  In science, one observes correlations and invents theoretical models that
describe them. In all sciences, besides quantum physics, all correlations are
described by either of two mechanisms. Either a first event influences a second
one by sending some information encoded in bosons or molecules or other
physical carriers, depending on the particular science. Or the correlated
events have some common causes in their common past. Interestingly, quantum
physics predicts an entirely different kind of cause for some correlations,
named entanglement. This new kind of cause reveals itself, e.g., in
correlations that violate Bell inequalities (hence cannot be described by
common causes) between space-like separated events (hence cannot be described
by classical communication). Einstein branded it as spooky action at a
distance. A real spooky action at a distance would require a faster than light
influence defined in some hypothetical universally privileged reference frame.
Here we put stringent experimental bounds on the speed of all such hypothetical
influences. We performed a Bell test during more than 24 hours between two
villages separated by 18 km and approximately east-west oriented, with the
source located precisely in the middle. We continuously observed 2-photon
interferences well above the Bell inequality threshold. Taking advantage of the
Earth's rotation, the configuration of our experiment allowed us to determine,
for any hypothetically privileged frame, a lower bound for the speed of this
spooky influence. For instance, if such a privileged reference frame exists and
is such that the Earth's speed in this frame is less than 10^-3 that of the
speed of light, then the speed of this spooky influence would have to exceed
that of light by at least 4 orders of magnitude.
