###### 22 February 2002

# In Search Of Extra Dimensions

##### by Kate Melville

"Reality is merely an illusion, albeit a persistent one," according to the late Albert Einstein. But, "if everything is an illusion and nothing exists," humorist Woody Allen has observed, "I definitely overpaid for my carpet."

Hang onto your carpet receipts:

Our understanding of reality - that is, a world where events happen over time within a three-dimensional space - may be turned on its head by the year 2005, scientist Maria Spiropulu said today during the American Association for the Advancement of Science (AAAS) Annual Meeting.

"The way we think about things is about to change completely," said Spiropulu. "This is truly a revolution in the way we understand our world."

Spiropulu, a 32-year-old scientist with the Enrico Fermi Institute at the University of Chicago, is hot on the trail of extra dimensions. She's using new methods to prove, experimentally, whether our reality is more complicated than we previously assumed.

"We are very close" to a new reality, she said. "Right now, we imagine space and time as a static question, and we solve equations as a function of space and time. But, what we're learning is that, at the very large scale or the very small scale, space and time are dynamic. What is happening at those scales, we cannot explain. So we have to wonder, do these scales hold some 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, physicist Gunnar Nordstrom (1881-1923) first introduced the notion of an extra dimension at the beginning of the 20th century. Perhaps, he thought, gravity happens in a realm we don't understand and can't mathematically define. Some 10 years later, Theodor Kaluza (1885-1954) and Oskar Klein (1894-1977) took Nordstrom's ideas another step forward: An extra dimension 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, Spiropulu explained. Physicists were energized in 1997 by the discovery of possible links between the standard model and "superstring theory"-the notion that a series of extremely tiny, vibrating strings may lurk beneath the level of quarks and leptons.

Researchers Nima Arkani-Hamed, Savas Dimopoulos, and Gia Dvali then caused further excitement, by suggesting that at least one of these tiny dimensions might, in fact, be large enough to measure. Still, no one has produced undeniable proof of superstrings, and many questions persist.

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."

Various scenarios or "frameworks" are emerging to describe a mysterious sister world where, as Alice in Wonderland once remarked, "nothing would be what it is, because everything would be what it isn't."

Our three-dimensional world includes the coordinates X, Y, and Z, extending infinitely throughout the universe. But, some researchers have proposed that extra dimensions may be finite, and compacted around a sphere, pole, or other geometrical shape. Others have said that quarks, the standard-model particles, may have "technicolor" cousins in another realm. Or, quarks and neutrinos may exist in a mirror-world, as "squarks" and "sneutrinos."

To learn more about what's happening at the very small scale, Spiropulu and her colleagues are staging high-energy particle collisions. Extra dimensions, she explained, would leave behind a "signature," and she hopes to detect it. The classic signature might be a graviton-the carrier of gravity-capable, perhaps, of trickling to another dimension. In her experiments, protons (the hydrogen nucleus is a proton) going at almost the speed of light smash head-on into anti-protons. "What comes out," she said, "is a graviton, escaping into an extra dimension, and leaving a viable signature in your detector."

In particle collisions, the conservation of energy and momentum can be measured, so that what goes into the initial experiment must jive with what's left over, post crash-test. "If it doesn't add up and you have significant imbalance," she explained, "that is a viable signal that there is an extra dimension where, if these theories are valid, gravity may become very strong, and other weird properties might kick in. The idea is that there may be a form of super-gravity in the extra dimension."

Spiropulu shared the latest experimental findings at the AAAS meeting, including a completely new-and what she described as "totally innovative strategy"-worked out by Harvard's Nima Arkani-Hamed and others for "dynamically generating an extra dimension and then testing it," rather than the opposite, more conventional strategy: Searching for proof of an assumed extra dimension.

"We're looking at some really neat, new ideas," she concluded. "We hope by 2005 to have great results on this topic."