What is grandfather paradox?
A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or a logical contradiction that is associated with the idea of time and time travel. In physics, temporal paradoxes fall into two broad groups: consistency paradoxes exemplified by the grandfather paradox; and causal loops.
The consistency paradox or grandfather paradox occurs when the past is changed in any way, thus creating a contradiction. If one were ever to go back in time and kill one's grandfather in his childhood, it would result in the time traveler's parents, and ergo the time traveler, not being born. If the time traveler weren't born, then he never went back in time to kill his grandfather in the first place. Therefore, he lives to offspring the time traveler's parents, and therefore the time traveler. There is, thus no predicted outcome to this. Consistency paradoxes occur whenever changing the past is possible.