Retardation in special relativity and the design of a relativistic motor

The Newton third law states that any action is countered by a reaction of equal magnitude but opposite direction. The total force in a system not affected by external forces is thus zero. However, according to the principles of relativity, a signal cannot propagate at speeds exceeding the speed of light. Thus the total force cannot be null at a given time. The above conclusion leads to the possibility of a relativistic engine in particular one that is based on a permanent magnet. The analysis is based on a previous paper in which we studied the relativistic effects in a system of two current conducting loops.