Time travel stories are cool because both the past and future are somehow more interesting that the present and because everyone wants a redo. But so far it appears we’re doomed to live consumed by regret in the eternal, boring present. Time marches on, inexorably and only forward. Or so we thought until Einstein came along. His special and general theories of relativity changed the way we think about time forever, and believe it or not, their raw equations permit time travel. They even tell us how to do it. So let’s review the possibilities, and decide how possible they really are.