In order to gain insight into the possible Ground State of Quantized
Einstein's Gravity, we have derived a variational calculation of the energy of
the quantum gravitational field in an open space, as measured by an asymptotic
observer living in an asymptotically flat space-time. We find that for Quantum
Gravity (QG) it is energetically favourable to perform its quantum fluctuations
not upon flat space-time but around a ``gas'' of wormholes of mass m_p, the
Planck mass (m_p ~= 10^{19}GeV) and average distance l_p, the Planck length
a_p(a_p ~= 10^{-33}cm). As a result, assuming such configuration to be a good
approximation to the true Ground State of Quantum Gravity, space-time, the
arena of physical reality, turns out to be well described by Wheeler's quantum
foam and adequately modeled by a space-time lattice with lattice constant l_p,
the Planck lattice.Comment: 56 pages, revised version to appear in General Relativity and
Gravitation (2000