The string landscape, black holes and gravity as the weakest force

We conjecture a general upper bound on the strength of gravity relative togauge forces in quantum gravity. This implies, in particular, that in afour-dimensional theory with gravity and a U(1) gauge field with gauge couplingg, there is a new ultraviolet scale Lambda=g M_{Pl}, invisible to thelow-energy effective field theorist, which sets a cutoff on the validity of theeffective theory. Moreover, there is some light charged particle with masssmaller than or equal to Lambda. The bound is motivated by arguments involvingholography and absence of remnants, the (in) stability of black holes as wellas the non-existence of global symmetries in string theory. A sharp form of theconjecture is that there are always light "elementary" electric and magneticobjects with a mass/charge ratio smaller than the corresponding ratio formacroscopic extremal black holes, allowing extremal black holes to decay. Thisconjecture is supported by a number of non-trivial examples in string theory.It implies the necessary presence of new physics beneath the Planck scale, notfar from the GUT scale, and explains why some apparently natural models ofinflation resist an embedding in string theory.Comment: 20 pages, LaTeX, 5 EPS figures; v2: minor correction