Ok, I might think of a better name for these. But these posts will be similar to CinemaSins or Cracked.com's lovely analysis of famous films. Today I'm reposting an email I recently sent my good friend Mike, and enhancing it with picture examples. Enjoy.
The problem? I think that the Back to the Future series doesn't obey its own time travel rules. Maybe this is old news and I'm just now getting this.
In the first movie, it's established that it's a Slow, Self-correcting Dynamic Timeline. Meaning, if Marty didn't help his parents get back together, he'd have never been born. But instead of instantly erasing him from existence when he saved his dad from getting hit by his grandfather's car (that's how his dad originally met his mom) They track this in the movie by watching his brother and sister slowly fade away in a photograph, since they're older than Marty, they would be "corrected" first. It's a slow correction, not instant, which was useful to give the main character time to fix things and thus is why I call it a Slow, Self-correcting Dynamic Timeline. Probably one of the easier time-travel theories to work with in fiction.
Anyways, in Back to the Future 2, that's supposedly not what happens at all. Suddenly everything is explained as Multiverse. Meaning, Biff goes back in time, and uses the Sports Almanac to create a tangent timeline, as explained by Doc Brown.
But. Buuut. You can't have both Multiverse and Slow, Self-correcting Dynamic Timeline at the same time. Gotta be one or the other, but I think the film tried to do both. But let's start with why it has to be one or the other.
Ok, let's say only Multiverse theory applies. How would Doc and Marty travel to alternate Hill Valley 1985? If Biff made that tangent universe, only he would experience the new changes. Heck, he probably then couldn't deliver the DeLorean back to the right future! He'd just land the DeLorean in Tangent Timeline Where Biff Rules All. So no DeLorean for Doc and Marty! Even if they somehow magically got the DeLorean back, they're using it to travel in the Good Timeline, right? Why would they suddenly appear in the Tangent Timeline Where Biff Rules All? There's no real reason for them to.
So now let's say only Slow, Self-correcting Dynamic Timeline. Biff hopped back to 1955, handed himself the Sports Almanac, causing a major impact in the timestream, a point where ripples happen. Doc explained this in the first movie, that the changes are fixed throughout time slowly like ripples in a pond. This is why Marty's older siblings are erased before him, as seen in the photograph. Anyways, Biff caused ripples. These haven't hit Hill Valley 2015 yet, which is why Old Biff can safely travel there and deliver the DeLorean so Marty and Doc can use it. But they have hit 1985 (probably while future hijinks were going down), which is why Biff Rules All there when Marty and Doc arrive. This even explains why the two of them can travel back to 1985 and 1955, because now it's all ultimately one timeline, no tangents to speak of. So then everything starts fitting: the only solution is to stop the ripples at their source, just like in the first movie, by going back to 1955 and preventing young Biff from using the Sports Almanac.
So ignore past Phil. The second movie actually obeys the original theory from the first movie, Slow, Self-correcting Dynamic Timeline. The real problem is that Doc Brown was off his rocker. He confused me. Because, in the second movie, he clearly explains Multiverse Theory, which as I just explained, does not apply.
Sad, because Doc Brown probably draws the simplest, best on-screen example of Multiverse theory that most of us learned it from.
That and the opening credits of Sliders.
But Phil, what about the third movie? Pfft. The third movie is just the same plot as the first movie, in the old west, with Doc Brown being more of a kid and Marty being more of an adult. The only time travel problem they deal with there is that they accidentally save a woman that should have died, because Doc is once again an idiot.
Heck. This whole series exists because of Doc's idiocy in inventing the time machine in the first place. Without his invention, Marty could've never accidentally screwed up time originally and we'd be fine. Gah.