  We conjecture a general upper bound on the strength of gravity relative to
gauge forces in quantum gravity. This implies, in particular, that in a
four-dimensional theory with gravity and a U(1) gauge field with gauge coupling
g, there is a new ultraviolet scale Lambda=g M_{Pl}, invisible to the
low-energy effective field theorist, which sets a cutoff on the validity of the
effective theory. Moreover, there is some light charged particle with mass
smaller than or equal to Lambda. The bound is motivated by arguments involving
holography and absence of remnants, the (in) stability of black holes as well
as the non-existence of global symmetries in string theory. A sharp form of the
conjecture is that there are always light "elementary" electric and magnetic
objects with a mass/charge ratio smaller than the corresponding ratio for
macroscopic extremal black holes, allowing extremal black holes to decay. This
conjecture is supported by a number of non-trivial examples in string theory.
It implies the necessary presence of new physics beneath the Planck scale, not
far from the GUT scale, and explains why some apparently natural models of
inflation resist an embedding in string theory.
