  Several authors have studied the generation of gravitational fields by
condensed-matter systems in non-extreme density conditions. General Relativity
and lowest-order perturbative Quantum Gravity predict in this case an extremely
small emission rate, so these phenomena can become relevant only if some strong
quantum effect occurs. Quantum aspects of gravity are still poorly understood.
It is believed that they could play a role in systems which exhibit macroscopic
quantum coherence, like superconductors and superfluids, leading to an
"anomalous" coupling between matter and field. We mention here recent work in
this field by Woods, Chiao, Becker, Agop et al., Ummarino, Kiefer and Weber.
New results are presented concerning anomalous stimulated gravitational
emission in a layered superconductor like YBCO. We model the superconductor as
an array of intrinsic Josephson junctions. The superconducting parameters are
defined by our preliminary measurements with melt-textured samples. We write
explicitly and solve numerically the Josephson equations which give the normal
and super components of the total current in the superconductor, and derive
from this the total available power P=IV. Then, assuming that the coefficients
A and B for spontaneous and stimulated gravitational emission are known, we
apply to this case the Frantz-Nodvik equation for a laser amplifier. The
equation is suitably modified in order to allow for a "continuous pumping"
given by an oscillating transport current. The conclusions are relevant for the
evaluation of gravitational emission from superconductors. We find that even if
the A and B coefficients are anomalously large (possibly because of the Quantum
Gravity effects mentioned above), the conditions for stimulated emission are
quite strict and the emission rate strongly limited by the IV value.
