Time Travel. James Gleick

too, says Zangwill. Just think what difficulties would be entailed. Our past had no Time Traveller barging through. A past that included a Time Traveller would be a different past, a new past. None of this was easy to put into words:

      Had he travelled backwards, he would have reproduced a Past which, in so far as his own appearance in it with his newly invented machine was concerned, would have been ex hypothesi unveracious.

      Then there’s the problem of meeting oneself. Zangwill becomes the first to notice this, and he will not be the last:

      Had he recurred to his own earlier life, he would have had to exist in two forms simultaneously, of varying ages—a feat which even Sir Boyle Roche would have found difficult.

      (His readers would recognize Roche as the Irish politician who said, “Mr. Speaker, it is impossible I could have been in two places at once, unless I were a bird.”)^fn1

      The book reviewers came and went, and before long philosophers got into the game. When they first took notice of time travel it was with a certain embarrassment, like a symphony conductor unable to look away from an organ grinder. “A frivolous example drawn from contemporary fiction,” said Professor Walter Pitkin at Columbia University, writing in the Journal of Philosophy in 1914. Something was stirring in science—the realm in which time was a measurable and absolute quantity known familiarly as t—and philosophers were uneasy. In the first years of the new century, when they turned to the subject of time, they had one thinker above all to contend with: the young Frenchman Henri Bergson. In the United States, William James, who might otherwise have been resting on his laurels as the “father of psychology,” found new vitality in Bergson. “Reading his works is what has made me bold,” James said in 1909. “If I had not read Bergson, I should probably still be blackening endless pages of paper privately, in the hope of making ends meet that were never meant to meet.” (He added, “I have to confess that Bergson’s originality is so profuse that many of his ideas baffle me entirely.”)

      Bergson asks us to remember how artificial is the notion of space as an empty homogenous medium—the absolute space announced by Newton. It is a creation of human intellect, he notes: “We may as well say that men have a special faculty of perceiving or conceiving a space without quality.” Scientists may find this abstract empty space to be useful for calculating, but let’s not make the mistake of confusing it with reality. Even more so with time. When we measure time with mechanical clocks, when we draw diagrams where time is an axis on a graph, we may fall into the trap of imagining time to be merely another version of space. To Bergson, time t, the time of the physicists, sliced into hours, minutes, and seconds, turned philosophy into a prison. He rejected the immutable, the absolute, the eternal. He embraced flux, process, becoming. For Bergson, the philosophical analysis of time could not be divorced from our human experience of it: the overlapping of mental states, the segue from one to the next that we experience as duration—la durée.

      He held time apart from space rather than commingling the two: “Time and space begin to interweave only when both become fictional.” He saw time, not space, as the essence of consciousness; duration, the heterogeneous tide of moments, as the key to freedom. Philosophers were about to follow physics down a new path, and Bergson would be left behind, but for now he was hugely popular. Crowds thronged his lectures at the Collège de France, Proust attended his wedding, and James called him a magician. “Dive back into the flux then,” cried James. “Turn your face toward sensation, that flesh-bound thing which rationalism has always loaded with abuse.” Here he parted company with physics.

      What really exists is not things made but things in the making. Once made, they are dead … Philosophy should seek this kind of living understanding of the movement of reality, not follow science in vainly patching together fragments of its dead results.

      Pitkin seems to have felt that he needed to rescue the poor scientists from Bergson’s onslaught. Described by Time magazine in a brief moment of fame as “a man of many ideas, some of them large,” he was a founding member of a short-lived movement that called itself new realism. In his 1914 essay he declared that he liked some of Bergson’s “conclusions” but despised his “entire method,” particularly the rejection of scientific process in favor of psychological introspection. Pitkin proposed to clear up the space-time conundrum by means of logical proof. He would embrace the physicists’ t and t′ and t′′ and yet he would prove once and for all that time is different from space. To wit: we can move hither and yon in space, but not in time. Or rather, we do move in time, but not freely: “a thing moves in time only by moving with all other things.” And how would he prove this? In a most unexpected way:

      To make the proof as simple as possible, I shall present it in the form of a sober criticism of one of the wildest flights of literary fancy which that specialist in wild flights, H. G. Wells, has indulged in. I refer, of course, to his amusing skit, The Time Machine.

      It was the first but not the last time that Mr. Wells’s amusing skit would impose itself upon the attention of this august journal.

      “You cannot leap back into the thirteenth century, nor can a man of that period hop into our own,” wrote Pitkin. “Mr. Wells would have us imagine a man at rest in the space dimensions, but moving with respect to the time of that space field. Very well! Let us do our noblest to play the game. What do we find? Something very disconcerting indeed. Something which, I fear, will make time-touring very unpopular among sedate people.”

      The traveler flies, not through abstract time (like the “pure space” of the geometer). He flies through real time. But real time is history: and history is the course of physical events. It is the sequence of activities, physical, physiological, political, and otherwise.

      Do we really want to go down this road? Must we look for errors of logic in a piece of fantastic fiction?

      Yes, we must. The practitioners of time travel, even in “pulp” magazines, were soon to work out rules and justifications that would make a Talmudist proud. What is allowed, what is possible, what is plausible—the rules evolved and varied, but logic must be honored. We may as well begin with Professor Pitkin, man of many ideas, some of them large, in the Journal of Philosophy.

      His argument would not seem very sophisticated to a typical teenage sci-fi aficionado circa 1970. In fairness, he recognizes that common human intuition about the world often fails to comprehend the strangeness of reality. Science keeps surprising us. Which way is up, for example? “It was held impossible ‘by the very nature of things,’” he notes, “that the earth should be a sphere, with people on the other side walking, heads downward.” (He might have added that Aristotle’s common sense revealed three and no more than three spatial dimensions: “the line has magnitude in one way, the plane in two ways, and the solid in three ways, and beyond these there is no other magnitude because the three are all.”) Could it be, he asks, that time travel merely strikes us as impossible “because of certain prejudices we entertain or certain facts and tricks of which we are still hopelessly ignorant?” Let us be open-minded. “[The] answer, whatever that may be, carries immeasurable consequences for metaphysics.”

      So Pitkin applies the tools of logic. These are his chief points:

      • As the time machine rushes through the years, everything ages rapidly, so the man in the machine should, too. “Nations rise and fall, tempests leap up, destroy, and subside, houses are built with toil and burned in the frenzy of sudden war, and so on.” As for the tourist, his clothes are unruffled and he scarcely ages a day. “How is this possible? If he has passed through a hundred thousand generations, why isn’t he a hundred thousand generations old?” Here is an obvious contradiction: “the first contradiction in the whole proceeding.”

      • Time goes at a certain rate, and this rate must be the same for everyone, everywhere. “Two objects or systems” cannot have “different rates of displacement or change in time”—obviously. Pitkin scarcely knew what devilishness Albert Einstein was conjuring in Berlin.

      • Traveling through time must obey rules of arithmetic, just like traveling through space. Do the math: “To traverse a million years in a few