Monday, 16 November 2015

quantum mechanics - Commutation of abstract O(3) generators and vectors



I've been given the following problem, and I'm quite lost with it.



Let L1, L2, and L3 denote the abstract o(3) algebras. You are given that A=(A1,A2,A3) and B=(B1,B2,B3) transform as vector operators of o(3).


Show that [Lj,AB]=0



I know that Lj=εjlmqlpm, and I can obviously determine the dot product, but I'm not sure where to go from there.



I do, however, know that A=1Ze2μ(L×p)+(1r)r, but I'm not sure how to integrate that into this problem.



Answer



A collection {V1,V2,V3} of operators on a vector space V is called an o(3) vector operator with respect to a representation ρ of o(3) acting on V provided [Vi,Lj]=iϵijkVk

where L1,L2,L3 are the representatives of the standard basis on o(3). This means, in your case, that merely knowing that Ai and Bi are the components of vector operators tells us that [Ai,Lj]=iϵijkAk,[Bi,Lj]=iϵijkBk
These commutation relations alone are sufficient to demonstrate the desired result using commutator identities.


As a tangential note, you will probably find the Wikipedia page on tensor operators to be generally, conceptually helpful for understanding this stuff. Also, a while back I asked the following question on physics.SE dealing with how to generalize and formalize the notion of tensor operators in a less basis-dependent way than how I defined them at the beginning of this answer. In case you're interested in math, here is that question:


Tensor Operators


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