In this paper, the authors have calculated the force on the tube, and stated that the magnitude of force on the magnet is same due to Newton's third law.
However, the third law is not valid under the special theory of relativity. In this case, are the forces still equal and opposite in magnitude due to some deeper reason? Is the net momentum carried by the fields is zero in this case?
Or is it just a low speed approximation? In electromagnetism, low speed approximations are not always valid (then magnetic force due to slowly moving charges would not arise)
In case it is a low speed approximation, how to find the force of the magnet on from an inertial frame where the tube is stationary?
