How Do Clebsch-Gordan Coefficients Help in Calculating Eigenvalues of J?

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Discussion Overview

The discussion revolves around the use of Clebsch-Gordan coefficients in calculating the eigenvalues of the total angular momentum operator J in quantum mechanics. Participants explore theoretical aspects, seek literature recommendations, and clarify conceptual misunderstandings related to angular momentum representations.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses a desire to understand how to calculate eigenvalues of J using Clebsch-Gordan coefficients, indicating a need for more detailed resources.
  • Another participant questions the assumption that eigenvalues can be derived from Clebsch-Gordan coefficients, suggesting that they are typically looked up in tables and that the eigenvalues are assumed known.
  • A participant mentions a resource that discusses the decomposition of the spin group into a Clebsch-Gordan series and relates this to the representations of SU(2), implying a connection between representations and eigenvalues.
  • There is a suggestion that calculating eigenvalues may involve determining eigenstates of a representation and applying J, but this is noted as potentially shifting the problem rather than resolving it.
  • Two participants later indicate that they resolved their misunderstandings regarding the coupled and uncoupled representations of angular momentum vectors using Clebsch-Gordan coefficients.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the method of deriving eigenvalues from Clebsch-Gordan coefficients, with some expressing confusion and others clarifying their misunderstandings. The discussion remains unresolved regarding the best approach to calculate eigenvalues using these coefficients.

Contextual Notes

Participants express varying levels of understanding regarding the relationship between Clebsch-Gordan coefficients and angular momentum eigenvalues, indicating potential limitations in their grasp of the underlying concepts.

djeikyb
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Trying to self-teach myself more quantum mechanics. Reading Zare's "Angular Momentum- Understanding Spatial Aspects in Chemistry and Physics". I don't really understand how to calculate eigenvalues of J using CG coefficients (I mean, I know the eigenvalues of J, just trying to calculate them a different way). Any suggestions of other textbooks that go into more detail?
 
Physics news on Phys.org
Bump.

Any literature suggestions at all?
 
The lack of help makes djeikyb very sad... :(
 
Why do you think you can get the eigenvalues from the Clebsch Gordan coefficients? AFAIK you look them up in a table, and there is is assumed that you know the new eigenvalues.

There is a sketch of the decomposition of the spin group into a Clebsch Gordan series, via the characters of the irreducible representations of SU(2) in "Quantenmechanik: Ein Grundkurs über nichtrelativistische Quantentheorie" by Norbert Straumann. Together with other stuff in that direction.

The hole idea is that J has to carry a representation of the complex rotations. It is given by the tensor product of the individual rotations of the particles. These tensor products decompose into different irreducible representations. Each representation goes along with one eigenvalue. If you know which representations are present, then you know which eigenvalues, are contained in a certain state.

Calculating the eigenvalues should be possible by calculating the eigenstates of a certain representation and then applying J. I don't see how that is helpful though. You just shift the problem from J do some other Operator D^j.
 
Yeah.
Turns out my problem was a misunderstanding on my part.

Got it all figured out now.
Thanks.
 
What I wasn't understanding was the coupled representation of two angular momentum vectors using a unitary transformation with Clebsch-Gordan coefficients and the uncoupled representation.

Got it now though, and now I can see that my question didn't make sense.
 

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