You might say we're living in a golden age of time travel. From television shows like Heroes, Lost, and Flash Forward to this summer's Star Trek movie, punctures in the space/time continuum are turning up all around us. As a physicist—and, perhaps redundantly, a science-fiction geek—I'm particularly sensitive to the pleasures of these mind-bending narratives. I'm also sensitive to their flaws. Most fictional accounts of time travel are rife with paradoxes, parallel universes, and plot holes that violate strict physical laws: Instead of exploring the limits of our understanding, they make a mockery of them.
That's why I'm so excited about the film adaptation of Audrey Niffenegger's The Time Traveler's Wife, which tells the story of Henry DeTamble, a man with a rare genetic disorder that causes him to skip around in time while his long-suffering wife, Clare, waits for him at home. The premise is no more or less plausible than that of, say, Back to the Future, in which a tricked-out DeLorean must reach 88 mph to jump into the past. But The Time Traveler's Wife follows through on its premise in a realistic way.
The notion that one version of time travel is more accurate than another might seem ridiculous on its surface, but physicists actually have rather a lot to say about how time travel should work. Some, in their more fanciful moments, have even devised ways to exploit Einstein's theory of general relativity to come up with "practical" models of time machines. Kip Thorne, in Black Holes and Time Warps, describes how wormholes can be successfully used to travel back in time, while in Time Travel in Einstein's Universe, J. Richard Gott does the same with gargantuan cosmic strings—threadlike concentrations of matter of almost unimaginable density and length—moving at close to the speed of light.
It's not fair to demand exact design specifications for a fictional time machine, but even if we sweep the technical details under the TARDIS, time-travel narratives ought to still abide by a few fundamental ground rules. Here's my list of the most important principles of time travel, real or imagined.
1) This is the only universe you've got.
In 1957, physicist Hugh Everett proposed what has become known as the "many worlds" interpretation of quantum mechanics. Quantum mechanics was one of the great breakthroughs of the 20th century, and it predicted, among much else, that the motions of electrons and other small particles are fundamentally random. Everett, then at the Pentagon, wondered whether the universe wasn't branching off into two nearly identical copies each time one of these random events occurred. Since there are lots of particles in the universe and they move around and interact very quickly, these parallel universes would multiply almost without limit.
The many-worlds interpretation provides a fertile basis for time fiction, via the ubiquitous Back to the Future model of alternate histories: Someone visits the past, teaches his father about believing in himself (or some similar nonsense), and thereby leaps into a parallel universe with a different (ideally, better) future than the first.
For a physicist, though, there's no reason to believe that a Back to the Future-style time machine is possible. For one, there's no evidence that Everett's parallel universes exist. (There's no direct evidence against them, either.) More importantly, Einstein's theory of general relativity—the branch of physics that might make time travel possible—doesn't take kindly to the idea. Every solution to Einstein's equations involves just a single universe. Maybe I'm being overly dogmatic, but I don't think it's unfair to insist that movies stay within the realm of what we currently know about how physics works.
In a rule-abiding time-travel narrative, there are no parallel universes—just a single timeline. The Time-Traveler's Wife follows this rule to a T, and there is a significant online presence dedicated to diagramming the unique, entangled history of Henry and Clare.
2) You can't visit any time before your time machine was built.
According toEinstein's picture of the universe, space and time are curved and very closely related to each other. This means that traveling through time would be much like traveling through a tunnel in space—in which case you'd need both an entrance and an exit. As a time traveler, you can't visit an era unless there's already a time machine when you get there—an off-ramp. This helps explain why we're not visited by time-traveling tourists from our own future. Futuristic humans don't drop in for dinner because we haven't yet invented time travel.
The time-machine construction clause is one of the most often overlooked of the rules of time travel and is the only real mar on the otherwise exceptional Terminator(1984), which proposes a single historical line (or loop) with no alternate universes. (Subsequent movies in the series revert to the parallel-histories model.) The Time Traveler's Wife very nearly gets it right: Since Henry isthe time machine, he can't visit any time before he was born. His daughter, on the other hand, bends those rules slightly: She manages to visit a time before her own birth but not so far back that her father hasn't been born, either. (We might take Henry's birth as the "invention" of time travel and the whole family as components of a single machine.)
3) You can't kill your own grandfather.
Supposing you've inherited a time machine from your grandfather. Presumably, you could pop back for a visit to thank him and/or commit retro-grand-patricide, couldn't you? Not so fast. To make the logic blindingly obvious, if you kill your grandfather, then you won't have been born, which means you couldn't have killed your grandfather, which (logically) means that you will be born.
If history is to have any consistency in a world with time travel, then the "grandfather paradox" (so named by writer René Barjavel) must be resolved. Physicists had little to say on this topic until the mid-1980s, when Igor Novikov of the University of Moscow used quantum mechanical arguments to develop what has become known as the "self-consistency theorem." Quantum randomness must obey well-established laws, and Novikov showed that the probability of producing a different future with a time machine was zero. To put it more simply: You cannot alter history in any way that changes it from what it always was.
So, try as you might, you can't kill your own grandfather, nor can you change history at all. The Terminator learned this the hard way, going back in time to prevent John Connor's birth by killing his mother. When a human travels back in time to protect her, the two fall in love—and she becomes pregnant with … John Connor. Ta-da.
There's no need for such finagling in The Time Traveler's Wife. Since Henry DeTamble serves as his own time machine, there's little chance of his preventing his own birth. Cf. rule No. 2.
4) You don't have nearly as much free will as you think you do.
Novikov's theorem can feel somewhat unsatisfying. As Kip Thorne writes, "something has to stay your hand as you try to kill your grandmother. What? How? The answer (if there is one) is far from obvious, since it entails the free will of human beings." The concept may be easier to grasp if you think about it in terms of inanimate objects: Imagine you shot a pool ball into a time machine and it emerged a moment beforeyou made the shot. Now suppose that you aimed the shot just right, so the outgoing ball (your ball, a second earlier) would block the original shot and prevent it from going into the time machine. This paradox, proposed by Joe Polchinski, then at the University of Texas, * turns out to be the same as the grandfather paradox, albeit with less profound implications.
Kip Thorne and his students worked out what looks like a compromise solution for the impossible pool shot. They argue that you'd line up your shot exactly, but as your ball headed toward the time machine, another one would fly out at a slight angle and graze its side. The first ball would still travel into the time machine but at a slightly different angle than you'd intended. Then it would come back out of the machine, a moment earlier, at the same barely skewed angle—and the Terminator-style loop would be complete.
If pool balls can be forced to succumb to their destiny, so can people. This fact is very easy to ignore if you don't know what the future will bring; it certainly seems like you've got free choice. But if you've already seen what your destiny is, then the future is already written. Making that self-consistent future play out is one of the great challenges of time-travel fiction.
In The Time Traveler's Wife, Henry and Clare enforce the (predetermined) future by giving each other instructions and hints about how things are supposed to happen. That gives them a feeling of free choice where none really exists. In a letter to Clare about their future, Henry explains, "I won't tell you any more, so you can imagine it, so you can have it unrehearsed when the time comes, as it will, as it does come."
Correction, Aug. 13, 2009: The original version of this article misspelled Joe Polchinski's name and suggested that he was still on the faculty at the University of Texas. (Return to the corrected sentence.)