OK, so I'm right in the middle of watching Interstellar and I've just seen the part where they have docked with the 'mothership' and they fire up their engines to get it to rotate. This is so they can generate a centrifugal force to simulate gravity. My question is - first the ship was not rotating and they took action to make it rotate. How could this possibly be? Angular momentum started at 0 and was increased without any apparent external torque being applied. Please could somebody help me out?