"Yes, and imagine a world with no hypothetical situations"
-Jasper Fforde
Do you believe in free will? Then you cannot travel into the future, even though our understanding of the limits of technological progress almost guarantee that we will have a mechanism for doing so one day.
Everyone knows that going back in time is ridden with paradox: if I return to the year 1955 and kill my grandfather, then I could not have been born, then I could not have made the trip into the past. Contrary to popular belief (and media misperceptions), you cannot change the past. This is not something we would have to worry about, even if we could travel into the past.
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