Over the recent holidays I spent one evening watching Cowboys and Aliens. The movie is ridiculous — of course it means to be, and the tragedy isn’t its irreverence but the fact it isn’t that good — but it is thought-provoking. Alien invasions stories are a hugely common trope, especially in film. It’s easy to understand why they pop up so frequently. Invasion stories feature suitably dramatic survival-of-humanity stakes, and importantly don’t require inventing a future society, like other forms of science fiction. Accessible, dramatic, conflict-oriented — what’s not to like?
Unfortunately, alien invasion stories are at best usually utterly implausible. Of course, this can be irrelevant — plausibility isn’t required to tell engaging stories, and is certainly much less important than relatable characterizations, drama, and the other tenets of good fiction. But it is an interesting challenge: how can authors tell stories about alien invasions in line with reasonable assumptions about aliens’ behavior and incentives?
The core implausibility about most alien invasion stories isn’t plucky humans triumphing over aliens capable of flying across the galaxy — suspension of disbelief exists for a reason — but instead aliens’ incentives. Of course, the only thing we can assume about alien civilizations is that, well, they’re alien. It’s very difficult to make any assumptions about how an alien civilization would be organized, what they would value, and how they would behave. But we are able to identify universal constraints, and extrapolate which of these constraints aliens’ incentives are bound by. No matter how alien, there are certain limitations that we can assume all technological civilizations are bound by. Civilizations appear to be universally limited to expensive, slower than light travel. The galactic scarcity of certain chemical elements is also universal, as is some degree of natural selection within and between species. Even if intelligent species are common they are likely be widely separated by space and time, and a plausible depiction of a populous galaxy must square with our failure to observe evidence of alien technological civilizations.
Most alien invasion stories depict aliens invading the Earth in pursuit of its resources. In Cowboys and Aliens [mild spoiler], the resource the alien aggressors are after is gold. This is, to put it politely, very implausible. While there are rarer chemical elements in the Milky Way, gold is scarce galaxy-wide, making it a potential valuable commodity for alien species. However, this is a tricky assumption in and of itself — humans’ high valuation of gold is not only due to scarcity, but also aesthetics. There’s no reason that alien sensory systems and psychology would necessarily value the shiny luster of gold. Worse, it’s difficult to believe that a civilization capable of routine interstellar travel faces any real scarcity. Even if the aliens had completely harvested their own system’s resources, there are considerably more cost-effective means of acquiring gold than stealing it from hardscrabble Westerners. Gold is likely to be present in most stellar systems, offering potential resources at lower travel costs than the journey to Earth (again, remember the ‘space travel is expensive’ assumption). More exotically, given the high cost of interstellar travel for even advanced species, harvesting resources from black hole accretion disks is likely more cost-effective for a suitably advanced civilization.
Other films are even worse. Battle: Los Angeles depicts aliens invading for [mild spoiler] Earth’s water, when any civilization able to travel between stars could trivially harness the energy to melt any minor icy moon. Avatar gets around this by inventing “unobtainum” so valuable it’s worth flying to the next start to get it — an example of MacGuffinite if there ever was one.The truth is there really isn’t any plausible rational reason for an alien invasion of Earth. Most simply, there’s no mineral resources found on Earth that can’t likely be had in any stellar system. It’s also unlikely that aliens would invade our planet for living space, because it’s similarly unlikely that any aliens we encounter require the same narrow environmental band that we do — aliens could just as easily “invade” the to-them inviting environs of Venus, Titan, or Jupiter. Really the only thing unique about Earth is its biosphere, but even if aliens valued Earth’s life they would be unlikely to invade in pursuit of it. Nuclear weapons are a comparably basic technology, and it’s a reasonable assumption that aliens would associate Earth’s obvious radio emissions with a mastery of nuclear technology. If prospective invaders could ascertain that Earth housed nuclear weapons, they would also know that its defenders could arbitrarily destroy the very biosphere the invaders were after.
So, assuming that interstellar travel is costly, it’s reasonable to say that while colonization is rational to select species, invasions are not — as long as we define “rational” as “fulfilling a reasonable cost-benefit ratio, expressed in resource terms”. But defining strategic rationality in even a solely human context is problematic. As M.L.R. Smith writes, citing F. Lopez-Alvez:
“To pass judgment on whether anyone is rational or irrational in political life is to assume that one exists in Olympian detachment with a unique insight into what constitutes supreme powers of reasoning (a self-evidently delusional position). The assumption of rationality, however, does not suppose that the actor is functioning with perfect efficiency or ‘that all rational decisions are right ones, merely that an actor’s decisions are made after careful cost–benefit calculation and the means chosen seem optimal to accomplish the desired end.’”
So while we can speculatively presume that aliens have no rational incentive to fight us for purely acquisitional rationals, this doesn’t rule out alien civilizations rationally valuing conflict. After all, strategic rationality rests on internal consistency, not a universal yardstick — the “desired end” varies. Perhaps alien civilizations are religiously driven to genocide (I’m using “genocide” in a species extinction context), or make the rational decision that the existence of any alien competitor is a potential existential risk that cannot be tolerated. As Charles Pellegrino has extensively argued, natural selection pressures would tend to select for aggressively competitive and preservation-minded civilizations. Aliens that expand far enough to encounter other species have powerful incentives to quickly destroy them.
But, importantly, there’s considerable ground between invasion and destruction; ground that doesn’t leave much room for exciting narratives. Again citing Pellegrino, a genocide-minded interstellar civilization would simply attack a potential competitor with a spacecraft accelerated to relativistic velocity. The capability to launch such an attack is a prerequisite of most plausible interstellar species, and would be nearly impossible to counter. So a plausible alien invasion story requires a delicate middle ground: aliens must not seek to destroy us completely, nor desire to capture and exploit the solar systems’ resources.
So where does that leave us? Luckily for humanity, it’s difficult to imagine a plausible alien invasion story because alien invasions themselves aren’t particularly likely. We haven’t observed evidence of technological civilizations in decades of looking, which means — among other possibilities — that they aren’t there at all and we’re alone in the galaxy (disconcerting), are deliberately preserving our ignorance (more disconcerting), or are actively hiding themselves out of fear of a dangerous universe (very disconcerting). But if we’re specifically looking for a plausible alien invasion, there are some possibilities.
A speculative example:
The conflict begins when humans detect the very bright exhaust plumes of alien spacecraft decelerating into the solar system. Even if the incoming aliens are equipped with extremely powerful antimatter reaction drives this deceleration will take decades, giving human civilization time to attempt to divine the aliens’ intentions and prepare for possible hostilities. The alien spacecraft are eventually revealed to be gigantic ships that, after decelerating, go into orbit in the outer solar system. Once they have arrived the aliens make an unexpected announcement. We are at war, but there are rules: we must fight, and we cannot use nuclear weapons.
If we violate these two rules, human civilization will be summarily destroyed by a relativistic rocket launched from outside of the solar system. However, if we follow the rules our civilization may survive.
This, of course, does not seem to make any sense. What are the aliens after? As the conflict progresses similar questions arise. The invaders seem to prefer to fight us on Earth, or in its close proximity. Fortunately for us, for a civilization capable of interstellar flight on a massive scale, the conflict seems bizarrely evenly matched: the aliens are certainly more advanced and their soldiers more formidable, but not so much that they instantly roll over Earth’s militaries. Even more fortunately, the aliens’ war strategy seems bizarrely uncoordinated. Eventually, humans deduct that the alien forces appear uncoordinated because they are — humans are facing not a unified force but competing, unaffiliated factions.
Why would the aliens behave this way? Clearly, this is not a rational way to win a military conflict. But it does make sense if they aliens aren’t seeking, in a strict sense, to win. After all, if genocide is best accomplished with a relativistic rocket and resource-motivated invasions don’t make sense, than any plausible invasion must be in pursuit of another outcome. In this speculative case, imagine a historically-martial alien species that, either by biological or social norms, selects leaders and social prestige through combat. If intelligent species are common enough to allow the practice to function, periodic limited wars would be a rational means of social organization, and an equitable way of periodically reordering the social hierarchy. Sure, this is an insanely costly means of social organization, but in strict combat and opportunity costs. But that doesn’t make it impossible, or even unreasonable for a post-scarcity species with limited individual self-preservation instincts and a fear of intra-species war.
These two rules are conductive to the limited war the aliens seek. Announcing that we must fight conveys that there will be no negotiated settlement, because, of course, the aliens aren’t interested in the outcome of the conflict, per se: they’re interested in the resulting redistribution of intra-society prestige, but not the actual military outcome. The prohibition on nuclear weapons limits the possibility that humans pose an existential risk to both the aliens, or ourselves. If nuclear weapons are allowed humans are likely to bomb ourselves into eventual radioactive extinction, regardless of the outcome of the war — something the aliens have no real interest in.
Taking this concept farther, why would the aliens have any interest in preventing inadvertent human extinction by excessive in-atmosphere nuclear weapons use? After all, they obviously place low value on human life if they don’t mind throwing us into a costly war simply to maintain their social order. One intriguing possibility is that perhaps the invaders have no qualms about destroying humanity, but fear punishment from a third-party species.
Given the vast age of the universe, the time span between the emergence of two species can measure in the millions of years — plenty of time for the elder species to gain an insurmountable advantage over its younger competitor (though this does not necessarily imply that the elder can prevent subsequent competitors from expanding as well, and complete galactic hegemony appears impossible). If this first species elects to expand, it has the enormous advantage of expanding into an empty galaxy and enjoys a later privileged position over younger civilizations in the local area.
If expansion is subject to such a massive first-mover advantage, the first long-lived first interstellar civilization to arise is in a position to force their own preferred norms on subsequent civilizations. This provides a reasonable pathway towards a local set of norms in a crowded galaxy, and avoid the collective action problem that otherwise hampers the emergence of restrictive norms. If this first-mover placed paramount value on preventing the genocide of intelligent species, this enforced preference would explain the reserved behavior of the alien invader. This prohibition is a milder version of the zoo hypothesis, expressed by William I. Newman and Carl Sagan as “imposing strict injunctions against colonization of or contact with already populated planets.” Simply fighting is permitted; completely destroying its inhabitants is not.
The presence of a local third-party hegemon would explain the invaders’ prohibition on nuclear weapons use. It would also make the aliens’ limited war practice considerably less risky. If the species they elected to target was actually a powerful civilization — aware of the local norm — for some purpose concealing its capabilities, the invaders would be presumably safe from existential retaliation.
This enforced norm means, of course, that the threat behind the invaders’ rules is a bluff — if humans refuse to fight the specter of an extinction-causing relativistic rocket strike is toothless. From the aliens’ perspective, this isn’t a problem. As long as humans are unaware of the powerful first-mover species, we’ll buy the threat. From a storytelling perspective (because, of course, that’s what alien invasion stories are about) this also gives a suitably dramatic out. Humans are suffering unbearable losses but puzzle out the nature of the galactic order, make a nail-biting, desperate decision, and call the bluff. Conflict ends, with only a minimum of the implausible deus ex machina invasion stories are prone to. Not bad, right?