  We model the settlement of the galaxy by space-faring civilizations in order
to address issues related to the Fermi Paradox. We explore the problem in a way
that avoids assumptions about the intent and motivation of any exo-civilization
seeking to settle other planetary systems. We first consider the speed of an
advancing settlement via probes of finite velocity and range to determine if
the galaxy can become inhabited with space-faring civilizations on timescales
shorter than its age. We also include the effect of stellar motions on the long
term behavior of the settlement front which adds a diffusive component to its
advance. The results of these models demonstrate that the Milky Way can be
readily 'filled-in' with settled stellar systems under conservative assumptions
about interstellar spacecraft velocities and launch rates. We then consider the
question of the galactic steady-state achieved in terms of the fraction of
settled planets. We do this by considering the effect of finite settlement
civilization lifetimes on the steady states. We find a range of parameters for
which the galaxy supports a population of interstellar space-faring
civilizations even though some settleable systems are uninhabited. Both results
point to ways in which Earth might remain unvisited in the midst of an
inhabited galaxy. Finally we consider how our results can be combined with the
finite horizon for evidence of previous settlements in Earth's geologic record.
Our steady-state model can constrain the probabilities for an Earth visit by a
settling civilization before a given time horizon. These results break the link
between Hart's famous "Fact A" (no interstellar visitors on Earth now) and the
conclusion that humans must, therefore, be the only technological civilization
in the galaxy.
