Strongly Coupled Grand Unification in Higher Dimensions

We consider the scenario where all the couplings in the theory are strong at the cut-off scale, in the context of higher dimensional grand unified field theories where the unified gauge symmetry is broken by an orbifold compactification. In this scenario, the noncalculable correction to gauge unification from unknown ultraviolet physics is naturally suppressed by the large volume of the extra dimension, and the threshold correction is dominated by a calculable contribution from Kaluza-Klein towers that gives the values for sin θw and αs in good agreement with low-energy data. The threshold correction is reliably estimated despite the fact that the theory is strongly coupled at the cut-off scale. A realistic 5d supersymmetric SU(5) model is presented as an example, where rapid d = 6 proton decay is avoided by putting the first generation matter in the 5d bulk.