"With the new Star Trek out, it’s long past time (as it were) that we laid out the rules for would-be fictional time-travellers. (Spoiler: Spock travels to the past and gets a sex change and becomes Kirk’s grandfather lover.*) Not that we expect these rules to be obeyed; the dramatic demands of a work of fiction will always trump the desire to get things scientifically accurate, and Star Trek all by itself has foisted half a dozen mutually-inconsistent theories of time travel on us. But time travel isn’t magic; it may or may not be allowed by the laws of physics — we don’t know them well enough to be sure — but we do know enough to say that if time travel were possible, certain rules would have to be obeyed. And sometimes it’s more interesting to play by the rules. So if you wanted to create a fictional world involving travel through time, here are 10+1 rules by which you should try to play.
0. There are no paradoxes.
1. Travelling into the future is easy.
2. Travelling into the past is hard — but maybe not impossible.
3. Travelling through time is like travelling through space.
4. Things that travel together, age together.
5. Black holes are not time machines.
6. If something happened, it happened.
7. There is no meta-time.
8. You can’t travel back to before the time machine was built.
9. Unless you go to a parallel universe.
10. And even then, your old universe is still there. "
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