Distnace between points and isometric mapping

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Discussion Overview

The discussion revolves around the concept of isometric mapping in the context of manifolds, specifically focusing on whether such mappings preserve distances between points. It touches on definitions related to distance, geodesics, and the implications for proper time and proper length in different types of curves.

Discussion Character

  • Conceptual clarification, Debate/contested

Main Points Raised

  • One participant defines isometric mapping as preserving the scalar product of vectors from tangent space, questioning if it also preserves distances on the manifold.
  • Another participant asserts that isometric mapping does preserve distances, linking this to the concept of integrated proper time along a worldline as a measure for ideal clock readings.
  • A different participant acknowledges the assertion but expresses uncertainty about the generality of distance definitions, emphasizing that proper time and proper length are more relevant in specific contexts.
  • Another participant raises concerns about the well-defined nature of distance, noting that there can be points on a manifold that are not connected by geodesics or can be connected by multiple geodesics, complicating the notion of distance.

Areas of Agreement / Disagreement

The discussion reflects both agreement and disagreement among participants. While some assert that isometric mappings preserve distances, others highlight complexities and limitations regarding the definition of distance in certain scenarios.

Contextual Notes

Participants note limitations in the definition of distance, including cases where geodesics do not connect points or where multiple geodesics exist, which may affect the clarity of distance measurements.

paweld
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We define isometric mapping so that its tangent mapping preserves
the scalar product of vectors from tangent space (the definition
doesn't refer explicite to notion of distance in the manifold).
Distance between two points of manifold is the length of geodesics
which joins them.
I wonder if it's true that isometric mapping preserves distances
on the manifold?
 
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It does. That's why integrated proper time is a "real thing" along a worldline which we can use to define as what an ideal clock should read.
 
Thanks, I thought so but I wasn't sure.
 
Keep in mind that the distance between points isn't well-defined in general. Instead we talk about the "proper time" of a timelike curve, and the "proper length" of a spacelike curve.
 
We might encounter even worse problems - sometimes there are points on the manifold
which cannot be connected by any geodesic or can be connected by more then
one geodesic. In such cases the notion of distance between these points isn't well defined.
 

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