BEIJING, March 8 (Xinhuanet) -- English science fiction writer H.G. Wells wrote in 1895 about time travel in his classic "The Time Machine."

    Some scientists say time travel makes good fiction, and that it will always be just that -- fiction.

    There are a handful of scenarios theorists have suggested for how one might travel to the past, said Brian Greene, author of the bestseller, "The Elegant Universe" and a physicist at Columbia University.

    "And almost all of them, if you look at them closely, brush up right at the edge of physics as we understand it. Most of us think that almost all of them can be ruled out," Greene said.

    "Space and time are tangled together in a sort of a four-dimensional fabric called space-time," explained Charles Liu, an astrophysicist with the City University of New York, College of Staten Island and co-author of the book "One Universe: At Home In The Cosmos."

    Space-time, Liu explains, can be thought of as a piece of spandex with four dimensions.

    "When something that has mass -- you and I, an object, a planet, or any star -- sits in that piece of four-dimensional spandex, it causes it to create a dimple," he said. "That dimple is a manifestation of space-time bending to accommodate this mass."

    The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.

    Mathematically one can go backwards or forwards in the three spatial dimensions. But time doesn't share this multi-directional freedom.

    "In this four-dimensional space-time, you're only able to move forward in time," Liu told LiveScience.

    The most developed approach to time travel involves a wormhole -- a hypothetical tunnel connecting two regions of space-time. The regions connected could be two completely different universes or two parts of the same universe. Matter can travel through either mouth of the wormhole to reach a destination on the other side.

    "Wormholes are the future, wormholes are the past," said Michio Kaku, author of "Hyperspace" and "Parallel Worlds" and a physicist at the City University of New York. "But we have to be very careful. The gasoline necessary to energize a time machine is far beyond anything that we can assemble with today's technology."

    To punch a hole into the fabric of space-time, Kaku explained, would require the energy of a star or negative energy, an exotic entity with an energy of less than nothing.

    Another popular theory for potential time travelers involves cosmic strings -- narrow tubes of energy stretched across the entire length of the ever-expanding universe. Leftover from the early cosmos, cosmic strings are predicted to contain huge amounts of mass and therefore could warp the space-time around them.

    Cosmic strings are either infinite or they're in loops, with no ends, said J. Richard Gott, author of "Time Travel in Einstein's Universe" and an astrophysicist at Princeton University. "So they are either like spaghetti or SpaghettiOs."

    The approach of two such strings parallel to each other, said Gott, will bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.

    "This is a project that a super civilization might attempt," Gott told LiveScience. "It's far beyond what we can do. We're a civilization that's not even controlling the energy resources of our planet."

    Mathematically, you can certainly say something is traveling to the past, Liu said.

    "But it is not possible for you and me to travel backward in time," he added.

    Maybe if there were a theory of everything, one could solve all of Einstein¡¯s equations through a wormhole, and see whether time travel is really possible, Kaku said.

    "But that would require a technology far more advanced than anything we can muster," he said. "Don¡¯t expect any young inventor to announce tomorrow in a press release that he or she has invented a time machine in their basement."

    "If you want to know what the Earth is like one million years from now, I'll tell you how to do that," said Greene, a consultant for "D¨¦j¨¤ Vu," a recent movie that dealt with time travel.

    "Build a spaceship. Go near the speed of light for a length of time -- that I could calculate. Come back to Earth, and when you step out of your ship you will have aged perhaps one year while the Earth would have aged one million years. You would have traveled to Earth's future."

    (Agencies)