D-branes, while gravitons, as closed strings, are free to swim through the bulk. Thus, composed of closed strings, we remain perpetually landlocked—with only our gravitational emissions able to plunge beyond our brane.
Because of gravity’s special ability to escape, its immersion in the bulk would effectively dilute it. The larger the bulk, the less contact with our brane it would have and the weaker it would appear. Gravity becomes the puny partner of the other forces.
String theorists soon realized that the relative weakness of gravity was one of M-theory’s strengths. From the time of Dirac, researchers have sought a solution to the question of why the other three sub-atomic forces are so much more powerful than gravitation. With the hope that M-theory would resolve this riddle, several groups set out to construct “brane-world models”: interplays of bulk and brane designed to replicate precisely gravity’s distinctive behavior.
In 1998 a team of Stanford physicists published one of the first and simplest brane-world scenarios. Known as the “ADD model,” for the initials of its designers Nima Arkani-Hamed, Gia Dvali, and Savas Dimopoulos, it offered a bold attempt to resolve the hierarchy puzzle and other issues. Remarkably—for the abstruse world of string theory—it stuck its neck out with clear, testable predictions.
According to the ADD scenario, everything we see in space— visible galaxies, quasars, and the like—resides on a D-brane. Separated from our brane by roughly a millimeter (1/25 of an inch) is a second shadow realm. In between, like the filling in a sandwich, is a thin layer of bulk, accessible only by gravitons.
The ADD team chose that particular thickness of the bulk to model the actual weakness of gravity compared to the other forces. Too much filling would create an indigestibly large discrepancy; too little would not produce enough of a bite to provide a distinction. Even for the best matching case, the researchers realized that their