I was doing some thinking about the paradoxes that arise during Back to the Future 2 when Marty, Doc and Jennifer travel to 2015 to see their future selves and help Marty's kid.

It was shown that BTTF (Back to the Future) uses the Multiverse theorem to solve any paradoxes of changing the past and creating a new future timeline.

I've created an info graphic to show all the jumps made in the BTTF series and the timelines they all created.

BTTF timelines and lines

Since Marty and gang went to the future at the start of Back To the Future 2, why would Marty, Doc, and Jennifer have a life and presence there if going into the future is going into a version of the future where the time travel had existed? If Marty were to travel, the timeline he visits in the future would be a timeline where the gap from his travels would be him not existing the whole time, as if he left the universe, ceased to exist in that timeline, then started re-existing again when he arrives in the future.

It was proven that that's how the timeline works in the first BTTF when they sent Einstein 1 minute into the future. The timeline where he travelled was as if the dog hadn't existed any more for 1 minute, then existed again when he arrived at the future date.

And since any travel back into the past creates a new timeline differing from any travelled future timeline, travelling back from the future to the present would not fix the paradox, it would just make a new timeline where both the time travels would exist.

So why was Marty able to see himself at all in the future that he visited at the start of Back to the Future 2?

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    Sounds like an open plot hole to me. Good luck getting a canonical answer.
    – Iszi
    Commented Dec 27, 2011 at 6:16
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    Rather than new universes being created only when they time travel perhaps they had always existed. Therefore, when Doc, Marty and Elizabeth travelled forward they ended up in a timeline/universe where Marty and Elizabeth never travelled forward in time. Although this breaks when accounting for future Biff remembering the flying DeLorean. Yea, good luck on that in-universe explaination.
    – Xantec
    Commented Dec 27, 2011 at 13:07
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    @gnovice That was addressed in a deleted scene. Future Biff faded out soon after he came back to 2015, and the future simply changed around Doc and Marty.
    – thedaian
    Commented Dec 27, 2011 at 16:36
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    Wibbly-wobbly, timey-wimey.... Commented Dec 27, 2011 at 18:01
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    There's no reason for Einstein to re-exist during that minute, since the Doc never sent him back, nor did he intend to.
    – Zommuter
    Commented Jun 15, 2015 at 19:10

12 Answers 12


There is a significant delay between a jump and the effect of the changes occurring. This lag of hours, even days (notice how long it takes him to start vanishing, despite turning his mother's head from his father at the start of 1955's plotline) is sufficient to explain the discrepancies.

Effects of timeline alteration take a long time to resolve... if they'd stayed in 2015 longer, perhaps future-dad would have started fading out. More importantly, if the Delorean had been stolen or destroyed in 2015 and it appeared unlikely that Marty and Doc would be able to return to the past, perhaps at that point future-Marty would have started failing and fading just like Marty did on stage in BttF 1.

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    Another way to look at it is intentions and plans. These have been shown to affect the timeline. Since Marty planned to return to 1985, he has a future self. Einstein (and, more importantly, Doc) never planned to go back in time 1 minute to avoid a discontinuity. Commented Dec 27, 2011 at 16:30
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    It would be cool if changes to the timeline propagated at a certain 'real' time rate, so a change in 1955 might not appear for a day, while a change in 1885 might not fully develop for two or three days.
    – Steve V.
    Commented Dec 30, 2011 at 0:31
  • @MyrddinEmrys Spot on. Look at what happened when Marty's parents kissed for the first time. Since it was the 50s, marriage and babies were all but inevitable.
    – user100501
    Commented May 22, 2018 at 19:48

There's no contradiction unless Marty, Jennifer, and Doc never travelled back to 1985. If they had decided to stay in the future, then there would indeed have been a problem.

I always felt that BTTF doesn't really handle time travel very consistently, sometimes switching to multiverse theory, sometimes to only one universe with ripple effect, sometimes one universe where everything is determined.

For the explanation I give to work, history should be immutable, i.e. Marty can travel to the future, but he could not have decided to stay there. He had to return so that he could get old and become old Marty. Problem is that this view of time travel conflicts with everything else in the movies.

If we use a multiverse-type theory, then we could say that Marty arrives in a future where he did not time travel to the future, but then you end up with another problem: what determines which universe you get in? Why doesn't he time travel to any other possible universe?

The ripple effect could work, but there was never any explicit mechanism stated for the rippling. In BTTF it was very slow, since Marty had a whole week to fix his parents' situation. In BTTFII, as soon as they go back to 1985, they arrive in the completely transformed 1985. Why?

Here's a previous related topic.

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    "In BTTFII, as soon as they go back to 1985, they arrive in the completely transformed 1985. Why?" It's because Biff had gone further into the past and altered the timeline there. They travel back to the timeline Biff had already altered.
    – Kevin
    Commented Jan 21, 2012 at 21:22
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    I think you missed the part where I said that the ripple effect is handled in a very inconsistent manner. Commented Jan 22, 2012 at 10:06
  • It was certain that Biff would get rich, but not that Marty's parents wouldn't fall in love. Also, as in Sliders, Doc knows his way around universes. Commented Jul 6, 2017 at 17:08

This was answered in the official BTTF FAQ which was done by the script writers, Bob Gale and Robert Zemeckis:

1.13: When Doc takes Marty and Jennifer out of 1985 and brings them to the future, how can Old Marty and Old Jennifer (and their family) even be in the future? Wouldn't their disappearance from 1985 instantaneously erase their future?

A: To be honest, yes, it very well should erase their existence from the future. This is, in fact, the ultimate paradox of Back to the Future Part II. We really thought about this one for a long time, but we finally decided that after the set-up of Doc saying "Something's got to be done about your kids," the audience would feel cheated if we went to the future and found out they didn't exist. You could, however, argue that existence of Old Marty, Old Jennifer and their kids in the future automatically proves that young Marty and Jennifer will eventually get back to 1985. The flaw in this reasoning is that Doc repeatedly tells us that the future isn't written, so why would this part of the future be "written?" Ah, but Back to the Future Part III may contain the answer to this question after all. When Doc spots the tombstone in 1885 and sees that the name on the photograph of the tombstone has vanished but the date remains, he says "We know this photograph represents what will happen if the events of today continue to run their course into tomorrow." That's a pretty big "if." And it suggests that time travel to the future always takes you to a future based on the events of the time you left — a logical extrapolation of what the future of that moment holds. Of course, the existence of free will allows for the possibility of infinite futures, which is what Doc says at the end of Back to the Future Part III: "Your future is whatever you make it." But time travel into the future takes you to the most likely future of the moment you left.

I think this projecting of the "logical extrapolation" of the moment you left can also explain why Einstein did not encounter an older version of himself--because Doc never had any plans to send him back to live out the minute he had missed, while it was always part of the plan that Marty would go to 2015, fix the problems with his kids, and then return to 1985 to live out his life normally, so the "logical extrapolation" took that into account.

Incidentally, given this answer it's interesting to speculate about whether the 47-year-old Marty in 2015 would have remembered taking some version of a trip to 2015 when he was 17. Maybe he would remember a more logical/probable version where the freak accident of Biff overhearing about the almanac didn't happen, so his memory would be that they helped out his kids in 2015 and then returned to 1985 and never did any more time traveling.


I've always looked at it like this: If anyone travels in time, i.e. gets in the DeLorean and travels back or forward in time, then TIME itself changes around them, never affecting their memories in any way.

An example from the first BTTF: Marty changes the lives of his parents, Biff, and his siblings but recalls none of those changes when he first sees them the next morning. He only recalls the way things were, because it's still possible for him to go back and change it all again to the way it was.

So keeping all that in mind, consider this: In some theories I've read from Starlog Magazine (in the 80s) "THE OTHER MARTY MCFLY", that OUR MARTY witnesses going back in time at LONE PINE MALL (at the end of BTTF1) is said to have gone to Marty's original time-line, where Doc is dead and his parents are still losers. When he returns to 1985 he has the DeLorean destroyed, and forgets about his adventure in time. Cut to 30 years later when (our) Marty/Doc/Jen visit the "future" (THE OTHER MARTY'S time line), unaware that any of those guys were even there!

  • Yr right. I was looking at imdb too quickly lol Commented Dec 27, 2011 at 19:30
  • There is the theory of the mobius, a twist in the fabric of space where time becomes a loop. Commented Dec 27, 2011 at 23:38

In your diagram you've missed out on one vital time-line. 2015 Biff travels back to 1955, alters the past, (with the racing book) and then travels back to his ORIGINAL 2015 time-line. The time-line is only seen to change later when Marty, Doc and Jen travel back to the then alternative 1985 time-line. The delayed reaction theory could explain it, but let's face it, they just screwed up.

  • That always bugged me too - how did Biff get back to the original 2015 when the past had changed? I always just figured that by the time old Biff gets back to 2015, Doc and Marty have already stopped young Biff from keeping the almanac. But of course, they also changed the future in BTTF3 as Marty didn't get into that accident, so didn't end up a deadbeat in the future, so the old Biff's future wouldn't have existed. So I agree - plot hole!
    – Nick Shaw
    Commented Apr 14, 2012 at 8:29
  • @NickShaw: You remember how strange old Biff acted when he returned to 2015? That's because he had changed the past 1955, and he -- the result of the original timeline -- started fading. Changes aren't instant.
    – DevSolar
    Commented May 22, 2014 at 12:21
  • @DevSolar - he acted strange at that point as he whacked himself in the stomach with his cane when trying to get it out of the Delorian after he'd returned - hence why the top of his cane was in the car for Marty & Doc to find later on. But an interesting thought - perhaps he would just fade out. Doesn't tie in with why Marty/Doc end up in a different 1985 though if they changed the past in the future in BttF3... :)
    – Nick Shaw
    Commented May 22, 2014 at 12:34
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    @NickShaw: The scene of old Biff fading was actually filmed (at about one minute in). The whacking was a symptom, not a cause. And the idea is that what they changed in BttF3 was basically Doc not getting killed, without screwing major things up again (like, making the name of Clint Eastwood stand for cowardly behaviour). So it's not the Clayton Ravine. ;-)
    – DevSolar
    Commented May 22, 2014 at 13:08
  • @DevSolar - ah, cool, hadn't seen those deleted scenes before. Good find!
    – Nick Shaw
    Commented May 22, 2014 at 13:32

Well, maybe time was aware that they had intentions to go back to 1985. In the first film, when Einstein goes 1 minute into the future, he doesn't ever go back 1 minute to where he left. If time were personified, it would basically say, "Screw it. I'm making a future of them if they never left to go to 2015," A better question is where was the Doc Brown that traveled to 2015 by himself and discovered that Marty's son would get into trouble? You could say that he went to October 22, because that is the day the newspaper was published. If that were the case, how would he know when it would stop raining? Did he look at the forecast from the day before for no reason?


The movies series doesn't work as a multiverse theorem example. I don't know where you got that from, if it's written somewhere please identify the source. It works on a single timeline with a ripple effect of sorts. When Marty goes back in time in the first movie he can erase his existence.

However differences made by time travel seem to erase from memories over time even from the time travelers. This explains why Marty's father and mother never find it strange that their third son looks acts and talks exactly like their old high school friend Calvin Klein and also why in the second movie Marty has a crappy boss because why else wouldn't he have asked his old friend Doc Brown to fix that mistake he made of getting into a race. Any changes made during time travel effect the entire timeline. The further back in time a change is made the longer it takes to hit the present. If Marty had been the oldest instead of the youngest he would have dissappeared in a few days not a week. Old Biff travels back to young Biff but is back within the same day so no time distrubance was made.

Marty and Jennifer have a future in in the second movie because they were not there an adequate amount of time to disolve from the past. In fact they were gone for about a minute in regular time. So for a visual imagine one line everytime a change is made the line gets a new branch and the old line stays steady and then becomes dashed an the dotted then gone.As long as no observable change happens in the timeline then time travelers moving through it create few ripples and no change happens. Soon the dashed dotted line is erased completely and all the exist is the one line.


It was proven that that's how the timeline works in the first BTTF when they sent Einstein 1 minute into the future. The timeline where he travelled was as if the dog hadn't existed any more for 1 minute, then existed again when he arrived at the future date.

Yes, but the 2015 future they were trying to prevent was what would happen if either Marty & Doc A) failed B) did nothing C) traveled to the future and failed D) traveled to the future, did nothing and came back

As was evidenced by Marty seeing himself in the past, if Einstein from 1 minute in the future had then time traveled back to the original time-departure point, he would eventually catch up again and see himself and possibly create a huge time-paradox.

So the 2015 future we see already exists based on the eventuality of Marty & Doc - at the very least - going to 2015 and returning back to 1985. If they did not return (successful or not), then the 2015 versions of them would start disappearing from existence.


It was not a mistake at all. When future Biff went back and gave the book to his past self, he returned to a future unchanged by his selfish, reckless behavior. This is correct because the true future had been corrected before biff even got back home to 2015. That is probably why he croaked as soon as he got back..he realized what he did was all for nothing. Had he been smart about it, he would have just killed himself right after getting the book to his younger self. Flying into a volcano or landing the Delorean in Hiroshima when the bombs dropped would have been effective in ruining the car beyond any possible repair, and there would have been no chance for a "Crazy wild eyed scientist or kid" asking about the book later. The REAL question is ..Why didn't Doc Brown just give Marty a note the week before his kid was going to do whatever, or give him a note in 1985 telling him not to open until 2015? Why was it so important to rush to the future? If Marty had been made aware of what his kid was going to do a year earlier, he could have just gradually taken care of the situation normally. He could have sent Jr. overseas or something to get him away from Griff. As for Marty not getting into the crash but seeing a future loser...That's a hole in the story i guess


I guess the ripple effect is just inconsistent because if you notice, when Marty chooses to not participate in the drag race with Needles near the end of BTTF3, Jennifer takes out the YOU'RE FIRED fax and it instantly erases. If that fax was from 30 years in the future, and it takes days for a change to effect events that far in the future, why did Jennifer see it erase so immediately after the drag race or at least Marty making the choice to not drag race? I guess one could say that Jennifer having the fax in 1985 brought it closer to the event in the timeline so it happened quickly but the fax is still from 30 years in the future despite being with Jennifer in 1985 so I am not sure I buy that.

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    I think certainty speeds up the changes. When Marty started to fade, he was still trying to hook up his parents. Commented Jul 6, 2017 at 17:33

the first movie was right because the Doc got shot & Marty was chased back to 1955 now he didn't know he arrived in 1955 until he arrived in Hill Valley & looked at the newspaper which read Saturday 5th 1955 when Biff entered & shouted at Mc Fly Marty was in disbelief on seeing just how young Biff was & the boy sitting next to his was his father Marty then followed him to where his mother had lived before you it George fell out of the tree & would have been hit by Sam's car this is until Marty jumped in & rescued George by doing that he ended up getting hit instead with Sam's car this should have altered everything but it didn't that was because Marty eventually brought them back together successfully it was near end the where Marty witnessed Doc been shot & he also witnessed himself been chased back to 1955 so there are questions relating to that Marty did that Marty fade from existence in 1955 & the answer would be no because in BTTF2 when the Doc & Marty went back to 1955 for the 2nd time the Doc warned Marty not to bump into your other self reminding him that he had been there already to bring his father & mother back together that was the same Marty that Marty had just witnessed 10 min earlier when he had arrived just arrived himself

  • 2
    This is quite hard to read at the moment. Could you edit in some punctuation and paragraphs?
    – TheLethalCarrot
    Commented Jan 25, 2019 at 17:35

Some artistic license has to be granted, but I think they manage to keep it reasonably consistent, which is the main thing, considering how much back-and-forth goes on.

If you look back to the photo of Marty, Linda and Dave in the first film. After Marty interferes with his parents' meeting, it begins to fade / disappear / whatever, right up to the point where that bully is trying to take Lorraine away from George.

But this doesn't really make logical sense - since it's from the future, shouldn't it "know" the final outcome? i.e. that OK, Marty will interfere with his parents' meeting, but also that he will encourage George who will then go on to knock out Biff, save Lorraine and stand up to that final bully?

I think the presence of Marty and Jennifer in 2015 follows the same logic. At the point we actually see them, the likelihood is that the visiting 1985 them will go back to 1985 and live out their lives as normal and get married in the Chapel O' Love, so the future that's presented is the one in which that happened.

  • It doesn't make sense to think that the timeline knows how things are going to turn out in the end, and shapes the present and future accordingly. If that were the case, then whenever Marty returns to 1985, it should be the same version, i.e. the final version, whereas in practice, we're shown at least three different versions of 1985 in the films. Also, if the older Marty we saw in 2015 was the final version, then he shouldn't have had a broken hand as a result of crashing his car into a Rolls-Royce, and he shouldn't have gotten fired from his job as a result of being manipulated by Needles. Commented Mar 31, 2023 at 13:33

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