“Close to the Speed of Light”: Dispersing Various Twin Paradox Related Confusions

Abstract

There is a big conceptual problem concerning the notion of 'speeds close to that of the velocity of light' if this notion is to be formulated within the framework of the Special Theory of Relativity. On the one hand, the velocity of light is defined as being the same at any point of any system of reference. On the other hand, the notion of speeds more or less approximating the velocity of light must have a system- of-reference-independent meaning if some alleged effects of the STR are to be explained. I will argue, contrary to familiar dynamic and geometric solutions to the so-called Twin Paradox, that the tension implicit in the afore- mentioned phenomena can be fully resolved only by adopting an ontologically absolute meaning for the term 'velocity of light'. The compatibility between the effects of speeds more or less close to the velocity of light with the invariance of the speed of light in all the systems of reference is based on the fact that these effects can be established only by comparing them directly. So, the change of the reference frame in the Twin Paradox has itself no significance for the difference in the aging of the twins. The fact that the twin who changes reference frames turns out to be younger results from the fact that he necessarily traverses a longer path in an absolute sense. The main point is that, given that the twins have first been moving away and then approaching each other by a uni- form velocity, we cannot operationally determine whether the twin who has turned out to be younger has aged more slowly during the moving apart or during the approach to his brother, though one or both of these two possibilities must be the case.