This is a continuation of a discussion started in the "Moons of Pluto" thread. Rather than hijack that thread, I'm starting a new one. NCC1701D brought up alternate realities, and I mentioned Doc Brown's explanation of the creation of an alternate reality in Back to the Future Part II
. Mirror responded that the film has a major problem in that regard, and I agree. In fact, all of those films suffer from some problems but, for the most part, the fake science is mostly valid.
For starters, there are typically two things that happen in Science Fiction when someone interferes with a time line, an they're either time ripples or the creation of an alternate reality. With time ripples, the effects of changing the time line ripple through like ripples on a pond and affect the time line from the point of change on forward. Lots of movies, including Timecop
and the Back to the Future
trilogy use time ripples. For those who don't like ripples, there's the creation of an alternate reality, or alternate time line. In that theory, when someone goes back in time and interferes, the time line is altered and that alteration creates a fork that continues parallel to the original time line. Both time lines still exist and they're separated by a dimensional barrier. An example of this is JJ Abrams' Star Trek
. His forked reality exists in parallel to the real Star Trek
reality. Typically, these phenomenons aren't used in the same movie or franchise (except Star Trek
) However, they're both used in Back to the Future Part II
and this creates some issues.
For starters, in the first film, when Marty goes back in time to 1955, he interferes with his parents first meeting. This puts their future and the future of him and his siblings in jeopardy, so the changes ripple forth and the picture he has of him and his siblings is altered as they begin to disappear in the order of their births, his brother, his sister and then him. Of course, in the end, he fixes things and makes them even better so that, when he returns to 1985, his parents are together and in a better place than they were in the film's beginning. This all happens in a single time line.
On to the second film. This film also uses the time ripple. When Marty goes forward to the year 2015, he fixes a mistake that his son makes that jeopardizes the future of his whole family and you see the results in the newspaper. However, while he's in 2015, Biff goes back in time to 1955 and changes the time line. However, instead of using the ripple effect, they decided to create an alternate reality. This immediately causes some issues. First, Biff creates the alternate time line, yet he's able to return to the future in the original time line. Yet, when Doc and Marty go back in time from the unaltered time line, they cross into Biff's alternate time line. For starters, why create an alternate time line then but at no other time in the trilogy? Also, how is it that the DeLorean, which is only a time machine, can apparently also cross dimensions into alternate time lines? You're talking about giving the time machine capabilities it didn't have before. In the other instances, when it travels through time, it stays in the same time line. While it's a fun film, these are interesting choices on the part of the filmmakers.
In the third film, they're back to time ripples only, no creation of alternate realities. I don't know if that's what Mirror had in mind when mentioning problems with the film, but that's what I've come up with.