vanhees71 said:
I'm unhappy about your formulation
The degrees of freedom and the Hilbert space you choose to describe them says something about what you intent to measure on the system.
because it seems to state a very common misconception about quantum mechanics. It seems as if you have an interpretation of the Heisenberg uncertainty relations in mind as if it would prevent from meausring the one or the other observable with arbitrary precision. ...
... One thing I could do is to explain the concrete example I had in mind:
When you describe a Stern-Gerlach experiment, your magnet may be described as fixed such that the magnetic field (except for its inhomogenity) points in z-direction, or you may describe it such that the magnet can be rotated around the particle beam. In the second case, you probably need to allow density matrices to describe the actual state of the incoming particles. But most introductory QM textbooks have not yet introduced density matrices at the point where they describe and analyse SG, so they typically go with the description using a fixed magnet. Independent of this, the silver atoms have more degrees of freedom in their quantum state than just the spin of the unpaired electron in the outer shell. But we won't describe those in our Hilbert-space for analyzing SG, because we don't intent to measure anything for which they would be relevant.