  The quite different behaviors exhibited by microscopic and macroscopic
systems with respect to quantum interferences suggest that there may exist a
naturally frontier between quantum and classical worlds. The value of the
Planck mass (22$\mu$g) may lead to the idea of a connection between this
borderline and intrinsic fluctuations of spacetime. We show that it is possible
to obtain quantitative answers to these questions by studying the diffusion and
decoherence mechanisms induced on quantum systems by gravitational waves
generated at the galactic or cosmic scales. We prove that this universal
fluctuating environment strongly affects quantum interferences on macroscopic
systems, while leaving essentially untouched those on microscopic systems. We
obtain the relevant parameters which, besides the ratio of the system's mass to
Planck mass, characterize the diffusion constant and decoherence time. We
discuss the feasibility of experiments aiming at observing these effects in the
context of ongoing progress towards more and more sensitive matter-wave
interferometry.
