Horava-Witten theory

The Horava-Witten braneworld is an 11-dimensional world. It can be schematically represented by 2-dimensional branes. It is acting to bind two parallel branes. Each of these branes has 9 spatial dimensions. The bulk has 10 dimensions of space, for a total of 11 spacetime dimensions. This is considered to be the first braneworld theory that was derived directly from string theory.  In the Horava-Witten braneworld, each of the branes will contain a different set of particles and forces. There is enough contained within these two branes to include the entire Standard Model. This would include us. All of the particles of the Standard Model can be understood as living on either of these two branes.

The 11th dimension, in this case, is bound between two branes. The Horava-Witten braneworld involves 6 unseen dimensions. Petr Horava and Edward Witten assumed that these 6 dimensions curled up into a Calabi-yau manifold. Thus, we are left with the effective 5 dimensions bounded by the 4-dimensional branes.