Perhaps the universe is connected, not as a cylinder or torus, but as a Moebius band or Klein bottle. Then parity transformations would be physically realizable, by transporting a system around the loop. Then again, we were once told that there is some barely marginal evidence that the universe has negative curvature, so maybe the universe is not just an ordinary doughnut, but a multiholed one. The hardest connectivity to comprehend is one in which the universe comes back on itself in the time direction. All of these models are consistent with general relativity, but I have no idea how they would fare in quantum gravity. __________________________________________________ Do you Yahoo!? Yahoo! Web Hosting - establish your business online http://webhosting.yahoo.com