The Fermi Paradox is the supposed inconsistency between the ease with which a spacefaring species could settle the entire Milky Way given billions of years and the fact that they are not obviously in the Solar System right now.
This, original form of the paradox was formulated most trenchantly by Michael Hart (more on him in Section 2.2 here) who called the lack of extraterrestrial beings or artifacts on Earth today “Fact A”. He showed that most objections to his conclusion stem from a lack of appreciation for the timescales involved (it takes a small extrapolation from present human technology to get interstellar ships, and even slow ships can star-hop across the Galaxy in less than its age) or what I’ve called the monocultural fallacy (positing a common behavior to all members of all extraterrestrial species, forever).
William Newman and Carl Sagan wrote a major rebuttal to Hart’s work, in which they argued that the timescales to populate the entire Galaxy could be quite long. In particular, they noted that the colonization fronts Hart describes through the Galaxy would move much more slowly than the speed of the colonization ships. They also argue that long-lived civilizations are anti-correlated with rapidly-expanding ones, and so they conclude that civilizations with very slow population growth rates are necessarily very slowly expanding. They conclude the Galaxy could be filled with both short-lived rapidly expanding civilizations that don’t get very far and long-lived slowly expanding civilizations that haven’t gotten very far—either way, it’s not surprising that we have not been visited.
We rebutted many of these claims in our paper on the topic. In particular, we argued that one should not conflate the population growth in a single settlement with that of all settlements. In particular, there is no reason to suppose that colonization is driven by population growth, resource depletion, or overcrowding, or that a small, sustainable settlement would never launch a new settlement ship. One can easily imagine a rapidly expanding network of small sustainable settlements (indeed, the first human migrations across the globe likely looked a lot like this).
Once this constraint is lifted, a second consideration makes Newman & Sagan’s numbers smaller. Most of the prior work on this topic exploit percolation models, in which ships move about on a static substrate of stars, but real stars move. Many of these papers also assume that the entire network of settlements have a similar behavior, and some posit they all might suffer a simultaneous culture shift away from settlement.
Jonathan Carroll-Nellenback at the University of Rochester with Adam Frank, and in collaboration with Caleb Scharf and me, has just finished work on analytic and numerical models for how a realistic settlement front would behave in a real gas of stars characteristic of the Galactic disk in the Solar Neighborhood.
The big advances here are a few:
- Jonathan has worked out an analytic formalism for settlement expansion fronts and validated it with numerical models for a realistic gas of stars
- Jonathan has accounted for finite settlement lifetimes, the idea that only a small fraction of stars will be settle-able, and explored the limits of very slow and infrequent settlement ships
- Jonathan has not assumed that settlement lifetimes or settlement behaviors are correlated. Rather, he assumed a simple, conservative set of parameterized rules for settlement and explored settlement behavior as a function of those fixed parameters.
In particular, the idea that not all stars are settle-able is important to keep in mind. Adam calls this the Aurora effect after the Kim Stanley Robinson novel in which a system is “habitable, but not settle-able.”
The results are pretty neat. When we let the settlements behave independently, Hart’s argument looks pretty good, even when the settlement fronts are pretty slow. In particular, one can have very limited range (no faster than our own interstellar ships but lasting a million years, or faster ships that can only travel about 1pc) and still settle the entire Galaxy in less than its lifetime because the front speed becomes limited by the speed of the stars, which carry settlements into range of new stars regularly and naturally diffuse throughout the Galaxy.
Jonathan explores a few regimes where Earth would not have been settled yet. He finds that it doesn’t take much—just a single settlement front with modest ship ranges and launch rates—to populate the entire Galaxy in much less than a Hubble time.
Also neat, is that Jonathan explores regimes where they have been here, but we just don’t notice because it was so long ago. Adam and Gavin Schmidt explored this possibility in their Silurian Hypothesis paper, and I did something similar in my PITS paper. The idea is that “Fact A” only applies to technology that has visited very recently or visited and then stayed permanently. Any technology on Earth or the Solar System that is not actively maintained will eventually be destroyed and/or buried, so we can really only explore even Earth’s history back in time for of order millions of years, and not very well at that.
So really, the question isn’t “has the Solar System ever had a settlement” it’s “has it been settled recently”. Jonathan shows that there is actually a pretty big region of parameter space where the Solar System is amidst many settled system but just hasn’t been visited in the last 10 million years.
Of course, there are still lots of other reasons why we might not have been permanently settled by a Galactic network of settlements—as we note in the paper:
Hart’s conclusions are also subject to the assumption that the Solar System would be considered settleable by any of the exo-civilizations it has come within range of. The most extravagant contradiction of this assumption is the Zoo Hypothesis (Ball 1973), but we need not invoke such “solipsist” positions (Sagan & Newman 1983) to point out the flaw in Hart’s reasoning here. One can imagine many reasons why the Solar System might not be settleable (i.e. not part of the fraction f in our analysis), including the Aurora effect mentioned in Section 1 or the possibility that they avoid settling the environment near the Earth exactly because it is inhabited with life.
In particular, the assumption that the Earth’s life-sustaining resources make it a particularly good target for extraterrestrial settlement projects could be a naive projection onto exo-civilizations of a particular set of human attitudes that conflate expansion and exploration with conquest of (or at least indifference towards) native populations (Wright & Oman-Reagan 2018). One might just as plausibly posit that any extremely long-lived civilization would appreciate the importance of leaving native life and its near-space environment undisturbed.
So our results are a mixed bag for SETI optimists: Hart’s argument that settlement fronts should cross the whole Galaxy—which is at the heart of the Fermi Paradox—is robust, especially because of the movements of stars themselves which should “mix” the Galaxy pretty well, preventing simply connected “empires” of settlements from forming. If Hart is correct that this means we are alone in the Galaxy, this is actually very optimistic for extra-galactic SETI, because it means other Galaxies with even a single spacefaring species should rapidly become endemic with them. Indeed, our analysis did not even include any effects like halo stars or Galactic shear which will make settlement timescales even faster.
On the other hand, there are a lot of assumptions in Hart’s arguments that might not hold, in particular that if the Sun has ever been in range of a settled system that “they” would still be here and we would know it. Perhaps Earth life for some reason keeps the settlements at bay, either because “they” want to keep it pristine or it’s just too resilient and pernicious to permit an alien settlement from surviving here. Is Earth Aurora?
The paper is here.