The expansion theorem in quantum mechanics states that a general state of a system can be represented by a unique linear combination of the eigenstates of any Hermitian operator.(adsbygoogle = window.adsbygoogle || []).push({});

If that's the case then that would imply we would be able to represent the spin state of a particle in terms of it's energy eigenstates for example however surely it's impossible to do this for a particle in an infinite square well for instance, since the energy eigenstates are independent of spin.

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# Problem with changing basis in Hilbert space

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