A remake of the movie ?The Time Machine,? inspired by the H.G. Wells novel, is in the theaters, which makes us wonder, once again, if time travel will ever be possible. This, in turn, brings up the question of whether time can be speeded up or slowed down. It turns out Einstein already predicted this in 1905 in his theory of relativity that states that time and space are joined in our universe as a four-dimensional fabric known as space-time. And both space and time change, or warp, as mass or speed is increased.

Travel fast and time moves more slowly. Increase the mass around you to near collapsible levels and you get the same effect. This phenomenon has already been proven, but only at minute levels. ?We have a hard time perceiving how time can bend just like other dimensions, so Einstein?s predictions seem strange,? says J. Richard Gott, author of the book ?Time Travel in Einstein’s Universe? and a professor of astrophysics at Princeton University. ?But this appears to be the world we live in.?

In 1975 Carol Allie of the University of Maryland synchronized two atomic clocks and placed one on a plane and flew it around for several hours while the other one remained on Earth. When the airborne clock was returned to Earth, she compared its time with the one that hadn?t moved and found that time had moved a fraction of a second more slowly for the clock on board the plane.

In other experiments, scientists have used particle accelerators to speed elementary particles to nearly the speed of light. They found the accelerated particles decayed slightly more slowly than ones that remained sitting in the lab. To measure the effect of mass on time, scientists placed atomic clocks at the top and base of skyscrapers. They found the clocks at the base, closer to the mass of Earth, ticked more slowly than those up high.

Gott believes that the same principles that make the clocks tick slower on planes and low on Earth means that time travel is possible. But physicists believe true time travel requires a very fast space ship. By riding on a spacecraft that can travel at speeds of two hundred million meters per second, or about four hundred and fifty million miles per hour, a passenger would experience significantly slowed-down time.

The slowed time would not be noticeable to the traveler, just as flying in an airplane doesn?t feel any different than sitting on Earth. But once the traveler returned, he?d find that those who remained on Earth had aged at a faster rate. Time would have slowed down for him. Time passed at its regular rate on Earth, while to the spaceship traveler, it crawled. So the traveler?s return to Earth is, in effect, a trip to the future.

Gott estimates that our most experienced time traveler is Russian cosmonaut Sergei Avdeyev who was in orbit for a total of 748 days during three space flights. Avdeyev?s prolonged travel made him a younger man by about one-50th of a second than those of us who have remained on Earth.

There are significant obstacles to building a spaceship that can travel at speeds close to the speed of light. It requires intense levels of energy that we currently can?t achieve. But Paul Halpern, a physicist at the University of the Sciences in Philadelphia and author of the book ?Time Journeys: A Search for Cosmic Destiny and Meaning,? believes this will be possible in the future. ?There?s no reason to think that in the next few hundred years, we won?t be able to reach these speeds,? he says. ?There?s no reason to discount the idea.?

If we learn how to travel into the future, we also need to know how to return by traveling to the past. According to Einstein’s theory, approaching the speed of light would theoretically slow down time, traveling at the speed of light would make it stand still and traveling faster than the speed of light would reverse time.

But Einstein also showed that traveling at or faster than the speed of light is impossible because mass at these speeds becomes infinite. British theoretical physicist Stephen Hawking says that means traveling back in time is impossible. However, other scientists think there may be a way to find ?shortcuts? to the past.

In the late 1980?s Kip Thorne of the University of California at Berkeley suggested that objects known as wormholes exist in space. These objects would be two connecting black holes whose mouths make up a tear in the fabric of space-time. By finding a wormhole and stretching it so one mouth extends light years away from the other, the wormhole could provide a passageway to a past or future point.

Thorne developed the theory after Carl Sagan had asked him whether there was a way he could send the heroine of his novel ?Contact? billions of miles to meet an alien and return home the same day. The book and the movie featured Thorne?s wormhole phenomenon.

However, the wormhole theory still has significant problems. Besides the problem of locating a real wormhole, scientists would also need to find a way to keep the wormhole?s entrances open long enough for a person to pass through. Quantum forces would cause the time machine to instantly squeeze shut. ?Time machines to the past are projects only a super civilization could attempt,? says Gott. ?It would require a civilization that has the resources of the galaxy at its command.?

Finally, there is the issue of the Grandfather paradox. What happens if a person travels back in time and kills his grandfather before the traveler?s mother is conceived? Does that mean the time traveler suddenly no longer exists?

Hawking says that time protects itself from such scenarios by preventing time travel to the past. Others suggest that the time traveler would simply enter a parallel universe that evolves along its own separate route in space. Halpern, says that the past, present and future may all exist at the same time and influence each other simultaneously in our universe.

Physicists say part of the reason why time travel may be difficult is our perception is limited to a fairly moderate world where the laws of physics are not pushed to their limit. Halpern says, ?Perhaps all of space and time exist at once and our travels through time are simply something that our conscious minds undertake. If we could break this force that is propelling us forward, maybe we could travel back in time.?

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