Hi I'm recently reading something about the moller scattering in QED, and I'm feeling a bit curious about how plane waves change their spin orientations (e.g. up to down)(I mean in the scenario that one is up and one is down). In the text it seems that it's because the electrons are indistinguishable, so it can end up in either final state. Therefore, I'm just wondering whether it is possible to distinguish the electrons through spin orientations. If this is not the case then I actually pretty curious about how the angular momentum is tossed between the two in order to flip the spin. Thanks!