Taming time travel

New work solves paradoxes by making the impossible impossible

Novelists and screenwriters know that time travel can be accomplished in all sorts of ways: a supercharged DeLorean, Hermione’s small watch and, most recently, a spacetime-bending hot tub have allowed fictional heroes to jump between past and future.


Physicists have proposed several ways of getting around the grandfather paradox (shown). In a new model, quantum probabilities simply rule out such situations. DEM10/ISTOCKIMAGES, adapted by E. Feliciano

But physicists know that time travel is more than just a compelling plot device — it’s a serious prediction of Einstein’s general relativity equations. In a new study posted online July 15, researchers led by Seth Lloyd at MIT analyze how some of the quirks and peculiarities of real-life time travel might play out. This particular kind of time travel evades some of its most paradoxical predictions, Lloyd says.

Any theory of time travel has to confront the devastating “grandfather paradox,” in which a traveler jumps back in time and kills his grandfather, which prevents his own existence, which then prevents the murder in the first place, and so on.

One model, put forth in the early 1990s by Oxford physicist David Deutsch, can allow inconsistencies between the past a traveler remembers and the past he experiences. So a person could remember killing his grandfather without ever having done it. “It has some weird features that don’t square with what we thought time travel might work out as,” Lloyd says.

In contrast, Lloyd prefers a model of time travel that explicitly forbids these inconsistencies. This version, posted at arXiv.org, is called a post-selected model. By going back and outlawing any events that would later prove paradoxical in the future, this theory gets rid of the uncomfortable idea that a time traveler could prevent his own existence. “In our version of time travel, paradoxical situations are censored,” Lloyd says.

But this dictum against paradoxical events causes possible but unlikely events to happen more frequently. “If you make a slight change in the initial conditions, the paradoxical situation won’t happen. That looks like a good thing, but what it means is that if you’re very near the paradoxical condition, then slight differences will be extremely amplified,” says Charles Bennett of IBM’s Watson Research Center in Yorktown Heights, N.Y.

For instance, a bullet-maker would be inordinately more likely to produce a defective bullet if that very bullet was going to be used later to kill a time traveler’s grandfather, or the gun would misfire, or “some little quantum fluctuation has to whisk the bullet away at the last moment,” Lloyd says. In this version of time travel, the grandfather, he says, is “a tough guy to kill.”

This distorted probability close to the paradoxical situation is still strange, says physicist Daniel Gottesman of the Perimeter Institute in Waterloo, Canada. “The thing is, that when we modify physics in this way, weird things end up happening. And that’s kind of unavoidable,” he says. “You’re dealing with time travel. Maybe you should expect it to be weird.”

In an earlier paper posted in May at arXiv.org, Lloyd and his team present an experiment designed to simulate this post-selection model using photons. Though the team couldn’t send the photons into the past, they could put them in quantum situations similar to those that might be encountered by a time traveler. As the photons got closer and closer to being in self-inconsistent, paradoxical situations, the experiment succeeded with less and less frequency, the team found, hinting that true time travel might work the same way.

The experiments were meant to simulate freaky paths through spacetime called closed timelike curves, which carry anything traveling along them into the past and then back to the future. Einstein’s equations predicted that travelers on a closed timelike curve would eventually end up back where they started. Although predicted to exist on paper, no such paths have been observed in the wild. Some physicists predict that these loops might exist in exotic regions where spacetime is drastically different, such as in the depths of black holes.

Despite its strange predictions, the new model forms “a nice, consistent loop,” says theoretical physicist Todd Brun of the University of Southern California in Los Angeles. The new papers make up “a really interesting body of work.”

These days, deciding which theory of time travel is best is largely a matter of taste. Until someone discovers a closed timelike curve in the wild, or figures out how to build a time machine, no one will know the answer, says Brun. “I don’t expect these will be tested anytime soon. These are ideas. They’re fun to play with.”

Back Story | MOVIE TIME
Time travel is a well-worn plot device in Hollywood, but few films seriously consider the paradoxes that a time traveler might face. Here are some that do, although not always in a serious manner.

Screen shot from movie, Back to the Future | Cinemaphoto/Corbis

Back to the Future (1985)

Marty McFly (Michael J. Fox) confronts a variant of the grandfather paradox when he goes back to 1955 and unintentionally prevents his parents from marrying.

Bill and Ted’s Excellent Adventure (1989)

Bill (Alex Winter) and Ted (Keanu Reeves) romp through the past collecting historical figures for a class report. The story contains several examples of the ontological paradox, in which information is conveyed to people in the present by visitors from the future, rendering it impossible to determine when the knowledge originated.

Twelve Monkeys (1995)

A convict (Bruce Willis) goes back to the 1990s to prevent an epidemic that has forced the surviving remnants of humankind into a dystopian underground existence. Though his efforts may ultimately advance the cause, his own fate is sealed.

Primer (2004)

Two engineers (Shane Carruth and David Sullivan) accidentally invent a machine that allows them to travel back in time, where they confront the paradox of interacting with their past selves.

Image: Cinemaphoto/Corbis

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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