Perhaps one of the questions for observers, like us who can ask questions, is why the Hilbert space of QM is complex? Maybe it is just because complex numbers are useful when describing waves, and there's that dang Schroedinger wavefunction, and deBroglie matter waves. But the same complex 2-vectors equally describe spin states (spin states have a wavelike nature??), electron spin is either parallel or antiparallel to the direction of motion. Or it's in a superposition of both. Which is why, if you have an incoming beam of electrons they're described by a mixed state, one which has equally probable outcomes for measurement. There are two more directions apart from the one defined by electron motion (i.e. linear momentum). If the electron isn't "moving", how many ways can the spin be in superposition? What's different when the superposition of spin states spans a pair of electrons? The math just "changes gear" so you have 4-vectors (otherwise called tensor products).