In Back to the Future I, when Marty comes back to 1985, he should not have seen his old, original self in the Twin Pines parking lot being pursued by the terrorists because 1). His actions in 1955 had significantly altered events and 2). He was now in a new, alternate universe. However, when returning to 1985, he should have come across his new, alternate self upon arriving at home. We know this because in the original universe, Marty's family is awkward and miserable, while in the new, alternate universe, Marty's family is cool and confident. The only people who don't seem to be affected by the new alternate universe are Marty and Emmett Brown. Biff, Marty's brother, sister and parents undergo personality changes because of Marty's actions while in 1955, but one glaring question remains: where is the new, alternate Marty? The old, original Marty is now living with his new, alternate family, sans his new, alternate self.

The Twin Pines scene is a paradox

because it happened only once but

takes place in both the original

and the new, alternate universe.

Brown had known all along that he would be shot by terrorists and was wearing a bullet-proof vest in both Twin Pines scenes. In fact, the Twin Pines scene is a paradox because it happened only once, but takes place in both the original universe and the new, alternate universe. Brown's prior knowledge of the event coupled with his exact replication of the Twin Pines scene seems curious to say the least. If Brown had known of the terrorist plot, he could have staged the demonstration for a day earlier, or gone into hiding, to avoid risking his time machine to terrorist damage. Additionally, his surprise at seeing the terrorists in the first Twin Pines scene was no surprise at all, for he knew in advance that they were coming. This leads one to wonder why a grown man would knowingly bring himself into a situation where he would be shot and subject this attempted murder to the eyes of a minor person. His not revealing any of this to Marty prior to the Twin Pines scene probably stemmed from his notion that telling people about the future could have "severe consequences" on events. "A man should not know too much about his own destiny." Ha. This coming from a man who knew 30 years in advance how, when, where and why he would be shot.

Contrary to the themes exhibited in Back to the Future I, time travel does not predicate dual universes. This leads one to think: which 1955 is the true 1955 -- The one without the time traveler or the one with the time traveler? Since they happened at the same time and at the same place, how is this possible? If time can be considered a straight line, only one instance of 1955 took place, and it included the time traveler. The old, original universe (with Marty's family growing up awkward and miserable) never even existed because of Marty's intervention in 1955. I'm going to argue myself into a hole if I continue doing this, so I'll stop.