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.)