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Why it is not even theoretically possible to travel in past but possible to travel in future ?

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closed as too broad by Phiteros, StephenG, Sir Cumference, J. Chomel, MBR Dec 11 '17 at 9:35

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    $\begingroup$ This question has cropped up on Physics SE before, but note that answering "why is X impossible ?" is often a problem in physics as you can really only prove (definitively) that something is possible. Mainstream theories do not make travel back in time possible because it e.g. requires conditions we believe are impossible or impossibly large energy levels to achieve. Traveling into the future is essentially how the universe is wired as far as we can tell. $\endgroup$ – StephenG Dec 10 '17 at 5:06
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The thing about time travel is it's hard to define. Technically we're constantly traveling into the future. Now, if you went at near-light speed for a long time and then stopped and came back, you would have the illusion of having time travelled into the future because time essentially went on without you. This is entirely physically possible, though not feasible of course. But past time travel, what does that mean? Does it mean rewinding all time for everything? Or one entity aging backwards? Or time goes backwards while you remain the same age, like the reverse of the future travel? In any case, unlike the future travel, there is no way, with our current model of physics, to do any of them. And I must reiterate, future travel by the means I described is only an illusion. You didn't move though time, time just went on without you. Obviously, this won't happen the other way.

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