The Bootstrap Paradox is a paradox of time travel that questions how something that is taken from the future and placed in the past could ever come into being in the first place. The bootstrap paradox is common in science fiction and takes its name from a short story by Robert Heinlein.
Imagine that a time traveller buys a copy of Hamlet from a bookstore, travels back in time to Elizabethan London, and hands the book to Shakespeare, who then copies it out and claims it as his own work. Over the centuries that follow, Hamlet is reprinted and reproduced countless times until finally a copy of it ends up back in the same original bookstore, where the time traveller finds it, buys it, and takes it back to Shakespeare. Who, then, wrote Hamlet?
A physicist working on inventing a time machine is visited by an older version of himself. The older version gives him the plans for a time machine, and the younger version uses those plans to build the time machine, eventually going back in time as the older version of himself.
Time travel, if possible, could result in some extremely strange situations.
The bootstrap paradox is the opposite of the classic grandfather paradox: Rather than going back in time and preventing oneself from going back in time, some information or object is brought back in time, becoming a “younger” version of itself, and enabling itself later to travel back in time. One then has to ask: How did that information or object come into being in the first place?