Proof Minkowski metric is invariant under Lorentz transformation

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Discussion Overview

The discussion revolves around the proof of the invariance of the Minkowski metric under Lorentz transformations. Participants explore the mathematical steps involved in demonstrating that the expression x² - c²t² remains unchanged when transforming coordinates between different inertial frames.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant expresses frustration in applying Lorentz transformations to the Minkowski metric, specifically struggling with the presence of the speed of light, c, in the calculations.
  • Another participant shares a resource that helped clarify the problem, suggesting that rewriting c²t² as (ct)² simplifies the understanding of the transformation.
  • A later post outlines a method involving linear combinations of the transformed coordinates, leading to the conclusion that the Minkowski metric remains invariant.
  • One participant reflects on a mistake made in their calculations, noting the importance of correctly distributing c² in the expressions.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to demonstrate the invariance, as some rely on external resources while others present their own methods. The discussion includes both successful and unsuccessful attempts at the proof, indicating a mix of understanding and confusion.

Contextual Notes

Some participants mention specific steps or transformations that may depend on assumptions about the definitions of the variables involved, such as the treatment of c in the equations. There is also an acknowledgment of arithmetic errors that can complicate the proof.

Who May Find This Useful

Readers interested in the mathematical foundations of special relativity, particularly those looking for clarification on Lorentz transformations and the invariance of the Minkowski metric.

SamRoss
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Ok, this should be an easy one but it's driving me nuts. When we take the Lorentz transformations and apply them to x2-c2t2 we get the exact same expression in another frame. I can do this math easily by letting c=1 and have seen others do it by letting c=1 but I have never seen anyone actually do it with the c's in there. It doesn't sound like it should be that hard but I just can't get it to work. Can anyone go through it?
 
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It's always the arithmetic isn't it :wink:? Glad it worked out!
 
The neatest way:

x' = γ(x - vt)
t' = γ(t - vx/c2)

Take linear combinations:

(1) x' - ct' = γ(x - ct - v(t - x/c)) = γ(1 + v/c)(x - ct)
(2) x' + ct' = γ(x + ct - v(t + x/c)) = γ(1 - v/c)(x + ct)

Multiplying (1) and (2) together, all the leading factors cancel and we get

x'2 - c2t'2 = x2 - c2t2

(Note that x ± ct are the eigenvectors of the transformation. Plus the factors in front are just the Doppler shifts.)
 
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Wow, I just realized I had simply forgotten to distribute the c squared to everything originally. Talk about dumb!
 

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