It would not be legitimate to attempt to apply the special principle of relativity, and to argue as though the wanderer had always been at rest; for by so doing we should be assuming that he had never suffered acceleration, and we should therefore have to credit his brother with having experienced this acceleration. The problem would be completely modified; and, indeed, the relative aging would then be reversed. But no argument against the theory could be invoked, for the simple reason that we should be considering a totally different problem, one in which the circumstances of acceleration were no longer the same. For the sake of completeness let us assume that it is now the wanderer who remains at rest, while the earth twin, the earth and the star execute a backward-and-forward motion, being of course subjected, in so doing, to acceleration when their velocity is reversed. Retaining the same numerical values, we should find that the erstwhile wanderer had waited twenty years, as before, for the return of his brother, but this earth brother would now conclude that his absence had lasted ten years instead of forty. So even from a purely numerical standpoint conditions would not have been completely reversed. Thus, in a general way, we see that when two observers leave each other, then meet again, one of the [Pg 238] two, if not both, must have been submitted to acceleration; otherwise they could never meet again but would wander farther and farther apart ad infinitum.[74] If we consider twins separating, then meeting again, the younger of the two will invariably be the one who has suffered acceleration. We need not be surprised at these very real effects produced by acceleration, for we must remember that acceleration is an absolute and not a relative like mere velocity. A body has one velocity or another, according to our point of view or choice of a frame of reference; but when a body is accelerated the choice of our frame of reference is irrelevant for the simple reason that acceleration betrays itself by physical forces and stresses which may exert palpable influences.