Regarding the time travel possibility, that seems simple because we think of earth as a fixed point. It's not. We're in constant motion, not just in our solar system but in the universe itself. So a visitor from the future would need to do a lot more than just flip a switch and watch the years roll back. Basically it's taking the kind of difficulties we see with spatial travel from point a to point b over incredible distances and compounding it with temporal calculations.
I'm not suggesting it wouldn't be possible. What I am suggesting is that it may be no less complicated than traveling between star systems.
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