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The century-old dream of time travel remains one of our greatest control fantasies—irrational and irresistible, supremely conducive to megalomania, born of morbid curiosity and mortal dread. Barring the odd literary anachronism (Rip Van Winkle’s big-sleep displacement, the Connecticut Yankee’s Camelot stopover), it wasn’t until 1895, when H.G. Wells unveiled The Time Machine, that the concept crystallized in the public consciousness, spanning nuts-and-bolts mechanics to abstruse metaphysics.
Wells constructed a launch pad for countless time-warp scenarios: destiny-tweaking second chances, prescience-exploiting schemes, cautionary dispatches from dystopian futures. The more enterprising fictions emphasized solipsistic absurdities. Via several carefully scheduled temporal hops (not to mention gender reassignment surgery and a disturbingly literal twist on Narcissus), the young man in Robert Heinlein’s short story “All You Zombies—” turns out to be his own father and mother—and the bartender to whom he’s confiding the whole sorry tale. Time travel went on to colonize our common dreams: in Harlequin romances, in the cathode memory banks of Star Trek and Dr. Who, and of course, at the movies, where time is lost and regained as a matter of course, and where beat-the-clock escapades have ranged from Chris Marker’s death-haunted curio, La Jetée, to Hollywood franchises by Robert Zemeckis and James Cameron.
Is theory finally catching up with fantasy? Consigned to sci-fi imaginations for much of the 20th century, time travel has emerged as a legitimate field of scientific inquiry in the last decade, bolstered by the Stephen Hawking-led pop-science boom. “In some senses, it’s still a bit wacky, but there’s definitely a respectable side,” says Paul Davies, professor of natural philosophy at Macquarie University in Sydney and author of the brazenly titled new book How to Build a Time Machine (Viking). “There’s a cottage industry of physicists who study time travel, not in the sense of patenting a design for a time machine but of trying to understand the causal structure of the universe.”
Ronald Mallett, a professor of theoretical physics at the University of Connecticut, has harbored an interest in the field since he was a young boy. “My father died of a heart attack when I was 10, and not long after that I read The Time Machine,” he says. “I thought if I could build a time machine, I could go back and warn him.” But as a physicist, Mallett didn’t even contemplate time travel research for years: “In many ways it was a conservative field, and I didn’t want to be considered a nutcase.”
There’s little danger of that now. Just as almost every sci-fi author has dusted off a time machine at some point, most big-name physicists (even the skeptical ones) have published papers in leading journals exploring the theoretical possibilities of time travel. (The worlds collide in Gregory Benford’s 1980 novel Timescape, with real-life members of the physics community scrambling to send a message into the past to avert an ecological disaster—Davies and some of his colleagues show up as characters.)
Wells himself studied biology, though he never claimed to be more than an amateur scientist. The Time Machine was conceived mainly as philosophical and social commentary, but its scientific insights are not to be underestimated. The culmination of a Victorian obsession with higher dimensions that also encompassed Edwin Abbott’s mathematical brainteaser, Flatland, Wells’s first novel hypothesized time as the fourth dimension a full decade before Einstein described a space-time continuum in his special theory of relativity. Which is not to say that Wells’s contraption is in any way a feasible archetype. His Time Traveller leaps into a quartz-lined carriage, cranks a lever, and hurtles through the time dimension while remaining fixed in the three dimensions of space—”fast-forwarding a cosmic movie,” as Davies puts it.
But in Einstein’s unified space-time, it’s impossible to move in time without also moving in space. (Cher’s linkage of chrono-manipulation—viz., “if I could turn back time”—with interstellar travel—to wit, “if I could reach the stars”—is, in at least one regard, scientifically sounder than Wells’s invention.) What’s more, the notion of an absolute landmark in space is meaningless, given the Earth’s orbit around the Sun, and the orbit of the solar system around the center of the galaxy. Paul J. Nahin, author of Time Machines, an extensive survey of time travel in science and science fiction, also notes that a Wellsian machine, which never gets out of its own way, runs the risk of colliding with itself.
Given the unyielding laws of physics, it may come as a surprise that travel into the future—or at least a restricted form thereof—is a proven fact. Einstein shattered Newton’s view of absolute time, “flowing equably without relation to anything external,” by identifying a “time dilation” effect. Time is stretched by motion, Einstein theorized, and moving objects age more slowly. On a high-speed trip, clocks tick slower: An astronaut’s wristwatch and biological clock, like his pulse rate, would slow down relative to someone who remains on Earth. The net effect is that he gains time on his journey. The higher the velocity, the greater the dilation factor. Consider an example given in Davies’s book: A round-trip flight to a star 10 light-years away at almost the speed of light (180,000 miles per second) would take just over 20 Earth years, but the astronaut would age less than three years on the trip, so upon her return, she has effectively traveled 17 years into the future.
Time dilation has been proven by placing atomic clocks aboard long-haul flights, but, Davies says, “people are often surprised, indeed disbelieving, when you tell them that simply by flying in an airplane you can jump ahead in time.” Not by much, admittedly. Our fastest spacecraft travels at a dismal 0.01 percent of the speed of light, and the resulting time warp is measured in imperceptible microseconds. J. Richard Gott, professor of astrophysics at Princeton, notes in his book Time Travel in Einstein’s Universe (Houghton Mifflin) that the most successful time traveler we know of is the cosmonaut Sergei Avdeyev, who was in orbit aboard the Mir space station for a total of 748 days. Allowing for Einstein’s principle of equivalence, which asserts that gravity slows time (i.e., a clock at a high altitude would tick slightly faster than on the surface of the Earth), Gott calculated that Avdeyev, whizzing around the planet at more than 17,000 miles per hour for more than two years, traveled into the future by approximately 1/50th of a second.
These numbers don’t bode well for aspiring century-straddlers. But even if engineering capabilities were to dramatically improve, facilitating high-speed propulsions through space and into the future, these would be strictly one-way trips: You could never return to the moment of your departure. Moving at a speed faster than light would allow travel to the past (it’s precisely what Superman does to save Lois Lane in the 1977 movie). But one of Einstein’s most famous results is that the light barrier is insurmountable—the ultimate speed limit. Physicists eventually realized that, rather than striving to attain superliminal speed, theories of backward time travel might be developed by considering the curvature of space-time (another Einstein conclusion) and contriving shortcuts where it would be possible to beat a light beam in a race.
In 1949, the Austrian mathematician Kurt Gödel observed that in a rotating universe, trajectories of light would loop in such a way that a time traveler could outpace them without having to surpass the light barrier. Available evidence suggests that the universe is not rotating, but Gödel’s model was a turning point all the same, in demonstrating that travel to the past is possible in principle. The next major landmark came, once again, from science fiction. If Wells can take credit for being the first to articulate this whole crazy idea, Carl Sagan’s 1985 novel, Contact, was the most important, albeit unintentional, catalyst of the last two decades. The book (later turned into a Jodie Foster movie) sends the heroine on a journey to the star Vega, 26 light-years away; Sagan’s first impulse was to shorten the trip by having her pass through a black hole.
His friend Caltech physicist Kip Thorne objected—with good reason. A black hole is a region of infinite density and intensely high gravity (generally thought to be left over from an inward-collapsing star), from which, by definition, nothing escapes. Thorne suggested that Sagan use a wormhole, essentially a black hole with mouths on either end, which could conceivably form a passageway between two distant swaths of space-time. He and his colleagues also investigated ways to counteract the lethal gravitational effects and make the tunnel safely traversable—the solution involved propping it open with “negative energy” or anti-gravitating matter. Contact posits a wormhole as a shortcut for space travel, but physicists soon realized that such a structure could also serve as a time machine; Thorne’s work—detailed in Black Holes and Time Warps: Einstein’s Outrageous Legacy—forms the basis for much current research. Another popular theory for reverse time travel, proposed by Gott in 1991, is predicated on cosmic strings: infinitely long strands of residual high-density matter that date from the Big Bang. Gott hypothesizes that if two strings moving in opposite directions zip by each other, a spaceship whose path traces a loop around them could arrive back where it took off before it left.
One might well wonder what tangible use any of this is to us. These are highly conjectural theories entailing extreme circumstances, and the jury is still out on whether space oddities like wormholes and cosmic strings exist in the first place. (They’re theoretically plausible, most astrophysicists agree, but there’s still no hard evidence.) Even if they do, the project of turning them into time machines, as Thorne and Gott have pointed out, would require the resources of a supercivilization. Davies’s book, in keeping with its mock-utilitarian title (which fittingly, though perhaps unwittingly, derives from pataphysics, echoing French absurdist Alfred Jarry’s 1899 Wells-inspired essay “How to Construct a Time Machine”), devotes a full chapter to the engineering of a wormhole time machine. His four-step guide for manufacturing a wormhole in a quantum vacuum and blowing it up to everyday dimensions is, to say the least, not your average science fair project.
Cynics have been known to scoff, “There’s speculation, there’s pure speculation, and there’s cosmology.” Cosmologists would respond that an elucidation of the nature of time is crucial to a complete theory of physics—the elusive “theory of everything” that reconciles Einstein’s general relativity (which explains gravity and the curvature of space-time) and quantum theory (which is focused on the subatomic world). Physicists describe the quest for this holy grail in grandly mystical terms—”reading the mind of God” is a favorite phrase. Michio Kaku, theoretical physicist at CUNY and author of Hyperspace, says the leading contender is the still incomplete solution known as superstring theory, which involves nine spatial dimensions. “String theory would settle once and for all the question of whether time travel is possible. If it is, it would allow us to make calculations about the stability of a wormhole.”
Mallett’s theory, which steers clear of stargazing, may seem the most down-to-earth option, even though its practicality is as yet inconclusive. The UConn prof proposes that a time warp could be created with a circulating laser beam: “It’s not just mass that can cause a bending of space-time. Anything that has energy can do that. So rather than deal with massive objects, where you have to worry about inertial forces, I thought, Why not look at circulating light?” Using a ring laser to produce a continuously rotating beam, Mallett calculated that at high enough intensities, space and time could be twisted in the circle within. “Of course the ideal result for an experiment would be if you turned on the circulating light and saw a little particle coming out that you hadn’t put in yet,” he adds with a laugh. “At the moment we’re just hoping to prove the twisting of space.” The Mallett machine might resemble “a long light cylinder”—a feature that the lifelong Trekkie says will no doubt please sci-fi aficionados. Davies is skeptical but intrigued: “There are gravitational effects associated with the setup he describes, but whether they achieve a twisting of time, I wouldn’t like to say. Stranger things have happened.”
The science of time travel has shed its air of outer-limits disrepute for the most part (an institution called the Time Travel Research Center, which claims to have a lab in Long Island, did not respond to requests for an interview), but there remains a hint of sheepishness within academia. Some physicists don’t talk of time machines, preferring the technical term “closed time-like curves.” Science fiction may have popularized time travel, but in providing abundant logic-shredding inconsistencies, it also bred deep skepticism. Wells himself was certainly mindful of the concept’s fundamental counter-intuitiveness. When his protagonist returns from A.D. 802,701, grimy and tousled, a stunned dinner guest exclaims, “A man couldn’t cover himself with dust by rolling in a paradox, could he?” The act of historical interference that proves potentially self-nullifying has become a pop-culture commonplace—from Back to the Future, where Marty McFly finds himself slowly disappearing after his teenage mother takes a sexual interest in him, to the Halloween episode of The Simpsons where Homer is transported to the prehistoric era via defective toaster and screws up evolution by sitting on a fish.
This oxymoronic vicious cycle is often dubbed the grandfather paradox. As in, if you go back in time to kill your grandfather, you wouldn’t have been born, and wouldn’t have been able to go back in time, and your grandfather would still be alive . . . Two diametrically opposed theories are commonly invoked as solutions. The first, the principle of self-consistency, argues that the time traveler was always part of the past. He fails to change it because he did fail; any attempt to do so is futile. The many-worlds theory permits free will, but its implications are no less disconcerting. Some scientists (notably Oxford physicist David Deutsch) propose a model in which a paradox bifurcates reality into the relevant alternate scenarios. Outlandish as it sounds, this hypothesis stems from the microworld of quantum mechanics (where, per Heisenberg, the behavior of subatomic particles is known to be undeterministic). All possible permutations of world history exist, alongside each other. Kaku says the latter conjecture is the dominant one among cosmologists: “When the universe was born, it was smaller than an electron, which is a quantum object that can exist simultaneously in many states. So the universe must also be a quantum object and exist in many states. Big bangs are happening all the time, even as we speak.”
Time travel is so rife with paradox (Gott’s book even uses Escher drawings to illustrate several concepts) that some physicists and philosophers argue these self-contradictions automatically rule it out. Nahin’s Time Machines offers several examples of apocalyptic sci-fi where a paradox represents the ultimate violation of nature and results in the universe being snuffed out. (He cites a “cosmic disgust theory” from John Varley’s novel Millennium, in which an attempted intervention prompts the wrathful creator to pen a disapproving note: “If you’re going to play games like that, I’ll take my marbles and go home. Signed, God.”) Instead of inspiring reverent fear before an almighty force, though, time-travel narratives increasingly bespeak a laissez-faire wish-fulfillment—witness their Reagan-era popularity. Hollywood chrono-trip arrogance reached its apex of entitlement in 2000’s Frequency: The hero, sending messages into the past by radio waves, doesn’t stop tampering until everything is exactly as he pleases.
Beyond the obvious logical dilemmas, the possibility of time travel uncorks distressing quandaries that range from the ethical to the theological. “Even scientists who acknowledge that you can’t rule out travel to the past would nevertheless like there to be a sort of meta-rule that would prevent it,” says Davies. “But we know of no such principle.” Stephen Hawking has proposed a “chronology protection conjecture”—an as-yet-unknown law of physics that would preserve causality and safeguard history from meddlers. (Jean-Claude Van Damme had another name for it: Timecop.)
Dissenters often point to the absence of time travelers in our midst. Why don’t we see tourists from the future descending on historic events? A sci-fi buff might point out that any self-respecting temponaut would (duh) show up in disguise. An astrophysicist would simply respond that no time machines have been built yet, and most of the blueprints we have for them stipulate that you can’t travel back to an era before the device was first switched on—i.e., before the “closed time-like curve” was formed. If a time machine were built tomorrow, we might suddenly find ourselves surrounded by visitors from the future. But any moment in the past, up until today, would be forever off-limits.
Some time-travel phenomena are internally consistent and yet utterly defy common sense. Consider objects (called jinn) that exist in closed time loops and have no point of origin, materializing from nowhere. Gott cites the gold watch in the movie Somewhere in Time that passes from old Jane Seymour to young Christopher Reeve in 1972 and (after he wills himself to 1912) back to young Seymour, who keeps it with her for the next 60 years before relinquishing it to Reeve. Jinn can also take the form of information transmission. A journalist on deadline could save himself a lot of work by leaping into the future, reading his published article (this one?), coming back, and writing it up—the content of the piece has essentially been pulled from thin air. Nahin stresses that loops like these, anti-rational though they may be, are not satisfactory rebuttals. On the contrary, “if time travel is possible,” he writes, “then it would seem that we will have to accept causal loops, too.”
In a 1998 paper, Gott and his Princeton colleague Li-Xin Li extended this circularity to the whole megillah, proposing—in what amounts to a cosmic extrapolation of the aforementioned Heinlein story—that “the Universe could be its own mother.” At the beginning—and the end—is a time machine; there is no earliest or latest event. A provocative answer to the conundrum of first cause, this theory relates to a controversial question Stephen Hawking posed in A Brief History of Time; describing the notion of “completely self-contained” space-time, he muses, “What place, then, for a creator?” Gott, a Presbyterian, writes in his book, “I would not pretend that a self-creating Universe is not a troubling notion—but perhaps we should find the Universe troubling.”
As in all matters temporal, things unavoidably turn fuzzy. The metaphysics here are inextricably linked to the philosophy of time—mysteries that have gripped thinkers from Aristotle (time is eternal in both directions) to Kant (the past cannot be infinite). Time travel also adds new twists to the age-old free will vs. omniscience debate, though Davies, who’s “not religious in any conventional sense,” points out it’s not worth getting too worked up just yet: “The possibility of time travel would be such a transformation of our understanding of reality it would transform our entire value system.”
So, in a best-case scenario, how far are we from achieving time travel? Davies says, “It seems to me the best hope for building a wormhole time machine is if our understanding of gravity is not yet the last word. Some people believe the next generation of particle accelerators could bring about dramatic gravitational effects at modest energies, and we would be able to sculpt space-time with those machines. I could well imagine a situation in which some sort of time travel is possible—you could send a particle back through time, say—but for one reason or another, humans never could do it.”
Mallett’s optimism is less guarded. “Once it can be done, even in the simplest situation, in the most primitive way, the engineering obstacles will be overcome,” he says. “Just think of the Wright brothers: All they did was get their plane to fly a hundred yards or so, and we had jet travel by the middle of the century. It’s only a matter of time before we find the right combination of things that permits it. I honestly believe this will be the century for time travel.”