Confusion in Lorentz Boost Equations: Minus Sign Needed?

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Discussion Overview

The discussion revolves around the interpretation of Lorentz boost equations as presented in S. Weinberg's "Gravitation and Cosmology." Participants are examining the necessity of a minus sign in the equations for the matrix components of the Lorentz transformation, focusing on the implications of different conventions for index placement and the definitions of velocity in different reference frames.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions whether a minus sign is needed on the left side of the equation for the Lorentz transformation matrix components as given by Weinberg.
  • Another participant notes that the sign of the term \(\gamma v_j\) suggests it may represent an inverse Lorentz transformation, depending on the convention used for index placement.
  • A third participant highlights a potential discrepancy in Weinberg's definition of velocity, suggesting it contrasts with the typical assumption found in most treatments of the subject.
  • A later reply indicates that replacing \(v\) with \(-v\) could yield the usual form of the transformation equations, but only under specific conditions where both frames are parallel.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of the minus sign and the interpretation of velocity in the context of Lorentz transformations. The discussion remains unresolved, with multiple competing interpretations presented.

Contextual Notes

Participants acknowledge the potential for typographical errors in authoritative texts and the variability in conventions used by different authors, which may contribute to the confusion surrounding the equations.

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On page 29 equations 2.1.20 and 2.1.21 of Gravitation and Cosmology by S. Weinberg he gives these expresions for matrix componentes:
[tex]\Lambda_j^0=\gamma v_j[/tex]
My question is: shouldn't there be a minus sign on left side of the equation?
 
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By looking at your latex source you seem to be writing

[tex]{\Lambda_j}^0 = \gamma v_j[/tex]
Now, I don't know Weinberg's convension for index placement for a Lorentz vs. inverse Lorentz transform matrices. Different authors use different convensions.

However, the positive sign of [itex]\gamma v_j[/itex] indicates that it must be an inverse Lorentz transform if [itex]v_j[/itex] is taken to be directed in the usual manner.

On the other hand, authors make typos and mistakes and proof readers don't catch them all. Just tonight, I'm reading some gr lecture notes by G. 't Hooft (nephew of G. 't Hooft, it seems), and it's peppered with plenty of them.
 
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Weinberg: "Suppose that one observer O sees a particle at rest, and a second observer O' sees it moving with velocity v."

In other words, he's saying that the line x = 0 corresponds to x' = vt'. This is the opposite of what most treatments assume, namely that x' = 0 corresponds to x = vt. So his definition of v is the opposite.
 
thank you both for your comments.
I understood the way Bill K says, but I needed a confirmation. I think we can replace v by minus -v only if both frames are parallel getting in this way the ussual form of the transformation equations.
 

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