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One of my favorite time travel versions in SF is that of Connie Willis: When you try to travel back in time, if you would have changed the timeline by any noticeable amount, you either (a) just don't go--the machine doesn't work--or (b) you go, but the timeline adjusts by like dropping you in Siberia 20,000 BCE or the center of the Pacific or something. Either way, the timeline is what it is, and you can go observe, if you're very careful not to change anything. Your time travel is part of the timeline, and obviously it didn't change anything significant because here we are...