Is the Lorentz metric compatible with the topology of flat spacetime?

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Discussion Overview

The discussion centers on the compatibility of the Lorentz metric with the topology of flat spacetime, exploring whether spacetime can be considered a metric space and how the properties of the Lorentz metric influence this classification. Participants delve into the implications of negative values in the Lorentz metric and its relationship with topological structures.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants argue that the Lorentz metric does not qualify as a metric in the traditional sense due to its allowance for negative values, which leads to confusion regarding the definition of open spheres in this context.
  • There is a suggestion that spacetime may not be a metric space because the Lorentz metric does not conform to the requirements of a standard metric, prompting questions about the nature of the topology of flat spacetime.
  • One participant points out that a topological manifold does not necessarily have a metric, and that spacetime is better described as a semi-Riemannian or pseudo-Riemannian space rather than a Riemannian metric space.
  • Another participant proposes that while spacetime can be viewed as a topological manifold with a locally Euclidean structure, the Lorentz metric introduces complexities that challenge the notion of using it to define open balls for a topology.
  • A reference to Chris Hillman's statement is made, emphasizing that the topology of Lorentzian metrics derives from the locally Euclidean structure rather than the indefinite bilinear form of the Lorentz metric.

Areas of Agreement / Disagreement

Participants express differing views on whether the Lorentz metric can be reconciled with the topology of flat spacetime, with no consensus reached on the nature of spacetime as a metric space or the implications of the Lorentz metric's properties.

Contextual Notes

Participants highlight limitations in the definitions and assumptions surrounding metrics and topology, particularly regarding the implications of negative values in the Lorentz metric and the distinction between topological and metric spaces.

dslowik
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Could someone clarify, and/or point me to some reference on:
Lorentz metric is not really a metric in the sense of metric spaces of a topology course since it admits negative values. If I use it to define the usual open sphere about a point, that sphere includes the entire light-cone through that point. so space time is not even Hausdorff?
 
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Is the topology of (flat) ST generated by a metric? i.e. is spacetime a metric space? If so, what is the metric? (Lorentz 'metric' goes negative -not a metric! is my confusion)
 
dslowik said:
Is the topology of (flat) ST generated by a metric? i.e. is spacetime a metric space? If so, what is the metric? (Lorentz 'metric' goes negative -not a metric! is my confusion)

Are you confusing topology and geometry? A topological manifold doesn't have a metric at all. As for being a metric space, technically, neither SR flat spactime nor GR spacetime are Riemanian metric spaces. Instead, they are semi-Riemannian or pseudo-Riemannian; specifically, this allows a metric signature other than ++++.
 
Right, a topological space may not have a metric.
A metric is additional structure on a set.
A set with a metric gives rise to a metric topology on that set. (I mean usual non-neg, symmetric, non-deg metric).

I am reading John Lee's Topological Spaces. There he defines a top manifold as:
1) topological space locally homeomorphic to R^n, 2) Hausdorff & 3) second countable.
So, assuming ST is a topological space (by this def), can't we use the local homeomorphism to R^n to define (locally) a metric on ST via pullback of the Euclidean metric on R^n? And likewise any topological manifold is a (local)metric space (but not general topological spaces).

It seems that ST is a topological manifold with a locally Euclidean metric. This describes its topological structure as a metric space. We than add further structure to this metric/topological space by adding the non-Riemanian Lorentz metric. Thus we are using one metric and corresponding open balls to describe the topology, and another metric to describe the 'physical' distance between points. the physical distance between some points is 0, which is a very different topology than the locally Euclidean one; but the Lorentz metric can't be used to describe open balls for a topology?

as Chris Hillman said:
"Lorentzian metrics get their topology from the (locally euclidean) topological manifold structure, not from the bundled indefinite bilinear form."
But it seems odd to me that we impose a locally euclidean topology, then use a quite different metric to describe physical separation of points..
 

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