Proton decay, black holes, and large extra dimensions

We consider proton decay in theories that contain large extra dimensions. I f virtual black hole states are allowed by the theory, as is generally the case, then proton decay can proceed via virtual black holes. The experiment al limits on the proton lifetime place strong constraints on the quantum gr avity scale M-qg (the effective Planck mass). For most theories, this impli es a lower bound of Mqg > 10(16) GeV. The corresponding bound on the size o f large extra dimensions is l < 10(6/n) x 10(-30) cm, where n is the number of such dimensions. Regrettably for most theories this limit rules out the possibility of observing large extra dimensions at accelerators or in mill imeter scale gravity experiments. Conversely, proton decay could be dominat ed by virtual black holes, providing an experimental probe to study stringy quantum gravity physics.