Note: 1. Whenever someone travels backwards through time, they create a new timeline. Timelines will be named in the order they are created (e.g., when Marty first travels through time and arrives in 1955, he creates Timeline B. Everyone indigenous to that timeline will be surnamed B, e.g., Doc B.) 2. Each timeline will be identical to the one that preceded it, with the exception of events altered as a result of the time travelers whose arrival created the new timeline.
It’s been pointed out many times before that the Marty that gets sent back at the end of BttF Part 1 is not the same Marty we know and love. This Marty was born in Timeline B, had cool parents, etc. The popular fan theory about this is that Doc Brown made Marty B disappear. The implications are more complex than that. If Marty B did not go back to 1955, then why don’t we have the same paradox that we’d have had if Marty had never been born? This led me to understand the flux capacitor. When Marty A goes back in time and creates Timeline B, the flux capacitor stops working. When Marty prevents his parents from meeting, he starts to disappear. This is because Timeline A and Timeline B are still causally connected. The flux capacitor allows multiple Timelines to exist, without being causally connected, therefore preventing paradoxes. Part 1 is the easiest to deal with. Marty skips to the future of Timeline B. Then Doc B tells him to get in the Delorean, right now!
In 2015, Marty’s girlfriend Jennifer sees future Marty. This is impossible. Marty A disappeared in 1985 and skipped straight to 2015. So who has been living Marty A’s life for the last 30 years? Marty B. Doc B isn’t cold-hearted enough to make the guy disappear forever, he just sent him a bit ahead, which is why he needed to get Marty out of the way as soon as possible (NOTE: In regards to the Rolls Royce accident that we see at the end of Part 3 and is mentioned in this sequence, it’s plausible that the accident would have occurred to Marty B, considering the chronology of the final scenes in Part 3. This poses no problem to the theory.) While there, Biff B steals the Delorean and travels back to 1985, creating Timeline C. One of the big questions people have is “Why didn’t Biff travel to the future of the timeline he just created, i.e. Timeline C?” Not only is the flux capacitor is keeping the timelines separate, 2015 B can’t collapse the same way 1985 A did because there are non-indigenous entities in 2015 B, namely, Marty A. So, Old Biff B gets back to 2015 B, and Marty A and Doc B go to 1985, only to discover that it’s 1985 C, (their arrival changing it to Timeline D). Marty A and Doc B go back to 1955 (creating Timeline E). In 1955 E, we see another instance of Marty A and the events from Part 1 play out as they already had (justifying point 2 of my intro notes). So, all seems well, then Doc B gets sent back to 1885 (creating Timeline F). Marty A then gets a letter from Doc B, saying he’s in 1885. Marty then finds indigenous Doc Brown (i.e., Doc E). You may ask, how did Doc B send a letter from Timeline F to timeline E? The flux capacitor is no longer working! Timeline E and F are still causally linked!
Doc E helps Marty A go back in time, creating Timeline G. Wild West stuff happens. The photos change because Timelines F and G are still causally linked. The good guys win and Marty goes to 1985 G. He goes to pick up his girlfriend (leaving the indigenous Marty w/o a car and no idea where his girlfriend is) and almost gets into an accident with a Rolls Royce. She takes the fax from 2015 B out of her pocket and sees it disappear. This is significant. Marty A and Doc B left Jennifer B (and the letter) on her porch in Timeline D. How’d she get here? My conclusion is that, without the flux capacitor, entities existing outside their native timeline will phase into new timelines, so long as the new timeline does not preclude the person from existing. Since there is no longer a flux capacitor, the letter is causally connected to Timeline F (i.e. the one after Marty retrieved the Sports Almanac and events could be assumed to be otherwise similar to Timeline B). Then time traveling trains, roll credits, and, assumably, Marty A and his doppleganger.