The abstract to Wiley's paper:
It has been widely acknowledged that self-replicating space-probes (SRPs) could explore the galaxy very quickly relative to the age of the galaxy. An obvious implication is that SRPs produced by extraterrestrial civilizations should have arrived in our solar system millions of years ago, and furthermore, that new probes from an ever-arising supply of civilizations ought to be arriving on a constant basis. The lack of observations of such probes underlies a frequently cited variation of the Fermi Paradox. We believe that a predilection for ETI-optimistic theories has deterred consideration of incompatible theories. Notably, SRPs have virtually disappeared from the literature. In this paper, we consider the most common arguments against SRPs and find those arguments lacking. By extension, we find recent models of galactic exploration which explicitly exclude SRPs to be unfairly handicapped and unlikely to represent natural scenarios.Also check out Karl Schroeder's review of the paper, in which he writes:
We also consider several other models that seek to explain the Fermi Paradox, most notably percolation theory and two societal- collapse theories. In the former case, we find that it imposes unnatural assumptions which likely render it unrealistic. In the latter case, we present a new theory of interstellar transportation bandwidth which calls into question the validity of societal-collapse theories.
Finally, we offer our thoughts on how to design future SETI programs which take the conclusions of this paper into account to maximize the chance of detection.
As possible explanations dwindle, we are being drawn inexorably toward the one explanation that is no explanation: that we really are alone. Why should this be? As Wiley shows, all it would take would be one alien species with our capabilities appearing, sometime in the past couple of billion years, and for that species to surpass where we are now technologically by, oh, say, a couple of hundred years... and the evidence for their existence should be present right here in our own solar system. It's an astonishing conclusion.
So are we alone? Well, there is one other possibility, at this point. I've lately been trumpeting my revision of Clarke's Law (which originally said 'any sufficiently advanced technology is indistinguishable from magic'). My revision says that any sufficiently advanced technology is indistinguishable from Nature. (Astute readers will recognize this as a refinement and further advancement of my argument in Permanence.) Basically, either advanced alien civilizations don't exist, or we can't see them because they are indistinguishable from natural systems. I vote for the latter.
This vote has consequences. If the Fermi Paradox is a profound question, then this answer is equally profound. It amounts to saying that the universe provides us with a picture of the ultimate end-point of technological development. In the Great Silence, we see the future of technology, and it lies in achieving greater and greater efficiencies, until our machines approach the thermodynamic equilibria of their environment, and our economics is replaced by an ecology where nothing is wasted. After all, SETI is essentially a search for technological waste products: waste heat, waste light, waste electromagnetic signals. We merely have to posit that successful civilizations don't produce such waste, and the failure of SETI is explained.
And as to why we haven't found any alien artifacts in our solar system, well, maybe we don't know what to look for. Wiley cites Freitas as having come up with this basic idea; I'm prepared to take it much further, however.