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