I’ve recently dreamed up a consistent model of time travel specifically designed to allow for good science fiction.
Up until now, I’ve typically used a branching-timeline model for the sake of simplicity, but it leaves much to desire in the way of plot: each time traveler finds themselves in their own, separate timeline, and there is eventually very little point in killing Hitler because he’s still alive in the original. To remedy that, I now introduce… [drumroll….]
The Ripple Model:
The central idea is an old cliche in lousy science fiction: you change the past, and the change propagates through the timeline until it hits the present. In most cases, it leads to absurdities like Back to the Future (“Oh my gawd, I’m fading! I have only hours of vaguely-defined metatime to make sure my parents meet, thereby leading to my conception, which is apparently not already guaranteed because only I have free will in this movie!”)
However, this approach can be made rigorous. Consider the following:
You leave 2010 and appear instantly (meaning that no subjective time passes) in 1930 in Germany. You shoot Hitler and immediately leave for France, 1940. Twenty seconds later, the world changes around you: the war-torn nation is replaced by a happy pastoral scene, but you are unaffected because your own history (starting from 2010 and including the time travel) is intact. You jump to the present, and find that WWII still happened. Seventy seconds later, the encyclopedia entry in front of you disappears, but you hardly notice, because a tiny fraction of a second after that, the ripple swallows your own remaining history and you disappear. Nearby, an alternative version of you that has never heard of Hitler or WWII carries on.