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Physicist throws time-travel theories a curve

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What do you get when you join a 1981 DeLorean, a "flux capacitor" and a digital dial set to Nov. 5, 1955? If you're the character of Dr. Emmett Brown in the 1985 movie Back to the Future, you've created a time machine.

The possibility of time travel has occupied the fantasies of philosophers, authors, children and directors. But to some physicists, it's more than pure fancy.

In the July issue of Physical Review Letters, Amos Ori, professor of physics at the Technion, the Israel Institute of Technology in Haifa, argues that the laws of physics don't stand in the way of building a time machine.

Ori hasn't created, or even designed, a physical time machine. He instead constructed a situation — a mathematical model — in which the laws of physics will make one for him.

"I write (the situation) mathematically. That doesn't mean that I know how to implement it practically." However, he says, if inhabitants of some highly advanced civilization could set up the conditions he describes, they might be able to travel in time.

The time machine Ori proposes isn't quite like the phone booth in Bill & Ted's Excellent Adventure or Back to theFuture's DeLorean. Ori's solution forms a closed, timelike curve. It's a bit like a song that never ends. Think of each musical note as a point in space. As you sing the notes, you move forward in time. You can travel around the curve — sing the song — but when you get to the end, you are also at the beginning.

"If you had a closed, timelike curve, that means that something could run around it forever and ever, always going to the future but always coming back to the beginning," says Ted Jacobson, professor of physics at the University of Maryland.

Ori isn't the first physicist to create a theoretical time machine. Unlike previous models, Ori's proposal doesn't require any unknown matter or energy. The previous theories were forced to use unrealistic negative energy to warp space and time.

Ori's plan requires absolute emptiness — a vacuum. That means that, in principle, a closed, timelike curve could even happen naturally, possibly through cataclysmic astronomical collisions in the abyss of space.

It is difficult for experts to imagine the exact conditions, but extremes in the universe, such as super-dense black holes, could create the conditions necessary for the formation of a time machine.

When Einstein declared that space and time were intimately connected, time travel became a physical possibility. If you could zip around like a beam of light, you wouldn't age, but physics won't let that happen. It remains to be seen whether physics will allow time travel, but Ori's work suggests it will.

Einstein opened the door for the scientific pursuit of a time machine, and physicists are searching for solutions. Ori says maybe it's possible, but "this isn't something that we are going to construct soon."

By Michelle Lefort

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