Search for Extra Dimensions

Traditionally, physicists have mathematically explained all that happens in the world by using a "standard model." In this system, all matter is made of lightweight "leptons" (such as electrons and neutrinos) and quarks. Three forces manipulate these particles: electromagnetism, and strong and weak nuclear reactions. But, this traditional approach doesn't explain gravity, the fourth force. The conventional rules of quantum mechanics have been successfully married with Einstein's Theory of Special Relativity, which explains the behavior of very fast objects -- but not with his Theory of General Relativity, the guidebook to gravitational force. Mathematical gobbledygook usually results from trying to combine quantum mechanics and general relativity. Consequently, we still don't know, for example, what happens to particles sucked into a black hole.

In an effort to uniformly explain all events, physicists introduced the notion of an extra dimension, and eventually proposed that it may be curled up like an unimaginably small ball, they said, on the order of the Planck scale -- the smallest unit of length in the universe (10 to the minus 33 centimeters).

The idea of an extra dimension was resurrected yet again in the late 1990s, as scientists began to ask whether Newton's Law of Universal Gravitation reliably predicts gravity's behavior below the centimeter scale, according to Maria Spiropulu, a 32-year-old scientist with the Enrico Fermi Institute at the University of Chicago. Since then, Spiropulu reported to AAAS attendees, experiments have shown that Newton's Law is valid down to the 200-micron level. That is, gravity "follows the rules" at that scale. But, the physical reality below this level remains a mystery. Somewhere within the Planck scale, or at extreme energy levels, an incredibly small extra dimension may finally combine gravity and electromagnetism, Spiropulu suggested.

"We're very close into the energies where we can see effects of a very low-energy Planck scale," she said. "If an extra dimension is mirroring the Planck Scale, that means that gravity and the electromagnetic theory is going to be unified tomorrow." Gravity, Spiropulu said, may soon be unified in an "unexplainable hierarchy of scale."

-- Ginger Pinholster

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