It turns out that time machines are just like every other machine; they don’t exist if they’re not invented. In 1992 Steven Hawking derived the “chronology protection conjecture“, which posits that “closed time-like curves” are impossible. Moving along a closed time-like path is like going for a walk in the woods and following a trail that returns you home yesterday. Second, closed time-like curves are impossible, but that’s not how time travel works. And third, time travelers don’t leave much evidence behind, because they couldn’t if they tried (and don’t when they do).