Time travel without grandpa-killing paradoxes

Wired : Einstein’s general relativity and other physics theories don’t explicitly rule out time travel. But any time traveler must confront one of the constraints that Einstein evoked to derive relativity: An effect cannot take place before its cause—or, more concretely, you cannot travel back in time and kill grandpa and your other progenitors. Wired‘s Laura Sanders describes a theory by MIT’s Seth Lloyd and his collaborators that allows time travel and prevents grandpa killing but at the cost of amplifying the probability of unlikely events.