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## The topology of spacetimes

 Quote by dextercioby Micromass, is it true that second-countable, Hausdorff and paracompact topological manifolds are metrizable ? If so, do you know a reference for a proof ? I've been looking for this fact for about an hour or so. Thanks!
In fact, if M is a Hausdorff locally Euclidean space, then second countable actually implies paracompact. So there is no need to add the paracompact condition. The proof of this fact can be found in Munkres: Theorem 41.5, page 257 (note that every topological manifold is in fact regular and Lindelof).

Also, a note on terminology. A topological manifold is defined as a locally euclidean, Hausdorff and second countable space. So the term "second countable topological manifold" has some unnecessary words (as does "paracompact topological manifold")

A more general result is the Smirnov metrization theorem. This states that any paracompact, Hausdorff and locally metrizable space is actually metrizable. A proof can be found in Munkres: Theorem 42.1, page 261 This proves in particular that every topological manifold is metrizable.

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 Quote by TrickyDicky Having said this your definition of singular spacetime might clear up something for me, is it defining something like a metric space that is not complete, that has missing points? Can this missing points be considered singularities? In that case things would start to make sense to me.
Because the idea is so attractive, over the decades, there have been a number of attempts to define spacetime singularities as missing points or adjoined boundaries, but many (all?) have had various problems.

The work of Susan Scott and collaborators has possibly shown the most promise, but I know very little about this stuff. An interesting recent paper:

http://iopscience.iop.org/0264-9381/28/16/165003/

Unfortunately, at this link, the paper is behind a paywall, and I can't find it on the arXiv. I have access to it, but I haven't a chance to look at it yet.
 micromass, this is just a technical note that I really don't think adds much to the main discussion and I have no interest whatsoever in arguing about it but just for the record and since you have insisted on it several times... You have said repeatedly that the Hausdorff condition is already required for topological manifolds, well I'm not the only one here that has pointed out that the reference text for this stuff, the 1973 book by Ellis and Hawking cites a few examples of topological manifolds that are not Hausdorff. Besides, I've consulted several standard texts on differential geometry and they also name the Hausdorff condition only when defining smooth/differentiable manifolds.

 Quote by micromass Third, Singular points on a manifold are not a concept depending on the topology.
Well, since there seems to be no commonly accepted definition of the singularity concept (only of singular spacetime), it is at the very least hard to say. It might not depend on the topology but it might be incompatible with it, just the same way a manifold won't admit certain metrics incompatible with its manifold topology.

 Quote by George Jones Because the idea is so attractive, over the decades, there have been a number of attempts to define spacetime singularities as missing points or adjoined boundaries, but many (all?) have had various problems. The work of Susan Scott and collaborators has possibly shown the most promise, but I know very little about this stuff. An interesting recent paper: http://iopscience.iop.org/0264-9381/28/16/165003/ Unfortunately, at this link, the paper is behind a paywall, and I can't find it on the arXiv. I have access to it, but I haven't a chance to look at it yet.
Thanks, I'm trying to clarify things with a book called "The Analysis of Space-Time Singularities" by C. J. S. Clarke. So far the impression I get is that this is a bit of a mined field.

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 Quote by TrickyDicky micromass, this is just a technical note that I really don't think adds much to the main discussion and I have no interest whatsoever in arguing about it but just for the record and since you have insisted on it several times... You have said repeatedly that the Hausdorff condition is already required for topological manifolds...
I didn't look back to see if he actually did (I doubt he did though) but yes it depends on the author. Some authors take Hausdorff as one of the conditions for a topological space to be a manifold and others don't. In the context of space - times if you want physically relevant ones you would probably include the condition.

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 Quote by TrickyDicky micromass, this is just a technical note that I really don't think adds much to the main discussion and I have no interest whatsoever in arguing about it but just for the record and since you have insisted on it several times... You have said repeatedly that the Hausdorff condition is already required for topological manifolds, well I'm not the only one here that has pointed out that the reference text for this stuff, the 1973 book by Ellis and Hawking cites a few examples of topological manifolds that are not Hausdorff. Besides, I've consulted several standard texts on differential geometry and they also name the Hausdorff condition only when defining smooth/differentiable manifolds.
It does depend on the author. I'm sure there are people who do things differently (although I would like to know which differential geometry texts you are talking about). But I think the standard definition is to require topological manifold to be Hausdorff. My posts try to reflect the standard position as much as possible. But yes, there are probably some authors who do things differently.

 Quote by TrickyDicky My confusion comes from not seeing how an structure that is supposed to act only locally can have global effects.
This is my original cause of confusion that began the discussion, still in the other thread. This happens for the metric tensor, the energy-momentum tensor, the Riemann tensor, etc.

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