Jim Holt writes: It was Albert Einstein who initiated the revolution in our understanding of time. In 1905, Einstein proved that time, as it had been understood by physicist and plain man alike, was a fiction. Our idea of time, Einstein realized, is abstracted from our experience with rhythmic phenomena: heartbeats, planetary rotations and revolutions, the swinging of pendulums, the ticking of clocks. Time judgments always come down to judgments of what happens at the same time — of simultaneity. “If, for instance, I say, ‘That train arrives here at seven o’clock,’ I mean something like this: ‘The pointing of the small hand of my watch to seven and the arrival of the train are simultaneous events,’” Einstein wrote. If the events in question are distant from each other, judgments of simultaneity can be made only by sending light signals back and forth. Einstein proved that whether an observer deems two events at different locations to be happening “at the same time” depends on his state of motion. Suppose, for example, that Jones is walking uptown on Fifth Avenue and Smith is walking downtown. Their relative motion results in a discrepancy of several days in what they would judge to be happening “now” in the Andromeda galaxy at the moment they pass each other on the sidewalk. For Smith, the space fleet launched to destroy life on earth is already on its way; for Jones, the Andromedan council of tyrants has not even decided whether to send the fleet.

What Einstein had shown was that there is no universal “now.” Whether two events are simultaneous is relative to the observer. And once simultaneity goes by the board, the very division of moments into “past,” “present,” and “future” becomes meaningless. Events judged to be in the past by one observer may still lie in the future of another; therefore, past and present must be equally definite, equally “real.” In place of the fleeting present, we are left with a vast frozen timescape — a four-dimensional “block universe.” Over here, you are being born; over there, you are celebrating the turn of the millennium; and over yonder, you’ve been dead for a while. Nothing is “flowing” from one event to another. As the mathematician Hermann Weyl memorably put it, “The objective world simply is; it does not happen.” [Continue reading…]