  It has been argued that self-replicating robotic probes could spread to all
stars of our galaxy within a timespan that is tiny on cosmological scales, even
if they travel well below the speed of light. The apparent absence of such von
Neumann probes in our own solar system then needs an explanation that holds for
all possible extraterrestrial civilisations. Here I propose such a solution,
which is based on a runaway error propagation that can occur in any
self-replicating system with finite accuracy of its components. Under
universally applicable assumptions (finite resources and finite lifespans) it
follows that an optimal probe design always leads to an error catastrophe and
breakdown of the probes. Thus, there might be many advanced civilizations in
our galaxy, each surrounded by their own small sphere of self-replicating
probes. But unless our own solar system has the extraordinary luck to be close
enough to one of these civilizations, none of these probes will ever reach us.
