Came here after the question was closed on Physics stack exchange.
An event requires 4 coordinates - spatial x,y,z, and time t. Multiple objects cannot occupy the same x,y,z at the same time t. Given this fact how can a subpart of the universe travel back in time? Won't it collide with something that was there at x,y,z at the moment t in the past? To explain it with a concrete example - cars pass through the intersection at different times. If they try to pass thru the intersection at the same time they collide. Pretty simple. Does this simple fact tell us that traveling backward in time is not possible? It may be that only the whole universe can rewind as that avoids the problem of collision.
And thinking about this more, this applies to traveling forward in time at the exact same coordinate x,y,z may not be possible. What are the implications of this to the twin paradox? Of course in the twin paradox, one twin has to travel away (change his/her x,y,z coordinates), and when they return they could avoid colliding by staying clear of objects at their original x,y,z.
What are the pitfalls of the about thought process?
BTW in Back to the Future when Marty goes to 1955, he must have implicitly adjusted the spatial coordinates to where the earth was and his town Hill Valley was on earth at that time of the day right?
BTW I am a science-literate person but not a physicist.