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In the Back to the Future trilogy, the character Marty McFly travels back in time, and meets his own parents. In doing this, he directly changes the outcome of many things including the relationship with his parents.

When he returns "home" from 1955, many things have changed. His entire family has more money, and they are much happier people. Biff Tannen no longer bullies Marty's father George, and Biff even details the family BMW. Marty also has his dream vehicle which happens to be a Toyota pickup truck.

This raises many questions. Since Marty returns back to his original time, the audience assumes that Marty is back home. This could not be further from the truth. His actions in the past changed everything. He actually has returned to an alternate universe. Sure, everything is better for him and his family, but this is not truly where he belongs.

Does the original version of the universe still exist where Marty simply disappears, never to return, along with the DeLorean?

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    I think they kept overwriting the timeline, rather than branching it and creating new universes. The original timeline is just gone. – Molag Bal Mar 15 '16 at 15:31
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    It seems like Doc talked about changing the timeline a lot, but never mentioned alternate universes or other realities. I think someone could put together some quotes to infer Doc's view of time travel. – Molag Bal Mar 15 '16 at 15:38
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    youtube.com/watch?v=gVx4OOcIRXg – calccrypto Mar 15 '16 at 15:40
  • @calccrypto - Needs posting as an answer – Valorum Mar 15 '16 at 16:06
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    See tvtropes. We're checking if Back to the Future presents trope 2 or 7. – Kos Mar 16 '16 at 10:42
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To your question "Does the original version of the universe still exist where Marty simply disappears", I would say the answer is "no". I think I'm not really disagreeing with Axelrod's answer since that answer is focused on whether Marty could recreate his original timeline, not on whether it continues to exist in parallel with the original, and Axelrod notes that timelines "cease to exist when overwritten". But I thought it might be worth adding an additional answer which expands on the idea that BTTF features a single rewritable timeline, rather than each trip to the past creating a new timeline which exists in parallel with the timeline the traveler had just come from.

The key is to remember the part of BTTF II where old Biff in 2015 steals the DeLorean, goes back to 1955 to give his younger self the almanac, and returns to 2015 afterwards, all while Marty and Doc and Jennifer are wandering around in 2015. After old Biff gives his younger self the almanac and sets the DeLorean to take him back to 2015, he should end up in the future of the world where his younger self gets rich using the almanac--the future of the hellish "1985A" we saw in the middle of the movie. In a parallel timeline model, Marty and Doc and Jennifer would remain in the original 2015, and simply find that the DeLorean was missing. Instead, BTTF writers Bob Gale and Robert Zemeckis explain in the Official BTTF FAQ that 2015 transformed around them:

1.9: When Doc and Marty are in 1955-A, Doc says they can't return to the future to stop Biff from stealing the DeLorean, because it would be the wrong future. But if that's true, how did Old Biff manage to get back to the same future that he left? Shouldn't he have come back to a different future?

A: As should be clear from the answer to the previous question, we believe Old Biff DID indeed return to a different future — a "2015-A," which would have transformed around Marty, Doc, Jennifer and Einstein (just as Doc explains how 1985-A would change into 1985 and instantly transform around Jennifer and Einstein). This would happen AFTER Old Biff returned with the DeLorean. For this reason, we made sure that Doc had caught Jennifer and exited the McFly Townhouse before Old Biff returned. Thus, by the time Marty and Doc are carrying Jennifer back to the DeLorean, there COULD be other residents in that townhouse — or perhaps the McFlys still live there. It is just as believable that the physicality of the neighborhood did NOT change as it is to believe that it did — so we didn't change it. We decided not to make anything of this idea because this is one of those difficult time travel concepts that general audiences have a real hard time understanding. (Try explaining this stuff to your mother and you'll see what we mean.) A detailed explanation of it would have slowed down the story, and most of the audience doesn't ever think about it. That's why we made certain things ambiguous and left various things open for interpretation in hopes that the possibility of at least one or two explanations would be better than a "definitive" explanation that you could find holes in. Let's face it, time travel is fantasy, so there's no way to "prove" anything. As filmmakers, we try to create a set of rule for our stories and stick by them, and stay consistent within the little "universe" that we've created.

And as they note, the same thing applies to Jennifer, since they left her on the porch in 1985A before they realized the timeline had been altered, then once they realized what had happened they decided to go back to 1955 (followed by the accidental trip to 1885), and when they returned to the new "fixed" 1985 Jennifer was still on that porch. Doc seems to know that time travel works this way, since when they're about to go back to 1955 he has this exchange with Marty:

Marty: Doc, what about Jennifer, what about Einstein? We can't just leave them here!

Doc: Don't worry Marty, assuming we succeed in our mission this alternative 1985 will be changed back to the real 1985, instantaneously transforming around Jennifer and Einie. Jennifer and Einie will be fine, and they will have absolutely no memory of this horrible place!

Also note something similar would have happened near the end of BTTF II when Marty is in 1955 and sees Doc in the DeLorean getting struck by lightning, then is almost immediately greeted by the Western Union guy with the message from Doc in 1885. In this case, there wouldn't have been a Western Union guy driving towards Marty with a letter for him when Doc was still hovering in the sky, but then when the lightning hits and Doc goes back to 1885, the "ripple effect" would have caused the world around Marty to change, including the Western Union guy materializing on the road near him (along with Doc's gravestone materializing in the graveyard and the older DeLorean materializing in the mine, as seen at the start of part III, and probably the Clayton Ravine transforming into the Shonash Ravine as discussed in the answer to question 1.18 from the official FAQ).

So, all of this seems pretty clearly incompatible with the branching-parallel-timelines model, and seems to indicate a single changeable timeline. To really make sense of this I think you have to imagine a second time dimension to keep track of "changes" to the timeline, or "meta-time" as it's termed on this page about time travel theories. Then you could have timelines that are "earlier" in meta-time and timelines that are "later" in meta-time. And when time travelers like Marty and Doc get to experience different timelines, we can assume that timelines they experience later according to their personal subjective time (which is also the order that movie viewers following their adventures see) are also later in meta-time.

Incidentally, the chart Axelrod posted, which originally comes from this deviantart page, can be understood as a graph which charts ordinary time on the horizontal axis (increasing from left to right) and meta-time on the vertical axis (increasing from top to bottom), with lightning bolts representing cases of the timeline transforming around Marty without him using a time machine (once in 2015 and the other in 1955 with the Western Union guy). The diagram Doc draws on a blackboard in this scene from part II could be interpreted in the same way, nothing that he says in that scene indicates the two timelines are coexisting in parallel, although a lot of viewers have interpreted it that way. This meta-time stuff is a little convoluted but doesn't lead to any obvious paradoxes, and it hangs together logically well enough that someone was able to design a simulation game about temporal war using this type of model (see here and here for an explanation of how it works).

One last point about meta-time in BTTF is that perhaps we shouldn't think of the timeline at each point in meta-time extending infinitely into the future. Instead, you could imagine a little arrow on the timeline marked "The Present" at each point in meta-time, and perhaps imagine that when time travel isn't happening, this arrow continuously moves forward along the timeline as meta-time passes (at a rate of one second per meta-second, say). That way, the future could still be in flux even if there was a definite past history at each point in meta-time. This would allow Doc to be correct when he tells Marty and Jennifer at the end of part III that "your future hasn't been written yet. No one's has." It would also help make sense of Gale and Zemeckis' answer to question 1.13 in the FAQ, where they say that although the future isn't written, jumping to the future takes you to the "most likely future of the moment you left", a "logical extrapolation" of that moment.

Thinking about what happens to "The Present" when someone makes a time jump, I would imagine the little arrow jumps along with them, and then starts moving forward along the timeline again from the point it jumped to. If the jump was to the past, it will be continuously over-writing that section of the timeline as it moves, like a tape that's been rewound to a certain point and then recorded over from that point on (do kids still know how tapes worked?), though perhaps the memories of time travelers will be immune to being rewritten at certain key points on the timeline (see my answer here for my ideas about how memory-preservation could work in BTTF).

  • in other words, the story is not consistant. – BЈовић Mar 16 '16 at 9:56
  • Of course, once Doc’s car got struck by lightning, the entire past changed, so the Western Union guy was driving towards Marty even before Doc’s car got struck… A meta time doesn’t need to be an actual thing in the universe. There might be a single space-time in which changes propagate like waves. And the meta time you describe is just the order of events in the memory of the time travelers, but unfortunately that doesn’t differ from false memory, ordinary non-time-traveling people can have (some researches conclude that most of human’s memory is between slightly wrong and dead wrong). – Holger Mar 16 '16 at 10:31
  • Let's face it, time travel is fantasy, so there's no way to "prove" anything. How dare you! – mgarciaisaia Mar 16 '16 at 14:05
  • @Holger - I don't think meta-time could just be order of events in memory, as opposed to a more "objective" sequence of different timelines, since the answer to question 1.18 of the official BTTF FAQ discusses some changes to the timeline that Marty and Doc never actually took note of, like the possibility that in the 1955 at the beginning of part III the ravine would still be called the "Shonash Ravine" rather than the "Clayton Ravine" since Clara Clayton had never fallen into it in that timeline. – Hypnosifl Mar 16 '16 at 20:32
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    "do kids still know how tapes worked" my 7 years old son who is used to mp3, internet and smartphones, was fascinated by an old tape recorder and how it worked and how he could overwrite just part of the recording (he does not now audio cut software until now). I ponder introducing him to a record player (the vinyl thing). :-) – Hothie Aug 16 '16 at 14:47
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Essentially, yes.

Marty's actions in the past changed his timeline. We address them as alternate timelines, but in fact they cease to exist when overwritten.

However, Marty can still go back to his original timeline. by stopping himself from making changes to the past, he could potentially clean the timeline back to its original form. This wouldn't make him disappear, as the first movie shows us that only methods which prevent his being born can result in his disappearance; his existence as a separate personality than that of the alternate timeline Marties is preserved.

That said, there's not a lot of easy ways for him to prevent his past self from modifying the timeline; the mere presence of a DeLorean, mad scientist, and 80s kid in the 50s has too much potential to backfire, in the long run; he'd have to find a way to stop the DeLorean from correctly getting to the past in the first place, without killing his old self. However, unless he somehow managed to get his old self to go somewhere else entirely, that would still leave a version of him in his original timeline.

BTTFT

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    Warning: do not attempt to read and make sense of this answer after having been awake for 30-odd hours and just finished a 14-hour flight. It won't work, and your brain will hurt. (Oddly enough, though, I just watched the movie on the flight… I wonder if I was on a DeLoeing 747…) – Janus Bahs Jacquet Mar 15 '16 at 17:34
  • Although… come to think of it… if he had managed to stop himself from ever going to the mall that night (say, sneaked in and unplugged his phone so Doc couldn't wake him up), then presumably Doc would have been actually killed by the Libyans, and none of the time travel would subsequently have taken place—so the ‘meta-later’ version of Marty that would have been stuck in his original timeline would disappear too… right? Wouldn't making sure he'd never time-travelled be equivalent to making sure he'd never been born in this scenario? – Janus Bahs Jacquet Mar 15 '16 at 17:44
  • @JanusBahsJacquet No. The meta-later Marty wouldn't disappear because all that matters is that there is a Marty who was born. He'd only disappear if he disrupted his own existence, IE if he prevented his parents from getting together. This is already addressed above. – Slacklord the Terrible Mar 15 '16 at 19:00
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    You say that the first movie “shows us that only methods which prevent his being born can result in his disappearance”, but what are you basing that on? I don't recall any other possible ways or the classic paradoxes of time travel (such as preventing an earlier version of yourself from ever time-travelling, or indeed killing it) being addressed at all. – Janus Bahs Jacquet Mar 15 '16 at 19:27
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    Only if you believe they are completely separate time lines rather than, as in Hypno's answer, changing versions of the same time line—in which case the changes between the different realities happen only after he returns. (And we don't have any reason to believe that, even in a parallel time line, rich-Marty wouldn't also hang out with Doc and be in the same place that night, do we?) – Janus Bahs Jacquet Mar 15 '16 at 20:57
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I would agree with Axelrod - "Marty can still go back to his original timeline by stopping himself from making changes to the past." But he can't accomplish this by stopping himself from going back in time - the older Marty needs to go back to 1955 to send Marty (prime) back to 1985 immediately after he got to 1955 for the first time.

Imagine a few weeks after BTTF 3... older Marty has collected all of the destroyed pieces of the delorean and all of the technical manuals that Doc Brown has left behind... and then he builds a new time machine. He takes an extra plutonium pellet with him back to November 5, 1955 just before 6:15am and waits near the barn at the Peabody ranch for himself to appear in the field and crash into the barn.

In the couple of minutes before old man Peabody comes out with the shotgun he would have to convince Marty prime to get in the passenger seat and then drive the delorean out of the barn avoiding the pine tree and get to a remote road that still existed in 1985. They power the delorean with the extra plutonium and set Marty prime's destination time for a couple of hours after he left on October 26, 1985.

Swearing Marty prime to secrecy and telling him that there is nothing he can do to prevent Doc's death - he then gets Marty prime to travel back to the future. The letter at the Western union office waiting for Marty in 1955 is erased from existance along with any evidence that Doc was ever in 1885. Additionally, older Marty and the new time machine will instantly vanish from 1955 as he would have never been there.

Marty prime will be returned to his original timeline... however older Marty's future will undoubtedly be very different. His family was never rich, he did not travel back to 1885 and learn from the mistakes of his ancestors, and most likely he will be linked to the events that occured at the mall involving him with Doc Brown and the terrorists and be arrested. Essentially Marty prime is screwed at that point.

But to answer the question - the original timeline is restored and the only changes are that old man Peabody had to build a new barn and file a police report.

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In Back to the Future, there are no such things as parallel universes. Besides, Marty is home at the end of Part III. Although the movie doesn't state it, its only common sense to assume that something during the trilogy, the memories of growing up with a sci fi author dad has fully caught with him as if he lived there and the memories feels as real as his old ones. In other words, by the end of Part three Marty remembers not one but TWO pasts. Both sets of memories feels very real to him also. The new is now where he belongs. Goodbye loser Dad and bitchy alcoholic mom. No more seeing those parents again. Marty no longer belongs there.

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