Lorentz Transformation Derivation: Assumptions Req'd?

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Discussion Overview

The discussion revolves around the assumptions required in deriving the Lorentz transformation, particularly whether the transformation from one coordinate system to another must be the same in reverse, with a simple correction for velocity. Participants explore the implications of this assumption and its relation to foundational postulates in physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if it is necessary to assume that the transformation from coordinate system ##\bf {x}## to ##\bf {x’}## is the same as from ##\bf {x’}## to ##\bf {x}##, aside from a velocity sign change, and seeks derivations that do not rely on this assumption.
  • Another participant suggests that transforming coordinates from one frame to another and back provides justification for the requirement of this assumption.
  • A participant asserts that this assumption is a consequence of the first postulate of relativity.
  • Another participant reinforces that the reciprocity property is also a requirement of the Galilean transformation.
  • One participant elaborates that both the Poincare and Galilei transformations can be derived from the first Newtonian postulate, along with assumptions about homogeneity and symmetry in space and time, leading to either Galilei-Newton or Einstein-Minkowski spacetime models.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and implications of the assumption regarding the transformation's reciprocity. While some assert it is a consequence of foundational postulates, others explore its derivation and implications without consensus on the necessity of the assumption itself.

Contextual Notes

The discussion touches on foundational principles of relativity and transformations, highlighting the dependence on specific assumptions and the implications of those assumptions for different models of spacetime.

Pencilvester
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In deriving the Lorentz transformation, is it required to assume that the transformation to get from coordinate system ##\bf {x}## to ##\bf {x’}## should be the same as that to get from ##\bf {x’}## to ##\bf {x}## (with the simple correction of flipping the velocity)? If no, could someone direct me to a derivation that does not assume this a priori? I’m having trouble deriving it myself without this assumption. If yes, what is the basis for this assumption?
 
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Consider what happens when you transform the coordinates of an event from the unprimed frame to the primed frame and then back again... that will be enough to justify this requirement.
 
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Pencilvester said:
is it required to assume that the transformation to get from coordinate system xx\bf {x} to x′x′\bf {x’} should be the same as that to get from x′x′\bf {x’} to xx\bf {x} (with the simple correction of flipping the velocity)?
That is a consequence of the first postulate.
 
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Haha, duh. Thanks!
 
Dale said:
That is a consequence of the first postulate.
It is also required of the Galilean transformation.
 
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Both the Poincare and the Galilei transformations can be derived from the 1st Newtonian postulate together with the assumption that time and space for any inertial observer are homogeneous (translation invariance in space and time) and that the space for any inertial observer is a 3D Euclidean affine space (implying that also rotations are a symmetry of space) and that the symmetry transformations of space and time together build a group. The "reciprocity property", i.e., that if an inertial frame ##\Sigma'## moves with velocity ##\vec{v}## wrt. to another inertial frame ##\Sigma## than ##\Sigma## moves with velocity ##-\vec{v}## relative to ##\Sigma'##, can be derived from these symmetry assumptions and needs not to be postulated. The analysis reveals that the only possible space-time models obeying these assumptions are either the Galilei-Newton spacetime or the Einstein-Minkowski spacetime of special relativity.
 

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