For my own convenience, and for the entertainment of any reader who wishes to revisit the issue, I here republish that part of my review of a time travel movie, where the theory and practice of time travel is discussed in detail.
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Time travel, in order to be time travel properly so called, must allow the time traveler to change the past and to visit hence foreknow the future.
The whole appeal, the whole point, of time travel is wish fulfillment. The one thing we humans cannot do, and can never do, is change the past and know the future. Such stories are speculations, or logic puzzles, about what might happen if we could.
In human experience, the past is fixed and the future is in flux. Because the future is in flux, we cannot foreknow it, not with certainty. We can guess or glimpse the future, perhaps, through human reason or divine revelation, or see shadows of what may be: but this does not fulfill the wish to escape the constraints of the human condition, and to have tomorrow’s winning lottery numbers today. One needs no fancy time-travel machine to visit a future that might be possible: one can dream up possibilities, as many as might be wished, sitting in an armchair.
Time travel, in sum, is the wish to change what cannot be changed, and to know what cannot be known.
The paradox is that if the past can be changed, then so can the present and the future, since these are determined by the past; therefore everything can be changed; whereas if the future is fixed, so must be the present and past, since these determine the future; therefore nothing can be changed.
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