henry (08/04/82)
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