The Paradox of Intercessory Prayer
Wikipedia says this about the Grandfather Paradox: the time traveller went back in time to the time when his grandfather had not married yet and killed him. As a result, the time traveller was never born when he was meant to be. If he was never born, then he is unable to travel through time and kill his grandfather, which means the traveller would be born after all, and so on. The same Wiki entry on the Grandfather Paradox also explains the "Novikov self-consistency principle [which] expresses one view on how backwards time travel could be possible without a danger of paradoxes. According to this hypothesis, the only possible time lines are those entirely self-consistent—so anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from happening, since this would represent an inconsistency. … Seth Lloyd and other researchers at MIT have proposed an expanded version of