Lowering $\alpha_{s}$ by flipping SU(5)

We show that the prediction for \alpha_s(M_Z) in flipped SU(5) is naturally lower than in minimal SU(5), and that the former can accommodate the full range of \alpha_s(M_Z) presently allowed by experiment. Our computations include two-loop (\delta_{\rm 2loop}) and light (\delta_{\rm light}) and heavy (\delta_{\rm heavy}) threshold effects. Unlike minimal SU(5), in flipped SU(5) the heavy threshold effects can naturally decrease the predicted value of \alpha_s(M_Z). We also show that the value of the proton lifetime into the dominant channel p\to e^+\pi^0 is within the observable range at SuperKamiokande, and should discriminate against minimal supersymmetric SU(5), where the dominant mode is p\to\bar\nu K^+.