Road to Reality
The vacuum is full of 'virtual' particles - particles that we feel only as forces such as inertial resistance to acceleration, nuclear confinement of charged particles, electromagnetism and gravity.
Aound any particle core some of these particles will be attracted, forming a polarised veil which acts like a dielectric, shielding the charge of the core. Electron cores tend to attract virtual positrons, leaving behind an outer zone of virtual electrons. This shields the real electron core charge by a factor of 137, the 'magic number' of QED. For the massive quarks, you get virtual quark pairs polarising around them. This limits the range of the colour charge of quantum chromodynamics. Gravity is just the shielding of a background pressure of the virtual particles in the vacuum. Because all energy has speed c, as per Einstein's E=mc^2, gravity goes at c.
We can visualise an emerging unified force as a progressive shielding effect by the polarised vacuum on the particle core. The strong nuclear force is the basic force, and gets progressively filtered down by the polarised virtual charges of the vacuum around the particle core until we get through electromagnetism, weak force, and finally gravity. No extra dimensions!
Dr Peter Woit has very interesting ideas on the problem of the actual particles themselves (I'm sticking to forces at present): http://www.arxiv.org/abs/hep-th/0206135