My dad and I enjoy the movie Back to the Future. As we watch it, he likes to point out continuity errors and plot holes in the story, like “Sure they know the minute lightning strikes, but they don’t know the second.” One of the things he consistently asks is “Where did the other Marty go?” at the end of the film when Marty (who was trapped in the past) goes back to the future a little earlier than when he left and observes another version of himself go back in time. So, what happened to that other Marty and why doesn’t he appear in 1985 with the first Marty? For the longest time, I haven’t been able to answer that question. Now, however, I believe I have the answer. There is no other Marty, but instead only one. Believing in the existence of another Marty that would come to join the first one hinges on a presumption of a multiverse-based time travel. Back to the Future, however, uses a form of time travel in which there is only one universe whose timeline is overwritten whenever time travel occurs.
In the story of Back to the Future, Marty gets into the DeLorean at the Twin Pines Mall in 1985 and travels back to 1955. He has his little adventure, and then travels back to 1985 and arrives 11 minutes before his departure. The car breaks down, and he runs to the mall, which due to the changes he did to past has now been called the Lone Pine Mall, and witnesses himself getting into the car and time travelling away. For clarity’s sake, I will refer to the Marty we have followed throughout the movie, who observes himself, as Marty-1 and to the observed Marty as Marty-2. My dad asks what happened to Marty-2, for presumably Marty-2 would have to return to 1985 at some point.
Confusion, I believe, arises from the assumption that Back to the Future takes place in a multiverse, and that time travel involves entering separate universes. Both Dad and I are familiar with the many-worlds interpretation of quantum physics, which is used in some time travel stories like Timeline by Michael Crichton or The Time Ships by Stephen Baxter. So, when Doc Brown in Part II draws a timeline skewing into a tangent to create an alternate 1985, the natural conclusion we make is that he is describing alternate universes. This, however, may not be the correct interpretation.
The time travel physics of Back to the Future seem to be based on there being a single universe with a time-space continuum that must be carefully looked after. When Marty inadvertently stops his parents from getting together in 1955, he starts being erased from existence. It takes action to repair the timeline to prevent a paradox. His actions in the past do affect the future, and though he seems to encourage people to do things they would do anyway (Goldie Wilson, Chuck Berry, his parents), he is not caught in a strange loop, as the 1985 he arrives in is not the one he left. The timeline is overwritten.
If Back to the Future used a multiverse, there would be no crucial need to avert paradoxes. Marty could break up his parents without being erased, as he would simply enter a separate universe where he doesn’t exist. He would be a traveler from a foreign universe, and thus be able to coexist with a universe that never had a native version of him exist. If he got his parents back together and then went into the future, it is possible he would have to deal with a Marty-2 showing up and there being two (or more) Martys in the same time. However, the only evidence for there being a multiverse is Doc’s diagram, and this may be a misinterpretation.
In Part II, 2015 Biff steals the time machine, gives his 1955 self the almanac, goes back to 2015, and runs off. Marty and Doc then retrieve their time machine without realizing anything was done with it. 2015 Hill Valley looks the same as before Biff tinkered with the timeline. Marty and Doc go to 1985 to find it turned into Hell Valley. Doc then draws his diagram. This has the appearance that Biff created an alternate universe, rather than overwriting the timeline.
This is a misconception, though. The reason why the Hill Valley 2015 Biff returns to in his present isn’t overrun with corruption is because the ripple effect hasn’t yet caught up with him. Biff, however, is groaning in pain as he runs off. Though unclear in the released film, a deleted scene shows explicitly that this is him being erased from existence as a result of his actions previously in the timeline. This seems to create a paradox in that Biff should never have acquired the almanac in the first place, but I think it is evidence of the writers making a mistake rather than evidence that Back to the Future really uses a multiverse. In any case, all that occurs in Part II can be considered an extension built onto what was originally thought up for the first film, and should not necessarily be considered as canon insofar as we are discussing the first film in its own context rather than the franchise as a whole.
As an extension to the thought experiment of the other Marty, my dad asks what would happen if Marty-1 should hypothetically get in Marty-2’s car and go back to 1955 with him. If this happens, would not the Martys quickly multiply, leading to several Martys continuously appearing in 1985? Well, no. Say Marty-1 gets in Marty-2’s car, goes to 1955 with him, and comes back together 11 minutes before Marty’s younger self (Marty-3) leaves for 1955. We won’t have any more Martys arriving. Marty-2 will just run into Marty-3’s car, they’ll leave, and we’ll be left with only one Marty. What happened to Martys 2 and 3? They become Marty-1’s past selves.
It’s a mistake to think of the Martys as separate entities. There is only one Marty McFly. It is not the case that we’re dealing with separate bodies that will always stay separate, as would happen in a multiverse, but rather person whose life along the timeline got some kinks in it. Usually a person’s life is like a straight line, but with time travel it’s like a person’s life is a loose rope that can be looped back on itself. A person’s older body can touch their younger body, but the looped rope is only one piece and it will eventually head off on its own once the loops end. That is, unless the timeline is a strange loop, but that’s not the way Back to the Future works.
In conclusion, Back to the Future time travel physics do not have the gap in reasoning that could lead to the existence of a bazillion other Martys showing up. The counterparts are not formed of newly created matter, but rather matter displaced that will eventually even out. Time travel is very confusing, but I think I have this figured out. Well, in Back to the Future, anyway.