String Theory: Brane Worlds
Ever since the inception of string theory, one of the major conceptual hurdles has been the addition of extra dimensions. With the introduction of all of these new objects, string theorists have begun exploring what they mean. One major step is the introduction of brane world scenarios, where our 3-dimensional universe is actually a 3-brane.
These extra dimensions are required so the theory is consistent, but we certainly don’t seem to experience more than three space dimensions. The typical explanation has been to compactify the extra six dimensions into a tightly wound object roughly the size of the Planck length.
In the brane world scenarios, the reason we perceive only three spatial dimensions is that we live inside a 3-brane. There’s a fundamental difference between the space dimensions on the brane and those off the brane.
The brane world scenarios are a fascinating addition to the possibilities of string theory, in part because they may offer some ways in which we can have consistent string theories without resorting to elaborate compactification scenarios. Not everyone is convinced, however, that compactifications can be eliminated from the theory and even some brane world theories include compactification as well.
Some specific brane world scenarios have been proposed, which offer some intriguing explanations for aspects of our universe, such as how to resolve the hierarchy problem. In fact, the idea of brane worlds allows you to consider the possibility of escaping our universe and traveling to a different universe on another brane!