Simple gamma matrices question

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Discussion Overview

The discussion revolves around the properties of gamma matrices, specifically addressing the claim that a matrix commuting with all four gamma matrices must be a multiple of the identity matrix. Participants explore the reasoning behind this statement, considering various approaches and methods of proof.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the validity of the statement that a matrix commuting with all gamma matrices must be a multiple of the identity, seeking clarification on the reasoning.
  • Another participant suggests using the completeness properties of gamma matrices to deduce the claim, proposing that a basis can be formed from the gamma matrices.
  • A different participant acknowledges the suggested method but expresses a desire for a simpler explanation, indicating that the topic may not be straightforward.
  • One participant proposes an alternative method of explicitly computing the commutator of an arbitrary 4x4 matrix with the gamma matrices, which leads to linear equations supporting the original claim.
  • Another participant raises a concern that this alternative method may depend on the choice of representation, questioning its universality.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the most elegant or straightforward method to prove the claim regarding the gamma matrices. Multiple approaches are discussed, indicating a lack of agreement on the best solution.

Contextual Notes

The discussion highlights potential limitations in the reasoning, such as the dependence on specific representations and the introduction of bases, which may not be universally applicable.

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Hi

I've just read the statement that a matrix that commutes with all four gamma matrices / Dirac matrices has to be a multiple of the identity. I don't see that; can anyone tell me why this is true?

Thanks in advance
 
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You can deduce this yourself using the completeness properties of gamma matrices. By considering (schematically) 1, \gamma, \gamma \gamma, etc. one can form a basis of matrices.

If you can decide:
1) that this basis is complete for 4x4 matrices
2) that nothing in this basis besides 1 commutes with all the \gamma
then you will have what you want.
 
Thanks.

OK, I also had this answer in mind (which I already found), but I thought that there is a simpler answer because this statement appeared at a point in the script where that basis has not yet been introduced. Perhaps I'll find a simpler answer, but I'm also fine with this one. At least I see that it's not that simple.
 
Yeah, I wouldn't necessarily claim this is the most elegant method. One could even explicitly compute the commutator of an arbitary 4x4 matrix with the four gamma matrices. This gives 4 4x4 linear matrix equations that will give the same conclusion. This is even less elegant but requires no mention of a basis.
 
Yes, but wouldn't this require a choice of representation? It would not require a choice of basis, but it wouldn't be canonical in the sense of independence of representation.
 

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