Temporal paradox
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A temporal paradox is an impossible situation in which a time traveller interferes with the timeline involved in his own existence.
The typical example is that of the grandfather paradox, wherein a time traveller goes back in time and kills his grandfather before his father is conceived. It is a paradox because if this occurs, he will never be born, and therefore never be able to travel back in time to kill his grandfather. This example is one type of causality loop. A variation on this paradox was explored in the 1985 film "Back to the Future" in which the protagonist goes back in time and interferes with his parents' first romantic encounter, thus putting his own existence into jeopardy.
Currently, temporal paradoxes are the domain of science fiction and philosophy. If a time travel device is ever invented, various schools of thought exist as to what would happen in the grandfather paradox. Some believe that time itself would not allow such a transgression, and that the timeline would be preserved (perhaps by not allowing time travel in the first place). This is the "timeline protection hypothesis." A similar theory states that time travel may in fact be a factor in making the universe the way it is today; actions of time travelers are responsible for the present situation. It is also possible that nobody with a time machine would actually attempt to cause any of these paradoxes out of fear that it would have dangeorus results.
Another idea is that any change in the timeline, even without personal interaction, while allowable, would cause a "butterfly effect" in the timeline. All history after the time the traveller visited would be affected by minute changes the traveller had made in the past, and the world several thousand years later would be completely different from the future the traveller had left. This has been coined the "timeline corruption hypothesis."
Another hypothesis is that there are an infinite number of universes, one for each possibility. So one universe would have a live grandfather,and another universe would have a dead one. Or else, maybe the universe would annihilate itself, for such a paradox would defy its laws.
Another theory concerning the classic grandfather paradox is that such an event would CREATE a new universe, one in which the aformentioned deed was committed. This would not effect the committer's universe, nor the committer themself.
There are also other hypotheses about this paradox and time travel in general.
