Alternate Representations of Lorentz Generators Mμν

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Discussion Overview

The discussion revolves around the representations of Lorentz generators \( M_{\mu\nu} \) and their various forms, particularly focusing on the uniqueness of these representations, including potential 2x2 matrix representations and connections to the group \( SL(2, \mathbb{C}) \). The scope includes theoretical exploration and mathematical reasoning related to Lie algebras.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the 4x4 matrix representations of Lorentz generators are not unique and inquire about alternative representations, including 2x2 matrices.
  • One participant asserts that there are infinitely many representations of the Lorentz algebra, providing an example of constructing 8x8 matrices using the Kronecker product.
  • Another participant suggests that since \( SL(2, \mathbb{C}) \) is the universal cover of the restricted Lorentz group, the Lie algebras are isomorphic, raising the question of deriving a 2x2 representation from \( SL(2, \mathbb{C}) \).
  • A participant provides specific definitions for the components of the Lorentz generators and presents a matrix representation that is infinitesimally close to the identity element of \( SL(2, \mathbb{C}) \).

Areas of Agreement / Disagreement

Participants generally agree that multiple representations exist, but the discussion remains unresolved regarding the specifics of deriving a 2x2 representation and the methods to achieve this.

Contextual Notes

There are limitations regarding the assumptions made about the representations and the dependence on the definitions of the groups and algebras involved. The discussion does not resolve the mathematical steps necessary to derive the proposed representations.

sshaep
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I know the representations of Lorentz generators M\mu\nu as 4X4 matrices.

This matrices satisfy the commutation relation(Lie algebra of O(3,1))

However I think these 4X4 matrix representations are not unique.

Is there any other representations satisfying the commutation relation? 2X2 matrix or

another?
 
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Yes, there are infinitely many representations of the Lorentz algebra. One trivial example is to form a set of 8x8 matrices by taking the Kronecker product of M with the 2x2 identity.
 
sshaep said:
Is there any other representations satisfying the commutation relation? 2X2 matrix or another?

Because SL \left(2 , \mathbb{C} \right) is the universal cover of the restricted Lorentz group, they have isomorphic Lie algebras.
 
George Jones said:
Because SL \left(2 , \mathbb{C} \right) is the universal cover of the restricted Lorentz group, they have isomorphic Lie algebras.

This means can I get the 2x2 representation of M\mu\nu from SL(2,C)?

But how? What I know is \sigma_{\mu}A^{\mu} _{\phantom{\mu}\nu}}=L\sigma_{\nu}L^{\dagger}
 
Take

<br /> M^{0i} = \frac{i}{2} \sigma^{i}, \ \ M^{ij} = \frac{1}{2} \epsilon^{ijk}\sigma^{k}<br />

Also, if you define

\omega_{i0} = 2 Re(\alpha_{i}), \ \ \omega_{ij} = 2 \epsilon^{ijk} Im(\alpha_{k})

then

<br /> 1 + \frac{i}{2}\omega_{\mu\nu}M^{\mu\nu} = \left( \begin {array}{rr} (1 + \alpha_{3}) &amp; (\alpha_{1} - i \alpha_{2}) \\ (\alpha_{1} + i \alpha_{2}) &amp; (1 - \alpha_{3}) \end {array} \right)<br />

represents elements of SL(2,C) infinitesimally close to the identity element.

regards

sam
 

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