Talk:Introduction to general relativity/Archive 1

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: 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)
 * 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.


 * 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)

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)

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: 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)
 * 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.

Streched rubber analogy
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)

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, 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 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!
 * 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.

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 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 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)
 * 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)

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)

Explanation of the physics of Einstein's gravitation
Hi Ladies and Gents, I just simplified the page as requested by EMS and in the process I explained Einstein's physics as it follows from Landau's "The Classical Theory of Fields". If you revert it then you better include this piece since otherwise nobody might understand how Einsteinian physics works and why it produces almost the same results as Newton's. Cheers. Jim 21:45, 28 March 2007 (UTC)


 * First of all, the simplification had been done. I will not say that there is not room for improvement in this article, bute replacing the inadequate with the incomprehensible is not an improvement.  Landau's math is assumming a level of knowledge about GR that is totally inappropriate to an introduction to the topic.  IMO, it does not belong anywhere in Wikipedia. --EMS | Talk 22:01, 28 March 2007 (UTC)

EMS - "inadequate and incomprehensible" is a good way of describing all those pages dedicated to gravitation (that's why I thought that one more won't hurt). Since in science only good way of getting the truth is to make an experiment and analyze the results, then we may ask how many guys understood gravitation reading those pages? Not having a previous knowledge of it, which is of course a necessary pre-condition for the experiment.

If it is more than zero then I'll be surprized. Because what I see here is a club that thinks that gravitational attraction (as sometimes it is called without understanding the phrase) is basically incomprehensible and so no effort can make it more comprehensible. Even if all the physics that is there is presented to them like this Landau's explanation. The club does not realize that the "geometric picture" must have its physical interpretation and the false analogies won't do. Understanding the physics of gravitation can't be replaced by even perfect "understanding of its math" (since then one "understands" only a false model).

As already Einstein and Feynman noticed the application mathematicians who think that they understand physics (there are no other than "applied", since "pure mathematicians" don't consider GR a science) "are not only bad physicists but also bad mathematicians". Einstein just were more polite than Feynman and expressed it as "exact math it is for sure not exact physics" (and that's why he "stopped understanding his theory when mathematicians started to explain it").

The reason is of course, what you already realized, that we rarely know physics, and all we have are just more or less accurate models which applied mathematicians don't realize yet (they all think that math=physics) and treat the universe as it were exactly described by their math. That's why "pure mathematicians" don't want to participate in GR project since they know (what "applied" don't realize) that this math is not exact. Without realizing all of this it is difficult to explain gravitation since, as Landau already said, "it is difficult to explain something that one does not understand himself".

That's why I see it as the most important problem with gravitation pages. This way the situation with gravitation is the same as described already by Feynman long time ago in his letter to his wife. And if you don't do something drastic it is going to stay this way forever. Or at least till the next breakthrough when astronomers finally realize that the universe doesn't expand and all their problems come from a defective math (the next "scientific revolution").

And now, after proving to myself that Feynman was right, I may turn to more important projects than explaining gravitation. Jim 07:07, 29 March 2007 (UTC)


 * You keep coming into these pages as if you are a high priest of gravitation whose word is to be treated as a Truth from Above. I have little use for "Truth", and even less use for the people who push it.  Usually, I get this type of thing from fellow independent researchers who don't like their own ideas being removed.  However, any rigidity in how things should be expressed is a bad thing in Wikipedia, and I have even seen successful academians get shoved aside for trying to impose their views here instead of contributing to a consensus.  You are in that same boat now:  That GR does correspond to Newton's laws in the weak field limit is an important point that should be made, and perhaps even demonstrated.  Even so, bringing in an obtuse batch of calculus without even describing the change in the coordinate speed of light on which the calculation is based (or the related POVs on how the energy changes) just plain does not work.  --EMS | Talk 16:37, 29 March 2007 (UTC)


 * :-) Jim 12:10, 30 March 2007 (UTC)


 * Dear EMS - Relax and don't worry about my knowing more than you do about gravitation. Most likely it is since I started learning about it long before you were born. It may be the reason for my understanding Einstein's theory better. And nothing here is mine anyway since all I know comes from Einstein. My possible contribution here is just noting one more identity (that I called "general time dilation" and that metric tensor is degenerated (contrary to what some editors say) since even non symmetricity of it was proposed by Einstein nearly 40 years before I noticed its obvious necessity.


 * But you don't even need to believe anything I say. Just check the results of observations. Check a few things that I describe (value of density of the universe, accelerating expansion of it and the value of the acceleration, reason for CMBR, explanation of the presence of near quasars, presence of dark matter, Pionieer "anomaly" ...) and try to come up with a different explanations than Einsteinian for all those results. When you have them send them to me to enjoy those possibly Newtonian or even quantum mechanical explanatons. I think it would be extremely interesting if they are to be explained differently than just trivial conclusions of Einstein's theory. So instead of complaining go to work and show the results that we all could enjoy. Before that you're just wasting time on trying to assert things that you can't prove like all those guys that Feynman wrote about in his letter to his wife and I know that you are an intelligent guy and not like one of those described by Feynman.


 * BTW, I didn't describe "the change in the coordinate speed of light on which the calculation is based" since I new for sure that the life of this text will be very short so why to waste good time on bad pojects. In a improbable case that this text would survive for some reason there was always time to beautify it and place all the necessary details into it. You could do it yourself since it is obvious what has to be done and what is not obvious can be found in Landau's textbook. Jim 10:38, 31 March 2007 (UTC)

The Eddington anecdote
I'm not at all happy with the sentence in the first paragraph: "This is done with the insight[1] that ..." in which the footnote refers to the anecdote about Arthur Eddington and "only 3 people understand relativity".

The Eddington anecdote and Einstein's insight seem totally unrelated. In any case, the Eddington anecdote must have happened long after Einstein's insight. The anecdote is nevertheless marvelous.

I would suggest something along the lines of "This is done with the insight that gravitation is not due to a force in the conventional sense, but is a fictitious force, (like centrifugal force or the Coriolis force), resulting from the curvature of space and time."

And, somewhere else, put in the Eddington story, perhaps in the context of "Relativity, both general and special, have a wide reputation for being difficult subjects to master. This was even more so in the early days. In fact, in the early 1920's ....."

Normally, I would just edit this in, in accordance with WP:BOLD. But this is outside of the areas I feel comfortable editing, and it has templates on it about its importance and visibility. What do people think? William Ackerman 16:53, 13 April 2007 (UTC)


 * That is astute. I edit the general relativity article more, but you certainly have my permission to be bold.  My only question is one of whether we need to mention the Eddington quote at all here.  That may be something which is better covered as part of the history of general relativity. --EMS | Talk 21:59, 13 April 2007 (UTC)


 * Done. Thanks.  I also rearranged a reference, and may have done it incorrectly (in addition to forgetting to put in an edit summary; sorry.)  I made the links when one clicks on the "[1]" work properly, copying the technique from another page.  But I still don't know what the standard format really is on references and notes.  And the fact that the reference to Hawking's book was an internal Wiki link rather than an actual ISBN link to a book made it more confusing.  If I have botched this, please fix it and accept my apology. William Ackerman 03:18, 15 April 2007 (UTC)


 * I think the current introductory paragraph is still problematic, to say the least. It states "This is done with the insight[1] that gravitation is not due to a force in the conventional sense, but is a fictitious force, (like centrifugal force or the Coriolis force), resulting from the curvature of space and time." In general relativity, only part of gravity is a fictitious force - and the part that isn't fictitious (cannot be transformed away by using a reference frame that is in free fall) is curvature - stating that the fictitious part is due to curvature is simply wrong. Correcting this statement will probably make it too technical - instead, I would propose to make a more general statement about gravity being part of the geometry of space and time.  The fictious force part could be dealt with later in the text.


 * I also think there are other, more general things the reader should know about gr at the beginning of a general introduction, for instance that it has many astrophysical applications (black holes, gravitational waves, cosmology). All in all, here's my proposal for the first paragraph:


 * General relativity (GR) is Albert Einstein's theory of gravity. First published in 1916, it is a generalization of Einstein's earlier theory of special relativity.  In contrast with Isaac Newton's law of universal gravitation, it posits that gravity is not due to a force in the conventional sense, but is an aspect of the geometry of space and time.  General relativity is one of the cornerstones of modern astrophysics, where it provides the foundation for our current understanding of black holes, gravitational lenses and gravitational waves.  It is also the basis of the standard model of cosmology.


 * Markus Poessel 20:18, 5 May 2007 (UTC)

Article overhaul
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)


 * 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)


 * As a logical next step, I've now started to add selected images to the text. Markus Poessel 10:30, 13 June 2007 (UTC)