Reverse engineering relativity?

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Discussion Overview

The discussion centers on the possibility of deriving the existence of an invariant speed starting from the Lorentz transformations and the Poincaré group SO(1,3). Participants explore whether this derivation is known and the implications of such an approach, considering the foundational aspects of relativity and its teaching.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question whether it is feasible to derive the existence of an invariant speed from the Lorentz transformations, suggesting that it might be an interesting but potentially flawed approach.
  • One participant argues that the property of absolute velocity is inherent to the Lorentz transformations, implying that starting from these transformations already assumes the existence of an invariant speed.
  • Another participant expresses skepticism about the educational value of this derivation, raising concerns about how to justify using the Poincaré group instead of Einstein's axiomatic approach.
  • A suggestion is made to find eigenvectors related to the transformations, with the expectation that they would lie on a cone through the origin, and to explore this in the context of 2D Minkowski space.
  • One participant mentions a paper by Ingo Kirsch that approaches the topic from a different direction, involving diffeomorphism invariance and symmetry breaking, and invites discussion on potential findings from this perspective.
  • Another participant highlights the existence of amateur aether theorists who accept Lorentz transformations but reject the postulates of special relativity, suggesting that proving a biconditional relationship between the postulates and derived results could help clarify ongoing debates.

Areas of Agreement / Disagreement

Participants express differing views on the validity and educational implications of deriving invariant speed from Lorentz transformations. There is no consensus on the approach or its merits, and multiple competing perspectives are present.

Contextual Notes

Some limitations include assumptions about the flatness of the manifold and the nature of the transformations being discussed. The discussion also reflects varying interpretations of the relationship between Lorentz transformations and the postulates of special relativity.

selfAdjoint
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Is it possible to start with the Lorentz transformations - I mean the whole Poincare group SO(1,3) - and derive that there exists an invariant speed? Is this a known derivation? I suppose we could assume a "flat" manifold with metric Diag(1,-1,-1,-1,).
 
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selfAdjoint said:
Is it possible to start with the Lorentz transformations - I mean the whole Poincare group SO(1,3) - and derive that there exists an invariant speed? Is this a known derivation? I suppose we could assume a "flat" manifold with metric Diag(1,-1,-1,-1,).

I am just wondering, wouldn't it be one heck of a coincidence that you start with "such" transformations ? I don't see the point in doing so, to be honest. However, isn't this quite easy to prove ? Just apply those transformations to two reference frames...

marlon
 
Besides, velocity being absolute is an inherent property of the Lorentz transformations, right ? They are constructed starting from the universal constant c and the homogenity of space...So if you start from the L transformations, you already have that property

marlon
 
It would be interesting SA.But it would be cheating.You cannot teach that.Students might ask you:"Why SO(3,1)"??What would you say ?What would be the reasoning behind not follwing the Einstein approach,the aximatical one?


It sort of reminds of the "deriving Schrödinger equation" threads.:wink:

Daniel.
 
selfAdjoint said:
Is it possible to start with the Lorentz transformations - I mean the whole Poincare group SO(1,3) - and derive that there exists an invariant speed? Is this a known derivation? I suppose we could assume a "flat" manifold with metric Diag(1,-1,-1,-1,).

Suggestion: Find the eigenvectors. (They should lie on a cone through the origin.)
It's probably easiest to leave off the translations.
To see that this is plausible, try the usual Lorentz Transformations in 2D Minkowski.

Edit: Furthermore, one should be able to show that if the eigenvalue corresponding to an eigenvector is not 1 or -1, then that eigenvector must have zero norm [with respect to the metric preserved by the transformation].
 
Last edited:
robphy said:
Suggestion: Find the eigenvectors. (They should lie on a cone through the origin.)
It's probably easiest to leave off the translations.
To see that this is plausible, try the usual Lorentz Transformations in 2D Minkowski.

Edit: Furthermore, one should be able to show that if the eigenvalue corresponding to an eigenvector is not 1 or -1, then that eigenvector must have zero norm [with respect to the metric preserved by the transformation].

Thanks robphy, that's what I was looking for. Here for the rest of you is my secret motivation. Ingo Kirsch of Harvard posted a paper, http://www.arxiv.org/abs/hep-th/0503024, in which he starts from diffeomorphism invariance on a non metric space and by repeated symmetry breaking gets down to SO(1,3) acting on the tangent space of a Lorenzian (1,3)-manifold. And supposing you came at it all from that direction, what could you find out and how would you go about it?
 
marlon said:
I don't see the point in doing so, to be honest.

I do! There are a whole gaggle of amateur aether theorists on the net who accept the Lorentz transformations, and yet do not accept the postulates of SR. They insist that there is no "if and only if" relationship between the two. By this insistence they maintain that their point of view is upheld by experiment every bit as much as the SR point of view. But if it can be proven rigorously that the postulates of SR and the derived results share a biconditional relationship, it would go a long way towards ending that debate.
 

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