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.
Which brings us to the present. This month we can enjoy the curious fact that Dreamworks' remake of the classic 1960 film The Time Machine is directed by H.G. Wells' great-grandson, Simon Wells. It retains the original film's Victorian decoration-a time traveler in tweed vest, a time machine with brass fittings and steam-era dials-as well as the Hollywood happy ending: Handsome Guy Pearce battles Morlocks in 800,000 A.D. and gets the girl, although both the girl and the special effects have been spectacularly jazzed up. As fascinated as he was by the conundrums of time travel, Simon Wells says he knew the darker end-of-history aspects of the original novel would not yield a blockbuster; he pitched the movie as "an essay into the nature of causality in which we blow a bunch of stuff up."
This is what the good time travel stories and the physics share: interest in the nature of causality. Time travel yields paradoxes with the narrative power of Greek myth, causing scientists to ask questions like, "What would happen if you journeyed back in time to kill your grandfather before you were born?" Well, a good story would happen, and that's what fuels the time travel fiction. What fuels the scientific interest is that time, since Einstein, has been entwined with the problems of gravity, quantum mechanics, and the search for a unified theory of everything. And time travel-into the past, where the paradoxes spring up-appears not to be in conflict with the laws of the universe.
If we're all time travelers, the problem is that we can't get very far using current technology. Spending just over two years in Mir's Earth orbit, going 17,500 miles per hour, put Sergei Avdeyev 1/50th of a second into the future, according to Princeton astrophysicist J. Richard Gott III, and "he's the greatest time traveler we have so far."
To move through time at a serious clip, you have to go very fast-say, 99.995 percent of the speed of light. At that rate, if you "go out 500 light-years, then come back at the same speed," Gott notes, "when you arrive back the Earth would be 1,000 years older and you would only have aged 10 years."
But to attain this sort of velocity you have to overcome what Michio Kaku, a professor of theoretical physics at the City University of New York, calls the "gasoline" problem: the need for an almost inconceivably powerful energy source. This problem crops up constantly in time-travel theory, from simple extrapolations of relativity (go really fast so you can travel to the future) to more complex ideas such as the wormhole engineering that Kip Thorne theorized for Carl Sagan.
"Back to the Future is one of my favorite time travel stories," says Kaku. "The DeLorean is energized by plutonium. But that's not enough. To really energize your time machine, you need what's called the Planck energy ... roughly a quadrillion times more powerful than our most powerful atom smasher." Physicist Paul Davies-author of How to Build a Time Machine, which will be released in the United States this month in happy synchronicity with the Wells movie-notes: "At the moment it looks like making a wormhole ... would require something like a particle accelerator bigger than the solar system. So if you just look at current technology, then it looks to be pretty hopeless."
The foundation of time machine physics is Einstein's General Theory of Relativity, completed in 1915, which describes gravity as a warping or curving of space and time by matter. "If," as Gott says, "space and time are curved, you can have a situation where space and time are sufficiently twisted that you can circle back and visit an event in your own past." (Picture an ant crawling along the edge of a piece of paper, trying to get from one corner to the far corner; then fold the paper so that the corners are brought together and the ant's journey is drastically shortened. To travel in time, just fold space and time in the same way; the time machine does the folding.)
"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.
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