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It is an interesting conundrum which really crosses the line between between physics and philosophy as to why it is that we can travel in space, but not in time (other than in one direction, and at a fixed speed. General Relativity describes the universe as consisting of 4 dimensional spacetime - 3 dimensions of space, 1 of time - and there's nothing "different" about the time dimension; it's mathematically equivalent to the spacial ones. And yet we can easily travel through the spacial dimensions, but not at all easily through the temporal dimension. Nobody really knows why that is.
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