I’m sure there have been many late night, pot-induced, dorm room discussions around this topic, but here’s my take on what occurred when a certain Mr. Marty McFly hit 88mph in his magic DeLorean. First, let’s recap:
1) In 1985, Marty McFly lived in Hill Valley, CA, which has seen better days. His mother, Lorraine, has a drinking problem, and his dad is tormented by his supervisor, Biff Tannen.
2) With the help of Doc Brown, Marty travels back in time to November 5, 1955, where he accidentally gives his dad confidence, causing his parents to live happily ever after. He also allows the coming celebrity of Chuck Berry and bathrooms were never safe again.
3) Marty returns to 1985 to find that his family is as happy as can be, his dad is a successful author and that Biff is now the spineless one. Marty has a new truck and the Doc has adapted the DeLorean to fly.
But…..Marty obviously didn’t return “home”, but that doesn’t mean his old home stopped existing. They kind of touched on this in the second one, but they kept it at one offshoot from the timeline. Marty’s original trip, though, should’ve caused multiple branches to occur and here’s why:
1) Marty returned to 1985 not knowing how things have changed. He was raised by entirely different parents, and if they ceased to be, then Marty’s memories shouldn’t exist.
2) The family talks about Marty’s life before he returned from 1955. That’s another Marty with another set of memories and a different upbringing. If this second Marty never travelled back in time, maybe there’s a metaphysical merging of their memories, much in the style of Quantum Leap.
3) But when Marty arrives in 1985, he sees another Marty flee to 1955(presumably. Why throw another variable in?). So another Marty travels back in time and either a) changes what he perceives as problems in the past, or b) changes nothing, or c) changes things back to the way they were
If (a) occurs, then another version(a third) of 1985 is awaiting the second Marty when he returns. If the third Marty travels back in time to flee the Libyans, which were present in the second 1985, then this will create a 4th, 5th, 6th, etc., 1985 until a Marty stops traveling back in time. All of these realities continue to exist due to what I stated in (1)
If (b) occurs, no new 1985’s appear, and the number of Earths stop at 2. In the first 1985, Marty has gone missing and is presumed dead.
If (c) occurs, a cycle is created around the two 1985’s which support each other’s creation, and Jennifer keeps changing from Claudia Wells to Elizabeth Shue.