It’s a longish post, but well worth your time.

In science fiction stories, there are a limited number of ways to explain the conundrum of how time travel can work in a world where there is both the appearance of free will and the appearance of cause and effect.

I doubt I can list all the various answers of the various imaginative authors who have attempted in an entertaining way to address the paradox. It makes for entertaining bull sessions by college students and philosophers, however.

But I can mention some basics:

In effect, the effort is to see how you can keep one or both of the appearances of cause and effect and of free will.

Wright mentions a number of methods (he calls them “options”) for dealing with the paradox:

1. Free Will Doesn’t Exist (You can go back in time, but you will do exactly what you did, because that is what happened. This is the Slaughterhouse Five answer. “The Moment is Structured That Way” say the Tralfalmadorians).
2. Cause and Effect Doesn’t Exist (You can go back in time, but in changing the past you only eliminate yourself).
3. The Universe Doesn’t Like Things Changed (You can go back in time, and change the past, but the universe will order itself so that it gets the desired result in spite of your actions. This is the Tralfalmadorians-on-Steroids).
4. Time Travel is Just a Scene-Change Device (Dr. Who, Quantum Leap, etc.)
5. Multiple Time Lines Without Consequence (the easiest solution, in which no matter how much mucking about you do, the only changes will be cosmetic, and you can always go back and re-do it, and you will discover that you in fact, already have)
6. Time is a Hard Drive Being Overwritten (By changing the past, you destroy the original timeline in which time travel was invented. So time-travel has the result of eliminating time-travel)

It’s great fun to consider, and I’ve always admired the Back the Future precisely because it makes Time Travel very difficult. You’ve got to have a Flux Capacitor and you’ve got to have 1.21 jiggiwatts of electricity and you’ve got to get that car up to 88 mph. Miss either one of these, and the time travel won’t happen.

I also especially enjoy the second movie because the plot takes it back inside the structure of the first movie, and revisits the exact same scenes without disturbing any of them, or indeed having anyone in those scenes notice that they’re being observed. It fascinates me. I’ve never seen a sequel deconstruct the first movie so entertainingly.