杏吧原创

SPACE may have a fifth dimension鈥攐ne 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.

鈥淚ncredibly, it could have gone completely unnoticed until now,鈥� says Raman
Sundrum of Stanford University in California. Physicists take extra dimensions
seriously because superstring theory, the best candidate for a 鈥渢heory of
everything鈥�, requires at least nine space dimensions.

鈥淭here are two ways the extra dimensions could conceal themselves from view,鈥�
says Sundrum. 鈥淥ne 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.鈥� 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 鈥渂ranes鈥�. And in
superstring theory, nature鈥檚 three non-gravitational forces鈥攖he
electromagnetic, weak and strong forces鈥攃an be naturally constrained to
operate only within a brane. Gravity is a problem, however. 鈥淕ravity is
intimately connected with the dynamics of space-time and so necessarily extends
into all extra dimensions,鈥� says Sundrum.

Gravity from bodies such as the Sun should theoretically spread
into this large extra space dimension, effectively diluting it within our
Universe鈥檚 brane. 鈥淚t would weaken with distance far faster than the
inverse-square law decline that we observe,鈥� says Sundrum.

But Sundrum and Randall have discovered this may not be so. 鈥淭he key is the
gravity of the brane itself, which is enormous,鈥� says Sundrum.

Gravity pulls on all sources of energy, including the energy contained in a
gravitational field. 鈥淐onsequently, the gravity of the brane pulls on the
gravity of objects like the Sun, preventing it from extending very far beyond
the brane,鈥� 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.

鈥淲hat鈥檚 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,鈥� says Randall.