On Monday we discussed "closed loop" time travel, so today we're going to take a look at a movie series where the characters are able to use time travel to change history: Back to the Future I-III.
Back to the Future plays rather fast and loose with time-travel logic as the series goes on. The first film is built around the premise that Marty McFly travels back in time from 1985 to 1955 and accidentally prevents his parents' first meeting in high school. In doing so, he has a week to fix things and get them together before time gets screwed up beyond repair and he misses his one chance to get back to his own time. Thus, the logic is pretty simple - Marty messed things up, and now he has to put it right. Pretty simple, right?
First, let me say that Back to the Future is one of my favorite movies, but I have a feeling that if it was released today, it would be nitpicked to death, as audiences are a lot savvier about time travel logic 25 years later. For example, in the process of getting his parents back together, Marty ends up creating a situation where his father George not only stands up to school bully Biff, but actually beats him up, and he ends up encouraging his father to take a chance on his own dreams of being a writer. This evolution is a sharp contrast to the way his father was in the original timeline. The 1985 George seen at the start of the film is a complete wimp who is still being bullied by Biff, but when Marty returns to 1985 at the end of the film, George is a successful writer, his parents' marriage is stronger and now Biff cows to George.
So Marty's visit changed some major stuff. Yet amazingly all of these major changes apparently had zero effect on the nights where Marty's parents conceived him and his siblings and despite their fortunes being much better in this new timeline, they still live in the same house. If the movie was held to strict logic, this probably would seem cheesy. However, because writer Bob Gale and director Bob Zemeckis crafted the film with a fun, almost whimsical tone, no one is likely to cry foul.
This fudging works because the script's own approach to time travel has been pretty loose. It helps that when Marty prevents his parents' meeting it's a big moment. It's not a butterfly effect-like set of dominoes like "Marty taking too long to place his order at the diner results in George not getting his food until five minutes later than he was meant to, which means that he leaves the diner later, which means that when he falls out of the tree after peeping into Lorraine's window there is no car to knock him out." Marty makes a BIG mistake, and then makes a BIG fix, so that keeps the audience focused on the broad strokes rather than the tiny details.
I do have to wonder about the fact that all of Marty's memories of growing up belong to a timeline that doesn't exist. Shouldn't the people living in the house be total strangers to him, more or less? What happens when his mother says, "Remember that summer up at Uncle Todd's cabin?" and Marty has no memory of it? And what happened to the Marty that those people knew? We've seen that he leaves this timeline much in the same way that "our" Marty did, but I'm left to wonder about his fate, and his life.
Another thing I find amusing in looking back at the films is how the second film was probably one of the first movies to really play with the idea of an alternate timeline. See, while Marty and Doc are visiting 2015, old Biff steals the time machine and uses it to deliver a sports almanac to himself in 1955. Thus, his younger counterpart has the results of every sporting event until 2000 and is able to put that to use by betting on the outcomes and amassing a significant fortune. Becoming a wealthy industrialist, he ends up corrupting the town and vastly altering the 1985 that Doc and Marty know.
The interesting thing is that the movie actually stops for two or three minutes so that Doc can literally pull out a blackboard and use it to deliver a lecture to Marty (and the audience) on how time travel has resulted in this alternate 1985. If I read that in a script today, I'd probably accuse it of overexplaining things, or talking down to the audience. I assume that at the time, there hadn't been many alternate timeline stories. so the filmmakers felt it necessary. (It's a Wonderful Life is the most obvious example I can think of. Can anyone think of any other major "alternate timeline" movies pre-1989?)
I also have to wonder about why Doc and Marty don't disappear when Biff alters time - and especially why the Delorean doesn't. In the alt-1985, Doc is committed in 1982, before he builds the time machine. Thus, there should be a paradox that results from the time machine not existing to cause all these problems. Wisely, the film avoids raising this issue at all. In the first film a big deal is made about how Marty's changes will cause him to be eventually erased from existence. It's an additional ticking clock. In Part II, Marty and Doc are merely working to set things right to prevent Biff's horrible future from coming to pass.
This is why I recommend that time-travel writers be aware of these little details, but do what they can to keep the audience from thinking on that micro-level. As cool as it is to think on the butterfly effect/chaos theory level of tiny changes producing huge results, if you open that can of worms then you're inviting the audience to nitpick a lot more closely.
Pick a set of time travel rules and stick to them. Then do what you can to keep things from getting too complicated.