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Image: "Timeline" time machine
Dressed in medieval garb, a team stands inside a machine that will transport them to 14th-century France, in "Timeline."
By Alan Boyle Science editor
msnbc.com

The science in science fiction doesn’t have to be possible — only plausible. That’s what makes time travel such a well-worn plot device in movies ranging from “The Time Machine” to “Back to the Future” to “Timeline,” the latest entrant in a seemingly ludicrous genre. The funny thing is, scientists say the idea of going back in time is looking less ludicrous and more plausible as time goes on.

“Timeline,” WHICH was adapted from Michael Crichton’s 1999 novel and opens Wednesday at theaters nationwide, is based on the premise that a Bill Gatesish techno-whiz has developed a machine capable of sending people back to 14th-century France.

The technological explanation takes up only a couple of minutes, just as the dino-cloning explanation did in “Jurassic Park.” When the machine revs up, all you see are flashes of light and pained looks on the faces of the time travelers.

Paul Nahin, a University of New Hampshire engineering professor who literally wrote the book on time travel in physics and popular culture, isn’t surprised that the movie pays relatively scant attention to the scientific details.

“I don’t go to a time-travel movie myself to be educated. I want to have a good time,” he said. “If I had a choice between a five-minute lecture on quantum physics and a 3,000-pound horse charging down on someone, I’d choose the horse.”

Crichton’s novel, which credits Nahin’s work in its bibliography, provides much more about the physics behind the flashes. This time machine works by breaking down an object — say, an actor like “Timeline” heartthrob Paul Walker — into a complete quantum mechanical description, then teleporting that description through a tiny wormhole in the space-time continuum. To wrap the book’s plot in plausibility, Crichton refers to real-life developments in quantum computing, quantum teleportation and cosmology.

Both book and movie downplay the fact that it would take the energy of several galaxies, godlike information-processing power and inconceivably good luck even to begin to do the things described. But it plays off the growing sense that turning back the clock might be theoretically possible after all.

“Over the last 10 years there’s been a sea change,” said Michio Kaku, a theoretical physicist at the City University of New York. “Ten years ago, you would be considered a lunatic if you proposed that time travel was possible. ... Now, the burden of proof has shifted to the cynics, who have to prove that it’s not possible.”

The battle over wormholes
Kaku laid out the possibilities for time travel in the book “Hyperspace” and more recently in an article for Wired magazine. One of the chief arguments in favor of time travel’s possibility is that there’s nothing which rules it out.

“In the laws of physics, if it’s not forbidden, it’s mandatory. This is pretty much proven every time,” Kaku said.

Most theoreticians say that if you can find a wormhole — that is, a shortcut between two distant regions of the universe’s curved space-time continuum — and figure out how to traverse it and move it around, you would have enough to make a time machine.

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In the past, the skeptics claimed that such a machine would explode the instant it was created.

“Since then, there have been experiments indicating that the machine does not explode,” Kaku said. If you could harness a rapidly spinning black-hole ring, for example, there might be a central gap through which you could step into another place in space-time. You could also try creating pairs of huge conductive plates, separated by the width of an atom, to take advantage of the universe’s negative energy ... or manipulating loops of cosmic string.

But there’s always a caveat: For example, no one yet knows how to find cosmic strings, or how to create sufficient quantities of negative energy. “Black holes are not preferable for time travel because they’re one-way trips. It’s like an elevator with only an ‘up’ button,” Kaku said.

For more than a decade, Cambridge physicist Stephen Hawking has been looking for a scientific reason to rule out the possibility of going backward in time — the evidence that would nail down what he calls a “Chronology Protection Conjecture,” based on quantum physics.

“That will be new physics,” Nahin said, “because right now nothing in physics would forbid time travel.”

Altering history?
If it turns out that time travel is indeed possible, what keeps the universe from imitating “Star Trek”? Would it be possible to assassinate Hitler, or save Lincoln? Could you shoot your own grandfather, and if so, where would that leave you? More germane to “Timeline,” could you alter the course of 14th-century history?

This goes to the age-old debate over causality protection — that is, the kinds of historical paradoxes that were just narrowly avoided in “Back to the Future.” In fact, Hawking has joked that his Chronology Protection Conjecture is an effort to “keep the world safe for historians.”

Explaining how the rules of time travel apply in the world of “Timeline” would give away the story, but suffice it to say that Nahin was generally satisfied with the way Crichton handled the issues in the book, and intrigued to hear about some extra twists that set up the movie’s ending.

Speaking more generally, Nahin and other scientists who allow for the possibility of time travel tend to set forth two arguments for causality protection:

History is history: One explanation is that changing the past would simply not be possible, based more on fundamental logic than on quantum physics per se: Anything that a time traveler was able to do would be accounted for in the historical record. If you were to plan an attempt to kill your grandfather, or to save Joan of Arc from being burned at the stake, history is already telling you that you didn’t go through with it.

“The fact that you can’t change the past doesn’t mean that you can’t affect it,” Nahin said. “We know Joan of Arc died, but you could very well be the person who threw the match on the wood.”

Many-worlds hypothesis: For would-be time travelers who aren’t satisfied with that deterministic explanation, the idea that our cosmos is just one of many parallel universes in a “multiverse” has given rise to a causality-preserving loophole.

Suppose that “the river of time” is able to fork into diverging rivulets: In this scenario, it is indeed possible to save Joan or kill your grandfather, but that change just sets your quantum universe going down a different fork in the river. If you follow that new fork, history could unfold differently. Joan would live on, or the person who turned out to be you (in a different universe) would never be born. But if you leap back to the fork you came from, nothing has changed.

“I tend to believe the many-worlds interpretation,” Kaku said.

This interpretation also makes choosing your destination infinitely more complex — and that could go a long way toward explaining why we’re not overrun with time travelers. The uncertainty that tends to accompany quantum effects would make it hard to find the precise fork you’re looking for. You might end up connecting with a universe where Joan never existed, where Earth never existed, or where stars and galaxies never formed.

Dude, where's my time machine?
Even under the many-worlds hypothesis, some universes are better than others — and that goes for time-travel movies as well. There are a couple of plot developments in “Timeline” that Nahin might quibble with, but nothing to compare with “Timecop,” the history-twisting tale that he ranks among the world’s worst time-travel movies.

“It had to be one of the most intellectually devoid films I’ve ever seen,” he said. “I wasn’t squirming over points that physicists could argue over. It’s just that the logic was all wrong.”

So if that’s the worst, what’s the best? Would you believe “Bill and Ted’s Excellent Adventure,” the 1989 slacker comedy starring Keanu Reeves and Alex Winter?

“They had time paradoxes galore,” Nahin said, “and every single one of them was handled correctly.”

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