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Aug 1, 2020 at 13:13 comment added Mr. Palomar Thanks for the response, it makes sense to me now.
Aug 1, 2020 at 11:43 comment added Philip @Mr.Palomar I'm not completely sure I understand, but yes to the first part. The question as to what determines a CSCO isn't (as far as I know) one that can be solved entirely mathematically. If we had "ignored" spin and thought that a system could be completely specified by the eigenvalues $| nlm\rangle$, then we'd be surprised by what happens when we place the atom in an external magnetic field: we would get more splittings in the spectral lines than expected! (Indeed this is why the "true" Zeeman effect is called "anomalous": it couldn't be explained without introducing Spin.)
Aug 1, 2020 at 11:24 comment added Mr. Palomar Thanks for the response. The question whether a set of operators forms a CSCO isn't merely a mathematical one, correct? For instance, when adding spin, what we're doing is we enlarge the ambient Hilbert space (tensoring with $\mathbb{C}^2$, I guess) so that there's more 'space' for an additional operator. Had we not enlarged it in the first place, $\big\{\hat{H},\hat{L}^2,\hat{L}_z\big\}$ would've been a perfectly fine CSCO. Do I understand that correctly?
Aug 1, 2020 at 10:10 history answered Philip CC BY-SA 4.0