Talk:Introduction to general relativity

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[edit] Thoughts

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)

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)

[edit] Too technical

Recently an anon editor has taken it upon itself to make this article into a rigorous treatment of the mathematical foundations of GTR, destroying the original purpose of the page, which was to provide a non-technical eaily accesible introduction to the subject for laymen. With a pile of technical intricasies like metric tensors and manifolds, this article is fast approaching the technical level of the original article. This needs to be stopped immediately and this article returned to its state as an introduction as the title suggests. Who is willing to do it? Loom91 07:25, 24 May 2006 (UTC)

I think that we need to analyze what this editor has done or it trying to do. I see him as having strengthened some of the pre-existing sections. For example, the "Einstein's treatment of gravitation" is now much better than the state I left it in. Also, the issues brought up in the newer sections are legitimate, but as written they say too much and tell the reader too little. It also does not help that this editor is not good at English [as evidenced by his excessive use of the work "the" (as in "the special relativity"), and his inability to spell "straight".]

My suggestions are to remove the section on math for now, and to move down and rewrite the section on extending special relativity. However, one to three carefully written paragravphs on the math of GR would be nice.

Part of the issue with the math section is that the writer does not understand what the metric tensor does, as it describes the squares of invariant intervals between coordinates. Curvature is not described directly by it. Instead, that is done by a related quantity called the Riemann tensor, and I am not sure that I even want to mention that. We have an article called mathematics of general relativity anyway, and the gory details may be found there.

Loom91 - Anyone can edit this encyclopedia. You are free to remove the offensive sections if you like, although I advise waiting a day or two to see who else responds. If you feel comfortable with it you can also extract the usable parts out of the extenion of SR section to create a shorter version on it, but if you have not worked with the Lorentz transformations for a while you may be better advised to let me take care of that. --EMS | Talk 15:52, 24 May 2006 (UTC)

[edit] Article improved, but still needs work

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:

1. The article structure needs to be reorganized. (I will do that soon).
2. 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.
3. The section of extending SR to cover non-inertial frames is needed, but needs to be rewritten.
4. 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)

[edit] Misinformation from User:Juansempere

I am approaching burnout trying to prevent misinformation from getting into the gtr-related articles, so others will have to deal with Juansempere (talk • contribs), who appears unwilling to accept correction regarding misleading statements I queried in his edits of Twin paradox. FYI:

In his edit of 28 May 2006, Sempere says " From this moment, we will use the word "gravitational" to refer to the fictious fields". This would guarantee nonsense, since tidal forces, spin-spin forces, etc. are certainly not "fictitious" as physicists use this term!

In a series of edits on 23 May 2006 (no idea if these have been reverted), I see Sempere claims

1. "If we had an infinite massive plane creating a gravitatory field the geodesics would behave as euclidean straight lines." In fact, the closest thing to a vacuum solution modeling "the field of an infinite thin plate" in gtr is something built using the so-called Taub plane-symmetric vacuum solution , and the behavior of geodesics in this does not accord with Sempere's claim. Actually, the very concept of an infinite thin plate is suspect in gtr (as one might expect, since this is a more realistic theory than Newtonian gravitation, so we shouldn't be suprised if some physically implausible idealizations familiar from Newtonian gravitation turn out to be difficult or impossible to realize in gtr).
2. "As we have seen before, spacetimes with null gaussian curvature are euclidean" and many similar statements completely ignore the crucially important local versus global distinction.
3. "This is normal, because the fictious nature of the observations in non-inertial systems. For example, in a rotating frame, we will see the whole universe in orbit around us. The farther things are the faster they will spin around us, and we will have to introduce a harder fictious force to explain such a weird orbit." Sempere insists on trying to explain (apparently) all gravitational effects as arising from "fictitious forces". This is utter nonsense, since of course tidal stresses are physical forces, spin-spin forces are physical forces, and so on. In particular, the curvature tensor represents physical forces, not "fictitious forces". (This is best seen using the Bel decomposition.)
4. "A tensor in mathematics is a Linear Function, given by a matrix, which given a four-vector (position and speed of a particle) will assign it inmediately the forces that are acting on it as a product of the matrix for the vector. With this instrument we will be able to describe any possible fictious field and to operate with it." Well, a tensor is a multilinear operator. A second rank tensor field can be represented (wrt some coordinate chart) as a matrix valued function on spacetime.
5. " This is called Gravitomagnetism and is the most general expresion of the fictious forces for an arbitrary-moving frame." Again, this is terribly misleading.
6. "Because of this curvature, the paths that objects in inertial motion follow can "deviate" or change direction over time." This is misleading because it confuses geodesic deviation (a local effect directly measuring curvature by comparing two "nearby" short geodesic paths) and light bending (a global effect).
7. "This identification of free fall and inertia is known as the Equivalence principle.)" It is very misleading to identify free fall and inertia, as I should think would be obvious.

These are only a few things I noticed at a glance. I haven't had the heart to read the article to see if these misleading/incorrect claims are still in the article; I hope not!

To sum up, please watch out for Sempere's edits, since they are so bizarre that I suspect a determined attempt to write hoax articles, to what end I can't imagine. BTW, I haven't had the heart to check whether he is also editing as an anon (in particular, as the Jazztel triple play services anon from near Madrid), but edits similar to those noted above should be reverted. ---CH 00:40, 15 June 2006 (UTC)

You are going to have to show me where any of these edits has survived the subsequest actions of other editors, including myself. I did some spot checking for the wordings cited above in the current article, and could not find any. What happenned is that I let Sempere edit this article for a while, correcting some of his most obvious errors. Then when he was done I used his work as a springboard for a rewrite of the article. So nothing of his has survived unscathed. --EMS | Talk 01:42, 15 June 2006 (UTC)

Firstly CH, WP:NPA. You have absolutely no ground for supposing Juansempre is trying to hoax us when all the mistakes you have pointed are at worst technical errors. In any case, I disagree with your assessment that Gravitation is not a fictitous force. What is actually implied by GTR is that Gravitational forces are no more real or fictitous than the Newtonian forces arising in non-inertial frames. Because of galilean equivalence, we are free to choose a reference frame in which Gravitation vanishes, therefore it can be called a fictitous force. Actually the concept of fictitous forces is redundant in GTR because non-inertial as well as inertial frames are on a physically equivalent footing because of the extension of the principle of relativity. Loom91 06:13, 15 June 2006 (UTC)

Loom - Chris (CH) is not claiming that Sempere is hoaxing us as much as he is claiming that Sempere is very much mistaken in what is being claimed in those edits. It is not a personal attack per se, but instead a technical going-over of Sempere's edits. That said, what seems to have happenned in that Chris got alarmed by Sempere's edits on another page, and looked at his contributions. There, he found a boatload of them here, and examined them. Unfortunately, Chris did not examine the article itself to see what of Sempere's edits had survived. So the red flags raised were not quite fair since the page has been dramatically reworked since Sempere completed his rewrite.

On the "ficticious forces" business: What is ficticious is the Newtonian force of "gravity". Gravitation is quite real in GR, and Chris points out. In addition, it does give rise to real forces, such as tidal stresses, which occur when different parts of a body are experiencing a differential acceleration across its length. In fact, Chris is a general relativity expert, and the issue with his writings is that at times they get too technical for the reader. This just happens to be one of those times. (BTW - The extant "ficticious force" business in the article was written by myself. So I agree with the gist of your complaint, but also see where Chris is coming from.) --EMS | Talk 03:47, 16 June 2006 (UTC)

Sorry for the confusion; indeed the link I provided in my comment above gives the 23 May edits by Juansempere (talk • contribs) which I criticized in my comment, but as Ed guessed, I failed to notice that the objectionable comment had been reverted before I came along. (Mea culpa; I plead exhaustion due to cruft patrol burnout.)

Loom, for tidal forces

1. not being fictious
2. being directly related to the tidal tensor (aka electrogravitic tensor, aka the "electric part" of the Riemann tensor, evaluated wrt a given observer)

see any modern gtr textbook, starting with Misner, Thorne, and Wheeler. See general relativity resources for full citations to several good graduate level textbooks. The fictitious force stuff is not precisely wrong, but it is only part of the story.---CH 06:07, 17 June 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. MP (talk) 13:06, 29 October 2006 (UTC)