THE GRANDFATHER PARADOX

The grandfather paradox is an example of a problem arising from the effect of time travel on causality, the idea that a cause must precede its effect. The paradox suggests that a cause is eliminated by its own effect, thus preventing its own cause and essentially becoming reverse causation

The classic analogy for this, and the one that gives the paradox its name, is a time traveler journeying back in time and killing their own biological grandfather before they can sire children. This means the time traveler could never have come to exist and, as a consequence, can't travel back in timeand thus can't kill their own grandfather. That means they then are born and can go back in time, hence the paradox. 

The grandfather paradox has been a trope of science fiction, appearing in Ray Bradbury's short story "A Sound of Thunder," the classic movie "Back to the Future" and many other works. But the grandfather paradox isn't limited to the realm of fiction. Philosophers and physicists began seriously thinking about the grandfather paradox when Albert Einstein's theories of special and general relativity suggested that time travel may be a theoretical possibility.