© Wikimedia Commons, CC Attribution-Share Alike 2.5 Generic license. Author: Lunch, 23 September 2007.
The string theories that correspond to quantum gravity, combined with the three other known forces, seem to require ten spacetime dimensions. We clearly live in four spacetime dimensions, so it is generally assumed that the other six dimensions must be curled up, or compactified, in a way that makes them impossible for us to see directly. The geometry of these compactified dimensions is complicated and difficult to visualize. The above image is a two-dimensional projection of a six-dimensional space that the compactified extra dimensions of string theory could occupy. (Unit: 4)