domenica 11 maggio 2008

He seeks a route through time

By Dan Falk
Globe Correspondent / May 12, 2008

Ronald Mallett lost his father to heart disease at the age of 10, an event that left him in utter despair. His depression lasted until he read "The Time Machine" by H.G. Wells and, a few years later, the theories of Albert Einstein - and he became determined to see his father again.
For years, Mallett, a physicist at the University of Connecticut, stayed in the "time-travel closet," as he put it, keeping his desire to build a time machine under wraps for fear of ridicule.
Today, with other established physicists speaking openly about time travel, Mallett is finally able to talk unabashed about his research. Not only that, he and other like-minded physicists are publishing their findings in peer-reviewed journals - something hardly imaginable just a decade ago.
Time travel, of course, has been a favorite topic for science-fiction writers for more than a century, from Wells's pioneering novel to the campy "Back to the Future" movie trilogy. But the scientific urge to investigate time travel is about more than sci-fi fantasies. Contemplating time travel is forcing scientists to confront some of the most profound issues in physics, from the nature of the universe's ultimate laws to fundamental questions about the nature of space and time.
Mallet's proposal seems innocuous enough: He's currently designing a table-top experiment using a ring of high-powered lasers. The idea is that light carries energy, and, as Einstein showed, energy is equivalent to mass - therefore beams of light can distort spacetime, just as large masses do. (The warping of spacetime by large masses was the prediction that made Einstein famous; the idea was confirmed when starlight was seen to be displaced by our sun during a solar eclipse in 1919.)
Inside Mallett's circle of laser beams, empty space would become "twisted" in much the same way that milk in a coffee cup begins to swirl when the coffee is stirred. If the beams of light are intense enough, the warping of space and time close to the beams could be severe enough to create a "loop" in time, Mallett says.
An object or particle that traveled along such a curve would, in theory, travel into its own past, just as walking around the block brings you back to your house. His goal is to send a stream of neutrons through the light beams - and, he predicts, transport them back in time by a tiny fraction of a second.
Mallett's work has brought mixed reactions. The head of his department at the University of Connecticut, William Stwalley, told New Scientist that although he was intrigued by the challenges of the experimental design, making any sort of time machine "seems like a distant improbability." Two physicists at Tufts University, meanwhile, recently wrote a critique of Mallett's theory, suggesting that any closed timelike curves he creates would need to be bigger "than the radius of the visible universe."
Of course, it's the philosophical problems that cause the most fuss - and which seem to inspire writers and filmmakers. The most famous dilemma is the so-called "grandfather paradox," in which a time traveler kills his grandfather, thus preventing his own birth. And yet, there's nothing in the known laws of physics that specifically prohibits time travel. As Case Western University physicist Lawrence Krauss has put it, "Einstein's equations of general relativity not only do not directly forbid such possibilities, they encourage them."
Most physicists suspect it would take extremely exotic structures - such as a black hole - to warp spacetime to the degree required to send something back in time. One favored scenario (at least on paper) is a so-called "wormhole," a pair of black holes that are connected via a kind of "tunnel" through spacetime. While a run-of-the-mill black hole would squash any traveler who entered it, a wormhole, in theory, could act as a "bridge" across distant points in space and time, depositing the traveler into another universe, or perhaps a distant part of our own universe, depending on where (and when) the two ends of the wormhole are located. While allowed by theory, however, there is no evidence so far that wormholes actually exist.
But imagine, for a moment, that time travel is possible. How could the paradoxes be resolved? One possibility is that the laws of physics somehow forbid such acts as the killing of one's grandfather, through a mechanism not yet understood (this is what Stephen Hawking has called the "chronology protection conjecture"). A more intriguing possibility is that a time traveler who alters the past would then experience a "new future" - a new universe, perhaps. In other words, if you kill your grandfather, you would find you now inhabit a different universe - with a different history - from the one you left behind.
"Our universe will not be affected by what you do in the past," Mallett said with confidence.
Time travel into the future, incidentally, is not as controversial: Thanks to the time-dilation effects described in the first part of Einstein's theory, known as special relativity, traveling into the future is as simple as moving quickly. The Apollo astronauts, for example, returned to Earth having aged a tiny amount less than their stay-at-home colleagues - because they had effectively traveled into the future (just a few hundredths of a second, mind you, because their speed was still a snail's pace compared with the speed of light).
Even if time travel is eventually shown to be impossible, that, too, would be a tremendously important discovery. As Hawking has put it: "It is important that we understand why it is impossible."
Dan Falk is a science journalist in Toronto. His latest book, "In Search of Time: The Science of a Curious Dimension," will be published by St. Martin's Press this fall.
Hometown: born in Roaring Springs, Pa., raised in the Bronx, now lives in East Hartford, Conn.
Education: BS from Penn State in 1969, PhD, also from Penn State, in 1973.
Favorite time-travel movies: "The Time Machine" (1960), "Deja Vu" (2007), Back to the Future (1985), "Planet of the Apes" (1968), "Somewhere in Time" (1980), "Frequency" (2000), "The Butterfly Effect" (2004), and "TimeCop" (1994) - "People ask me all the time: 'Wouldn't there be abuses if time travel were possible?' I like this movie because it addresses how time travel would have to be regulated like everything else in our lives."
His 2007 book: "Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality."
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