But quantum physics doesn’t let you calculate the momenta of A and B once they collide. Instead, the quantum wave function connects A and B in a strange way. Because of their collision, A and B share a single wave function, rather than having their own individual wave functions. But that shared wave function doesn’t say what the particles’ momenta are before a measurement is made. It simply ensures that, once A’s momentum is measured, B’s momentum will always be equal and opposite.
