# The great beyond

By Marcus Chown SPACE may have a fifth dimension—one more than the four we are familiar with, say two physicists. And it could be infinite, unlike the tiny extra space dimensions that have been proposed in the past. “Incredibly, it could have gone completely unnoticed until now,” says Raman Sundrum of Stanford University in California. Physicists take extra dimensions seriously because superstring theory, the best candidate for a “theory of everything”, requires at least nine space dimensions. “There are two ways the extra dimensions could conceal themselves from view,” says Sundrum. “One is if they are rolled up far smaller than an atom. The other is if the Universe is confined to a kind of lower-dimensional island within higher-dimensional space.” An infinitely thin two-dimensional piece of paper would form such an island within normal 3D space. This latter possibility has now been explored by Sundrum and his colleague Lisa Randall of Princeton University, New Jersey. Remarkably, superstring theory requires lower-dimensional islands, or “branes”. And in superstring theory, nature’s three non-gravitational forces—the electromagnetic, weak and strong forces—can be naturally constrained to operate only within a brane. Gravity is a problem, however. “Gravity is intimately connected with the dynamics of space-time and so necessarily extends into all extra dimensions,” says Sundrum. Gravity from bodies such as the Sun should theoretically spread into this large extra space dimension, effectively diluting it within our Universe’s brane. “It would weaken with distance far faster than the inverse-square law decline that we observe,” says Sundrum. But Sundrum and Randall have discovered this may not be so. “The key is the gravity of the brane itself, which is enormous,” says Sundrum. Gravity pulls on all sources of energy, including the energy contained in a gravitational field. “Consequently, the gravity of the brane pulls on the gravity of objects like the Sun, preventing it from extending very far beyond the brane,” says Sundrum. Crucially, with gravity confined to the brane, the force is undiluted and displays the familiar inverse-square law. And the mechanism for confining gravity works no matter how big the extra space dimension. “What’s so amazing is that the theory mimics familiar four-dimensional gravity so well that it would be very difficult to tell that there is an extra dimension,