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time travel in back to the future December 22, 2006

Posted by d.w. in time travel.

back to the future is a pretty funny movie with a really funny premise. it’s about this kid that time travels back to the 50’s and gets a crush on his mom — “but you’re so ho… you’re so ho… you’re so thin” — and then she gets a crush on him — “isn’t he a dream?” — and then they make out and he almost isn’t born. he also finds out his dad is a total pervert and a creep, but that he likes to write science fiction. plus they pack it full of time travel jokes like “do you have no concept of time?” and don’t forget that george and lorraine name their son marty after their friend from the 50’s named marty who happens to be their time-travelling son. my favorite joke, though, is that the 1955 doc brown pays off the police officer when the cop finds out he doesn’t have a permit for his “weather experiment.” but i’m less interested in the jokes and more interested in their take on how time travel works and the effects of their choice on the structure of the universe. the basic principle in back to the future is that time is mutable. the specific details of how it changes can only be surmised from events in the movie, unless you just take doc brown’s explanation in back to the future 2 to be the way time changes. while portions of the movie support his story, there are significant problems with it in other parts of the movie. in fact, there seem to be two slightly different, but conflicting, ways that “history is going to change.”

first, i’ll summarize doc brown’s explanation. doc brown says that when marty time travels from 1985 to 1955 he creates an alternate timeline. the first timeline ceases to be exist — his brother and sister disappear completely in that picture– so when marty goes back to 1985, he actually goes back to a different version of 1985 which i’ll call 1985b. that’s why he’s so confused by everything when he arrives in 1985 — he’s in 1985b and things are different from 1985a. but it’s normal to everyone else in 1985b because they are the b-versions of themselves and they grew up in 1985b whereas he’s from 1985a. when marty sees doc brown get shot and watches himself time travel to escape the libyans, he’s actually watching the b-version of himself time travel and he takes marty-b’s place in 1985b. one could assume that marty-b time travels and changes things slightly to create a 1985c and so forth.

this version of time travel doesn’t hold up for certain portions of the movie. for instance, why would marty start to disappear while he’s playing the guitar? he’s never in danger of not existing according to doc brown’s model because only the b-version of marty is in danger of not existing. as soon as marty time travels he creates an alternate time-line, so his brother and sister should disappear out of the picture immediately. he should be the only entity to survive from 1985a. also, why is 1985b almost exactly the same as 1985a? except for the differences in his family — his mom is thin and cool, his dad is successful, biff doesn’t push anyone around, his brother and sister have better jobs and are better groomed, he has that sweet truck, his parents are cool with his trip to the lake with jennifer — 1985b is an exact copy of 1985a. his bedroom is identical — same alarm clock, same books, same bed. the same movie (orgy — american style) is playing in the theater. the same bum sleeps on the bench. marty-b is friends with doc-b. the same situation occurs at the parking lot. his parents live in the same house in the same neighborhood. i don’t get that one. if george mcfly is a successful author and they’re not as poor anymore, why do they still buy the same crummy house? and why doesn’t marty have his own guitar equipment? they buy him a truck, why can’t they buy him a guitar and an amp? and why is he still friends with doc brown? probably because his parents don’t support his musical ambitions.

all these problems suggest an alternate explanation. the alternate model of time would say that when marty travels back in time and changes things, he actually changes his own history and timeline. there would be only one timeline and marty would alter it. at the beginning of the movie marty says, “history is going to change,” and he would be exactly right. also, lorraine says, “if grandpa wouldn’t have hit him, none of you would’ve been born,” and that would really be true. then it totally makes sense for him to start disappearing because his existence actually is in danger. i still don’t get why most things would be the same in 1985 when he returns, but someone could probably convince me it’s fine with very little effort.

here’s the problem with this model: since marty changes his own history, his memories should instantly change and he should remember growing up with thin, cool parents. instead, he’s confused when he gets back to 1985.

so either marty can’t disappear while he’s playing guitar, or he can’t be confused when he gets back to 1985.  if he disappears then his memories should change over to the new ones.  if his memories don’t change he shouldn’t disappear.  both things can’t happen.  either he’s a different marty than marty-b and he never is in danger of not existing or his memories should reflect his new childhood, etc.

what about a mix between the two models? the only model i could come up with is that there is a 1985a and a 1985b but that 1985a continuously deforms into 1985b. so 1956a would turn into 1956b faster than 1985a would turn into 1985b. it would also have to hold true for spacial distances. places closer to hilldale would transform faster than europe, for instance. while this technically disagrees with doc brown’s model, an argument could be made that his is the same model but he gave the simplified version of the explanation to marty. the problem with this model is one of timing. if marty starts to disappear, it means that 1985a changes to 1985b pretty fast. but when he gets back to 1985b he’s still confused. so they would still have to do away with the disappearing part. like if doc brown told him that if they didn’t get back together marty would disappear in a week or a month or something. instead, doc brown says, “your sister will follow and if we don’t repair the damage you’ll be next.”

the time travelling problems in back to the future stem from the writers’ insistence to employ multiple contradicting models of time travel in order to make the story more exciting. while they succeed in the excitement part, i’m going to have to say that they blew their chances of making a time travel movie that wasn’t totally impossible.

back to the future time travel = unrealistic, not possible, illogical