Another fictional example : I recall a short story where a scientist traveled back to ancient Greece, and posed as a scholar from India to impart as much modern knowledge ( mostly mathematics ) as he thought they could handle. When he returned to his own time, expecting a more advanced world, he found himself trapped in a medeviel technology world. It turned out that he’d basically crushed their spirits; when confronted with how little they actually knew, and how far they were “behind India”, they just gave mathematics and abstract knowledge in general up as not worth trying for.
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3. Time travel is impossible. (This is included purely for the sake of completeness – I’ve never read an SF story based on that assumption.)
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Well, there’s they Niven story Rotating Cylinders and the Possibility of Global Causality Violation where the laws of physics allow time travel, but the universe always stops any attempt to do so. Some sort of disaster always causes the collapse or destruction of those who attempt it.
Some other versions of time travel :
1 : Only unrecorded/unknown details can be changed. I recall a time traveller who arranged that his life in the past be recorded, so it was impossible to kill him - that would change history. Someone tried to shoot him, and the bullet slowed . . . stopped, and fell, glowing white hot as the bullet’s kinetic energy “spontaneously” turned into heat.
2 : You can change the past, but only a small region surrounding the traveller changes. You could travel back in time and shoot someone, and as soon as you travelled far enough away from the murder the change would be erased. It was mainly used as a form of sociopathic entertainment; you could run around raping and torturing and murdering people, and all evidence of your crimes would vanish when you left.
3a : Time travel is possible, but the past you reach won’t be our past. Basically, you have a universe constantly flowing “forward” from the Big Bang, with each moment being in effect a seperate world, So you travel back a thousand years, and you end up in a what amounts to an alternate universe ( but not a seperate time line; sequential, not parallel ) that’s a thousand years younger. Any changes you make won’t affect us, because the changes only travel forward at one second per second.
3b : You can change the past, but since the change only flows at the normal time rate it never reaches us. Another version of serial alternate universes, but in this one you can change “our” past, more or less.
4 : You can travel into the past or future, but nothing’s there. The present has moved on, and the past and future don’t exist.
5 : You can only travel into the future.