The Great Filter

Charlies Diary recently took on the issue of the "Great Filter", which is a corollary of Fermi's Paradox.

In a nutshell, Fermi's paradox asks why in such a huge universe as old as ours, we haven't yet encountered interstellar life yet.  One possible answer is the "Great Filter", i.e. some systemic mechanism by which intelligent life usually crashes and burns before it can generate interstellar civilization.

Charlie explores the concept of "Griefers", an early interstellar civilization that systematically stamps out would be interstellar civilizations.  But, his suggested mechanism, involving technological exotica like Von Neumann probes, Matrioshka brains, Dyson spheres and Nicoll-Dyson beams are way over the top.

My own inclination is for narratives that are more prosaic and more inevitable, along the lines of historical examples of the smallpox and Columbus,of  kudzu, and of megafauna extinction upon first contact with modern humans scenarios.

1. Assume that there is some ecological envelope in which planets, or at least parts of planets and moons, are "habitable" by living beings.

2. Assume that some forms of life are more robust than others.  One kind of organism may be able to survive almost anywhere, while another may require ideal Goldilocks conditions.

3. It follows that out of all of the different kinds of life that evolve independently on billions and billions of worlds over billions and billions of years, that there must be some species that is maximally robust, i.e. that is capable of living in a broader range of conditions than anything else.  Interstellar cockroaches, if you will.  This might even be something very much like humans.

4. Suppose further that you select not just for a species that is maximally robust (humans might qualify) but also for maximal efficiency (humans definitely don't qualify).  This species is so efficient over such a broad range of habitable environments that it is very prone to outcompeting all other forms of life in most of that range, like kudzu in the American South.  Species that co-evolve with it might be able to keep it in check (just as African predators that co-evolved with humans did not experience megafauna extinction), but in the absence of very specific adaptations this species becomes ubiquitous on almost all habitable planets.

Also note that ecological dominance in a new habitat need not happen quickly.  This species can be perfectly effective as a Great Filter even if it takes ten million years, on average, for example, to come into its own within an ecosystem from a starter colony beginning.

Let us call this species, without loss of generality, "griefers".  Griefers could be a nano-scale species akin to viruses and bacteria.  Indeed, very small individual griefers are rather favored, because it takes much less energy to send a probe with a colony of nano-griefers to another star, than it does to do the same with a larger organism.

5. Assume that this species selected for its maximal robustness and efficiency has a dormant spore stage that can endure prolonged periods in non-habitable environments such as the exterior of interstellar space craft.

6. Assume that some species on the griefer home world attains the ability to send out interstellar probes that can identify habitable worlds, self-replicate, and send new small probates to newly identified habitable worlds around new stars.  This may very well not be the griefers themselves.

7. Assume that the interstellar griefer delivery species, unaware of the havoc griefers can wreck on species that have not co-evolved with it, do not sanitize their first batch of probes and that self-replication of probes requires a habitable environment.

If the griefer delivery species manages to reach a level where it can send out self-replicating probes on this model contaminated with griefers just once, with a single probe launch from the home world, in the entire history of the universe, then the entire universe will be sterilized by griefers in due course and a Great Filter has come into being.

8.  Note that griefers and their companion species on the griefer homeworld need not be perfect to give rise to a Great Filter sufficient to explain Fermi's paradox.  A Great Filter with 99% or 99.9% efficiency would be hard to distinguish in practice, from a perfect one.

9.  Also, even if a griefer is incapable of destroying a well established advanced technological species, it could perhaps, make all other potentially habitable worlds within a few parsecs of the well established species, uninhabitable going forward, making the bridge that must be crossed to transition from being a mono-stellar species to an interstellar species insurmountable.  Indeed, there is no need for the companion species that inadvertently hurls probes infected with griefers to other habitable worlds to ever manage to get any of their own kind to another planet.

10.  This is also a phenomena that can play out in parallel, with multiple independent source worlds that each wipe out a different part of the range of habitable worlds.  For example, one kind of griefer could destroy life on nearly frozen ice/methane worlds, while another kind of griefer could wipe out Venus-like very hot and toxic worlds.