Unseen Dimensions Could Explain Weakness Of Gravity
In just two years, a new proposal for solving some of science's most persistent puzzles -- most important, how to include gravity in an explanation of the fundamental forces of nature -- has become the hottest theory in physics.
Now, in an article titled "The Universe's Unseen Dimensions," Nima Arkani-Hamed of the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), Savas Dimopoulos of Stanford University and Gia Dvali of New York University explain their revolutionary ideas in the August, 2000, issue of Scientific American.
The notion sounds deceptively simple: besides the familiar three dimensions of space there may be other dimensions, too small to see yet perhaps as large as a millimeter.
Arkani-Hamed and his colleagues came up with the theory to explain why the Standard Model of particle physics can give a common explanation for all the forces of nature except gravity.
Another way of phrasing the conundrum is to ask, why is gravity so feeble? Although we think of gravity as strong -- we can get hurt if we fall down -- compared to electromagnetism, gravity is astonishingly weak. It takes the gravity of the whole Earth to hold a pin on a tabletop; a toy magnet can lift it easily.
Perhaps gravity only seems weak, however. If electromagnetism and the forces that act on quarks are confined to our familiar three dimensions of space and one of time, while gravity is free to propagate in all dimensions, we would experience only part of its effects.
It's a worldview that opens bizarre landscapes -- including possible parallel universes coexisting with our own, in real space and near enough to touch. Our familiar world may inhabit a kind of "membrane" in a multidimensional "bulk" of possible universes.
So strange is this picture that "the most extraordinary thing about the theory is that it didn't die an immediate death," says Arkani-Hamed, who is a member of Berkeley Lab's Physics Division and an assistant professor of physics at the University of California at Berkeley. "It explains a lot and raises a lot of possibilities, yet it contradicts no experimental results."
Many other attempts to explain the Standard Model's shortcomings have been made, but the new theory has an enormous advantage over them all: it can easily be tested in giant particle accelerators already under construction and in tabletop experiments already underway. Indeed, the theory's predictions will most likely be proved or disproved within a decade.
Proof will signal the biggest upheaval in fundamental physics since Isaac Newton saw the apple fall in 1665. Scientists have long assumed that G, the gravitational constant Newton devised to calculate the attractive force between masses at different distances, is fundamental and unchanging.
Arkani-Hamed and his colleagues suggest we have little reason for assuming that G is fundamental.
"It has only been measured down to about a millimeter," he says. "What if gravity is actually as strong as the other forces at distances we haven't measured yet?"
While in three spatial dimensions, gravity obeys an inverse square law -- if you halve the distance between masses, the gravitational attraction between them quadruples; cut the distance to a third, and the force increases nine times -- in four spatial dimensions, gravity increases or falls off as the inverse cube of the distance. With each additional dimension, the inverse law increases.
Two extra dimensions need only extend about a millimeter for gravity to be comparable in strength to the other forces.
There may be more than two extra dimensions, but even if there are seven -- a number favored by string theorists -- the distance across which the strength of gravity would rapidly increase would be as wide as a uranium atom, still vast compared to the tiny "rolled-up" dimensions of string theory.
The new theory of Arkani-Hamed, Dimopoulos and Dvali is not string theory, but they have shown that their theory of gravity in extra "large" dimensions is not only compatible with string theory, it offers interesting solutions to many outstanding questions.
For example, there may be other membranes in the bulk -- real worlds less than a millimeter from our own -- which communicate with ours only through gravity; invisible masses confined to these parallel worlds could be the universe's mysterious dark matter.
"Or instead of invoking parallel universes, we might live on a folded universe," Arkani-Hamed suggests. "In this view, 'dark matter' might be just ordinary matter, because the light from a star on a fold only one millimeter away might have to travel billions of light years along the wall to reach us. Although we feel its gravity, we haven't seen it yet."
Moreover, if the force of gravity increases dramatically at short distances, it may be possible for the next generation of accelerators -- such as Europe's Large Hadron Collider scheduled to begin operation in 2005 -- to create black holes, regions smaller than the radius of the extra dimensions where gravity is so strong that nothing can escape.
Since small black holes quickly evaporate by Hawking radiation (orphaned members of pairsm of virtual particles whose partners are swallowed by the hole, and which carry off some of the hole's mass), this low-energy radiation from a high-energy collision would be an unmistakable signal that a black hole had been created.
"All the old mysteries of the Standard Model can be addressed in this theory, and there is no conflict with either supersymmetry or string theory," says Arkani-Hamed. "If we do the experiments, we have a good chance of seeing evidence for or against these ideas in the next ten years." - By Paul Preuss
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