One of the main reasons why time travel is thought to be impossible is that it could cause paradoxes. The favourite type of paradox that is used to illustrate this is the Grandfather Paradox. This can be put in the form 'What happens if I go back in time and shoot my grandfather?' The strict determinists would say that my existence is proof that I could not have done it. There are those who would say that I would disappear as soon as I did the deed (think "Back to the Future 1" here). A more esoteric possibility would be that I continue to exist but if I go back to the future I would end up in a future where I did not exist - this was the plot line underlying Poul Anderson's Time Patrol sequence. An even more esoteric possibility is that both time-lines exist; one where my grandfather was not shot, which would be shut off from me and another where my grandfather died. It's this one that Simon Hawke used for his Time Wars stories.
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