The reasons why time travel is supposed to be impossible

The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer Nathaniel Schachner in his short story Ancestral Voices and by Rene Barjavel in his 1943 book Le Voyageur Imprudent (Future Times Three).
time travel

The Mystery of John Titor

The paradox is described as follows: the time traveler goes back in time and kills his grandfather before his grandfather meets his grandmother. As a result, the time traveler is never born. But, if he was never born, then he is unable to travel through time and kill his grandfather, which means the traveler would then be born after all, and so on.