Can different paths in spacetime have the same separation?

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Discussion Overview

The discussion revolves around the concept of spacetime separation and whether different paths can yield the same separation between points in spacetime. Participants explore the implications of integrating infinitesimal separations along various paths, particularly in the context of curved spacetime, and consider the nature of spacelike, timelike, and null separations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that integrating an imaginary infinitesimal separation along different paths could lead to cancellations, raising questions about physical significance.
  • Others argue that the separation between fixed points can be spacelike, timelike, or null depending on the metric coefficients along different paths.
  • A participant suggests that in curved spacetime, the four-dimensional distance between points depends on the path taken, despite the infinitesimal separation being defined for the pair of points.
  • Another viewpoint is that while the path may influence the nature of the integral, the value of ds² could be considered path-independent, depending only on the coordinates of the points.
  • Some participants note that in flat spacetime, the distance between two points is path-dependent, contrasting with the potential for varied metric coefficients in curved spacetime.
  • There is a discussion about the implications of mixed paths and whether they can be physically meaningful, with some suggesting that only paths that are consistently timelike, null, or spacelike are meaningful.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of spacetime separation and the implications of different paths. The discussion remains unresolved with no clear consensus on the significance of integrating along various paths or the implications of mixed paths.

Contextual Notes

Limitations include the dependence on the definitions of spacetime intervals, the potential for varying metric coefficients in curved spacetime, and the unresolved nature of integrating along different paths.

  • #151
Anamitra said:
A set of differential equations should have a unique solution set corresponding to a given set of boundary conditions. We may try out different techniques--but the aim is to find a solution set that fits into the boundary conditions.If we can do this--the job is done.We can get the correct solution from a set of infinite solutions.

It is easey to see that the boundary conditions do nothing for you. Suppose one solution consistent with them. Do any of uncountably infinite coordinate transformations, you are still consistent with them *and* with your set up.

You really need to let go of the idea of coordinate grid having any meaning (separate from a metric; or unless defined with a fixed operational definition). If you consult books on GR, you will find 100% unanimity that coordinates by themselves are meaningless. More, that points in spacetime have no meaning; only material objects and measurements have meaning.
 
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  • #152
DaleSpam said:
Consider the family of helical paths:
\left(ct,R \; cos(\omega t)+R, R \; sin(\omega t), 0\right)
Where \omega=2\pi/T

This helix connects the events (0,0,0,0) and (cT,0,0,0) with a smooth path. Those events are also connected by a straight timelike path.

The only objection I have with this is that the "T" in \omega=2\pi/T is the period around the helix, whereas the "T" in (cT,0,0,0) is the time to decay for the muon. Otherwise what you've said is right on.
 
  • #153
Yes, they are the same. Why do you object to that?
 
  • #154
DaleSpam said:
Yes, they are the same. Why do you object to that?
Actually I don't object. It's just that it wasn't specified in the original post. Your restriction that the muon exists such that it moves exactly one circle of the helix in its lifetime took some thinking for me to accept, considering that individually muons decay randomly. - But this is just an example of a curved path in spactime, not really about muons, right?
 
  • #155
kg4pae said:
Actually I don't object. It's just that it wasn't specified in the original post. Your restriction that the muon exists such that it moves exactly one circle of the helix in its lifetime took some thinking for me to accept, considering that individually muons decay randomly. - But this is just an example of a curved path in spactime, not really about muons, right?

Please, the muon was introduced to possibly clarify issues around what types of paths can exist between two events. Nothing else besides one post in this whole thread (before yours) deals with muons. The issue Dalespam was clarifying is long since settled, and even for that, the muon was really irrelevant.
 

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