A physical interpretation of gravitational instantons

A gravitational instanton solution found by Eguchi and Hanson some time ago is found to describe a special set of states in the Hilbert space of quantum gravity. We show how states of this type have to be normalized and how expectation values of operators are to be computed in such states. It then becomes clear that these states actually describe physical particles much like solitons. Being unstable they decay into large numbers of ordinary particles. (orig.)