125 lines
6.7 KiB
Plaintext
125 lines
6.7 KiB
Plaintext
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Playing fast and loose with time: a new study suggests that time travel
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is not quite impossible.
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by William F. Allman
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Time travel has popped from the annals of science fiction into the
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realm of scientific respectability at least in theory, that is.
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By combining two well-established principles originally outlined by
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Albert Einstein nearly a century ago in his theories of relativity,
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three theoretical physicists have proposed a novel scheme that appears
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to permit a limited sort of time travel. The scientists' result is
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not antything close to a blueprint for building the sort of time
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machine that science-fiction heroes are forever leaping into to
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explore the lives of cave men or peek into the future. But it
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suggests that either Einstein's equations are wrong or that the
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universe is governed by some very different principles from what
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physicists have long assumed.
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The notion that the causes of an event taking place in the present
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could be eliminated by traveling into the past and changing history
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has always made physicists (not to mention stock speculators and
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newspaper publishers) squeamish about the idea of time travel;
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physicists have thus held that there must be laws that provide a kind
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of "cosmic censorship" that prevents moving through time, and thus any
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tampering with the sequence of events. Various theoreical scenarios
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for time travel have been cooked up in the past, but in each case
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physicists have found physical laws that blocked it.
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The new study, however, appears not to violate any known physical
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laws. "We're asking, if you take Einstein's equations far enough,
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will they get you in trouble?" says Michael S. Morris of the
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University of Wisconsin, one of the co-authors of the new study.
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"We're suggesting that maybe they will, because at some level we may
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have to abandon either Einstein or causality"-the' basic principle of
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physics that one event
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Time flies. The authors' hypothetical time "machine" starts with
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Einstein's famous, and apparently paradoxical, discovery that time is
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not constant throughout the universe, but rather varies depending on
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the velocity at which the timekeeper is traveling. While to an
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astronaut, a clock on board a speeding spaceship will appear to tick
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at a normal rate, to a stationary observer on Earth, the clock will
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appear to be marking time very slowly. This apparent slowing of time
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applies not only to clocks but to everything in the spaceship,
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including the astronauts, and has given rise to what is known as the
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"twin paradox": When the speeding astronaut returns to Earth, he will
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have aged less than a twin brother he left behind. This bizarre
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time-slowing effect has been demonstrated by researchers who
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synchronized two highly accurate atomic clocks and then flew one of
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the pair aboard a high-speed jet for hours. When the two clocks were
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reunited, the airborne clock had fallen slightly behind its stationary
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twin.
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The second Einsteinian principle that goes into the time machine is
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the warping of space by gravity. This idea is often explained by
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analogy to a worm crawling over a sphere. To the worm, the world is
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flat. Crawling from one spot on the sphere to another in what seems
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to be a straight line, the worm doesn't realize that it is actually
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tracing out a path that curves in three-dimensional space to follow
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the sphere's contour. The worm also doesn't realize that it could
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take a shortcut by burrowing through the core of the sphere.
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Similarly, what appears to us as three-dimensional space is, according
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to Einstein's theory of relativity, bent in dimensions that we cannot
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easily imagine, and a "wormhole" could connect two seemingly distant
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points in space.
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To turn a wormhole into a time machine, say the physicists,
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technicians in some "arbitrarily advanced" civilization could harness
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the gravitational forces necessary to make one end of the hole move
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back and forth at extremely high speed while keeping the other end
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stationary. Like the astronaut in the twin paradox, this rapidly
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moving end of the its stationary counterpart. By entering the "older"
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end of the wormhole and coming out the "younger" end, a person could
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theoretically travel from the present to the past.
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Keeping the door open. While the physicists' calculations suggest
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there's nothing to prevent all this from happening, they admit that no
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one has actually ever seen a wormhole. Calculations by Morris and
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co-authors Kip Thorne and Ulvi Yurtsever of the California Institute
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of Technology imply that a wormhole can be kept open only under some
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strange and exotic physical conditions. Phenomena that under normal
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circumstances exist only in infinitesimally small particles for
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infinitesimally short periods of time would have to exist throughout
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the wormhole for as long as it remained open.
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Even if a wormhole time machine were possible, it wouldn't permit the
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kind of time travel fancied by H. G. Wells and others who have mused
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on the possibility of traveling far into the past or future. Even
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though members of an advanced civilization might be able to cause one
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end of a wormhole to age less slowly than the other, at best they
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would only be able to make time stand still at the instant the tunnel
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was created, not move it back any further into the past, and they
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couldn't move the other end ahead into the future.
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There's at least some circumstantial evidence that the more
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far-reaching kind of time machines envisioned by sciencefiction
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writers will never be created, even by the most technologically
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advanced civilization imaginable. If such a machine were ever to be
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built, a traveler from the future probably would already have shown up
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here by now.
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