A temporal paradox, time paradox, or time travel paradox is an apparent contradiction or logical contradiction 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.
A grandfather paradox is a potential logical problem that would arise if a person were to travel to a past time. The name comes from the idea that if a person travels to a time before their grandfather had children, and kills him, it would make their own birth impossible. So, if time travel is possible, it somehow must avoid such a contradiction.
A causal loop is a theoretical proposition in which, by means of either retrocausality or time travel, a sequence of events (actions, information, objects, people) is among the causes of another event, which is in turn among the causes of the first-mentioned event.