What Keeps Time Moving Forward?; Scientific American Time; Special Editions; by John Matson; 4 Page(s)

Physicists often describe the fabric of the universe we inhabit as four-dimensional spacetime, comprising three dimensions of space and one of time. Yet whereas we spend our days passing freely through space in any direction we wish (gravity and solid obstacles permitting), time pushes us along, willingly or not, in a single, predetermined direction: toward the future.

This is the arrow of time—life carries us from the past, through the present, and into the future. Back to the Future plotlines notwithstanding, no one knows how to reverse the arrow—how to move backward in time—and the logical paradoxes that would result from such a trip into the past render it a thorny proposition at best. (Thanks to a prediction of special relativity called time dilation, travel into the distant future is relatively easy: just move really, really fast.)