Vector parallel to our instantaneous direction of travel

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Discussion Overview

The discussion revolves around the behavior of a vector that is kept parallel to the instantaneous direction of travel along a closed path on a sphere. Participants explore the implications of this concept, particularly in relation to the idea of parallel transport and the differences between spherical and Euclidean spaces.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant claims that tracing a closed path on a sphere results in the vector not pointing in the same direction upon return, questioning whether this is universally true.
  • Another participant suggests that starting at the north pole and walking along a specific longitude can lead to returning to the north pole with the vector pointing in a different direction.
  • A different viewpoint argues that the original claim should use "may" instead of "will," as there exist closed paths that can return the vector to its original orientation.
  • One participant discusses the impossibility of parallel transporting a vector in Euclidean space while noting that it is possible on a sphere, but emphasizes the need for smooth paths in certain contexts.
  • Another participant acknowledges a misunderstanding regarding parallel transport in their earlier contributions.
  • A participant reiterates the idea that a sharp bend in the path could be smoothed out to return the vector to its original direction, suggesting that the smoothness of the path affects the outcome.
  • One participant contends that the requirement for the path to be smooth is not necessary for the original claim to hold, indicating a potential misunderstanding of the author's intent regarding parallel transport.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of the vector along closed paths, with some supporting the original claim and others providing counterexamples or alternative interpretations. The discussion remains unresolved regarding the implications of smoothness and the nature of parallel transport.

Contextual Notes

The discussion includes assumptions about the nature of paths (smooth vs. closed) and the definitions of parallel transport in different geometrical contexts, which are not fully resolved.

touqra
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"If we trace out a close path on a sphere, requiring that we always hold some vector parallel to our instantaneous direction of travel, at the end of our trip, the vector will no longer point in the same direction as it did at the time of departure."

I did a small test with what he claims. But it wasn't true at all.
Did I miss something?
 
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start at the north pole and start walking down the lne iof 0 degrees longitude, surely you can see that is is easy to end upi walking back to the north pole with the vector pointing in some other direction? eg loop round and approach from the line of 90 degrees longitude.
 
He should have said "may" instead of "will", as you can find closed paths that do return the vector to its original orientation.
 
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why is this impossible on euclidean space? move the vector round the edge of a (smoothed off if we want to not worry about sharp undifferentiable bends) square it comes back pointing at 90 degrees. of course if the closed paths are required to be smooth at all points, including where it joins up (ie a smooth non intersecting map of S^1 into the space) then it is impossible in any space. it is impossible to parallel transport a vector to a different on in R^n, and possible on S^2, but that isn't what the question was after.
 
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Ah, good catch. I was thinking of parallel transport when I added that.
 
matt grime said:
start at the north pole and start walking down the lne iof 0 degrees longitude, surely you can see that is is easy to end upi walking back to the north pole with the vector pointing in some other direction? eg loop round and approach from the line of 90 degrees longitude.

But then, if I were to loop around and come back via the 90 deg longitude, I would have a sharp bend at the point where 90 deg is connected to 0 deg longitude. So, if I were to "smooth out" this sharp bend, I would finally end up with a vector pointing at the same direction as the initial direction.
 
you don't have to smooth out the final join though. it doesn't say the path is smooth and closed, just closed. as i said, if the path has to closed and smooth (homeomorphic and infinitely differnetiable image of S^1) then it is wrong.

i don't know what the author is getting at though, as this is true in any space. it sounds to me like they're getting parallel transport messed up.
 

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