Ever since Einstein, physicists have regarded the universe as four- dimensional. In addition to the three physical dimensions -- length, width and height -- there exists time, which is treated mathematically as though it were equivalent to the other three. But there is one important difference: while humans can travel freely in any physical direction -- up and down, left and right, back and forth -- they can go only forward in time, never backward.
Still, there is nothing in the laws of physics that says time cannot run backward. Einstein's equations of motion work equally well, mathematically, when the direction...
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