Most people are familiar with the idea of wormholes in space. Try to imagine the universe as a balloon. When you pick two spots on the balloon and push toward the center, you can make those two points meet with no space between them. To imply that idea was true for the universe would mean that two points far from each other could be traversed in one step. The normal immense distance between the earth and the moon, or even another galaxy could be skipped by taking a shortcut. Now imagine time as a ribbon with ripples. The same idea could be applied to time. What if you took the ribbon at the base of two ripples and joined it? The longer distance between those two points in time, now out in a loop, could be skipped by passing where the ribbon touches. There are a few problems though. When travelling through time, one can only travel forward. Travelling backward seems impossible because changing something would have an effect on your opinion back when you came from. That would change your motivation for going back in the first place, and you wouldn't make the change at all. For example, if you went back in time and successfully prevented the assassination of JFK then back when you came from JFK would have lived and your motivation would not exist. If JFK were alive, then you would never have the idea to go back in time to prevent his death. If you don't go back however, he would have died which would once more provide the motivation. Going backward ties the ribbon in knots; the action itself might be possible, but to do it would be to condemn yourself to a life in limbo while the rest of existence is living two lives.