Planck level physics (10^-35 m)

The most plausible candidate for a TOE is that of string theory, unfortunately this subject has become rather confused and very technical. The "evangelical" theory of strings demands that there should be 10 dimensions of spacetime together with supersymmetry that relates bosons and fermions to each other in a somewhat prejudiced manner. In recent years it has also become popular to invoke a dual symmetry that allows the 6 'compactified' dimensions of spacetime to rearrange themselves somewhat similar to an amoeba suffering from indegestion :-)
Perhaps all of these properties of string theory are true and unavoidable. For my part I am content with the property that strings do not suffer from the inconsistent renormalisation of Feynman diagrams that arise with point-like gravitons in attempts to formulate theories of quantum gravity.
The generalisation of the Feynman diagrams in the standard model gives rise to the so called "world sheets" of perturbative string theory. Altough these world sheets do not require renormalisation they are not a complete description of a TOE since at very high energy (called the Planck density) when one sums up all the diagrams they do not sum up to a finite value. This is known as a blowup of the interaction (in QED such a blowup occurs at an energy of 10^280 MeV so a TOE should presumably fix this up)
The solution to this is thought to be a non perturbative formulation of the theory.

The best such approach looks to be a theory where the strings interact not only with joining and splitting of each other but also by forming knots and links. Such a theory would also require that space has only 3 dimensions since knotting and linking is not possible in higher dimensions, the extra degrees of freedoom would cause spontaneos decomposition. In analogue to the Bohr model of an atom a string can vibrate in progressively higher modes. The ground state of a string should have zero mass and the first excitation level should have a mass of 10^19 GeV (Planck mass) and a length of 10^-35 m (Planck length).
The higher states should have masses that are proportional to the square root of the excitation level and a length that is proportional to the inverse of this length. In this scheme the massless particles of everyday physics should be very large contrary to experience. One can speculate that dynamic effects are present and that interactions of the massless states with the background massive states causes a compactification of these zero mass strings rather than of nonexistent extra dimensions. Such dynamic effects would cause small masses to these particle states in the Mev and GeV range of electrons and quarks.