Can Geodesics Be Inflectional in Euclidean Space?

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Discussion Overview

The discussion revolves around the concept of geodesics in Euclidean space, specifically whether a geodesic can be described as "inflectional." Participants explore the definitions and properties of geodesics, particularly in the context of perturbations and variations in length, while questioning the implications of these properties in different geometrical contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions if a geodesic can be inflectional, suggesting that geodesics are generally stationary rather than extremal.
  • Another participant rephrases the question, discussing the stationary property of geodesics and the effects of perturbations on length in Riemannian and Lorentzian manifolds.
  • A third participant expresses uncertainty about the equivalence of their question to the previous one, indicating a need for precision in framing questions.
  • Concerns are raised about whether a general theorem exists that prevents the variation in length from being smaller than O(ε²) or if it could potentially be O(ε³).
  • One participant suggests that if a surface has a sufficiently high order of contact to the tangent spaces along a geodesic, the variation could indeed be smaller than O(ε²), but questions the appropriateness of the term "inflectional."

Areas of Agreement / Disagreement

Participants express differing views on the nature of geodesics and the implications of perturbations, with no consensus reached on the concept of "inflectional geodesics." The discussion remains unresolved regarding the existence of a theorem that would limit the variation in length.

Contextual Notes

Participants acknowledge the complexity of the definitions involved and the potential for varying interpretations based on different mathematical perspectives. The discussion highlights the need for clarity in terminology and the assumptions underlying the questions posed.

humanino
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I am trying to figure out if a geodesic can be inflectional (in euclidean space...). I am not sure it even makes sens, from the definition of a geodesic, but it seems to me that a geodesic will not in general be extremal, but only stationnary.

Is there a general theorem preventing a monster such as an "inflectional geodesics", or do you have a beautiful example, or am I just obviously on the wrong track here ? :smile:
Thank you for any help.
 
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Rephrasing the question...

Your term "inflectional geodesic" is I nonstandard, but I think I see what you are asking. Let me rephrase it.

The stationary property says that if we consider a geodesic arc with endpoints P,Q, then making small perturbations of "size" \varepsilon[/tex] (keeping the endpoints P,Q) will change the length by only O(\varepsilon^2). For an arc in a Riemannian manifold, this length change will in fact be an <i>increase</i>, whereas for a timelike arc in a Lorentzian manifold, it will be a <i>decrease</i>. So the question was: could there be some exotic signature with the property that some such perturbations of some particular geodesic arc increase the length, while others decrease it?<br /> <br /> (As an example of local versus global distinction: as we can see by considering great circles on a globe, &quot;small&quot; is essential in the above! <i>Globally</i> there may very well be more than one geodesic arc between P,Q, with different lengths.)
 
Chris Hillman said:
could there be some exotic signature with the property that some such perturbations of some particular geodesic arc increase the length, while others decrease it?
Well, I was not thinking about something that elaborate. I am not sure that my question is equivalent (or even is implied by, or implies...) yours. So my first lesson would be to be more precise in my questions if I want an expert answer :smile:

Let me try to rephrase.

I am concerned with geodesics on surfaces embeded in euclidean spaces, so I am thinking in a physicist's manner. I should try to switch to the mathematician point of view, and think in terms of metric directly. You said the variation is O(\varepsilon^2). My rephrased question could be : "Is there a general theorem preventing that the O(\varepsilon^2) vanishes anywhere (between an arbitrary pair of points), whatever the metric, or could it happen that the variation is O(\varepsilon^3) (what I called inflectional) ?"
 
Last edited:
Well, if your surface has sufficiently high order contact to the tangent spaces all along some geodesic arc, then sure, I suppose the variation could well be even smaller than O(\varepsilon^2). I don't think that "inflectional" would be a good term at all for this kind of thing, however. And I can't understand the first alternative you tried to describe, so I guess I still don't know what the question is.
 

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