Is time travel possible? In 1943, a science fiction writer called René Barjavel posited the following paradox.
A man travels back to the past and kills his biological grandfather before he meets his grandmother. Thus, his grandparents would not have sired a son (the man’s father) or daughter (mother), which then suggests the man could not have been conceived. If so, who killed the grandfather? As there was no one to kill the grandfather, he would have had a child and the man would ultimately be born, travelling back to the past and killing his grandfather. This paradox suggests that time travel is impossible.
Some people use the parallel universe theory to argue against the paradox. They suggest that as soon as the man travels to the past to kill his grandfather, an alternate universe is created where the grandmother meets a different man and the course of time is changed. This is a valid theory but the grandfather paradox still holds strong in disproving time travel. However, the grandfather paradox only states that travelling back in time is impossible; it says nothing about time travelling to the future.