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The laws of physics seem to allow time travel, but no one has had much hope of building an actual time machine because it would take such exotic conditions and materials.

Now, physicist Amos Ori of the Technion Israel Institute of Technology in Haifa has come up with a potentially more practical time machine design. Unlike most previous proposals, this one requires only normal matter and the vacuum known to exist in space, says Ori.

One type of time travel occurs routinely here and now: our inexorable one-way drift into the future. Einstein's special theory of relativity revealed the possibility of accelerated travel into the future. Suppose a person spends a year in a rocket that's traveling slightly less than the speed of light. Because motion at such enormous speeds drastically slows the clock for the traveler, that person could return to Earth to find that many years had elapsed at home. In that way, a traveler could leap into the future.

Retreating into the past is another matter, but one that relativity theory also suggests might be possible. The theory shows that gravity curves space-time and slows clocks. That's why time-travel theorists have proposed that regions of space-time might naturally, or by human intervention, be made to curve back onto themselves. Someone moving around such a loop could travel back in time .

A new version of such a loop is what Ori proposes in the July 8 Physical Review Letters. The loop would form within an empty, donut-shaped region of space-time enveloped by a sphere of normal matter, he says. The distortion of space-time in the central donut would result from other huge nearby masses, perhaps including a black hole, or from interference of gravity waves propagating through the donut. To return to the past, a traveler in a rocket would zip around inside the donut, receding a little further into the past with each orbit, Ori says.

Ken D. Olum of Tufts University in Medford, Mass., is skeptical that Ori's concept could succeed. Until now, scientists have called for using prodigious amounts of an exotic entity known as negative energy, which theorists expect to exist only in minuscule quantities, for time machines. In 1992, Stephen W. Hawking of Cambridge University in England proved a theorem that rules out time machines built without negative energy, Olum notes. Ori counters that Hawking's analysis involves certain conditions that don't apply to his concept.

Igor D. Novikov of the Niels Bohr Institute in Copenhagen finds Ori's concept of a donut-shaped core "very original and probably without unrealistic parts." The new proposal "is a valuable contribution to studies of potential time machines," he says.

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