"General relativity makes a number of big predictions, including the idea that mass warps space-time. If you missed my column on falling into a black hole, the effect of this is that time runs slower near a massive body than far away. You could build a time machine, of a sort, by hanging out near the edge of a black hole, allowing the universe to age must faster than you, and then returning home...to the future. But since you were going to wind up in the future if you just waited long enough, this is kind of stretching the definition of time travel. What we really want to do is go into the past. If G.R. allows us to bend time, will it let us fold it back on itself?"