There is also no way he could have met himself in the future.
Edit: Unless the time machine was capable of visiting parallel universes.
Edit: Unless the time machine was capable of visiting parallel universes.
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.
† 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.
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.
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.
[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).
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.
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.
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.