Shortest arc between two points in polar coordinates

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Discussion Overview

The discussion revolves around calculating the shortest arc length between two points in polar coordinates, specifically within the context of a Euclidean plane transformed from Cartesian to polar coordinates. The scope includes theoretical considerations of metrics and geodesics.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how to compute the shortest arc between two points in polar coordinates, given a transformation from Cartesian coordinates.
  • Another participant asks for clarification on the metric being used in the polar plane.
  • A participant describes deriving a metric from the Cartesian coordinates, resulting in a diagonal matrix representation.
  • One participant suggests that the shortest arc corresponds to the image of a straight line under the isometry.
  • A participant expresses uncertainty about the original question, concluding that in a flat plane, the shortest arcs are straight lines, and proposes converting the parametric representation of a straight line in Cartesian coordinates to polar coordinates.
  • Another participant affirms that computing geodesics remains consistent regardless of the metric, agreeing with the previous participant's conclusion about the straight line approach.

Areas of Agreement / Disagreement

There is some agreement on the idea that the shortest arcs in a flat plane are straight lines. However, the initial question about the computation method and the implications of the metric used remains somewhat contested.

Contextual Notes

The discussion includes assumptions about the nature of the metric and the implications of curvature in determining shortest paths, which are not fully resolved.

mnb96
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Hello,
If we consider a Euclidean plane \mathbb{R}^2 with the ordinary inner product, and we "distort" it through a cartesian->polar transformation, how should I compute the shortest arc between two points (r,\theta) and (r',\theta') ?
 
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mnb96 said:
Hello,
If we consider a Euclidean plane \mathbb{R}^2 with the ordinary inner product, and we "distort" it through a cartesian->polar transformation, how should I compute the shortest arc between two points (r,\theta) and (r',\theta') ?
What metric are you using on the polar plane?
 
I am using the metric I derived from the equations
x=r cos(\theta)
y=r sin(\theta)

From those I got:

M = diag(1,r^2)
 
I guess I'm not sure what you're looking for. The shortest arc will be the image of an honest straight line under the isometry.
 
Ok. I guess my original question was meaningless.
As far as I could understand, computing a shortest-arc length makes sense only on surfaces whose curvature changes locally. The \mathbb{R}^2 plane is flat, so the shortest arcs between two points are always straight lines.

Basically, all I have to do is to consider the straight line connecting the two points (in cartesian coordinates), and convert its parametric representation into polar coordinates.

Is this correct?
 
Computing geodesics is the same no matter what the metric does. It's just particularly easy here.

So yes, you're correct.
 

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