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.
Read more about Grandfather Paradox: Other Considerations
Famous quotes containing the words grandfather and/or paradox:
“My grandfathers, my grandmothers and my mother hardly ever spanked at all. My grandfather said that if you spanked the little ones, you made them scared and they couldn’t think. My great great-grandfathers used to use the double rope, but they never hit you; they would just barely miss you with that rope. Afterwards, they would go easy. They would take this boy or girl and talk very softly and kindly to them, and these youngsters would listen.”
—Max Hanley (20th century)
“A good aphorism is too hard for the teeth of time and is not eaten up by all the centuries, even though it serves as food for every age: hence it is the greatest paradox in literature, the imperishable in the midst of change, the nourishment which—like salt—is always prized, but which never loses its savor as salt does.”
—Friedrich Nietzsche (1844–1900)