What properties does Baym use to derive the L commutation relation?

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Discussion Overview

The discussion centers on the derivation of the L commutation relation as presented in Baym's Lectures on Quantum Mechanics. Participants explore the mathematical properties and identities involved in the derivation, particularly focusing on vector cross products and the implications of non-commuting operators.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions the validity of an identity used in the derivation, noting that the non-commutativity of position (r) and momentum (p) might affect the outcome.
  • Another participant suggests a specific vector identity involving cross products that could lead to the desired result.
  • A later reply proposes using Levi-Civita symbols and their properties to verify the identity in question, asserting that the unit vector n commutes with both r and p.
  • One participant confirms that the identity can be verified using the suggested mathematical approach, indicating a successful resolution of their earlier confusion.

Areas of Agreement / Disagreement

Participants express differing views on the validity of certain mathematical identities due to the non-commutativity of operators, but there is a general agreement on the utility of specific vector identities to resolve the issues raised.

Contextual Notes

The discussion involves assumptions about the properties of vector operations and the commutation relations of quantum mechanical operators, which may not be fully resolved in the exchanges.

Who May Find This Useful

Readers interested in quantum mechanics, particularly those studying operator algebra and vector calculus in the context of quantum theory, may find this discussion relevant.

univox360
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In Baym's Lectures on Quantum Mechanics he derives the following formula

[n.L,L]=ih L x n

(Where n is a unit vector)

I follow everything until this line:

ih(r x (p x n)) + ih((r x n) x p) = ih (r x p) x n

I can't seem to get this to work out. What properties is he using here?
 
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Can you show

0 = n x (r x p) + r x (p x n) + p x (n x r) ?

What you want would follow from this.
 
I understand, but is this identity valid since r and p do not commute? This identity is constructed using B(AC)-C(AB) which seems to change order of operation...
 
Try using

[tex]\left( A \times B \right)_i = \epsilon_{ijk} A_j B_k[/tex]

and

[tex]\epsilon_{ijk}\epsilon_{iab} = \delta_{ja}\delta_{kb} - \delta_{jb}\delta_{ka}[/tex]

in the three terms in your expression (repeated indices are summed over). Also, n is a triple of numbers, and so commutes with r and p.
 
I have now done the calculation. The identity can be verified by using
George Jones said:
Try using

[tex]\left( A \times B \right)_i = \epsilon_{ijk} A_j B_k[/tex]

and

[tex]\epsilon_{ijk}\epsilon_{iab} = \delta_{ja}\delta_{kb} - \delta_{jb}\delta_{ka}[/tex]

in the three terms in your expression (repeated indices are summed over). Also, n is a triple of numbers, and so commutes with r and p.
 
Yes, using that theorem this works. Thanks so much!
 

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