  In this work we review some concepts developed over the last few years: that
the gravitational vacuum has, even at scales much larger than the Planck
length, a peculiar structure, with anomalously strong and long-lasting
fluctuations called "zero-modes"; and that these vacuum fluctuations behave as
virtual particles of negative mass and can interact with each other, leading to
the formation of weakly bound states. The bound states make up a continuum,
allowing at each point of spacetime the local excitation of the gravitational
vacuum through the coupling with matter in a coherent state. The spontaneous or
stimulated decay of the excited states leads to the emission of virtual
gravitons with spin 1 and large p/E ratio. In this paper we also expand and
discuss in physical terms several important new details concerning the
zero-mode interactions, the dynamics of virtual particles with negative mass
and the properties of virtual gravitons.
