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Scientific proposals for time machines are always a letdown: If you had an infinitely long cylinder. If you could get a glob of stuff with negative energy density. If it could resist quantum-mechanical fluctuations. Physicist Amos Ori of the Israel Institute of Technology in Haifa hasn't exactly invented the flux capacitor used in Back to the Future, but his latest idea has a couple things going for it: finite size and no need for questionable negative energy to prop it open. He reasons that a doughnut-shaped vacuum containing a certain doughnut-shaped gravitational field in otherwise ordinary space constitutes a time machine. Fly your spaceship into the toroid and exit at any time of your choosing, as far back as the creation of the time machine. The remaining questions are whether it is stable and how to set one up—Ori does not know how but speculates that possibilities include focusing gravitational waves or whirling some really massive object around. Catch the July 7 Physical Review Letters for more.
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