Good discussion here of how modern technologies (3D printing) could be applied to building our own self-replicating space probes, which in turn inevitably leads to a Fermi paradox discussion. Replicating probes are the way to go because they cover more territory faster. Authors cited in the article place lower and upper bounds on time to colonize the galaxy with self-replicators from 3.75 to 300 million years. Either way, it's difficult to square an apparently empty galaxy with these numbers, or indeed with the feasibility of such an endeavor. In fact, Frank Tipler (who made the upper bound estimate here) went so far as to argue that this has to mean there are no other intelligent aliens.
This version of the Fermi paradox can be solved by any of these:
- Most likely: they're around, and we haven't looked enough, or don't recognize them, or they avoid us because they're more interested in their own replication than in contacting other intelligent species, for some strange reason. We should expect that any self-replicating probes we find will have undergone natural selection to be primarily good at making copies of themselves, and secondarily at performing whatever mission their initial long-ago designers created them for.
- We're over-optimistic, and self-replicating probes are not feasible.
- These time estimates are not conservative enough.
- We really are the only intelligent species, or at least life is extremely rare.