Hi all! So rumor has it, that the spin of a particle is preferably aligned opposite to the direction of the particle's momentum, whereas an antiparticle spins along the direction of motion. For the sake of simplicity let us assume that the (anti-)particle is massless and hence it is an eigenstate of both helicity and chirality operators. My question reads as follows: "How is the conception of such definite, predetermined spin direction compatible with the uncertainty princple, which forbids the simultaneous knowledge of all spin vector components?" I suspect that most textbooks refer only to the z-component of spin but they do not bother to elucidate that detail. If that's not the case, I would appreciate any kind of explanation, involving spectral theory concerning Dirac's solutions or relativistic redefinition of the spin operator.