The grandfather paradox is the big one in time travel stories. You go back in time, you kill your grandfather, you’re never born. But if you never exist, how can you go back and kill your grandfather (I have, by the way, no idea why it’s grandfather rather than say your father or mother)? Similarly, once the past has been changed, why do your characters not remember it as always being true (RA Lafferty’s “Thus We Frustrate Charlemagne” has fun with people constantly trying to change the past, and never realizing they’ve already done it).
I don’t think this is actually that big a paradox. I’ve always thought the simplest solution was to assert that if you’re back in the past, you’re disconnected from the consequences of time travel. If you change your past, you’ll still exist as you were. When you return to your present, you’ll be in a world where “you” don’t exist. For example, at the end of Scooby Doo: Mystery Incorporated, the team discovers as the result of the time changes they’ve wrought, their entire lives are different, but they still remember everything the way it used to be.
Of course that still doesn’t explain how in (for example) Running Against Time, Robert Hays friends in the present remember the original past, before he went back to try and stop Oswald shooting JFK (need I say it goes horribly wrong?). As a result, they’re able to monitor Hays’ impact on the time stream by looking at old newspapers and comparing the headlines with their memories.
One possible solution is that just being aware time is changing makes it possible to remember. In STEIN’S;GATE: Burdensome State of Deja Vu, Okarin has the ability to sense the timelines changing, but Cristina suggests deja vu results from everyone having a small ability to “read Stein” as they call it.
Another paradox defuser, as I noted here, is to have some kind of tech that neutralizes the effect of time changes.
One common solution is to say that the protagonist (or villain) didn’t change anything, they just created a new branching timeline. Which makes sense, but (as noted at the link in the previous paragraph) it’s really unsatisfying in movies. Having seen Robert Loggia drunk and guilt-ridden for most of Against Time, I want to know the ending (he’s happy, healthy, and his tragedy has been erased) affects him, not that it created a new Loggia and the old one is still miserable in the original timeline.
Yet another possibility: Magic. Or God. In Repeat Performance, it’s obvious something supernatural and powerful allowed Joan Leslie her one-year do-over. Presumably anyone/anything that plays at that level won’t have trouble keeping her memory intact.
Of course, lots of time-travel movies don’t have any theory, they just go with maximum dramatic effect. But I’m impressed b the movies that do make an effort.
