The positive energy theorem precludes the possibility of Minkowski flat space
decaying by any mechanism. In certain circumstances, however, large quantum
fluctuations of the gravitational field could arise---not only at the Planck
scale, but also at larger scales. This is because there exists a set of
localised weak field configurations which satisfy the condition int d4x
sqrt{g}R = 0 and thus give a null contribution to the Einstein action. Such
configurations can be constructed by solving Einstein field equations with
unphysical dipolar sources. We discuss this mechanism and its modification in
the presence of a cosmological term and/or an external field.Comment: LaTeX, 8 page
