I was doing some thinking about the paradoxes that arise during Back to the Future 2 when Marty, Doc and Jennifer travel to 2015 to see their future selves and help Marty's kid.
It was shown that BTTF (Back to the Future) uses the Multiverse theorem to solve any paradoxes of changing the past and creating a new future timeline.
I've created an info graphic to show all the jumps made in the BTTF series and the timelines they all created.
Edit: When I say Elizabeth or Liz, I mean Jennifer, lol...
Since Marty and gang went to the future at the start of Back To the Future 2, why would Marty, Doc, and Jennifer have a life and presence there if going into the future is going into a version of the future where the time travel had existed? If Marty were to travel, the timeline he visits in the future would be a timeline where the gap from his travels would be him not existing the whole time, as if he left the universe, ceased to exist in that timeline, then started re-existing again when he arrives in the future.
It was proven that that's how the timeline works in the first BTTF when they sent Einstein 1 minute into the future. The timeline where he travelled was as if the dog hadn't existed any more for 1 minute, then existed again when he arrived at the future date.
And since any travel back into the past creates a new timeline differing from any travelled future timeline, travelling back from the future to the present would not fix the paradox, it would just make a new timeline where both the time travels would exist.
So why was Marty able to see himself at all in the future that he visited at the start of Back to the Future 2?