The twin paradox and the principle of relativity

Abstract

The twin paradox is intimately related to the principle of relativity. Two twins A and B meet, travel away from each other and meet again. From the point of view of A, B is the traveller. Thus, A predicts B to be younger than A herself, and vice versa. Both cannot be correct. The special relativistic solution is to say that if one of the twins, say A, was inertial during the separation, she will be the older one. Since the principle of relativity is not valid for accelerated motion according to the special theory of relativity B cannot consider herself as at rest permanently because she must accelerate in order to return to her sister. A general relativistic solution is to say that due to the principle of equivalence B can consider herself as at rest, but she must invoke the gravitational change of time in order to predict correctly the age of A during their separation. However one may argue that the fact that B is younger than A shows that B was accelerated, not A, and hence the principle of relativity is not valid for accelerated motion in the general theory of relativity either. I here argue that perfect inertial dragging may save the principle of relativity, and that this requires a new model of the Minkowski spacetime where the cosmic mass is represented by a massive shell with radius equal to its own Schwarzschild radius.