for National Geographic News
As any self-respecting science fiction fan knows, wormholestheoretical shortcuts through space and timemake for excellent time travel portals.
The latest movie to transport people into the past is this summer's A Sound of Thunder, based on the classic 1952 Ray Bradbury novella. In it, a group of hunters build a time machine, which looks like a wormhole of sorts, to travel back to the dinosaur era. There, things go awry when one hunter kills a butterfly, which completely changes the course of history.
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The movie was widely panned by critics and seems to have quickly slipped out of theaters. But the questions it raisesthe mystery of time and the possibilities of traveling through itremain among the thorniest in physics, keeping a growing number of scientists occupied.
It's not like scientists are looking for a way to actually travel through time. But some believe that theorizing about how it could be donemaybe by using a wormhole in spacewill help them understand and perhaps even revise the laws of physics.
"Traversable wormholes are extremely useful as gedanken experiments"the term describes experiments that can be reasoned theoretically but are impractical to carry out"to probe the limitations of general relativity," said Francisco Lobo, an astrophysicist at the University of Lisbon in Portugal.
Albert Einstein's relativity theory set the speed of light as the universal speed limit and showed that distance and time are not absolute but instead are affected by one's motion.
A clock in motion will always appear to run slowly compared with one at rest, because time is relative to the speed at which a body is moving. That fact would, in theory, allow for time travelat least if you have a very fast spaceship.
Consider this: If an astronaut travels into space for six months at a substantial fraction of light speed and takes another six months to return to Earth, he would land in the future.
While a year will have elapsed on the astronaut's clock, tens of thousands of years may have gone by on Earth, depending on how close to light speed the astronaut traveled.
"The bottom line is that time travel is allowed by the laws of physics," said Brian Greene, a Columbia University physics professor and the author of The Fabric of the Cosmos: Space, Time, and the Texture of Reality.
But the laws of space and time as Einstein laid them out may be revised by the quirky rules of quantum theory. Quantum theory describes the microscopic randomness that fills the universe.