I have a basic question about what spin invariance means. If we were to use a classical example, if the spin of a basketball was invariant, what would that mean? Would every frame measure the same revs/min? Newtonian mechanics predicts that every frame would measure the same revs/min, but sr predicts that every frame would measure a different revs/min. So, would every frame measure a basketball travelling near the speed of light as having the same revs/min? Or, the same number of rotations, over different periods of time?