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Suppose you have two electrons in the singlet state (+=spin up, -=spin down):

lψ>= l+>l-> - l->l+>

Now if you measure the spin on the first electron the explanation is (I think) that this collapses one electron onto l+> or l-> such that the wave function becomes:

lψ>= l+>l-> (for measuring +)

But I don't understand this. How is this form of wave function consistent with the antisymmetrization requirement for the wavefunction for electrons?

Actually I'm also generally confused by how measurement in quantum mechanics is precisely defined. Suppose I want to measure the total spin. How does one do this? Is measurement of a quantity defined as the interaction of the quantity with an object large enough to exhibit classical behaviour.