In a previous post I discussed the possibilities of replicators being aided in their diffusion between star systems by close passes; I also pointed to an interesting paper by Forgan et al looking at projected travel times of interstellar probes, based on powered vs. gravity-assist travel. A new paper (on which Forgan is the senior author) looks again at this problem using a Monte Carlo simulation approach, and gets an interesting but as always frustrating answer.
First, the paper would be valuable if all it contained was the excellent review of prior work done on this NP-hard version of the traveling salesman problem. (You think planning your road trip taxes a server?) In particular, they point to a prior paper arguing that von Neumann probe expansions could be slowed or stopped by mutant probes where a predator-prey dynamic evolves in the population and the probes hunt themselves to extinction. I had previously written about self-replicating probes becoming cancerous (a statistically more likely outcome) and turning to expansion above any other mission they previously had; but this only makes the Fermi Paradox more vexing rather than solving it.
Their approach was to simulate the galaxy as having 1 star per cubic parsec (so no local "backwaters"), with self-replicating probes moving at approximately the speed of Voyager I, 1000 m/s. They then compared replicating and non-replicating probes; not surprisingly, self-replicators were much much faster, exploring 100% of a box of 100,000 stars in at most 30 million years. In the replicator condition, the number of probes is no longer the constraint, and contact can occur in (possibly massive) parallel rather than serially. Of note: the authors also include a requirement for communication of which stars have already been explored, although if we assume a robustly replicating probe, this is unnecessary. ("Robust" means that the time and ability to replicate enough probes to successfully reach the neighboring stars are small relative to travel time.)
The authors correctly note that this short time frame makes the Fermi Paradox more vexing. So the alternatives with respect to self-replicating probes are:
1. They're here and we haven't found or noticed them yet.
2. They're not here, because
2a. Replicating probes are not possible or not effective (they can't be made at all, or they mutate)
2b. intelligence capable and willing to build such probes appears more rarely than once every 30 million years in a volume of 100,000 stars (they choose not to, or they don't evolve in the first place, or they wipe themselves out before they create them)
2a seems unlikely because we already know that replicators can develop through natural selection. This leaves us with 1 and 2b. It is certain that we have only the barest knowledge of the rest of our solar system, and gravity wells are expensive to get out of. My prediction is that we'll find evidence of such probes on low gravity bodies (asteroids, comets, small moons) as we continue to explore, but that once we find something strange, it will take time for us to understand what we're looking at.
Hat tip to Ben Weaver.