From: utzoo!decvax!cca!gwyn@BRL@sri-unix Newsgroups: net.physics Title: Re: paradoxical twins Article-I.D.: sri-unix.3306 Posted: Thu Sep 16 20:11:53 1982 Received: Fri Sep 17 03:29:37 1982 From: Doug GwynIn all discussions like this, it is important to specify carefully just what is measured and how. A diagram often helps, if you don't take any distances on it as "absolute". Acceleration is not relevant. The twins paradox can be demonstrated by using three reference frames, one "stationary" (earth-bound) and two moving in opposite directions with the same speed with respect to the "stationary" frame. Then you can (1) transfer your attention from the earth-bound frame to an "outward-moving" one, (2) after a suitable interval transfer your attention to an "inward-moving" frame, and finally (3) transfer your attention to the starting point when it arrives. This thought-experiment is equivalent to the space-flying twin's itinerary, but all accelerations have been removed so that any time-dilation cannot be ascribed to an object being affected by pseudo-gravity forces or whatever. The key to the paradox when cast in this form is the availability of suitable "universal" time references in each frame; e.g. use a maser clock or something similar. Given such clocks, the time dilation claimed for the twin paradox does in fact occur in accordance with special relativity. Yet none of the clocks ever feels an acceleration. Special relativity does not answer the question, "Why can't the earth-bound observer, by symmetry, claim HE is younger than the space traveler?", except by pointing out that the situation is NOT symmetrical since by no stretch of the imagination can the space traveler (out and back) consider that he is constantly in an "inertial frame". In fact, a minimum of TWO inertial frames must be used, with a changeover in point of reference at the turn-around point of the journey. Accompanying this change of viewpoint, one must also insist that the REST OF THE UNIVERSE has suddenly started moving differently. Quite apart from questions of Mach's principle in General Relativity, the introduction of an "outside force" acting on the space traveler to turn him around (or, equivalently, to give the rest of the universe a shove) violates symmetry in a fundamental way. The detailed treatment of the problem necessarily involves agents and effects outside the province of Special Relativity. However, the conventional analysis from the viewpoint of the earth-bound frame fits within the theory's constraints and is therefore accurate.