Sorry, obvious to a mathematician who thinks about dz/dt=iz and realises that “exponentiation is time-evolution”. At that point it’s just “if you rotate for just long enough to turn half-way round, you’ll be pointing backwards”.
Sorry, obvious to a mathematician who thinks about dz/dt=iz and realises that “exponentiation is time-evolution”. At that point it’s just “if you rotate for just long enough to turn half-way round, you’ll be pointing backwards”.