he theory of relativity, laid down by Albert Einstein in the first part of the 20th century, tells us that the faster you go, the slower time passes for you as compared to the rest of the universe. As countless science-fiction stories have shown, starships moving close to the speed of light become one-way time machines, rocketing into the future at a furious pace while time inside the machine passes syrupy-slow. Other tickets to the future include suspended animation (the Rip Van Winkle effect), the distorted spacetime near a black hole (what I call the "gravitational fossil" effect) and simply living a long time, until the world around you has changed.
I would include the science-fictional gimmick of "stasis fields" as well—special force fields which slow or halt the flow of time inside—except that no one has ever described a plausible method for building one. Recent experiments with ultracold matter have been able to slow and even stop the propagation of light waves, so perhaps this points to a future possibility, but for now the idea is little more than an interesting plot device.
But anyway, yeah, there are a number of promising methods for reaching the future if you really want to. Unfortunately, getting back to your starting point—or jumping backward into your own past—is not quite so easy. Still, from Edward Page Mitchell's 1887 The Clock That Ran Backward to Warner Brothers' latest H.G. Wells remake, humans have dreamed of traveling backward in time, either to observe history or else to tamper with it. Be honest: we all have moments we'd like to undo.
More than just a theory
Possible? Actually, yes—modern physics teaches that in our four-dimensional universe, time can fold back on itself in peculiar ways. Leaping across it is not easy—most proposals involve unbelievably long and heavy cylinders spinning unbelievably fast, or flying through the center of a spinning black hole, or traveling the long way around the universe until you return to your starting point. It may also be possible to send energy and particles backward; one longstanding theory declares that antimatter is simply normal matter moving backward in time. This opens the possibility of sending signals back, as in Gregory Benford's novel Timescape, or James P. Hogan's slightly earlier Thrice Upon A Time. Difficult? Very definitely. But not impossible.
In the classical, Victorian view of H.G. Wells' day, time is simply another direction—one we can't see or freely move through, but which is otherwise very similar to the up-down, left-right and front-back dimensions we already know. Thus, the past and future are completely real, completely solid, and actually located very close by. Different points in time are like different playing cards in a deck: we turn them over one by one, but all the other cards still exist, piled up in a predetermined order.
Can the deck be stacked in a new, perhaps more desirable way? Can the cards themselves be altered? There are two classical views on this. First, that no changes are possible, i.e., that a traveler or message simply becomes (or is destined to be, or always has been and always will be) a part of the static, unchanging sculpture of history. (In the words of Larry Niven: "If you go back in time to shoot Jesus, the gun will definitely jam.").
The second view is that the traveler can alter the events of the past. There are a couple of variations on this theme: that only inconsequential events are changeable (as in Connie Willis' "Fire Watch" series), or else history has an "inertia" or springiness which is difficult to overcome (as in Mark Twain's A Connecticut Yankee in King Arthur's Court and James Cameron's Terminator series).
Free will can be costly
An often overlooked consequence of these theories is that if time travel is possible, and the future is infinite (or at least very long), then the total number of time travelers must be quite large. This implies that as the future unfolds, all the interesting events and periods in history will gradually become polluted with chrononauts. Or rather: time travelers from various future dates always have been and always will be a part of those events, while "true" historical figures are a tiny minority, just barely sufficient to keep the names and dates and gene pool rolling along as they're supposed to. And frankly, this idea takes all the fun out of history, turning millennia of life-and-death struggle into little more than a crowded museum display.
Another unpleasant corollary is that if the past and future are unchangeable, then free will does not exist. We believe ourselves to be conscious and self-determining, but in fact we're more like characters in a story, our every thought and action laid down by the dreams of some four-dimensional creator. John Calvin, the Christian theologian of absolute predestiny, would be so pleased.
Fortunately, with the advent of chaos theory at the end of the 20th century, these models began to seem rather quaint. Long-range weather forecasting is mathematically impossible, because every moment is so exquisitely sensitive to the events and condition of all the moments before it. You can simulate weather for a few days, but changing a single number in the twelfth decimal place will completely rewrite the 10-day forecast. In other words, if you travel 10 days back in time—even for an instant—your mere presence in the past will disturb small air currents which disturb large air currents which disturb major weather systems. And when you return to your proper time, the weather may be completely different than it was when you left. So probably will the sports scoreboards and stock market—that crosstime investment scam may not work out as well as you'd hoped.
This leads to a different sort of time-travel theory: the destroy-the-future model. The small changes wrought by time travelers can not only bring rain but erase people and families (as in Robert Zemeckis' Back to the Future series), change the course of nations (as in Ray Bradbury's classic "A Sound of Thunder"), rewrite human history from end to end (as in Richard Garfinkle's 4-D adventure All of an Instant) and even extinguish the entire human species (as in the parodical "Time and Punishment" segment of The Simpsons' "Treehouse of Horror V").
For me, this theory is much more exciting than the Calvinist one, because it brings free will back into the picture, with literally the entire future at stake. Knowing the relative merits of our world today, do you risk going back to kill Hitler? For all you know, this could trigger a nuclear armageddon in 1968.
Defusing the power of paradox
It could also trigger something even more sinister and confusing: a paradox. The classic example is going back and murdering your own grandfather while he was still a child. In addition to the pain and suffering of his family, this causes your birth to be erased from the future, which in turn causes you not to have traveled back to commit the murder. But if you didn't commit the murder, then you are born, and do travel back ...
The universe as we know and love it does not contain any paradoxes, and probably cannot contain them, so dozens of stories have been wrapped around the various elegant or zany or creepy methods by which such a paradox loop could be (must be?) avoided or broken. And though it doesn't play well in fiction, chaos theory provides another safety valve: in the disrupted future, you probably aren't born anyway. Your parents may still meet, and may even still procreate on the proper occasion, but what are the odds that the exact same sperm cell finds its way to the egg again? That the childhood events which shaped you will all turn out the same?
And it turns out that quantum mechanics provide an even cleaner solution: parallel universes. These are not a comic-book invention, but a serious scientific theory which accounts for the otherwise inexplicable behavior of very small things. It's the only theory which can make sense of quantum computing, for example. The idea goes: every event with more than one possible outcome produces a branching point, where all the possibilities come true in separate universes. From there, chaos theory takes over and the universes begin to diverge, with small changes snowballing into large ones. The traveler through quantum time can never change his or her own past, because the very act of traveling creates new branch points and new universes. In this one, you don't arrive. In that one, you do, disrupting the entire future, possibly including the invention of the time machine itself. But it doesn't matter—there is no paradox—because the time machine is also a universe-jumping machine (a la Sliders), and the universe where the machine originated is not changed, and by definition cannot be changed.
Thus, the future and past are both unwritten, or infinitely written in a sea of infinite possibility, and new universes are created all the time. In fact, they can be created to order: quantum megalomaniacs can go back and win any war they please, constructing a future where everything goes their way (remember "Biff, the luckiest man in the world"?). And really, while the residents of such universes will assuredly suffer, they were also created for that purpose, while their original copies in their original universes live on unaffected. So what harm is done here? What is the morality?
Remember, to the best of our understanding, time travel is possible, so these speculations are not completely idle. We probably should have some philosophical positions lined up, just in case, so when a doorway opens up spilling futuristic Nazi soldiers into our world, we at least know what crime to charge them with. Quantum jaywalking?
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog, Asimov's, Science Fiction Age and other major publications, and his novel-length works include Aggressor Six, the New York Times notable Bloom, and The Collapsium.
