A New Look at Simultaneity

Abstract

It is generally believed that an invariant notion of a global present or "Now" cannot be defined in special relativity, because of the relativity of optical simultaneity. I argue that this may be a non sequitur since it is not necessarily the case that the psychological "Now" should be thought of as associated with constant time slices in spacetime. By considering a science fictional version of the Twin Paradox due to Robert A. Heinlein, I argue that it is psychologically plausible to associate the common specious present of several observers in relative motion with certain hypersurfaces of proper time of those observers corrected for acceleration history and relative motion in an obvious way. If this is correct then the relativity of optical simultaneity may be simply irrelevant to the question of the relativity of a globally distinguished "present".