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                     The science and the fiction of time travel are weird. But the science is weirder.

                                                                   by Scott Mowbray


When H.G. Wells sent the hero of The Time Machine into what Wells called "futurity," it was on a grim 30-million-year round-trip to pretty much the end of Earth time, when the last, poorest excuses for life were flopping around like squid under a darkening sun. Wells wasn't the first writer to imagine time travel, but he advanced the idea that a machine, rather than an angel or a bonk on the head, could accomplish it, and he pushed his machine to the limit. It moved through futurity like a bucking bronco: "I have already told you of the sickness and confusion that comes with time travelling," Wells' hero remarks. "And this time I was not seated properly in the saddle ..."

To Wells in 1895, time was a dimension much like forward and back, or up and down, but he gave no clue as to how the machine might move a human being through the fourth dimension into the future: He just wanted to get there. Einstein offered an answer seven years later, in 1905, with his Special Theory of Relativity. Time, by Einstein's equations, was not a fixed property of the universe (moving in one direction at the same rate for everyone, which was Newton's view), but a relative property of things in motion. A clock in motion ticked slower than a stationary clock; a moving clock traveled into the future relative to the clock at rest. It turned out that we'd been time traveling all along, we just didn't have clocks precise enough to show it. (Later we built such clocks, and they did.)

So began a century of strong, almost gravitational, attraction between physics and fiction, as both orbited around an idea that seems fantastic whether tackled by Rod Serling or by Einstein's heirs. The neatest, and certainly the most famous, example of the synergy may be that of Carl Sagan and his novel Contact. In the early 198os, Sagan turned to a physicist friend, Kip Thorne of Caltech, when he needed to jump a character through space. Thorne developed a theory whose byproduct was, essentially, a blueprint for a time machine that would require a "supercivilization" to build. Sagan's book became a movie starring Jodie Foster. Thorne's so-called wormhole theory was published in the eminent journal Physical Review Letters.

Scientists tell us it's technically possible. Here's a how-to guide for the ambitious tinkerer.by Michael Moyer

Start with a Black Hole ...

The physical possibility of time traveL is something of a catch-22. Any object that's surrounded by the twisted space-time that time travel requires must by its very nature be fantastically perilous, a maelstrom that would inevitably tear apart the foolhardy traveler. So physicists have labored to create a theoretically acceptable time machine that's free from nasty side effects like certain death. Their starting point: black holes.

Black holes are famous for sucking in everything around them—including light—and never letting go. But black holes have other characteristics, namely the way they bend nearby space-time. A black hole is infinitely dense, which means that it pulls the fabric of space-time to the breaking point—creating a deep pockmark, complete with a tiny rip at the bottom.

Many have wondered what lies on the other side of this rip. In 1935, Einstein and his colleague Nathan Rosen developed a scenario in which the tiny rip in a black hole could be connected to another tiny rip in another black hole, joining two disparate parts of space-time via a narrow channel, or throat. The Einstein-Rosen bridge, as the notion was then called, looks like a black hole attached to a mirror image of itself.

This bridge—a sort of back door leading from the interior of one black hole into another—is today known as a wormhole. Such a portal could in theory create a shortcut through space-time—just the thing a time traveler would need if he wanted to cheat Father Time out of a few million years.

Next, Modify the Wormhole ...

The problem with wormholes is that the channel created between two black holes is minuscule, smaller than the center of a single atom, and remains open for only a fraction of a second. Even light, the fastest entity in the universe, would not have enough time to pass through. And no matter how sturdy his spacecraft, our traveler would inevitably be ripped apart by the black hole's immense gravitational forces. Because of these and other problems, the Einstein-Rosen bridge was for many years thought of as a geometric curiosity, a theoretical quirk that could never be of use to even a fictional time traveler. Einstein's equations might allow for wormholes, but the universe certainly did not. All that changed in the 1980s, however, when a physicist at the California Institute of Technology devised a better way to use wormholes as time machines.

( For things like Wormholes to exist, the universe must be curved. To imagine a wormhole, consider again a 2D piece of paper. Curve it into a saddle; a wormhole would be the path through the air from one side of the paper to the other. Instead of walking all the way around the saddle, you could take a shortcut through a higher dimension. )

If Einstein and Rosen are the architects of the space-time shortcut, then Kip Thorne of Caltech is its structural engineer. Starting from the rough sketch that Einstein and Rosen left behind, Thorne created an algorithm that describes in strict mathematical terms the physics of a working time machine. Of course, actually building Thorne's time portal would require a technological prowess that is at least many centuries away. But his work proves that time travel is possible—at least in theory.

Thorne's problem was finding a way to hold open the wormhole's channel, or throat, long enough for an explorer to pass through. Ordinary matter won't do: No matter how strong it is, any scaffolding made of matter cannot brace against the crush of space-time. Thorne needed a substance that could counteract the squeeze of a black hole. Thorne needed antigravity.

Instead of contracting the space around it, as ordinary matter does, antigravity—or negative energy, as it is sometimes called—pushes it apart. In theory, antigravity would be placed inside a wormhole's throat, opening it wide enough for an astronaut, or possibly even a spaceship, to pass through. Antigravity does the trick; the problem is finding it. Einstein first postulated the existence of antigravity on cosmic scales in 1915, a conjecture proven correct eight decades later. But Einstein's antigravity is wispy and dilute, a spoonful of sugar dissolved in the Pacific Ocean. Opening a wormhole requires a regular torrent of antigravity.

The best current candidate for creating concentrated antigravity is called the Casimir effect. Because of the quirks of quantum mechanics, two flat metal plates held a hair's width apart generate a small amount of negative energy. That energy, multiplied many times over, could in principle be used to create a traversable wormhole. The widening, meanwhile, would dilute the strength of nearby gravity, preventing the traveler from being torn apart.


Once the antigravity scaffolding is holding open the portal, the traveler passing through would emerge in a distant place. But time travelers, of course, want to journey not just geographically but temporally. So Thorne's next step was to desynchronize the two regions on either side of the wormhole.

To do this, he applied an old trick of Einstein's. A major consequence of Einstein's Special Theory of Relativity is that time slows for objects that move quickly. Thorne applied this principle to one of the two black holes that make up a wormhole. Imagine lassoing one of the black holes—perhaps by trapping it inside a cage of negative energy—and towing it around the universe at close to the speed of light. That black hole, and therefore that end of the wormhole, would age more slowly than the stationary end of the wormhole. Over time, the black holes would become desynchronized, two objects connected through the wormhole but existing in different eras. An explorer who entered the stationary end of the wormhole would exit the moving end, many years earlier than when he departed, making the wormhole a true time portal.

Or Try It on a Shoestring

The most recent development in the physics of time travel came in 1991, when Princeton astrophysicist J. Richard Gott III suggested that hypothetical objects called cosmic strings might enable an astronaut to travel backward in time. Cosmic strings are long, thin objects that some cosmologists believe coalesced out of the universe's very earliest days. They are infinitely long, no wider than a single atom, and so dense that a few miles of a single cosmic string would outweigh Earth itself.

Gott's proposal relies on idealized versions of cosmic strings. In order to be em-ployed in the service of a time traveler, two cosmic strings, perfectly parallel and traveling at nearly the speed of light, must whiz past one another like two cars traveling in opposite directions on a highway. As the strings pass each other, space-time would become profoundly distorted by the influence of these fast-moving filaments. A savvy time traveler, waiting in a nearby spaceship, could exploit those distortions by flying around the coupled strings. If he timed it just right, the twists in space-time would enable him to return to his starting point before he began—making the voyage a one-way trip back in time. Which means that, according to the laws of physics, journeys through time are conceivable, if rather difficult to arrange. It may be only a matter of time.


It takes a lot of gravity to significantly warp time. A black hole has such enormous gravitational force that it creates a tear in space-time itself, but a black hole is no portal because it will suck a would-be time traveler into the cramped quarters of infinite density, forever. A properly engineered wormhole, however, theoretically creates a passage between two black holes that leads to another place in the universe through the space-time tear; a bit of galactic-scale fiddling with one end of the wormhole turns it into a time machine (more on wormhole engineering).

"What Kip Thorne and his colleagues noted," says Gott, "was that if you moved the wormhole mouth correctly, Jodie Foster (in the film version of Contact) could have come back before she left... . Jodie Foster would have been waiting at the same spot to shake hands with Jodie Foster when she arrived." The notion of creating a hopeless causal loop in time is childishly easy to understand. In Back to the Future, Michael J. Fox finds himself fading from existence after journeying back in the souped-up DeLorean and attracting his mother's romantic interest at a time when she was supposed to be falling in love with Fox's future father. (According to one academic paper, this "is the first science fiction film to make explicit the incestuous possibilities that have always been at the heart of our fascination with time travel.")

Davies says the paradoxes of time travel have repelled some physicists, who were afraid of being ridiculed. Igor Novikov, a Russian astrophysicist who has written extensively on the subject, says that for decades, "very serious mathematicians, very serious physicists were not brave enough to declare that time travel is possible."

Many resolutions to the paradox have been proposed. One simply maintains that the universe won't let paradoxes be created: If you try to kill your grandfather, you won't be able to. You'll change your mind, or the gun won't go off, or you'll be a lousy shot. This notion, it will be noted, has serious implications for the existence of free will. Other approaches say there really are no paradoxes; every problem can be solved mathematically without producing a paradoxical inconsistency.

But the paradox problem does not bother David Deutsch, a theoretical physicist at Oxford University. Deutsch dislikes the violent aspect of the grandfather paradox, which he says is only confused "by the issue of human conflict that people put into the story to make it more interesting." Deutsch has instead created a gentler paradox.

In this experiment, a person watches a time machine to see whether a copy of himself emerges on, say, Tuesday. If it does not, on Wednesday he journeys back in time one day—emerging from the time machine on the same Tuesday when he had not emerged before. The experiment can be reversed: If, in the opposite scenario, he does emerge on the Tuesday, he simply waits until Wednesday and chooses not to get in the time machine. In either case, a paradox is created: The time traveler is there on Tuesday and not there at the same time—a phenomenon that, intriguingly, echoes some of the fundamental mysteries of particle behavior at the quantum level.

Indeed, Deutsch reaches into quantum mechanics for an explanation of the paradox. He is a leading proponent of the many worlds theory, in which multiple new universes are triggered by each quantum event. If two subatomic particles collide, for example, one of the particles does not go either right or left, but rather it goes right in one universe and left in another universe that is instantly created in the so-called multiverse. In the Deutsch experiment, one universe contains the lab in which the experimenter did not journey back, another the lab in which he did: paradox resolved.

In any case, a hundred years after the Special Theory of Relativity, time travel is theoretically possible, and therefore theoretically in the future of human development. So the big question is: When will we have a machine to test the theories? Wormhole engineering doesn't offer much promise, since it involves near-light-speed travel, antigravity, and the like. But Paul Davies is hopeful. He says we would begin by sending a particle, rather than a human being, back in time to test the paradoxes. Emerging theories about the nature of gravitation already suggest, he says, that "it might be possible to reconfigure space-time with energies that could be accessible by the next-generation of particle accelerator." If so, "We're talking about more like 100 years," Davies says, "than about who knows how many years."

> Reported by Peter Kobel

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Wormholes, time travel and quantum gravity

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