"Eu acho que está bem estabelecido que para a premissa básica na Trilogia Back to the Future funcionar, nós temos um sistema de linhas de tempo alternativas."
Se por "linhas de tempo alternativas" você quer dizer linhas de tempo paralelas coexistentes, para que a linha de tempo "anterior" não seja apagada quando a linha de tempo "nova" for criada, então não, Back to the Future não usa essa teoria. Em vez disso, ele usa o modelo de uma única linha do tempo que pode ser reescrita toda vez que uma mudança é feita, e essa reescrita pode fazer com que o mundo se transforme em viajantes do tempo através do efeito cascata (como o que aconteceu quando Jennifer foi deixada na varanda alterado '1985-A' e Doc tranquilizou Marty dizendo: "Não se preocupe, Marty, assumindo que teremos sucesso em nossa missão, essa alternativa de 1985 será alterada de volta para o 1985, transformando-se instantaneamente em torno de Jennifer e Einie". essa mudança ondulatória na linha do tempo também pode apagar um viajante do tempo da existência, nos casos em que ele não deveria ter nascido na linha do tempo alterada, ou não deveria ter vivido até a idade atual. Tudo isso fica razoavelmente claro no FAQ Oficial de Volta para o Futuro dos criadores Bob Gale e Robert Zemeckis, quando eles falar sobre por que o velho Biff estava originalmente planejado para começar a desaparecer quando retornou a 2015, e também por que ele retornou ao mesmo 2015 em que Marty, Doc e Jennifer estavam, em vez deles permanecerem no "original" 2015 enquanto o velho Biff retorna para um 2015-A alternativo (o futuro do infernal 1985-A criado por ele ter dado a ele mesmo o almanaque esportivo em 1955) como seria de esperar na coexistente teoria cronológica paralela:
1.8: What happened to old Biff when he staggered out of the DeLorean in 2015?
The Secrets of the Back to the Future Trilogy A: Our intention regarding old Biff was that upon his return to 2015, he would be erased from existence because he had changed his entire destiny by giving his younger self the Sports Almanac. (Probably, Lorraine shot him sometime around 1996!). After old Biff clutches his chest and staggers (the same symptoms that Marty exhibited in Back to the Future when he was beginning to be "erased"), we actually filmed him falling onto the street and vanishing, and we previewed the movie this way (see The Secrets of the Back to the Future™ Trilogy). However, the vast majority of the audience did not understand it, so we decided to cut it out, leaving the answer ambiguous, and subject to various interpretations — besides the above explanation, you can believe that Old Biff had a heart attack from the shock of time travel of from flying the car, or from something that happened to him in 1955.
1.9: When Doc and Marty are in 1955-A, Doc says they can't return to the future to stop Biff from stealing the DeLorean, because it would be the wrong future. But if that's true, how did Old Biff manage to get back to the same future that he left? Shouldn't he have come back to a different future?
A: As should be clear from the answer to the previous question, we believe Old Biff DID indeed return to a different future — a "2015-A," which would have transformed around Marty, Doc, Jennifer and Einstein (just as Doc explains how 1985-A would change into 1985 and instantly transform around Jennifer and Einstein). This would happen AFTER Old Biff returned with the DeLorean. For this reason, we made sure that Doc had caught Jennifer and exited the McFly Townhouse before Old Biff returned. Thus, by the time Marty and Doc are carrying Jennifer back to the DeLorean, there COULD be other residents in that townhouse — or perhaps the McFlys still live there. It is just as believable that the physicality of the neighborhood did NOT change as it is to believe that it did — so we didn't change it. We decided not to make anything of this idea because this is one of those difficult time travel concepts that general audiences have a real hard time understanding. (Try explaining this stuff to your mother and you'll see what we mean.) A detailed explanation of it would have slowed down the story, and most of the audience doesn't ever think about it. That's why we made certain things ambiguous and left various things open for interpretation in hopes that the possibility of at least one or two explanations would be better than a "definitive" explanation that you could find holes in. Let's face it, time travel is fantasy, so there's no way to "prove" anything. As filmmakers, we try to create a set of rule for our stories and stick by them, and stay consistent within the little "universe" that we've created.