and you shall know them by their trail of artefacts…
As Wiley points out in this paper, even if the lifetime of an interstellar civilization is short; even if they’re all doomed; there is no credible argument as to why they couldn’t create self-reproducing probes (SRPs) to investigate the entire galaxy that, collectively, outlive the originating civilization. This is the very scenario I paint in Permanence. SRPs are a cheaper solution than one-off expeditions. In fact, SRPs are so efficient a solution to exploration and colonization that, plugging in some highly conservative numbers of how many civilizations there might be out there, Wiley shows that hundreds to billions of such probes should actually be here, in our solar system, right now!
In 1981, Frank Tipler put forth an argument that extraterrestrial intelligences do not exist, based on the absence of von Neumann probes. Given even a moderate rate of replication and the history of the galaxy, such probes should already be common throughout space and thus, we should have already encountered them. Because we have not, this shows that extraterrestrial intelligences do not exist.This is thus a resolution to the Fermi paradox—that is, the question of why we have not already encountered extraterrestrial intelligence if it is common throughout the universe.
A response came from Carl Sagan and William Newman. Now known as Sagan’s Response, it pointed out that in fact Tipler had underestimated the rate of replication, and that von Neumann probes should have already started to consume most of the mass in the galaxy. Any intelligent race would therefore, Sagan and Newman reasoned, not design von Neumann probes in the first place, and would try to destroy any von Neumann probes found as soon as they were detected. As Robert Freitas has pointed out, the assumed capacity of von Neumann probes described by both sides of the debate are unlikely in reality, and more modestly reproducing systems are unlikely to be observable in their effects on our Solar System or the Galaxy as a whole.
Another objection to the prevalence of von Neumann probes is that civilizations of the type that could potentially create such devices may have inherently short lifetimes, and self-destruct before so advanced a stage is reached, through such events as biological or nuclear warfare, nanoterrorism, resource exhaustion, ecological catastrophe, or pandemics due to antibiotic resistance.
Simple workarounds exist to avoid the over-replication scenario. Radio transmitters, or other means of wireless communication, could be used by probes programmed not to replicate beyond a certain density (such as five probes per cubic parsec) or arbitrary limit (such as ten million within one century), analogous to the Hayflick limit in cell reproduction. One problem with this defence against uncontrolled replication is that it would only require a single probe to malfunction and begin unrestricted reproduction for the entire approach to fail — essentially a technological cancer — unless each probe also has the ability to detect such malfunction in its neighbours and implements a seek and destroy protocol. Another workaround is based on the need for spacecraft heating during long interstellar travel. The use of plutonium as a thermal source would limit the ability to self-replicate. The spacecraft would have no programming to make more plutonium even if it found the required raw materials. Another is to program the spacecraft with a clear understanding of the dangers of uncontrolled replication.