The problem is that once he leaves 1985 to go to 2015, from everyone else's perspective he would have effectively disappeared. He (and Jennifer) wouldn't be around anymore to grow up and have a family, so there'd be no future Marty or Jennifer to visit. At most, Marty could visit his parents, who would have continued to exist the whole time and would be pretty shocked to see him after he went missing for 30 years.
In order for Marty to go to the future and see himself, he'd need to continue to exist in 1985 somehow. I'm not sure how to reconcile him getting into the DeLorean with Doc and yet simultaneously going back into his house to live the rest of his life normally.
A parallel universe seems like the only way for this to work, despite adding a bunch of extra complexity. It also conveniently allows old Marty to be unaware (to some extent) of young Marty's visit, because old Marty would have lived a life where he didn't take that trip.
> In order for Marty to go to the future and see himself, he'd need to continue to exist in 1985 somehow. I'm not sure how to reconcile him getting into the DeLorean with Doc and yet simultaneously going back into his house to live the rest of his life normally.
This is something I've mentally struggled with since the second film came out. His leaving 1985 temporally is an exit event; he ceases to exist in his own timeline. The only explanation I could come up with is that he does indeed visit an alternate timeline where Doc never created the time machine and therefore Marty never goes back in time to "fix" his father's resolve. There are hints about that; Old Marty is still unsure of himself and allows his boss to run all over him, his son took on his and his father's bad traits, etc. The only way this works is if Doc is unable to visit the same timeline going forward, and can only do so going backwards. The problem becomes, if every forward time travel event leads to an alternate timeline (as is necessary for existential reasons), and any changes made to his own past timeline also cause a new timeline to branch off, why is Doc so worried about fixing the past in order to fix the future? They aren't his past and future anymore, just ones he created by traveling through time in the first place.
"why is Doc so worried about fixing the past in order to fix the future? They aren't his past and future anymore, just ones he created by traveling through time in the first place."
Is it possible that an inventor brilliant enough to have built a time machine doesn't subscribe to the parallel-realities hypothesis, and is unaware of how his invention really works? We'd need to answer this question to get to why Doc Brown cares so much.
This is the most consistent explanation I've heard yet, but it doesn't seem to square with BTTF I's strong suggestion that Marty is traveling within the same universe. For instance, the fading photograph implies that whatever universe Marty visited in 1955 is causally connected to the 1985 universe in which the photograph was taken.
>He (and Jennifer) wouldn't be around anymore to grow up and have a family, so there'd be no future Marty or Jennifer to visit. At most, Marty could visit his parents, who would have continued to exist the whole time and would be pretty shocked to see him after he went missing for 30 years.
They did that all after he got back. He left and returned at the same point, so from the perspective of an observer outside his timeline he was never actually gone.
> He (and Jennifer) wouldn't be around anymore to grow up and have a family, so there'd be no future Marty or Jennifer to visit.
Why? They could travel to the future, meet their future selves, then travel back to the past, live their lives and in the future meet their time-travelling selves.
That's true. BTTF isn't normally predeterministic [1] though, but maybe the desired result could be achieved through a couple of history rewrites. For example, young Marty visits a weird future where old Marty doesn't exist, then he goes back to the original present to live normally. We can then envision a replay of young Marty's experience in the future, wherein things fade in/out around him to create a world where old Marty exists too.
[1] By predeterministic I'm thinking of something like Crichton's Timeline, where the characters visit the past not to change it but to fulfil it (i.e. there is no rewriting or branching, their existence in the past is a constant).
A good possibility, but Back To The Future's time travel doesn't require 'immediate' consistency. Could be that changes are propagating at 2†, so Marty can meet his future self and watch him fade into a future self that is experiencing his present choices.
† Because the propagation would be occurring in sec/sec, the units cancel, so it's just a number -- I jest, those aren't the same 'seconds' unit, you you see the point.
Substantially faster than 2, but somewhat inconsistently. Marty's brother started fading after only 9-10 hours. Marty doesn't start fading until nearly the week is gone. Old Biff starts fading after an hour or so? The tombstone fades immediately, and the matchbook and newspapers change within a few minutes of Marty stealing back the almanac for the last time, or possibly immediately after it's burned.
The more I consider this the less it seems like there even IS a rate of propagation, because if it was _only_ sec/sec delays then things would change instantly, not fade in and out.
Wikipedia would have you believe that "parts per million" is a dimensionless coefficient, by a similar argument. It's not true; for example, grams of gold is not the same unit as grams of seawater.
Indeed, their argument is belied by the old intro to Chemistry technique I learned -- start with what you know, and keep multiplying by 1, or things that are true expressed in fractions, like 3.28m/ft, stuff like that. That example fact is context-less (although it has dimension), but most aren't, so it's not "gram" as a unit, but "gram of gold in solution", otherwise you will get completely screwed up (particular if you don't specify "in reagent X" as part of the unit context -- wowzers the errors can jump out at you pretty quickly).
1. Marty leaves timeline to go to 2015 2. Marty meets Marty-F in the 2015 3. Marty returns to 1985 and grows up to be Marty-F.
Of course this means that he knew, but that's okay.