Grand Father Paradox

THE GRANDFATHER PARADOX

The grandfather paradox was a proposed by the science fiction writer René Bariavel in his book Le Voyageur Imprudent as paradox of time travel. The paradox says that the time traveler went back the time his/her grandfather had not married yet. At that time, the time traveler kills his/her grandfather, and therefore, the time traveler is never born when he/she was meant to be.

The Novikov self-consistency principle and Kip S. Thorne expresses one view on how backwards time travel could be possible without a danger of paradoxes. According to this hypothesis, the only possible time lines are those entirely self-consistent so anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from happening, since this would represent an inconsistency, for example, that if some time traveler killed the child who lived in his old address, this wouldn’t necessitate that the child was not the time traveler's younger self, nor the younger self of anyone alive in the time frame that the time traveler came from.

There could be "an ensemble of parallel universes" such that when the traveler kills the grandfather, the act took place in a parallel universe where the traveler's counterpart never exists as a result. However, his prior existence in the original universe is unaltered. Succinctly, this explanation states that: if time travel is possible, then multiple versions of the future exist in parallel universes. This theory would also apply if a person went back in time to shoot himself, because in the past he would be dead as in the future he would be alive and well.

According to the Nonexistence theory, if one were to do something in the past that would cause their nonexistence, upon returning to the future, they would find themselves in a world where the effects of (and chain reactions thereof) their actions are not present, as the person never existed. Through this theory, they would still exist, though.

Some science fiction stories suggest that any paradox would destroy the universe, or at least the parts of space and time affected by the paradox. The plots of such stories tend to revolve around preventing paradoxes.

A less destructive alternative of this theory suggests the death of the time traveler whether the history is altered or not. In this theory, killing one's grandfather would result in the disappearance of oneself, history would erase all traces of the person's existence, and the death of the grandfather would be caused by another means (say, another existing person firing the gun); thus, the paradox would never occur from a historical viewpoint.

While stating that if time travel is possible it would be impossible to violate the grandfather paradox, it goes further to state that any action taken that itself negates the time travel event cannot occur. The consequences of such an event would in some way negate that event, be it by either voiding the memory of what one is doing before doing it, by preventing the action in some way, or even by destroying the universe among other possible consequences. It states therefore that to successfully change the past one must do so incidentally.

The grandfather paradox has been used to argue that backwards time travel must be impossible. However, a number of hypotheses have been postulated to avoid the paradox, such as the idea that the past is unchangeable, so the grandfather must have already survived the attempted killing; or the time traveler creates or joins an alternate time line in which the traveler was never born.

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