  According to some authors, gravity might be an emergent phenomenon in a
fundamentally flat space-time. In this case the speed of light in the vacuum
would not coincide exactly with the basic parameter "c" entering Lorentz
transformations and, for an apparatus placed on the Earth's surface, light
should exhibit a tiny fractional anisotropy at the level 10^{-15}. We argue
that, most probably, this effect is now observed in many room-temperature
ether-drift experiments and, in particular, in a very precise cryogenic
experiment where this level of signal is about 100 times larger than the
designed short-term stability. To fully appreciate what is going on, however,
one should consider the vacuum as a true physical medium whose fundamental
quantum nature gives rise to the irregular, non-deterministic pattern which
characterizes the observed signal.
