Various science fiction authors have observed that a system (universe) in which it is possible to
- Travel back in time
- Change something
- Return to the (possibly modified) "present"
is not stable. With all possible time travelers from the entire future available to meddle, eventually (in terms of iterations of the above actions, not "elapsed time") someone will make a change that causes time travel to not be invented. From that system state, no one will travel backwards in time and make a change... so the "time travel never invented" state is the
only stable state for the system!
Xenophon