then I felt obligated to add a third category so as not to perpetuate that bit of naughty claim-jumping, and then I had to write something to explain what the new crazy category title meant, and then I had to explain the sort of theory that fitted this category, and THEN I ended up alienating everyone here by making them think I had set out to use the page for self-promotion. Which really wasn't the idea. If MTW hadn't "bent" the definition to suit their purposes, I wouldn't have felt the need to explain what had been omitted. I suppose another option would have been to drop the "problematic" metric/non-metric categories altogether and just used a single bulletted list, maybe hope that someone else would write a separate page on categorisation issues.

I don't really care too much who takes over the page or what they do with it, or what it's called (help yourself), but I do think that some sort of central listing page perfoming this function ought to exist. I'd personally prefer a page that includes current GR along with the other theories, but if nobody can think of a better title, then perhaps that "GR alternatives"-type title ("GR and alternatives"? still clumsy) might be a decent compromise. I think that its a measure of the profound definitional inadequacies of this subject that we don't even seem to have an agreed generic name for these theories (other than the "general theories ..." bit that seems to upset people here so much).

ErkDemon 22:06, 20 August 2005 (UTC)

[edit] Chris Hillman's comments on revised version

Hi, Erk, good, we are making progress, and I think I am beginning to understand better your motivations/concerns!

I also am trying (over time) to revise the Wipedia articles in the Category:General relativity to give a better portrait of modern gravitation theory. In particular, I think we both want to see an intelligent, timely, well-informed, and well-organized discussion of the successes and failures (theoretical and observational) of the most important theories. And I think we both would agree that one of the most common misconceptions is that the only theoretical reason to be unhappy with gtr is that it is not a quantum theory; another "classical objection" is that gtr is notoriously awkward in the way it handles the exchange of energy-momentum between the gravitational field and all the rest (teleparallel gravity may offer a better way, although in it's current state that theory has disadvantages of its own).

(My goal on Wikipedai is not to write a textbook, but to help orient readers and to lead them to appropriate resources for more in-depth study.)

You wrote: if anyone can think of a better title for a listing page for theories similar to GR, then please suggest away. "Alternative theories of relativity"? No, not specific enough to gravitational theory. "Alternative gravitational theories" wouldn't specify that the principle of relativity should be involved. "Alternative relativistic theories of gravity" is a bit long-winded. "GR alternatives"? Maybe, but it would be nicer to have a list page that referred to a set of theories that included Einstein's GR.

We already have such a category, Category:Theories of gravitation. We urgently need a companion article (with the same name) offering a thorough comparision of gtr with competitors, including discussion of classical theoretical motivations for exploring alternatives. I think this article should be transformed into such an article under a title like "Classical theories of gravitation", and it should discuss not only the stuff you (ErkDemon) are concerned with in the present (second major revision) of this article, but other issues as well.

Both I and EMS seem to have nominated you to begin writing the new article :-/

If you decide to avoid quantum gravity (which might be wise), it could be called Classical theories of gravitation. Just be sure you do not use the nonstandard and confusing term "general theories of gravitation".

Perhaps you could organize discussion of theories motivated in part by an attempt to more fully/clearly realize some kind of "Mach principle" into a section of the article called something like "Machian theories", but be sure to emphasize that this term is not standard, and that the concept is also nonstandard but useful in this article. If you do a very good job of sorting out the extent to which various theories realize various "Mach principles" (see the article by Bondi cited in the Mach's principle), your term may even become something more than a nonstandard term of convenience. Aim high, say I!

A quibble: the first paragraph of the second major revision of this article is still misleading by overemphasizing the important of "Mach principle"s in motivating gtr; this was certainly one of the major motivations, but there were several others. But in any case, for most readers I think it is more important to stress how gtr fits into modern physics, not into Einstein's views in 1915. I agree that Mach principles have always been controversial, and I hope you will improve the existing article to better explain why (and also to explain that there are many distinct principles, not all very precisely formulated).

Why can't we leave well enough alone? Because a "quick allusion"s to gtr's shortcomings will be misunderstood by almost all readers, especially since there is presently no adequate survey on Wikipedia comparing gtr's theoretical advantages/disadvantages with other theories. Or even in the research literature. If we are going to mention shortcomings (and of course we should), we must clearly and fairly explain them.

The reason for the lack of a really good review is certainly not any concern about "rudeness" (since when were scientists ever concerned with being polite to the detriment of advancement of their field?), but the conceptual difficulties involved and the fact that gtr has been so much more intensively studied than its principal competitors that mastering even the gold standard theory (gtr) probably daunts many would-be commentators.

BTW, I myself planned to write a well-organized survey of theoretical advantages/disadvantages of gtr somewhere on Wikipedia, since I think I know the literature fairly well and can give a much more accurate/balanced account than can be found here at present. But I wouldn't get to that for a loooong time, which is another reason why I think you should make a start on approximating an adequate article of this nature.

About acoustic analogs (?): I may have misunderstood what you had in mind. If you are talking about the work of Matt Visser and others on physical analogues such as acoustic black holes and optical black holes, then you would probably agree with my frequent assertion that one of the major lessons of this work seems to be that "black hole thermodynamics" is a very general concept, not really part of gtr at all. I myself have been planning to eventually ensure that Wikipedia clarifies this point in some appropriate place, but I wouldn't get to this for a looong time, so again I'd encourage you to go for it, using Visser's book-length review at the Living Reviews website as a guide.

You wrote: some of the more enthusiastic GR proponents seem to have been busily subclassing and redefining new GR-specific variations of general terms to try to make them more GR-specific.

I have no idea what a "GR proponent" might mean in physics; gtr is our current gold standard theory of gravitation, but its classical shortcomings are more or less well known to the researchers who actually use this theory (close acquaintance breeds, certainly not contempt, but a fair appreciation of both advantages and disadvantages). We're all on the same side, struggling with the same intellectual issues.

A minor quibble: gtr should not be regarded as "coming in two varieties"; please see the article on Lambdavacuum solutions.

Finally: to sum up the situation, I think EMS and I agree that you should retitle and greatly expand this article, or at least retitle it and flag it as a stub which needs substantial improvement.

CH (talk) 20:30, 21 August 2005 (UTC)

[edit] Retitle and third major revision

Hi all, I see Erk gave me permission to move the article, so I did that (due to wikiserverstress this is incorrectly attributed in history--- this is frustrating, but not really my fault!). OK, Erk, now it's up to you! :-/

CH (talk) 21:19, 21 August 2005 (UTC)

[edit] Todo list and students beware

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

Sadly, I am now abandoning this article to its fate. See User:Hillman/Archive for the last version I edited. I emphatically do not vouch for anything you might see in more recent versions.

Good luck in your search for information, regardless!---CH 23:02, 30 June 2006 (UTC)

[edit] This article has been superseded

My apologies for starting the writing of the new article "Alternatives to general relativity" before reading this one. The topics covered are almost exactly the same; the new article covers them in much greater depth. I'll leave it up to you to decide whether or how to merge the two, and under which of the two headings to merge them. I don't mind if all of the new article is moved into this one, so long as the introduction to the new article remains where it is. On the other hand, perhaps this article could be scrapped, I didn't find it easily. I still have a few editing details on the new article so give me another two or three days before modifying it. Mollwollfumble 05:42, 30 August 2006 (UTC)