Grandfather Paradox

The grandfather paradox is a proposed paradox of time travel first described (in this exact form) by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent (Future Times Three). The paradox is described as following: the time traveller went back in time to the time when his or her grandfather had not married yet. At that time, the time traveller kills his or her grandfather, and therefore, the time traveller is never born when he or she was meant to be.

Despite the name, the grandfather paradox does not exclusively regard the impossibility of one's own birth. Rather, it regards any action that makes impossible the ability to travel back in time in the first place. The paradox's namesake example is merely the most commonly thought of when one considers the whole range of possible actions. Another example would be using scientific knowledge to invent a time machine, then going back in time and (whether through murder or otherwise) impeding a scientist's work that would eventually lead to the very information that you used to invent the time machine. An equivalent paradox is known (in philosophy) as autoinfanticide, going back in time and killing oneself as a baby.

The grandfather paradox has been used to argue that backwards time travel must be impossible. However, a number of hypotheses have been postulated to avoid the paradox, such as the idea that the past is unchangeable, so the grandfather must have already survived the attempted killing (as stated earlier); or the time traveller creates - or joins - an alternate time line in which the traveller was never born.