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At the end of the first Back to the Future movie, Marty comes back ten minutes early, and sees the events from the start of the movie playing out (him running to the DeLorean and going back to 1955). Then, he's sent back home, but his family is different from the original timeline.

Remember - Marty can't be sent to the original timeline when travelling in the DeLorean with the lightning bolt because he already altered his parents' lives. He could only go to the alternate timeline (the timeline of his family being successful).

In other words, since Marty already altered his parents' lives in 1955, when he hops in the DeLorean in 1955 to go back to the 80s, he's cursed by only being able to go to a alternate timeline.

Maybe, it's a coincidence, and the alternate, better timeline and the original timeline just seem to have the exact same events play out. But, I'm curious how this is possible?

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    "The alternate, better timeline and the original timeline just seem to have the exact same events play out," is exactly it.
    – Buzz
    Dec 19, 2021 at 2:12
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    Oh wait. I just remembered. When Marty comes back to 1985 at the end of BTTF, he sees Doc was wearing a bulletproof vest. That happened because of the 1955 letter ("Don't open this until 1985.") I forgot about that. Thanks
    – Fuad Ahmed
    Dec 19, 2021 at 2:23
  • BTTF creators Bob Gale and Robert Zemeckis wrote an FAQ here that doesn't address this specific question, but in the answer to question 1.19 it does talk about the idea that the timeline has a sort of "self-preservation instinct" that causes important events (especially those that would lead to a paradox if they didn't occur) to tend to still happen in an altered timeline as long as the changes aren't too drastic.
    – Hypnosifl
    Dec 19, 2021 at 18:38

3 Answers 3

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From what we see of Marty's post-timeline change family (the one that exists because of Timeline 1's Marty changing how his parents meet), it looks none of the changes prevented Marty and Doc Brown meeting.

We never explained it in the movie. But the history of the characters that Bob Zemeckis and I created is this:

  • For years, Marty was told that Doc Brown was dangerous, a crackpot, a lunatic. So, being a red-blooded American teenage boy, age 13 or 14, he decided to find out just why this guy was so dangerous. Marty snuck into Doc’s lab, and was fascinated by all the cool stuff that was there. when Doc found him there, he was delighted to find that Marty thought he was cool and accepted him for what he was.

    Both of them were the black sheep in their respective environments. Doc gave Marty a part-time job to help with experiments, tend to the lab, tend to the dog, etc.

And that’s the origin of their relationship.

In fact, as his father is a Science Fiction author in Timeline 2, it may be that Timeline 2's Marty had a different motivation, but resulted in a similar outcome. I'm not clear if Timeline 2's Doc Brown would have recognised either Marty 'yet'1 when they first meet, but that could have 'stabilised' that event between timelines.


1 Whether the Doc Brown of timeline 2 remembers Marty appearing to him in the 50s of timeline 1 or 2 is unclear based on how history can be re-written. Did timeline 1's Doc Brown only continue with time travel research because of future-marty? Or is that only what happens in timeline 2? It's left unclear, as all time travel plots should be.

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    I think this answer is correct, and that the way time travel works in the BttF universe is that it's possible to make timeline changes, as long as they aren't paradoxical. In other words, Marty could change the timeline, as long as he didn't do so in ways that prevented his birth or prevented him from going back in time in the first place. The changes he made were subtle enough that his family still existed - their circumstances are just peripherally improved because of Marty's influence on the younger version of his father.
    – tbrookside
    Dec 19, 2021 at 15:15
  • @tbrookside Actually BTTF allows paradoxical changes too - if you are lucky to have a photo it will allow you to slowly watch yourself being destroyed. The universe allows you to exist until it is certain you won't. The same rules can be seen in Looper (which was a good film shame the director decided to destroy star wars later). In Looper a crime gang wants to kill an older version of someone so they chop off the legs of his young version and his legs disappear while running. No concern about paradox just a timeline with continous update and move on. Dec 19, 2021 at 17:59
  • @lucasbachmann do we actually know what would have happened if Marty faded from existence?
    – AncientSwordRage
    Dec 19, 2021 at 18:13
  • @AncientSwordRage I think just a very confused dance hall. BTTF2 was completely wiped out by Marty getting over "chicken" and yet Doc still is in the past and has a family so the movie's events seems left intact. I suspect the important thing is identifying when "now" is as far as BTTF or Looper is concerned. But as far as actual people disappearing the video game I think did that more. And BTTF2 future Biff disappeared off camera as I understand it - though it looked like a heart attack - so if true he still got the almanac back. Obviously we can't litigate BTTF too much in comments Dec 19, 2021 at 18:53
  • "Timeline 2's Marty had a different motivation, but resulted in a similar outcome" -AncientSwordRage. Maybe, when Timeline2's Marty hopped in the Delorean and went to 1955, he made an alternate timeline himself?
    – Fuad Ahmed
    Dec 20, 2021 at 2:23
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AncientSwordRage gave a great out-of-universe answer. Here is an in-universe one.

Because Marty had already told him the flux capacitor worked, and left a note telling him what to do (in a round-about way).

After meeting Marty in the past Doc Brown had 2 extra bits of information.

  1. The Flux capacitor for sure worked.

  2. Terrorist were going to shoot him the night Marty goes back in time.

Doc Brown now has more information about the "shot by terrorist" timeline than any other, so it's to his advantage to keep those events the same since he can prepare for it.

Refined plutonium is dangerous and hard for individuals to get, including Doc Brown. He already knows the plutonium he gets from the terrorist works, and he knows when they will attack. It's in Doc Brown's best interest to keep at least that part of the time-line intact.

He's seen as a crack-pot, so it'd be difficult to get a large university or government agency to back his work. His options for plutonium suppliers are pretty thin - better to get a bullet-proof vest than gamble on a brand-new time-line

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    This makes sense. Since, Doc knows what'll happen, everything will play out same, it's just at Lone Pine Mall and not Twin Pine Mall. It all aligns. 1955 Doc of the alternate timeline knows about the flux capacitor and the terrorists, so in 1985 same events play out, but he survives.
    – Fuad Ahmed
    Dec 20, 2021 at 2:28
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    Not only is it hard to get plutonium from a university or government agency, there’s still a clear benefit for all people in ensuring that the terrorists get rid of their plutonium.
    – Holger
    Dec 20, 2021 at 10:42
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The events at the beginning of the film took place at Twin Pines mall. The end took place at Lone Pine mall. (Note: the DeLorean killed a pine tree at the 1955 farm)

Now this is purely personal but I like to think Rich Marty went back in time and made his parents losers. But that's not part of the answer. The two versions of events are just astoundingly similar.

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    I forgot about that aswell. Lone Pine Mall could probably be an indicator that Marty is no longer in the original timeline. Thanks for pointing that out.
    – Fuad Ahmed
    Dec 19, 2021 at 2:27
  • So you think there are a pair of time loops (like a time figure-eight) that alternate between loser and successful McFly families?
    – Barmar
    Dec 19, 2021 at 17:16
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    The more amazing thing is that parents with such different personalities and life trajectories in the timelines would still give birth to the same 3 children. But this is the kind of "suspension of disbelief" that's needed for practically all multiple-timeline stories (e.g. the Star Trek mirror universe).
    – Barmar
    Dec 19, 2021 at 17:18
  • @Barmar yes that's what I described. But technically BTTF doesn't work like that so I'm not being serious. (There appears to be only one universe with continuous updates on most likely potential events and actual events.) Though some other science fiction could do that figure 8 and be internally consistent. Dec 19, 2021 at 17:25
  • I also had some detailed speculations in this answer about hypothetical "rules" for BTTF time travel that would avoid the issue of rich Marty from the lone pine timeline doing something different when he got to 1955--basically, whenever a time traveler appears at some date, their memories could get overwritten to match the "earliest" version of the timeline where they appeared at that date.
    – Hypnosifl
    Dec 19, 2021 at 21:12

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