From: utzoo!henry Newsgroups: net.space Title: acceleration is relative Article-I.D.: utzoo.2332 Posted: Tue Aug 3 19:12:09 1982 Received: Tue Aug 3 19:12:09 1982 Comparing the direction of "down" will not reliably distinguish between gravity and acceleration. Consider the acceleration of the surface of a sphere expanding at an increasing rate. This is a pretty exact simulation of not just the force but also the shape of gravity. The "twin paradox" is not a paradox because the supposed symmetry of acceleration is not present. One twin is accelerating with respect to the large, gravitating mass of the universe; the other is not. Henry Spencer utzoo!henry