the maddening weirdness of time travel

“You’re dealing with time travel. Maybe you should expect it to be weird.”

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.

sciencenewsBut 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, New York.

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. 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.”