  For about the last 60 years the search for extraterrestrial intelligence has
been monitoring the sky for evidence of remotely detectable technological life
beyond Earth, with no positive results to date. While the lack of detection can
be attributed to the highly incomplete sampling of the search space,
technological emissions may be actually rare enough that we are living in a
time when none cross the Earth. Here we explore the latter possibility and
derive the likelihood of the Earth not being crossed by signals for at least
the last 60 years to infer upper bounds on their rate of emission. Under the
assumption that technological emitters are distributed uniformly in the Milky
Way and that they generate technoemissions at a constant rate, we find less
than about one to five emissions generated per century with 95% credible level.
This implies optimistic waiting times until the next crossing event of no less
than 60-1800 years with a 50% probability. A significant fraction of highly
directional signals increases the emission rates upper bounds, but without
systematically changing the waiting time. Although these probabilistic bounds
are derived from a specific model and their validity depends on the model's
assumptions, they are nevertheless quite robust against weak time dependences
of the emission rate or nonuniform spatial distributions of the emitters. Our
results provide therefore a benchmark for assessing the lack of detection and
may serve as a basis to form optimal strategies for the search for
extraterrestrial intelligence.
