Talk:Introduction to general relativity/Archive 1

message: First of all, I will reiterate my feeling that this page is baiscally a good idea.  However, I must empahsize that by it's title it has a simple mission:
:To state what Einstein's theory of gravitation is.
This is not the same thing as describing general relativity.  We already have a general relativity article, and I have more than once pruned GR stuff out of the old gravity article.  However, this article is more than a errant section turned into an article.  It already to some extent acts as a bridge between the topics of gravitation and relativity.  SC got it off to a good start in that regard.  If this article sticks to its mission, it can act as this bridge and play that role well.
As I see it, the goal of this article must be to state as clearly and non-technically as possible just what it was that Einstein did.  So it should touch on the topics of the equivalence principle, geodesics in general relativity, spacetime curvature, and perhaps the Einstein field equations.  Note that none of this can be covered in depth, or even to the level that it is addressed in the general relativity article itself.  This article must contain the "broad brush" treatment, being a sort of teaser for the GR article itself.  If this article should start going into real depth it will become redundant.  Instead it must support and compliment the existing GR article as well as the gravitation article.
The big things that need to be dealt with at this time IMO are:
* The section on the testing of general relativity:  There already exists a tests of general relativity article, and much of this material is reiterated there.  That section should either be merged into the  "tests ..." article or removed, and in either case replace with a much briefer reference to the topic.
* The description of the impetus behind general relativity is not correct:  The speed of gravity was a minor consideration.  The equivalence principle and the revelation that it could not explain the Earth's gravitational field without spacetime being curved are the real genesis of this theory.
* Some mention of the comological issues are appropriate:  It is an interesting irony that the same theory that give us the gravitation between massive bodies also gives us the (apparently accelerating) expansion of the universe.
* Sectionalization to outline the relevant major concepts:  This would provide a more readable article and a framework for fleshing it out.
Finally, the big thing for any editors to remember is that the subject of this article is how Einstein's theory explains gravitation, and not the theory itself. --EMS | Talk 04:53, 16 March 2006 (UTC)
replies: text: One issue a thorough rewrite of this article could explore is the question of whether or not gtr is a theory of gravitation (mainstream view) or something more/other than that (Einstein's view, shared by some even today).  There are some fine quotes from Synge pithily expressing the mainstream view.  OTH, I have sketched a highly informal argument based on Ted Jacobson's derivation of the EFE, black hole thermodynamics, and the question real the real EFE please stand up? (c.f. energy conditions), to the effect that gtr can be viewed as the minimal framework for theories of gravitation, which remarkably turns out to be a theory of gravitation.  Rather as if the classical thermodynamics led to a unique theory of matter.---CH 04:16, 25 March 2006 (UTC)
thread_title:  Thoughts 

message: The latest changes are an improvement, but this article is not yet in the state is needs to be in to be good.  Here are the needed improvements as I see them:
# The article structure needs to be reorganized.  (I will do that soon).
# The section now called "Einstein's treatment of gravitation" needs to be cleaned up.  What is says is good, but the grammar is awful.  Once again, I will take care of that.
# The section of extending SR to cover non-inertial frames is needed, but needs to be rewritten.
# The section on geometry needs to be condensed much more.  People need to get a sense of what is being done in GR here, but it must be on a very high level accessible to the average reader.
Remember that we already have a full general relativity article.  The value of this article is as a basic introduction to the topic.  The more technical it gets, the more is comes the resemble the general relativity article and the less useful it is. --EMS | Talk 21:50, 24 May 2006 (UTC)
thread_title:  Article improved, but still needs work 

message: I added the section entitled the streched rubber analogy because I felt that for an introduction to general relativity it was far too technical. This is a widely accepted analogy and makes it much easier to understand how gravity occurs. I am completly open to suggestions about it. I was especially wondering if there was a better title I could use and whether there is a better place I can put it.
--Billy 22:50, 3 January 2007 (UTC)
thread_title:  Streched rubber analogy 

message: 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. MP   (talk) 13:06, 29 October 2006 (UTC)
thread_title: Relativity Wikiproject

message: I have integrated the sections "Einstein's treatment of gravitation" and "Extension of special relativity to non-inertial frames of reference", which were really talking about much of the same thing, into a new section "From special to general relativity" describing the road from acceleration to geometric gravity. I've also added a reference for this whole section (Renn, Einstein's Life and Work in Context) which is written at a similar level as this general introduction.
In integrating the different parts of the text, I have shortened some of the descriptions from "Extensions of special...".  I think a general reader unfamiliar with inertial frames and the Doppler effect would have a very hard time understanding the quick one-paragraph derivation of the gravitational redshift - so the choice is either to elaborate, which would have made the section much too long in the absolute sense (this is meant to be an introductory overview, after all) and in the relative sense (other equally important sections should have gotten more space, then), so I've opted for shortening it and adding a link to a web document that explains further details.
Similar for the bending of light - potential conflict of interest notice: I'm citing an online article of my own, simply because I know of no other article for a general audience addressing this issue (the original Ehlers/Rindler article is too technical).
I've also inserted the new introductory paragraph that I had proposed on the talk page yesterday.
At that point, part of the section "The geometry of gravitation" was redundant, and I thought that other parts weren't quite correct, so in part II of my changes, I've overhauled the section "The geometry of gravitation", renaming it "Geometry and gravitation", removing redundancies relative to the previous section, and restructuring into three subsections "Probing the gravitational field" (about test bodies, incorporating some material from the former "Einstein's treatment of gravitation" section) as well as "Sources of gravity" and "Einstein's field equations" incorporating material from the previously previously existing "The Einstein Field equations".
In looking at the resulting version, I thought that the remaining section on "Experimental tests" did really contain material falling into three different categories, and also that there was not enough information on astrophysical applications and on the problem of quantum gravity. Consequently, in part III, I have overhauled the remaining section on "Experimental tests", splitting what was there into three sections: "General relativity and observation" (a brief summary of solar system tests, including laboratory tests, with a pointer to pulsar tests), "Astrophysical applications" (brief pointers to gravitational lenses, black hole physics, gravitational wave astronomy and cosmology) and "Beyond general relativity?" (quantum gravity, Pioneer anomaly and the like).
As I think that the Kopeikin results aren't really suitable for an introduction (not only because they are controversial, also because, if they are included at that level of detail, then a great number of other equally or more important items should get more space, making the text much longer; too long for an introduction) I have removed them.
All in all, there are now five main sections: From sr to gr (history/motivation, equivalence principle, inertial forces); Gravity and geometry (geometrical interpretation, sources, Einstein's equations); General relativity and observation (summary of predicted effects, tests); Astrophysical applications (black holes, gravitational waves/lenses, cosmology); Beyond general relativity (quantum gravity, cosmological modifications, anomalies).
Admittedly, the article has become somewhat longer.  The first part (From sr to gr and Gravity and geometry) is about a page longer than before.  The new "beyond gr?" and the astrophysical applications add more length, but I think the topics are important enough - crucial to getting a proper overview of gr, I'd say - to warrant that. If the article can be streamlined/shortened while still keeping the text intelligible, reasonably complete and correct, all the better. Markus Poessel 16:10, 6 May 2007 (UTC)
replies: text: I've now started the next phase of the article overhaul by adding references to the first section, "From special to general relativity". Time permitting, I will be adding references to the other sections, as well.  Markus Poessel 15:27, 12 June 2007 (UTC)
replies: text: As a logical next step, I've now started to add selected images to the text. Markus Poessel 10:30, 13 June 2007 (UTC)
thread_title: Article overhaul