Proving invariance of scalar product

Click For Summary

Discussion Overview

The discussion revolves around proving the invariance of the scalar product of two four-vectors under Lorentz transformations. Participants explore the concept through comparisons with simpler cases, such as the invariance of scalar products in two-dimensional vector spaces under rotations.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant asks how to prove that the scalar product of two four-vectors is invariant under Lorentz transformations.
  • Another participant suggests starting with the simpler case of proving invariance for the scalar product of two vectors in a plane under rotation.
  • A participant expresses uncertainty about the process for the four-vector proof, indicating a need to apply the Lorentz transformation to both vectors and check if the scalar products remain equal.
  • A later reply confirms the approach of checking the equality of scalar products after applying the transformation.
  • One participant reports successfully proving the four-vector invariance and identifies a similarity with the rotation proof, noting that earlier confusion stemmed from using incorrect transformation formulas.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the proof process for the four-vectors, as there is ongoing exploration and clarification of the steps involved.

Contextual Notes

There is mention of potential confusion regarding transformation formulas, which may affect the understanding of the invariance proof. The discussion does not resolve these uncertainties.

Gabor
Messages
3
Reaction score
0
Hi everyone,

How would I go about proving that the scalar product of two four-vectors (A,B) is invariant under a Lorentz transformation?
 
Physics news on Phys.org
As a warmup, you might try to prove that the scalar product of two vectors in the plane is invariant under a rotation.
 
Okay... I could do that for 2 vectors (x1, x2) and (y1, y2) in a plane.

As for the four-vector proof, I'm not even sure I'm doing it right... My understanding is that I have to take the scalar product of the two vectors A and B. Then I have to apply Lorentz transform to both vectors and calculate the scalar product of A' and B'. For invariance, these two scalar products should be equal?
 
yes.

How did you do the problem for the dot product of vectors in the plane?
 
I figured out the proof for the four-vectors. Now I see the similarity between that and the rotation proof. Turns out I was using the wrong transformation formulas for my vectors and that's why things didn't add up. Thanks for your help!
 

Similar threads

Replies
3
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 17 ·
Replies
17
Views
4K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 7 ·
Replies
7
Views
1K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 11 ·
Replies
11
Views
5K