Nonlocal Ghost Formalism for Quantum Gravity
  The quantum theory of a free particle on a portion of two-dimensional Euclidean space bounded by a circle and subject to non-local boundary conditions gives rise to bulk and surface states. Starting from this well known property, a counterpart for gravity is here considered. In particular, if spatial components of metric perturbations are set to zero at the boundary, invariance of the full set of boundary conditions under infinitesimal diffeomorphisms is compatible with non-local boundary conditions on normal components of metric perturbations if and only if both the gauge-field operator and the ghost operator are pseudo-differential operators in one-loop quantum gravity. 
Giampiero Esposito