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time travel - Why was Marty present in his jump to 2015 in Back to the Future 2?



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?



Answer



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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