Talk:Asymptotically flat spacetime

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Help with this template Comments: edit – history – watch – refresh Geometry guy 01:57, 15 June 2007 (UTC)

To-do list for Asymptotically flat spacetime:	edit · history · watch · refresh
Tasks for expert editors: write adequate articles on Penrose diagrams, conformal compactification and revise/move/delete the presently absolutely awful conformal infinity) using these, expand this article to include a coordinate-free definition, perhaps following Wald or Frauendiener.

[edit] Need coordinate-free description of AF

I think this description: "An asymptotically flat spacetime is a spacetime in which the metric approaches the Minkowski metric very far from the gravitating object."

is too confining, because it is coordinate-dependent.

I would tend to say:

"An asymptotically flat spacetime is a spacetime in which the geometry approaches that of Minkowski space at large distances from the source or sources of gravity."

That allows more arbitrary coordinates. Your definition would not even allow Minkowski spacetime with spherical coordinates for the space part. We need to move from coordinate-dependent statements to more geometry-dependent statements were possible in my opinion. Pdn 18:15, 4 Jun 2005 (UTC)

I may have misunderstood what you wrote, but I think my definition would still include Minkowski spacetime by default (it's just using a different coordinate system - Minkowski in spherical polars approaches Minkowski in spherical polars). Whatever the Minkowski metric looks like in a given coordinate system, one can always bring it into the standard form (diag{1,1,1,-1}) using a non-singular coordinate transformation. However, I do agree that a more geometric definition is better. Mpatel 11:51, 5 Jun 2005 (UTC)

Thanks for fixing it up. Actually, the original writing said "Minkowski metric" which is diag(1,-1,-1,-1,-1) or its negative depending on which signature you use. Spherical polar coordinates have some powers of r and sine squared in the metric. So although I am perhaps close to splitting hairs, I believe I was right insofar as the accuracy of description goes. Of course, in a more general case, one might run into Minkowski spacetime even better disguised, so one has to look to geometry, not to the metric. A good example is the bookFoundations of Radiation Hydrodynamics Author(s): Mihalas, Dimitri; Mihalas, Barbara Weibel. ISBN: 0195034376. Pub. Date: 1/1/1985. Oxford Univ Press. This book has metrics, tensors, and Christoffel symbols galore and purports to be general-relativistic, but the Riemann tensor is in all cases zero. Of course, by correctly using the tensors and so on in flatspace, one may (sic) make the material portable to G.R. but that's not guaranteed. Pdn 15:27, 5 Jun 2005 (UTC)

[edit] Major rewrite

This article is presently in an awful state. I got sick of seeing it so messed up, so am trying to completely rewrite it. Naturally, this turns out to involve writing or rewriting other articles, and I am being called away in the middle of the work. I'll try to finish it later tonight, so am leaving the flag. Note that this article cannot even pretend to be acceptable until a definition is given!---CH [[User_talk:Hillman|(talk)]] 22:46, 18 November 2005 (UTC)

Wikipedia is too darn slow/unstable for me to continue. I'll have to wait for better service to try to pick up the thread.---CH [[User_talk:Hillman|(talk)]] 01:11, 21 November 2005 (UTC)

[edit] Mark Roberts

The version of this article prior to the heavily rewritten one by myself was due in part to User:Markdroberts, who cited his own preprint, which I think most experts in the field would agree is quite weakly/incoherently argued and even, in places, misleading. In my revision, I left in the citation, in fact I devoted much space to discussing the objections which Roberts raises, but I tried to give a more balanced discussion, with citations which in my view better represent the state of the art regarding rotating fluid solutions.

This raises yet again the problem of academic mavericks (or at least academically trained mavericks) writing biased articles on technical subjects or introducing bias simply by citing only a very small and non-mainstream portion of the relevant literature. This example is particularly troubling because even physicists with other specialities than classical gravitation might not realize at a glance how misleading the previous discussion was.

Disappointed, CH [[User_talk:Hillman|(talk)]] 01:11, 21 November 2005 (UTC)