Danger - physics lecture gobbledegook ahead
There are two major models which attempt to extend physics beyond the so-called Standard Model; supersymmetry (SUZY), and string theory. There are several possible forms of each. Both models are extensively studied theoretically, neither has any experimental support as yet.
Supersymmetry does not require a particular number of dimensions beyond the 4 we perceive (three spatial dimensions plus time), but allows for the possibility of up to 11 spatial dimensions.
String theories requires more than the three spatial dimensions and time. The most widely-studied string theory, the membrane or M-theory, requires 11 spatial dimensions. (This is the one Kurch mentioned. Always knew you were a Brane-y fellow, Kurch!!) The related superstring theory requires ten, while the bosonic string theories require 26 dimensions. (Bosons are a class of subatomic particles, defined as having integer spins, that includes the photon, the name for a particle of light.) All these string theory dimensions are for flat space; if space is curved, the numbers go up.
My beginning students often ask why we only perceive three spatial dimensions if there are more than three. The basic answer is, we don't know for sure. There are two popular ideas. One is that the universe is smaller in the other dimensions than we can perceive. If the universe were trillions of light-years across in "our" three dimensions, but the size of an electron in the others, we would not perceive the others. Another idea is specific to the M-theory of string theory, and says that the universe we perceive is only a three-dimensional (+1 for time) subset of the whole.
The question my students ask most often, though, is "What makes us believe this ****?" I spend a great deal of time on that in the class for non-majors. We don't believe either of these yet (although emotions among researchers can run high). Both models developed from a mathematical explanation called the Standard Model. The Standard Model is a set of equations. The several solutions to the equations describe the properties of known subatomic particles, which gave us confidence in the equations, but there were more solutions than there were particles. When new particles were discovered, and they turned out to have the properties the equations predicted, the mayor of Physicsville declared a holiday, and there was much rejoicing, Robin's minstrels sang, etc.)
But there are properties of the universe not covered in the standard model, so after the rejoicing, the mayor put all the physicists back to work on extending the Standard Model. SUSY and string theory are possible extensions. Each makes more predictions (new particles and properties of the universe), but our technology hasn't yet been able to check any of the new predictions. [Not quite true, we've already eliminated a few minor versions of each.] That's why physicists get so excited about new particle accelerators and telescopes - they offer the chance to check the predictions and maybe decide if SUZY or string theory is the way to go, or if we're way off base and have to look in a new direction.
Wikipedia has decent articles on the Standard Model, and on both supersymmetry and string theory, but they assume you remember some of your modern physics class. If you've followed the discoveries of quarks in popular magazines like Scientific American, they should give you the gist.
http://en.wikipedia.org/wiki/Standard_Model
http://en.wikipedia.org/wiki/Supersymmetry
http://en.wikipedia.org/wiki/String_theory
🙂 E. Bunbury
