In June 2016, the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, hosted a conference on the subject of “Time in Cosmology.” One of the questions debated by the attendees was the reality of time. Do past, present and future exist in any meaningful sense? Any expectation that physicists might have a comprehensive answer to that venerable question would not have survived exposure to even a single seminar. “If I bang my head against the wall,” physicist Avshalom Elitzur told journalist Dan Falk, “it’s because I hate the future … The future does not exist. It does not! Ontologically, it’s not there.” Others begged to differ.
The nature of time, in other words, remains as mysterious to modern science as it seemed to Aristotle and St. Augustine, and as wonderfully perplexing as it probably still seems to an undergraduate philosophy student after a couple of bong hits. What, then, are we to make of time travel? By all rights, the phrase ought to be meaningless—an undefinable noun embedded in an irrational metaphor. But the idea is everywhere. We all know what it means … or at least, we know roughly what to expect when a movie hero or a cartoon character climbs into a time machine.
James Gleick’s Time Travel asks how that happened: how a wholly fantastic and apparently counterintuitive concept of time became a cultural trope so ubiquitous as to be unavoidable.
It was the writer H.G. Wells, Gleick says, who first stole time from theologians and philosophers and delivered it to the literate public as a plaything. He did that in his seminal short novel The Time Machine (“one of those books you feel you must have read at some point, whether or not you actually did”), by literalizing the metaphor of time as space. Wells imagined time as a physical dimension like length or breadth, except that one cannot wilfully travel in it—unless, somehow, one could. To lend verisimilitude to this premise, Wells cited not philosophy or theology but Victorian science: “Scientific people,” Wells wrote, “know very well that Time is only a kind of Space.” This is a journey by machine, not a divine revelation or a mystic vision. The Time Traveller (the only name Wells gives his protagonist) sallies into the future with the implicit blessing of the Enlightenment.
Wells was fascinated by his era’s keystone discoveries in geology and biology. Take the long view, and biological species blur into one another in the kinetoscope of Darwinian evolution; the crust of the Earth is crushed and folded into mountains and valleys; primordial sea beds surge to Himalayan heights. The Time Machine extrapolates those processes into the future. By the year “Eight Hundred and Two Thousand odd” the human race has become two distinct species, the passive Eloi and the Morlocks who prey upon them. But not even this is a final state. Change over time is inexorable and annihilating. Eventually the narrator arrives at a future in which humanity is extinct: “All the sounds of man, the bleating of sheep, the cries of birds, the hum of insects … all that was over.”
This was a new way of thinking about time, grandiose and gorgeously melancholy, and as fantasy it was well-nigh irresistible. Science fiction, the genre Wells unwittingly helped to found, returned to the subject for the next century, working out the logical implications. If I murder my grandfather, do I still exist? If I kill a butterfly in the Jurassic era, will that act, amplified by the passage of time, render the present day unrecognizable? The genre murdered more than a few grandfathers and butterflies in its exhaustive exploration of those questions. In a sense, Gleick suggests, science fiction crowdsourced a philosophical problem, making of itself a sort of pulp-paper Wiki in which even an idea-starved commercial writer or a geeky 16-year-old fan might contribute a trope, a meme, a novel twist.
Gleick knows the genre intimately and does a fine job of delineating its century-long pas de deux with time travel. But if science fiction writers were first to the ball, others arrived soon after. A few years after Wells invented his metaphysical machine, Newton’s classic conception of time as a fixed frame of reference was overturned by Einstein’s relativity. Time became spacetime—not exactly “a kind of Space” but indivisible from it, no longer fixed but flexible, contorting to preserve the apparent absolute velocity of light for any observer in any state of motion. The physicist Hermann Minkowski built on this idea in his 1908 paper “Raum und Zeit” (“Space and Time”) to propose a relativistic “block universe,” in which there is no such thing as the present save from an observer’s point of view, and in which no such point of view is more privileged, more authentically now, than any other. Was yesterday’s breakfast less real than today’s lunch? Is today’s lunch more real than tomorrow’s dinner? According to Minkowski, no. “For us devout physicists,” Einstein said, “the division between past, present, and future is only an illusion, if a stubborn one.”
Moments of time were thus reduced to points on a graph with four axes. This was a wholly deterministic view, one that accommodated a certain philosophical fatalism but had no use for such poetic foolishness as time travel. The concept was simple, concise and easy to understand—too much so, perhaps. Gleick describes the mathematician Kurt Gödel presenting his friend Einstein with the gift of a surprising calculation on the occasion of Einstein’s 70th birthday: Einstein’s field equations allow for the possible existence of “closed timelike curves.” A closed timelike curve, Gleick explains, “loops back upon itself and thus defies ordinary notions of cause and effect: events are their own cause.” Which would seem to raise the old science-fictional question of paradoxes and grandfathers, but Gödel was untroubled. “Time travel is possible,” Gleick quotes him as saying, “but no person will ever manage to kill his past self … The a priori is greatly neglected. Logic is very powerful.” Time travellers everywhere must have breathed a sigh of relief.
Simple determinism also began to run into trouble at the subatomic level. Quantum mechanics described fundamental particles that seem to behave as if time has no preferred direction, turning ideas of cause and effect into mere parochial conceits. Time might be emergent, some suggested, a phenomenon that appears only at larger scales of reality. Even more provocatively, Hugh Everett’s many-worlds interpretation of quantum mechanics proposed that all possible futures are equally real, instantiated in alternate worlds somewhere beyond our accessible fraction of time. Once again, philosophers and science fiction writers sharpened their pencils. As of 2016 these questions remain hotly debated but essentially unresolved, stranded on the murky border between science and metaphysics.
Gleick is a gifted explainer, and he conducts the reader through these controversies with an erudition that is sometimes astonishing—one wonders, has he googled every poet who ever invoked the word time?—but never intrusive or intimidating. He leaves us at last on the shoals of the present day, where the idea of time travel is equally at home in children’s cartoons and in peer-reviewed journals of science, in novels of literary ambition and in Facebook memes, “in the pop songs, the TV commercials, the wallpaper.” Literature and film and the internet have commingled past and future so completely that the concept of purely linear time has begun to seem passé, which is to say archaic, a notion whose time has passed. “In the wired world,” Gleick writes, “creating the present becomes a communal process … Images of the past, fantasies of the future, live videocams, all shuffled and blended. All time and no time.” We are all time travellers now.
Again, or not. We would do well to remember that virtually every technological innovation has been said in its day to abolish time (electrical lighting), or to speed it up (the telegraph), or to take it captive (the motion picture camera). But time continues to dictate its own pace and to defy captivity as effectively as it defies understanding. For a contemporary reader, Gleick’s book is an admirable, erudite and deeply entertaining guide to the way we think about time. Had a copy been transported into the hands of H.G. Wells, he would no doubt have found it astonishing. Readers a century or two from now may find it merely quaint.