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In Back To The Future Part 2, Doc Brown says that if young Jennifer (from 1985) were to meet her older self (from 2015), it could create a paradox that would unravel the fabric of the space-time continuum and destroy the entire universe.

Back in Part 1, at the Enchantment Under The Sea dance, Marty actually begins to fade out while playing "Earth Angel" with Marvin Berry and his band. Fortunately George kisses Lorraine at that moment and reinstates Marty, as well as Dave and Linda in the photograph.

If Marty had faded out completely, would that have resulted in a universe-destroying paradox, as there would have been no Marty to pilot the DeLorean back to 1955 and interfere with George and Lorraine's first meeting? Or would the universe have found some way of preserving itself?

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    That would be scary, especially if you believe (as some people do) that there are millions of inhabited worlds out in space. Imagine millions of alien mad scientists, driving their cars up and down the timelines, trying to create paradoxes. – user14111 May 26 '17 at 8:38
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    But of course, the Jennifers do meet, and apart from them both fainting, nothing bad happens. Certainly, the universe persists. – Darren May 26 '17 at 13:06
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    The whole universe? Probably not. I believe The destruction might in fact be very localised, limited to merely our own galaxy. – user13267 May 26 '17 at 23:44
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    "Great Scott!!" – Hagen von Eitzen May 27 '17 at 12:28
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    @StefanMonov According to Wikipedia, it's both: "Larry Niven wrote a short story, 'Rotating Cylinders and the Possibility of Global Causality Violation', that borrowed its title from Tipler's paper." – nobody May 27 '17 at 19:11
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I think it's important to note that having young Jennifer meet her older self isn't said to definitely create a universe ending paradox, but that it could create a universe ending paradox. As mentioned elsewhere in the answers and comments, they do meet, momentarily, and simultaneously pass out with no other ill effects to themselves or the universe.

So, from this point I think we can apply some sort of Law of Least Effort: the Universe will only be destroyed to the minimum extent required to resolve the conflict.

When we look at the Jennifers, they could have met and had a lovely lunch together without causing any universal catastrophes. However, when you put the two of them together, they become a danger to the rest of the universe. The potential for any paradoxical conflict becomes higher because the two of them are together. Odds are most of those conflicts could be resolved without much hassle - millions of butterflies flap their wings without ever causing a hurricane, but it could happen.

What's more likely to happen during that hypothetical lunch between the Jennifers is that Old Jennifer is constantly having her history just slightly rewritten as Young Jennifer is influenced by what she sees to make small future choices differently. If Young Jennifer sees that Old Jennifer enjoys chicken salad even though she herself has never had a taste for it, perhaps she'll start to eat it earlier than she otherwise would have. Maybe this ultimately means nothing. But maybe Young Jennifer gets salmonella from improperly prepared chicken salad that she wouldn't otherwise have eaten, meet a nice doctor, break up with Marty and have an entirely different life. Would Old Jennifer fade away in the middle of the lunch? Probably. At least. It could be much more drastic. The entire future could be completely different, depending on how much relied on that particular choice.

Perhaps the entire future could fade away, with our time travellers in it. Whoops. Now we've started a cascade, because now the past point where they left from has to resolve the conflict of them missing, because the future they went to no longer exists. Maybe they'll just be gone, and that's that. Maybe not. Maybe, when everything is said and done, the whole of existence will be gone.

So, that's a rough idea of the mechanism at play in the paradox that could be created when the Jennifers meet.

It's a slightly different scenario when Marty is in the past and fading away. He's changed the future by interfering in his parents meet cute. He's fading away because he's potentially changed the future enough that it prevents his birth. This can be resolved completely by his fading away. Nothing else needs to happen to fix it - the future that he's from, for all intents and purposes, has been obliterated, but he's the only errant element missing from it, and there's a new future to take its place, the future from this new series of events.

What we don't see, but what is probably happening simultaneously, is that the DeLorean is also fading from the timeline as he is. If Doc still developed the time machine, and input the date in 1955, and managed to take Marty's place in the DeLorean (or some other helper did so instead), then what might be happening is that as Marty is fading out, the other time traveler and their version of the DeLorean could be fading in.

That could cause some cascading problems. Because now you're inserting someone else into the history that would have been doing different things that would not have been done in Marty's timeline. Probably that can be resolved by small changes to the timeline - after all, how much could be changed in a week - but maybe they can't, especially if that other person created conflicting interactions with the now missing and unobtainable Marty. I don't think you can assume that "the Universe" would keep whoever it was from going back into that time, but at the same time, maybe Doc was killed and no one went back in time in the new future. In that case, there's nothing left to fix, once Marty is gone, he's gone and the paradox has been resolved.

What I wouldn't expect is that Marty's disappearance would unwrite any of the impact he had had on the timeline - he existed in 1955 until he didn't anymore. His fading away happened at a specific point in time, if his fading away would have had a broader impact it wouldn't have just been him that faded but everything changed by his presence would have faded at the same time - notably the guitar, or as he faded, Marvin would have faded into place on stage since his hand would not be hurt. That suggests that there's no paradoxical loop by which he could cause his own undoing, and then undo that cause because he's no longer there to cause it. That's not how paradoxes work in BttF. If he faded away, the guitar would have dropped to the floor, because he had been there to hold it and then disappeared at that point in time.

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    Lots of excellent answers, but I think Jason's final paragraph clinches it for me - Marty wouldn't have just faded out, but Marvin would have been fading in with an uninjured hand, playing guitar on "Earth Angel". But he didn't. – Wallnut May 30 '17 at 8:30
  • Glad you liked it, this was a fun one. – Jason May 30 '17 at 15:04
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I think the case of Old Biff applies here.

In BttF Part II Biff from 2015 goes back to 1955 and changes the timeline. He then goes back to the future (roll credits), where he walks off. Offscreen, Old Biff actually disappears, because he has created a timeline where he dies before 2015.

This is the same paradox you've mentioned; Biff created a world where he died before going back in time, which means he shouldn't have been able to go back in time, so the universe should explode. But it doesn't explode; there's a movie and a half that proves it (we can assume the universe resolves paradoxes in time-traveler-time, since Biff travelled sixty years into the future before feeling the effects of his changes).

From this, I can think of two options: one, destroying the time traveler is enough. The universe can go from there with a single paradoxical scar, confident in the fact that the rest of time makes sense. Option two is more sinister: the universe, unsatisfied with a single life, begins to destroy more of the timeline, fading away more and more paradoxes until nothing is left.

In a later part of the above linked page, the paradox between the Jennifers is explained:

Doc Brown surmises that if a paradox were indeed to occur, the result could be cataclysm of some sort. On the other hand, since a time paradox never truly does take place in the films, it could mean that there's some sort of "self preservation" mechanism in the cosmos which prevents a paradox from ever happening. Perhaps then, this is the reason that both Jennifers faint — to prevent a potential paradox!

In the case of Old Biff, Doc and Marty go back in time to sort out the inconsistency before any 'cataclysm' happens; this could be another instance of the 'self preservation mechanism'. In the case of Marty fading away in Part I, evidence shows there'd be enough time for Doc to take Marty's place in the time machine, go back in time a few days, and prevent Marty from altering the timeline in the first place.

So, in either case, it seems likely the universe would have survived.

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    There's another possibility: Doc is actually wrong about the universe being destroyed by a paradox. The paradox in part 2 is pretty deep as in alternate 1985, Doc is put in an asylum, and Marty has been living in Biff's tower. It's extremely unlikely in that universe that they would ever have had a chance to build the time machine. They also created a paradox in going to the future to stop Marty's kid from being arrested, because once that event has been changed, they would no longer have a reason to have gone there in the first place. – Kai May 26 '17 at 17:01
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    @Kai All the paradoxes in Part 2 are resolved by the end of the film. And them traveling into the future doesn't create a paradox, because if they don't have a reason to go, they won't go, and thus they will have a reason to go, so they'll go. So they always have a reason to go to the future. I think generally, the BttF universe gives a lot of leeway for paradoxes, as long as things are set mostly right again within a few days it'll turn out all right. – DaaaahWhoosh May 26 '17 at 18:04
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It is impossible to give an in-universe answer

All talks of time paradoxes are Doc's scientific speculations, which may or may not be true. Nobody had such experiences before, and time travel theory is based on many assumptions. We can speculate as much as we want on what could happen if Marty dissolves because events that led to his birth in the past did not happen as a result of his own actions. Maybe he would cease to exist, and that's all. But then, his disappearance could cause a chain of events with unpredictable results.

Out-of-universe, a time paradox was obviously not planned by the movie creators. It is a constant danger to keep the audience hooked.

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    And Marty was named after himself, too. – Wildcard May 27 '17 at 1:48
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Probably not. It's not directly disclosed in the movie and there's no consistent framework disclosed so that you could extrapolate towards an answer, but given the events that are observed in the movies without serious consequence, it seems unlikely.

Propagation of events in BTTF is very inconsistent across the series.

1) BTTF 1- Marty fails to get his parents to fall in love and his entire existence, including the fact of his time travel, begins to disappear. The problem is that the 1955 Marty came from a timeline where his parents did get married and produce him as a child. So in this model of time travel, there's only one timeline and what happens to one Marty happens to all Martys- however, their actions remain even as they disappear. Even though Marty disappears from the entire timeline, Marty's earlier interference with his parents meeting isn't undone. Presumably Marty could have continued vanishing with no other ill consequences.

2) BTTF 2- Old Biff goes from 2015 to 1955 and causes a chain of events that result in the intervening 60 years of Biff's life being significantly different, with major effects on the surrounding world and several main characters who exist in the 2015 universe dying significantly earlier. Yet Old Biff emerges from the time machine in 2015 as if nothing had ever happened. In fact, none of the effects of his trip become apparent until after Doc and Marty go back to 1985. Marty's parents still exist, Biff isn't rich or in control of the town, and so on. In this model of time travel, events don't propagate into the future if it would ruin a good reveal in the next scene.

3) BTTF 2- Marty from BTTF2 is in 1955, observing Marty from BTTF1 while that Marty is experiencing the symptoms of his pending non-existance... but BTTF2 Marty never fades out or experiences any other symptoms of not existing. In this model of time travel, all Martys are the same Marty regardless of timeline... unless they're in different movies.

4) BTTF 2- Marty and his gf exist in the future despite having arrived there via time machine and never returned. In this model of time travel, events don't propagate through the timeline if it would prevent the plot from happening.

Anyway, it's not consistent and none of the above events seem to have resulted in any negative consequences, let alone destruction of the universe. My feeling is that unless the writers wanted to take the movie in that direction, it wouldn't happen. It might almost happen as a way of creating suspense, but I doubt it would actually happen. It's just not that serious a movie series.

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Depends on your version of time travel. I could literally code up a computer program with built-in (albeit simulated) working time travel. Would anything bad happen? No, it would just continually run the same moment in history over and over with each iteration going from paradox to no paradox. Sometimes (if you get lucky and the paradoxes don't cause so much lag that it crashes) the paradoxes somehow manage to fall into a stable time loop that resolve all of them simultaneously. Would it be cataclysmic? Maybe not. Perhaps Marty's mother or father marry someone else and go on to have a different son. That son then winds up with Doc Brown and takes the machine back in time in the same exact situation (except now it's the Russians cause he's gotta rush to the past and make sure he's born).

So, it might just be that is Marty fails, then someone new is born that takes his place and winds up stopping his parents from being married. In essence, he's setting up the universe to either be trapped in a perpetual loop or he is setting up the foundation for some kind of weird back to the future with the plot twist of actually being the reason the parents get married instead of the opposite.

Of course, nobody can ever actually prove these things. It's always speculation. I'm just applying my own interpretation of time travel. It could be that their universe isn't built to handle any temporal loops (energy can be neither created nor destroyed among other things) and just instantly fades out of existence. Or maybe all of history collapses and time itself ceases to exist. You live, die, and age all in the same exact moment while you're also being conceived and born along with everyone else who has ever lived.

Doc Brown is guessing that a cataclysm will occur but more likely he's just warning that the butterfly effect will result in cataclysmic results for Marty's future. It's really hard to tell if he is referring to a relativistic universal black hole big crunch end, or just irreparable damage to future events resulting in Marty working the car wash with Biff.

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