Grandfather paradox

The grandfather paradox is a paradox of time travel. Suppose you traveled back in time and killed your biological grandfather before he met your grandmother. Then you would never have been conceived, so you could not have traveled back in time after all. Now did you travel back or not?

This paradox has been called the "Pogo Paradox" in the Star Trek universe, named for Walt Kelly's Pogo's saying "We have met the enemy and he is us."

The grandfather paradox has been used to argue that backwards time travel must be impossible. However, other resolutions have also been advanced, usually by authors of fiction who want to write about time travel into the past while maintaining a veneer of scientific plausibility.

For instance, there could be "parallel universes" (see Everett many-worlds interpretation) and when you travel back in time and kill your grandfather, you do so in a parallel universe in which you will never be conceived as a result. However, your existence is not erased from your original universe. Alfred Bester's short story "The Men Who Murdered Mohammed" uses this premise.

Another resolution holds that, if one was to travel back in time, the laws of nature would simply forbid them from doing anything that could later result in their time travel not occurring. This theory might lead to concerns about the existence of free will (in this model, free will may be an illusion). This theory also assumes that causality must be constant: i.e. that nothing can occur in the absence of cause, whereas some theories hold that an event may remain constant even if its initial cause was subsequently eliminated. It is also possible that the time travellers intended action might be completed, but never successfully enough to result in cancellation - see Novikov self-consistency principle.