Running of Newton's Constant and Quantum Gravitational Effects

Newton's gravitational constant is shown to be a running coupling constant, much like the familiar running gauge couplings of the Standard Model. This implies that, in models with appropriate particle content, the true Planck scale, i.e. the scale at which quantum gravity effects become important, can have a value different from 10^19 GeV, which would be expected from naive dimensional analysis. Then, two scenarios involving this running effect are presented. The first one is a model which employs huge particle content to realize quantum gravity at the TeV scale in 4 dimensions, thereby solving the hierarchy problem of the Standard Model. Secondly, effects of the running of Newton's constant in grand unified theories are examined and shown to introduce new significant uncertainties in their predictions, but possibly also to provide better gauge coupling unification results in some cases.