A spacefaring species could easily settle the entire Milky Way given billions of years. Yet the fact is that there is no obvious one in our solar system right now. The supposed inconsistency between these statements is the Fermi Paradox, named for the Nobel Prize-winning physicist who supposedly first formulated it. In a trenchant formulation of the Fermi Paradox, American astrophysicist Michael H. Hart called the lack of extraterrestrial beings or artifacts on Earth today “Fact A.” He showed that most objections to his conclusion—that a spacefaring civilization could have crossed the galaxy by now—stem from either 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 our galaxy in less time than the galaxy’s age) or else the dubious assumption that all members of all extraterrestrial species will avoid colonizing behaviors forever (an example of what I’ve called the monocultural fallacy).
William Newman and Carl Sagan later 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 described through the Milky Way might move much more slowly than the speed of the colonization ships if their population growth rates were so low that they only needed to spread to nearby stars very rarely. They also argued that being a long-lived civilization is inconsistent with being a rapidly-expanding one, so any species bent on settling the galaxy would not last long enough to succeed. In other words, they reasoned that 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.
Being a long-lived civilization is inconsistent with being a rapidly-expanding one.
In a 2014 paper on the topic, my colleagues and I rebutted many of these claims. In particular, we argued that one should not conflate the population growth in a single settlement with that of all settlements. There is no reason to suppose that population growth, resource depletion, or overcrowding drives the creation of new settlements, 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).
Another factor affects Newman and Sagan’s numbers on timescales and colonization-front speeds. Most of the prior work on this topic exploits percolation models, in which ships move about on a static two-dimensional substrate of stars. In these models, a star launching settlement ships can quickly settle all of the nearby stars, limiting the number of stars it can settle. But real stars move in three dimensions, meaning that they can carry their orbiting settlements throughout the galaxy, and that a settlement will always have fresh new stars to settle if it waits long enough.
Jonathan Carroll-Nellenback, at the University of Rochester with Adam Frank, not long ago finished work, with Caleb Scharf and me, on analytic and numerical models for how a realistic settlement front would behave in a real gas of stars, one characteristic of the galactic disk at our distance from the galactic center. The big advances here are a few:
- Carroll-Nellenback validated an analytic formalism for settlement expansion fronts with numerical models for a realistic gas of stars.
- He 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.
- He also explored a range of settlement behaviors to see how galactic settlement fronts depend on them.
The idea that not all stars are settle-able is important to keep in mind. Adam Frank 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 slow. Even if all the ships have a very limited range (only able to reach, say, the nearest stars to Earth) and even if they are no faster than our own interstellar ships today (like Voyager 2), we find they can still settle the entire Milky Way in less than its lifetime, supporting Hart’s version of the Fermi Paradox.
Carroll-Nellenback also explores regimes where they have been here, but we just don’t notice because it was so long ago. Frank and Gavin Schmidt explored this possibility in their Silurian Hypothesis paper, and I did something similar in my paper on “prior indigenious technological species” in the solar system. 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 Earth’s history back in time for something like millions of years, and not very well at that.
The question, in other words, isn’t, “Has the solar system ever been visited?” It’s, “Has it been settled recently?”. Carroll-Nellenback shows that there is actually a pretty big region of parameter space where the solar system is amidst many settled systems but just hasn’t been visited in the last 1 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 … including the Aurora effect … 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.
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, then 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 consider the existence of halo stars, which do not rotate with the galactic disk, or the fact that stars closer to the center take less time to go around—both will make settlement timescales even faster than we calculate.
On the other hand, there are a lot of assumptions in Hart’s arguments that might not hold. In particular, his assertion that if the sun has ever been in range of a settled system then it would have been settled and the settlers would still be here. Perhaps Earth life for some reason keeps the settlements at bay, either because “they” want to keep Earth life pristine or it’s just too resilient and pernicious for an alien settlement to survive. Perhaps Earth is Aurora?
Jason Wright is an astronomer at Penn State University and a member of the Center for Exoplanets and Habitable Worlds. He blogs at AstroWright, where this article was originally published. It has been slightly adapted. Follow him on Twitter @Astro_Wright.
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