  We report on work in progress in quantum field theory about possible
interactions between coherent matter, i.e. matter described by a macroscopic
wave function or a classical field, and a certain class of vacuum fluctuations,
called "zero-modes of the Einstein action". These are little-known virtual
masses present in the vacuum state of quantum gravity. A couple of equal masses
of this kind can be excited by an oscillating coherent source with frequency f
and decays to its ground state emitting a virtual graviton, which can propagate
and transfer momentum p to ordinary matter. The virtual masses recoil in the
emission, and this amounts to a transfer of momentum -p to the vacuum; this
momentum can be passed in turn to some matter, or not. The energy hf for the
process does not come from the vacuum, but from the coherent source. The ratio
hf/p is of the order of 1 m/s. This model was developed to explain experimental
results showing the emission of anomalous high-momentum radiation from certain
superconductors, sometimes with a strong recoil of the emitters. The recoil is
energetically quite efficient, at least at small power, and could be exploited
for propulsion. It has not been tested in space, however, and our model cannot
yet predict if the recoil is affected by the presence of near matter. (Another
model predicts that it is not.) We also briefly mention a possible application
of the anomalous radiation itself and we evaluate the (large) electric and
magnetic field strength needed to produce an effect equivalent to that of a
superconducting emitter.
