Unique Distance on Pseudo-Riemannian Manifolds: Riccardo's Question

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The discussion centers on establishing conditions for a unique and well-defined concept of distance on pseudo-Riemannian manifolds, specifically through the maximum length of geodesics connecting events. A critical requirement is the existence of a maximum or minimum length for geodesics between two points. The term "normal neighborhood" is introduced, indicating regions where a single geodesic connects any two points, with references to Kobayashi and Nomizu for further exploration. The distinction between spacelike and timelike geodesics is noted, with unique geodesics typically found in spacelike-separated scenarios.

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Rick89
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What conditions do we have to put on a pseudo-Riemannian manifold in order for a unique and well defined concept of distance between events to be meaningful? I'm thinking about for example max. length of geodesic connecting two events. We have to require one such maximum or minimum length to exist (condition on number and lengths of geodesics between two points...). Anyone knows an exact way of dealing with this?
Thanx
Riccardo
 
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The name for a region in which only a single geodesic connects any two points is a normal neighborhood. There are theorems that such things always exist around every point, although I don't know the precise statement. Kobayashi and Nomizu is the usual reference I see cited. Most differential geometry texts will probably work, though.

I do not know of any general results that allow an estimate of the "size" of a normal neighborhood. I can't even imagine how such a theorem might even look.

In a practical sense, geodesics are usually unique when connecting spacelike-separated points. Timelike geodesics "focus" much more easily, and overlap all the time.
 

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