The front-runner for a "theory of everything," which would corral all the known forces and particles into a single equation, is a mathematical tangle of "strings" that wander through 10 dimensions. Seen in our three-dimensional world, strings would appear as particles. Unfortunately, string theory is generally considered untestable: The seven extra dimensions are usually assumed to curl up so small that no conceivable experiment would detect them.
Now some physicists are proposing a watershed: Some of the dimensions could be as large as a millimeter and might be easily probed. "It's really profound," says University of Michigan, Ann Arbor, physicist Gordon Kane. "It's hard to say it strongly enough."
One group, who presented their work last week at the International Conference on High Energy Physics in Vancouver, Canada, looked at the possibility of loosening up the seven dimensions that the particles of the everyday world don't inhabit. Stanford University physicists Nima Arkani-Hamed and Savas Dimopoulos, with Gia Dvali of the International Center for Theoretical Physics in Italy, showed that if gravity exists in these extra dimensions, its effects could be easily detected, since gravity would be millions of times stronger at short distances than traditionally assumed. At least two tabletop experiments are planned to probe gravity's strength at distances down to a micrometer.
A second group, Keith Dienes, Emilian Dudas, and Tony Gherghetta at CERN in Switzerland, has shown how allowing a curled up dimension to unravel a bit allows three forces (the electromagnetic, strong, and weak) to unify into a single force at an energy a trillion times lower than previously thought. "Everyone thought the extra dimension would destroy the unification" and the theory, Dienes says, but add it and "bingo, the three forces unify almost immediately." If true, the strength of the three forces would have almost the same strength when probed at the Large Hadron Collider under construction at CERN.
The reaction to these ideas is mixed: "It's crazy, but it's not that crazy," says John Schwarz, a string theorist at the California Institute of Technology. Still, "it's a long shot," he says. Schwarz and others suspect an extra, "large" dimension for gravity might be inconsistent with various astrophysical measurements or with the current picture of the big bang. So far no one has been able to kill the idea. And, comments Juan Maldacena, a theorist at Harvard University: "In this field, any idea that is not obviously false is interesting."