By Taylor Marvin
One of the more interesting questions about the universe is the apparent rarity of intelligent life. It is reasonable to suspect that given the vast size of the universe and apparent frequency of rocky planets intelligent civilizations are common in galactic habitable zones, even disregarding the possibility of exotic biologies. However, humans have not encountered aliens and observed no evidence of these civilizations, despite the fact that evidence of both extant and extinct sufficiently advanced civilizations should be apparent across galactic distances. This is especially puzzling because today’s humans are not far from the technological requirements — conservatively, fusion drives and generation ships — required to colonize a significant portion of the galaxy.
This puzzle — if aliens are common, where are they? — is termed the Fermi Parodox. Scientific America author Ian Crawford elegantly summarized the possible solutions to the paradox:
“There are only four conceivable ways of reconciling the absence of ETs with the widely held view that advanced civilizations are common. Perhaps interstellar spaceflight is infeasible, in which case ETs could never have come here even if they had wanted to. Perhaps ET civilizations are indeed actively exploring the galaxy but have not reached us yet.
Perhaps interstellar travel is feasible, but ETs choose not to undertake it. Or perhaps ETs have been, or still are, active in Earth’s vicinity but have decided not to interfere with us. If we can eliminate each of these explanations of the Fermi Paradox, we will have to face the possibility that we are the most advanced life-forms in the galaxy.”
There’s a lot to explore here, but I’d like to focus on two of the four potential answers: that intelligent civilizations are chose not to expand through the galaxy, or are somehow prevented from doing so. Importantly, it appears that this “prevention” is not based on an inherent difficulty of interstellar colonization. Again quoting Crawford:
“Any civilization with advanced rocket technology would be able to colonize the entire galaxy on a cosmically short timescale. For example, consider a civilization that sends colonists to a few of the planetary systems closest to it. After those colonies have established themselves, they send out secondary colonies of their own, and so on. The number of colonies grows exponentially. A colonization wave front will move outward with a speed determined by the speed of the starships and by the time required by each colony to establish itself. New settlements will quickly fill in the volume of space behind this wave front.”
In a famous 1998 paper “The Great Filter – Are We Almost Past It?”, economist Robin Hanson suggests that humans do not observe aliens because life encounter a “great filter between death and expanding lasting life” that prevents it from colonizing the galaxy.
“No alien civilizations have substantially colonized our solar system or systems nearby. Thus among the billion trillion stars in our past universe, none has reached the level of technology and growth that we may soon reach. This one data point implies that a Great Filter stands between ordinary dead matter and advanced exploding lasting life.”
Either intelligent life evolves extremely rarely, or it is extinguished before expanding. While Hanson believes this filter is best explained by the presumed rarity of the evolution of intelligence, he provides a fascinating description of social hypothesis that explain the theorized short lifespan of intelligent civilizations. Interestingly, as humans appear to be relatively close to interstellar capability, this suggests that — rejecting a biological Great Filter mechanism — that humans are also close to encountering the Great Filter.
Confounding the puzzle, Hanson argues that evolutionary theory suggests that civilizations that do arise tend towards expansion, making their absence harder to explain:
“In general, it only takes a few individuals of one species to try to fill an ecological niche, even if all other life is uninterested. And mutations that encourage such trials can be richly rewarded. Similarly, we expect internally-competitive populations of our surviving descendants to continue to advance technologically, and to fill new niches as they become technologically and economically feasible.”
Hanson argues that energy constraints, desire to outpace potential competitors, and concerns over local disasters would motivate even sedentary civilizations to expand — the galaxy is not full of hermit civilizations. Similarly, the finite lifespans of main-sequence stars would eventually force all civilizations that reach the end of their sun’s life to expand or die. This suggests that most intelligent civilizations eventually expand, leading it to the Fermi paradox — if intelligent civilizations are common and expansionist, why don’t we observe them?
There are three broad possibilities: aliens are expansionist but hide, either on purpose or inadvertently; civilizations are routinely destroyed before they can expand; or that civilizations elect not to expand.
Because evidence of advanced civilization is typically thought to be detectable on galactic scales, if expansionist civilizations exist in our galaxy something must be preventing us from detecting them. Typical explanations include that we have detected but cannot recognize evidence of very alien extraterrestrial civilizations for what it is, by chance aliens avoid technology detectable over vast distances, or that the galaxy is dangerous and technological civilizations are actively hiding.
Another possibility: rather than electing not to expand, planets are somehow routinely prevented from developing interstellar civilizations. Science fiction suggests a few fictional answers. In Alastair Reynolds Revelation Space, nascent interstellar civilizations inevitably attract the malevolent attention of the “Inhibitors”, dormant machines left over from an early interstellar war, or, more fancifully, in Charles Stross’ A Colder War ill-advisedly meddle with H.P. Lovecraft’s monsters. Other commonly theorized dangers are nuclear or biological warfare, or environmental disaster. More exotic theorized perils include civilization-destroying experiments with strong artificial intelligence, or attracting the attention of rapacious hidden aliens (I find this unlikely).
Another potential “Great Filter” mechanism is that alien civilizations do arise, are not prevented from expanding but instead elect not to. There are numerous explanations for this tendency. An early, widespread alien civilization could have imposed a “no-expansion” norm on following civilizations; Reynold’s long-lived Inhibitors could be considered a particularly violent way of enforcing this norm across deep time. Civilization could be universally cautious, and avoid expansion at all costs for fear of attracting the attention of hidden malevolent aliens; however, it is difficult to reconcile this with the death of stars — why would a solar system-bound civilization fear a potential danger over certain death at the end of their sun’s life? Alien civilizations could also universally prize preserving the natural state of the galaxy, though again it is doubtful that this naturalistic impulse would survive the death of civilization’s stars. Or, advanced civilizations could universally embrace virtual reality or lose physical form while somehow avoiding the resource and survivability incentives to expand.
Another potential solution is that advanced civilizations commonly arise, but are prevented from expanding due to for purely economic or organizational reasons; in this case, the solution to the Fermi paradox would be the “it is too expensive to physically spread throughout the galaxy” hypothesis. As Hanson notes, there are numerous problems with this theory; most notably, evolutionary pressures tend to select expansionary traits in successful or long-lived societies. However, I’d like to examine this possibility in more detail: why would civilizations chose not to expand in the absence of external pressures (previously set non-expansion norms, fear), innate non-expansion traits (tendency towards naturalism), or disinterest (move to virtual reality without a local resource constraint, etc.)?
There are clearly long-term benefits to galactic expansion. Civilizations that do expand would have access to much greater energy resources and vastly increased security. However, it is important to remember these benefits are collective, long-term benefits, and species with finite lives have little reason to invest in the extreme long-term. If we restrict our discussion to human-like species composed of reproducing, autonomous, sentient individuals, it is possible to argue (speculatively!) that the drive for galactic expansion largely vanishes. Interstellar colonization is a collective effort that likely fails a human-based cost-benefit test scaled around a few human generations; when rational short-lifespan individual utility maximizers are the decisionmakers, under conditions roughly similar to foreseeable future humanity interstellar colonization seems unlikely. It is even possible that individual species like our own would be unable to organize interstellar expansion when motivated by the impending death of their sun.
I am not arguing that the “it is too expensive to physically spread throughout the galaxy” is a particularly convincing universal solution to the Fermi paradox, but instead that economic constraints are a more likely explanation for supposing that near-baseline humans will not expand widely in the foreseeable future than astronomical or social-triggered destruction.
Of course, “conditions roughly similar to foreseeable future humanity” benchmarked on the early 21st century certainly leaves a lot of leeway for future humans, not to mention other species broadly similar to our own. That said, we can broadly speculate about the qualities of expansionist species with biology (again, reproducing, autonomous, sentient individuals) similar to our own:
- Exponential reproduction: In the last half century the world total fertility has fallen precipitously, from a mean of 4.95 in the 1950-55 era to 2.36 today. This fall is well understood, and is associated with the advent of birth control, rising incomes, and women’s’ increased social empowerment and education. But importantly, falling total fertility is only possible because birth control allows sex to be decoupled from reproduction, and the human reproductive drive is a sex drive. It’s entirely possible that an alien species would have a reproductive, rather than sex, drive that negated the entire idea of birth control and made exponential population growth difficult to avoid. Massive population growth could be a powerful incentive to invest in interstellar expansion.
- Extreme life extension: I’ve previously wondered if human’s falling birthrates would prevent humanity from ever investing in space colonization — after all, barring some catastrophe living in off-world will in the medium-run always been more expensive and uncomfortable than living on Earth. If humans don’t have a pressing reason to leave in large numbers, they likely won’t. While human colonies off of the Earth would significantly improve the survivability of the human species, it’s difficult to imagine this is a sufficient reason to motivate investing in these colonies. However, medical advances resulting in extreme life extension would undo the population control gains from stable world total fertility and again raise the specter of global overpopulation, perhaps prompting investment in off-world colonization. The same logic could apply to other species.
- Competing local societies: As Hanson notes, competition creates strong pressure to expand into unexploited niches. Competition among local societies could create incentives to expand in otherwise non-expansionistic species. However, it is difficult to imagine sufficient competition among human-like species to prompt interstellar expansion while avoiding local war that destroys the capability for extensive interstellar travel, though perhaps strong prohibitions on armed conflict could avoid this.
- Innate expansionistic tendencies: To move into more speculative factors, it’s possible to imagine alien species with an innate desire to expand — just as human behavioral evolution appears to have favored aggression. An innate desire for expansion would motivate investment in colonization beyond that justified by human cost/benefit calculations.
- Low/High risk tolerance: Space exploration is risky, both in direct risk and its high opportunity cost. Space colonization is much more risky. It’s conceivable that a species with a higher innate psychological tolerance for risk would elect to invest in risky expansion for reasons that don’t make sense to humans. Conversely, an species with a tolerance for risk much lower than humans could judge the long-term security of space colonization worth the risk and opportunity cost. Lifespan could conceivable play a role as well; assuming species consisting of sentient individuals, longer-lived species could either have lower (more to lose) or higher (boredom) tolerance for risk than humans.
- Extreme technological advancement: All of these previous traits alter the benefit side of an expansion cost/benefit ratio. However, extremely advanced technology developed for other purposes could justify expansion by radically reducing the cost of expansion. For example, self-replicating von Neumann machines could make expansion much cheaper. This relative affordability could prompt highly advanced species to expand when they otherwise would elect not to.
If this theory holds (and I’m not entirely convinced that it does; for example, extreme life expansion could be very common even in relatively young intelligent species), we would expect human-type civilizations that do expand to be dominated by those with innate high population growth, or extremely high technological capabilities (i.e. no expensive generation ships or warp drives). More speculatively, we could expect the most expansionist species to be those where policy is not set by individual utility maximizers. These “non-individually rational” species could include hive minds a la Star Trek’s Borg, machine races, or something else entirely.
If we accept the argument that species composed of short-lived, individual utility maximizers are not particularly inclined to expansion, and these civilizations tend to not delegate social decisions to non-individual utility maximizing actors like “God computers”, then a potential solution to the Fermi Paradox is that civilizations with the expansionist traits listed above arise only rarely. This, however, does not address the problem that expansionist societies would tend to out-compete and displace non-expansionist societies.