UPI (Sep 18, 1996) -- Several years ago, an "impossible" idea -- time travel -- began to look possible. Physicists tinkered with physicist Albert Einstein's equations and discovered that with a little tweaking, plus a trainload of good luck and a couple of trillion dollars, they might build a "time machine."
Step inside it and you could return to an earlier time, as did H.G. Wells' hero in his classic tale, "The Time Machine."
It was a neat idea, but now it's turning sour. As researchers have learned, Mother Nature has erected all kinds of wild, weird barriers to time travel -- barriers that may never be breached.
For time travel, "the situation is a hell of a lot messier than it was a few years ago," acknowledges physicist Matt Visser of Washington University at St. Louis. He recently wrote a 400-page technical book on time travel, entitled "Lorentzian Wormholes: From Einstein to Hawking," published by the American Institute of Physics.
Ever since Wells' masterpiece hit bookstores, time travel has been a staple of science fiction. Hollywood and the TV networks make zillions off time-travel fables such as "Back to the Future," "Timecop" and "Quantum Leap." The stars of these shows hurdle centuries as casually as if they were changing subway cars.
Want to see the dinosaurs? or witness Columbus' landing in America? or return to Dallas on Nov. 22, 1963 to see if JFK was killed by one gunman -- or two? "No problem," the science-fiction writers declare, "just step aboard my time machine!"
Einstein objected to the notion of time travel. Long before his death in 1955, he pointed out that if time travel were possible, then you could, in theory, go back in time and shoot your younger self. And if that were feasible (Einstein asked triumphantly) then how could you
have existed in the first place? Rather than try to reconcile such paradoxes, he concluded that time travel is impossible -- as impossible as having your cake and eating it too.
Still, like skeptical shoppers poking at suspicious-looking tomatoes, physicists enjoy tinkering with physical equations to see how much abuse they can take. In the 1980s, physicists Kip Thorne, Michael Morris and Ulvi Yurtsever of California Institute of Technology began wondering if they could use Einstein's equations to design a time machine -- on paper, of course. They drew inspiration from an idea in Carl Sagan's outer-space novel"Contact."
Sagan's heroes traveled to the stars via a "wormhole," an hypothetical tunnel through space and time that allows instant travel between distant points. Step into a wormhole and you would emerge in (say) another planetary system.
Thorne and his colleagues played with Einstein's equations and realized not one but two amazing things. First, the equations implied that wormholes might, indeed, exist and could be "constructed" in various ways, e.g. by generating intense gravitational fields to "warp" space. For now no one has a clue exactly how to do this.
Second, a wormhole might also permit travel through time. Step into the wormhole at, say, 12:00 noon and you might emerge at 11:59 a.m., a minute earlier. To make this happen would require a huge amount of scientific trickery -- e.g. moving one end of the wormhole while the
other end remains still.
Still, the bottom line was sensational: Despite Einstein's objections, time travel might be possible after all. Sadly, there was one hitch: You couldn't travel back to a moment older than the machine itself.
For example, if you created the time machine at 6:00 pm, then you couldn't enter it at, say, 7:00 p.m. and fly back to 5:59 p.m. You could travel back to 6:45 or 6:30 or 6:00, but no earlier. So you can forget about joining an expedition to hunt Tyrannosaurus rex or to interview Queen Victoria.
Nonetheless, other researchers were astounded by the possibility of any kind of time travel, even in this limited form. Thrilled, they investigated alternate designs for time machines.
The noted astrophysicist J. Richard Gottof Princeton University suggested weaving a time machine from cosmic "strings" -- hypothetical string-shaped objects that may float through space, millions of miles long and weighing as much as many suns. They're so massive that they would, according to Einstein's equations, alter the "fabric" of space and time in their vicinity. Gott said that by flying in just the right path near two cosmic strings, a rocketship could hurtle back in time.
Meanwhile, at the University of California at Berkeley, physicist Yakir Aharonov proposed a time machine similar to a giant balloon: It would instantly inflate or deflate, altering local space and time and hurtling an occupant through time.
Into this theoretical mania entered Stephen Hawking, British astrophysicist extraordinaire. Almost totally paralyzed by a neurological disorder, Hawking keeps busy calculating and communicating with others via a keyboard on his wheelchair.
Skeptical about time travel, he proposed what he half-jokingly called the "Chronology Protection Conjecture": i.e. Nature forbids time travel in various ways -- say, by generating so much radiation within a wormhole that it "boils" away before a spaceship can pass through it.
Besides, Hawking asked, if time travel is possible, then why haven't we been invaded by hordes of tourists from the future?
Time-travel buffs countered Hawking's objections with even more grandiose theories. The strangest is the "multiple universe" hypothesis, according to which our universe is just one of countless universes, many of which are virtually Xerox copies of each other.
That is, someone exactly like you, or almost exactly like you, exists in another universe somewhere. Now as you recall, Einstein opposed the notion of time travel because it would allow paradoxical events, e.g. you killing your younger self.
But that's no problem, claim multiple-universe fans. In their view, when one travels back in time, one is actually traveling into the past of a different, albeit similar-looking, universe. So when you travel back in time and kill "yourself," you're not really killing "you"; rather, you're killing an exact or near-exact copy of yourself!
Outlandish as it sounds, two Oxford University professors defended this notion in an article, "The Quantum Physics of Time Travel," for the March 1994 issue of Scientific American.
Still, like kids who gorge themselves sick on ice cream, even theoretical physicists can swallow only so much weirdness before they have to give up and lie down for a while. And judging by recent trends, the time-travel theorists are losing their former self-confidence.
Once optimistic about wormhole schemes, Visser now says "my gut feeling is that Hawking's Chronology Protection Conjecture is correct and that there is no time travel." Even Thorne agreed last year that for various technical reasons, wormhole voyagers would likely "get
fried" by various physical phenomena.
"If you watch 'Star Trek' and listen to all their babbling about wormholes," Visser adds, "you might wonder: What would we have to do to build one of these wormholes, which they talk about so happily?
"Well, we now know how really hard it is to build one of these things. It's clear now that it's just not technologically possible in the foreseeable future."
Copyright 1996 by United Press International
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