Talk:Congruence (general relativity)

Still needed
When I have time, I still need to add material on null geodesics. Eventually I plan to add links to examples in Category:Exact solutions in general relativity, once that category is sufficiently populated. Eventually, there should be sufficiently many examples and figures to make the often misunderstood concepts of expansion, shear, and vorticity understood.---CH (talk) 10:18, 19 September 2005 (UTC)

New sections (articles ?)
You've mentioned above the need for discussion of null congruences. A suggestion: for better organisation, it may be an idea to have separate sections (or articles, if there's much to discuss) for timelike and null congruences, as the maths may get slightly heavy. ---Mpatel (talk) 08:44, 17 October 2005 (UTC)


 * I'm not very happy with the organization either, in fact that's part of the reason why I ground to a halt while I regroup (wait for fresh inspiration while trying to finish other tasks). Oh well, I hope we'll sort it all out in the end.---CH  (talk) 07:30, 18 October 2005 (UTC)

Merger proposal
I think that timelike congruence should be merged into this article, unless we can actually create a worthwhile article on timelike congruence (I know I certainly can't). ---Mpatel (talk) 12:49, 19 October 2005 (UTC)


 * Hmmm... some things are easier to do for timelike congruences, e.g. matching across a hypersurface orthogonal to some irrotational congruence; maybe that's why I was apparently planning to discuss null and timelike congruences separately. Anyway, OK by me to merge them.  ---CH 04:39, 25 January 2006 (UTC)


 * Oh, I see, in the current version of this article I never got around to discussing the kinematics of null congruences.  I remember now that I became unhappy with writing myself into a corner in the organization as I was writing this article, precisely because my attempts to start in on null congruences all seemed awkward from the standpoint of style.---CH 04:44, 25 January 2006 (UTC)


 * Even better, except for the first few paragraphs the present version is about timelike congruences, and since I was consistently opening each article with a section called something "physical interpertation", I remember now why from this point of view might make sense to have three articles, a short one on congruences in general which discusses some common properties and points at two more detailed articles on timelike and null congruences. The advantage is this maintains consistency with a general vision for organizing articles for our project.  The disadvantage is that it seems awkward in this case because the common features ought to be discussed after the interpretation, yet they must go in the article which points at the two specialized articles which discuss the interpretation.  Yercchch, that's awkward. ---CH 04:50, 25 January 2006 (UTC)

Ok, I now think it's better to have separate articles for timelike and null congruences. Here's an idea:


 * Perhaps a small intuitive discussion of a congruence should go somewhere at the start (along with the definition of a general congruence).


 * Then the section, 'Types of Congruences' can briefly discuss the timelike and null congruences (without giving much mathematical details) and have 'Main articles: timelike congruence and null congruence' at the start. This discussion should, with sufficient skill, be able to combine the common features and the interpretation in one go.

I propose that this is all that the article should contain (of course, with references and possibly external links), as then the timelike and null congruence articles can discuss further technicalities. MP  (talk) 08:52, 28 January 2006 (UTC)

Since timelike congruence has been tagged for merger for over a year and doesn't really have any content, I've simply redirected it. If someone wants to expand it, feel free to un-redirect. —Quarl (talk) 2007-02-25 09:56Z 

Edits by User:JarrahE
Actually, I misread something so I didn't need to completely revert these changes, and have reintroduced some of Jarrah's changes. The errors I found in Jarrah's "corrections" were:
 * 1)  plus a scalar θ is wrong (should of course be scalar function times identity operator, which is awkward to explain other than by writing out the frame components as I did a moment later in the original.
 * 2) acceleration of the volume was wrong (Jarrah noticed this and corrected it himself),

Matters of taste:
 * 1) I prefer see BLAH for justification of these claims to "see BLAH for a justication, the justification, justifications in the plural".
 * 2) etc.

OK, so looks like only one small error plus the one which Jarrah caught.

Thanks for your cleanup, Jarrah. Sorry if I seemed to jump all over this, but from past experience, I know that well-intentioned editors can very quickly mess up a page very badly and safest bet is to restore to last good version and check proposed changes systematically. Apart from matters of taste most of your changes wound up surviving my scrutiny.---CH 23:20, 24 April 2006 (UTC)


 * Chris -


 * I'm sorry if I've introduced errors onto the page. I still haven't figured out what these errors are, I thought you said they'd be listed on the talk page (maybe I should be more patient), but I guess I should trust you since this is all a bit off-topic for me.  But anyway, the long equations were getting cut off by my browser (and you had a comment on top of the talk page asking to reformat them) so I did, splitting them into two lines.  Does this make them all somehow incorrect?  I do this in my own papers all the time, I think everyone does.  Anyway, I'm not interested in an edit war, you can format them as you like, I just wanted to say that putting a lot of terms on the same line as you'd been doing means that people like me can't view the whole expressions. JarahE 23:06, 24 April 2006 (UTC)

Impatient indeed: your edits conflicted with mine! ---CH 23:20, 24 April 2006 (UTC)


 * Don't worry about it. I've done worse when people have changed pages that I felt attached to.  As a show of good will, I won't defend my comment on the fact that a symmetric tensor can be decomposed as a traceless tensor and a scalar, as it all seems kind of semantic (I meant decomposed in the sense of Young tableaux, you meant decomposed in the sense of writing a matrix as the sum of two summands ... unfortunately I made the unfortunate word choice plus instead of and making it seem like I was using your notion of decomposition and would therefore need tensor with the identity matrix). Anyway, I found your article to be very useful, which is the reason that I cleaned up the grammar and tried to reformat it for my antiquated browser. JarahE 23:45, 24 April 2006 (UTC)

Related article created
I've created the stub optical scalars. MP  (talk) 19:57, 21 May 2006 (UTC)

Edit by 219.77.131.175
An anon using IP corrected a typo in the equation for the quantity I called Jdot. We need to watch out for highly malicious vandalism in which some anon changes a correct equation to an incorrect equation, but this edit seems legitimate. Thanks to the anon, and please register to make it easier for us! ---CH 19:23, 2 June 2006 (UTC)

Students beware
I created the original version of this article, did a good deal of work on it, and had been monitoring it, but I am leaving the WP and am now abandoning this article to its fate.

Just wanted to provide notice that I am only responsible (in part) for the last version I edited; see User:Hillman/Archive. I emphatically do not vouch for anything you might see in more recent versions.

Good luck in your seach for information, regardless!---CH 23:07, 30 June 2006 (UTC)