This entry will spoil one of the major plot elements of the new Harry Potter film, The Prisoner of Azkaban (and I assume the book too, though I haven't read it), so don't read any further if you don't want to ruin it. It might also ruin my favorite episode of Andromeda and one of the most interesting elements of Babylon 5, but you can avoid that by skipping the last part (when I address other movies and TV shows) if you just want to read what I have to say about Harry Potter and the philosophy of time involved.
The major plot device of the film is a time travel sequence that took what seemed to me to be about the last half hour. Harry and Hermione traveled back to an early point in the story and went through a number of events that had happened earlier. J.K. Rowling shows that she understands the metaphysics of time far better than most science fiction authors, since the result is a completely consistent story with all the loose ends tied up at the end. Everything Harry and Hermione do had already happened, and you can rewatch the movie and see how it all fits with everything we'd seen before. You didn't know what was really going on the first time, but nothing changed. We just saw it from a different perspective, and there were two Harrys and Hermiones during that segment of time.
The main issue here is that the past already happened and can't be changed. If you're changing it, it's not the past that you're changing. What you're doing is creating (or just going to) an alternate past of another timeline. In other words, you're simply doing what they did in Sliders but moving to an earlier time as well. That's not time travel. With real time travel, what would happen is that you would go back and fulfill what you already did (but that the earlier version of you may not have known it happened or may not have known it was you doing it, which was one of the key features of the Harry Potter story). It's already true that it happened once you begin your trip back. It's just that you haven't done it yet in your own personal timeline.
My primary premise here is that you can't change the past any more than you can change the future, but you can't change the future. You can cause things to happen in the future, but that doesn't change it. It brings it about. It's not as if the future, which hasn't happened yet, was one way, and then you did something to change it. You can also cause things to happen in the past (if time travel is possible or if God, who knows the future, responds to your prayer for something to have happened in the past that you don't yet know about and causes something to happen in the past). Causing something to happen in the past doesn't change the past. It brings it about, just as causing something in the future brings it about.
I've only seen a few other cases where this is done right. One was Terry Gilliam's (of Monty Python fame) Twelve Monkeys. Gilliam and co. played up most of the interesting aspects of the correct metaphysics of time, but saying anything else would seriously ruin the story. Babylon 5, of course, had the famous time travel story that spanned three seasons. In the second half of the third season, some characters went back to the events of a first season episode, and we saw the other side of what seemed really strange at the time. Then one major character realized he was a major figure in the history of one of the civilizations in the story from 1000 years back, and he went back in time to fulifll his destiny and become the legend he (and we) had been hearing about for years. When Andromeda was still under the helm of Robert Hewitt Wolfe before the idiots who own the show fired him and destroyed it completely, they had one absolutely brilliant time travel story in which they go back to Dylan's time and fulfill the past that Tyr had remembered from his childhood by destroying most of the Nietzschean fleet, and they had another that was also consistent, I think. There was a fun one later on that still was all wrong, and they've done some pretty dumb things with time in later seasons also. Stargate SG-1 had one episode that did it absolutely right, and then they ruined it with a followup that made no sense metaphysically speaking (but was still a great episode). I'm pretty sure both Bill and Ted movies turned out ok. Isn't it great that those are in the group that did it right?
To this date, the only Star Trek story that I know of (and I've seen most of what they've put forth) that gets the metaphysics right is Star Trek IV. Well over half of the other time travel stories (and there are many) are really fun, but Star Trek writers tend to be metaphysically incompetent when it comes to time travel. Some of their other stories have some great philosophical stuff, but not these. The X-Files only tried it once and failed miserably. The same is true of Farscape. The first four Planet of the Apes movies were fine, but the fifth, I think, led to trouble, depending on how you take the very end. The reworking with Marky Mark seemed to me to be fine until the end, which I still can't make any sense of on any theory of time. I think the first Terminator movie might be ok, but the second and third failed miserably in terms of their metaphysics.
Back to the Future is just awful, and the second and third movies are even worse (though very, very fun). Timecop is similar. At least Quantum Leap didn't contradict itself all the time or leave all sorts of things unexplained, though the main premise has problems. Depending on your interpretation of how things worked, Minority Report was either totally awful (if it involves libertarian freedom and changing the future, which is impossible) or excellent on almost everything (if it involves a fixed future that the precogs are fallible in predicting) except with one time loop with no explanation for why it should be there (a common problem in Star Trek). The latter interpretation is less common, but I prefer it.
Feel free to challenge me on any of these claims or to seek clarification on my reasons for evaluating them this way. I'm just recording my conclusions at this point.
[If you want to read the original comment thread from this post at its old location, you can read it in the archiv e of this post at the Wayback Machine.
Jeremy Pierce is a philosophy professor, Uber/Lyft driver, and father of five.