"Back to the Future" (the book) and the Butterfly Effect

[Max Sinister]

You all know the geek classic BttF, I guess. (If not, try to fix that gap.) The book is lesser known, although Ryan North (of Dinosaur Comics fame) riffed it in his tumblr blog (yes, these words weren't invented yet when I was born). It has flaws (as he is pointing out), but also contains a great depiction of the Butterfly Effect, on p. 134-36.

You can read it here, but I'll quote Doc Brown here: “That’s a tiny item but it could happen. Let’s suppose the theater operator is looking at the balance sheet one day and thinking about selling. Maybe that extra fifty cents pushed the receipts from $999.75 to $1000.25. That is, it goes from three figures to four figures. That might be the psychological difference between keeping the theater or selling it. So, influenced by the extra fifty cents, he keeps it. Not long afterward, when the theater would be closed if he’d sold it, a fire starts and some people are trapped inside. One of them is a youngster who’s destined to become President of the United States – except that now he’s dead.”

“And I did it,” Marty muttered. “I killed him with my fifty cents.”

[mspence]

I think they overestimated the butterfly effect. The theory is that history can't be changed through time travel. You might be able to create a new TL but you'd still be stuck in the original. The new TL would exist as a separate alternate reality, not a continuation of the old one. So Marty wouldn't have gotten his cooler parents or new house & truck when he got back (note that there's no mention of whether Jennifer's life changed or not.)

[Max Sinister]

We'll have to be very careful which theories we assume and are talking about, or we'll get into a mess of a discussion.

If it works as you said, then it'd be the case that Marty always has been there in 1955, even if nobody would have suspected that (besides Doc Brown who's in the know). After all, everyone knows that time travel doesn't work...

OTOH, chaos theory says that with almost certainty a TL where Marty visited 1955 (even in case he didn't do as much as in the movie) would start to diverge from one where he didn't, to the point that they'd be even more different than the two TLs in the movie.