The unification of the electroweak force and the strong force with the gravitational force in a so-called " Theory of Everything " requires an even higher energy level which is generally assumed to be close to the Planck scale of 10 19 GeV. In theory, at such short distances, gravity becomes comparable in strength to the other three forces of nature known to date. This statement is modified if there exist additional dimensions of space at intermediate scales. In this case, the strength of gravitational interactions increases faster at smaller distances and the energy scale at which all known forces of nature unify can be considerably lower. This effect is exploited in models of large extra dimensions.