The Economics of Alien Invasion, Updated
By Taylor Marvin
I saw it. At the bottom of the page is what they got wrong.
Here’s the trailer for the upcoming film Battle: Los Angles:
This may or may not be a good movie. But it did get me thinking: if intelligent, advanced alien civilizations exist, what incentives would they have to invade Earth?
Though it’s impossible to reasonably speculate about extraterrestrials’ motivations, there are some universally constant factors that would influence the behavior of any technological civilization, no matter how alien:
- Space is extremely big, and the distance between even a relatively nearby alien civilization and us means that travel between the two would take at least decades. Barring a major hole in our understanding of physics faster than light travel is impossible, meaning that for even a very advanced civilization interstellar travel would be extremely costly and time consuming. Given the enormous energy requirements of interstellar flight, even for a energy-rich civilization that had intensively industrialized its solar system these costs would be very significant, and it’s been speculated that interstellar travel could turn out to be so expensive that no economy could reasonably support it. This logic would still apply to alien civilizations with vastly different available resources and economic systems than we’re used to. Even an enormous multi-world alien economy would face the problem of scarcity and costly interstellar travel would have to compete for resources with other projects. If aliens do choose to aggressively develop interstellar travel they must have an extremely powerful incentive to do so, especially give the huge opportunity costs of building an interstellar fleet.
- An alien civilization would have to be very close by to know about humanity at all. The Earth’s presence is obvious across great distances, and any alien civilization interested in our planet would probably be able to infer the presence of some form of life on Earth by the high concentration of free oxygen in our atmosphere. However, it would be much harder to detect our technological civilization. The main evidence of our civilization detectable across interstellar distances is our radio emissions, and it seems likely that even a vastly different alien civilization would be able to interpret radio emissions from Earth as a sign of intelligence. However, because humans have only been emitting lots of noise in the electromagnetic spectrum for about the last hundred years radio waves from Earth have only had time to travel roughly 100 light years, expanding in a sphere around the Earth at the speed of light. That implies that only an alien civilization 100 light years from Earth would be able to detect human technology. It’s unlikely that, given the vast size of the galaxy and presumed rarity of habitable planets, an advanced civilization would lie so close to ours. However, it isn’t impossible — within only a 21 light year radius of Earth lie 100 stars, some with planets. Despite this it seems unlikely that an advanced civilization would lie so close to us, because we would probably be able to detect it. It seems unlikely that an alien species experiencing technological development would not have emitted significant electromagnetic radiation during its history, emission that would be detectable by us. This, combined with an assumed 200+ year lag period between the advent of alien radio and interstellar space-flight technologies, strongly implies that there are no technological civilization within at least two hundred light years of Earth and no alien intelligence aware of our presence.
- There are few plausible incentives aliens would have to invade the Earth. Science fiction authors and filmmakers typically offer the Earth’s resources as motivation for aliens to mount an invasion of our planet. This doesn’t seem likely. Most of the resources Earth offers are common in the universe; water is ubiquitous, rare metals are a normal component of asteroids, and solar systems with terrestrial planets suitable for mining seem to be common. Given the very high cost of interstellar travel, it’s hard to imagine any physical resource found on the Earth that couldn’t be more cheaply exploited closer to an expanding alien civilization. However, one unique feature of Earth is its climate and atmosphere. If an alien species had a similar physiology to Earth life and planets with oxygen atmospheres and liquid water turn out to be rare in this section of the galaxy it’s possible that our biosphere could motivate an alien invasion. However, this doesn’t hold up to scrutiny. The only reason an alien civilization would send a military force, rather than a colonizing mission, to Earth is if they knew an intelligent species lived here and judged our technological abilities to be a threat. If an alien aggressor was aware that Earth harbored a technological civilization, it would be reasonable to assume the aliens also recognized human’s nuclear weapons capability, a fairly basic technology a sufficiently advanced civilization would likely be aware of. The threat of a human nuclear defense of the planet would remove the incentive to invade Earth for its atmosphere or biosphere — resources that would be destroyed by nuclear warfare. An irradiated Earth would be of much less utility to even a radiation-tolerant species, making the notion of an invasion driven by a desire for alien living space or biological resources unlikely.
The sheer vastness of space is what ultimately makes the prospect of an alien invasion unlikely. Even if an advanced civilization was located relatively close to our solar system, the limited speeds of even extremely advanced space travel would mean that the flight time of an invasion fleet would be measured in centuries, if not millennia. For a civilization motivated to mount an extremely costly invasion whose prospective gains would be realized thousands of years into the future would require a species with the ability to think over the extremely long term without significant discounting future returns to present costs. The necessary long flight times of interstellar travel also have interesting tactical implications — a fleet that arrived at Earth would be armed with at least centuries-old technology from the aliens’ own perspective, with the possibility that human weaponry had advanced significantly during their long journey. The lack of clear gains from successfully occupying Earth, combined with the extremely high costs of developing sufficient interstellar travel capabilities, suggest that mounting an invasion of Earth would not be in the interest of most imaginable alien economies.
However, not all wars are motivated by economic concerns. The medieval Crusades are thought to have been at least partially motivated by the need for an outlet for Europe’s large number of otherwise unoccupied professional soldiers and overpopulation pressures, and it is possible to imagine similar conditions motivating an alien civilization. And of course we have really no real basis to speculate about what incentives an imaginary alien civilization might face. We can state that interstellar travel would probably be very costly for any civilization, but it’s possible that we’re wrong. Similarly, an alien species could put enormous value on Earth for reasons we can’t even comprehend, enough value to justify the extreme costs and long time horizons of an invasion. However, given the inherent expense of interstellar flight, likely large separation between intelligent civilizations, and universal prevalence of resources there seem to be few incentives for an alien invasion, even for an extraterrestrial species. We’re lucky.
Update: I’m a sucker for science fiction, so I saw the movie Friday. Unsurprisingly, the filmmakers chose not to incorporate my theories on the cost-effectiveness of alien invasion. Specifically (spoiler alert!):
- The aliens are willing to travel hundreds of light years, a journey that will take at the very least centuries (and that’s assuming extremely advanced propulsion technology like antimatter thrusters) to find water, the most common multi-element molecule in the universe? We know that water itself is extremely common in the universe, and we know that solid planets with surface temperatures to permit liquid water exist, because we’ve already found one. Even if the Battle: Los Angeles aliens somehow couldn’t find a water source closer to home, why would they need to invade Earth for it? The movie attempts to address this- a television reporter claims that Earth is “the only known source of liquid water in the universe.” Even if this turns out to be true, the aliens shouldn’t need to find liquid water — if you can generate enough energy for interplanetary travel, you can melt Pluto into as much liquid water as you want.
- The aliens’ invasion force is mostly composed of infantry. This doesn’t make much sense — if you truly want to exterminate Earth’s population, an orbital bombardment is a much cheaper way to do this. What are the aliens planning to do, individually shoot 7 billion people? This is not an efficient use of resources.
- Similarly, the aliens choice to use living troops rather than robots is interesting. It’s clear that the aliens don’t have some cultural taboo against constructing robots, as their air force is entirely drones. It’s also possible that, unlike humans, the alien species doesn’t have an aversion to losing soldiers.However, even if the alien society was willing to suffer preventable casualties utilizing live troops would be enormously costly for an invasion force, because living beings require biological inputs during the interstellar voyage to Earth. Even if the aliens put their troops into some kind of hibernation during the centuries to millennia-long voyage, they would still have to provide their troops with an atmosphere, requiring more massive, complex ships. This would significantly increase the cost of constructing an invasion fleet. Considering the resources necessary to transport and individual alien soldier to Earth, they aren’t expendable, and certainly have better things to do than hunt down Aaron Eckhart. Additionally, the aliens fight us on roughly equal terms — their weapons are similar to ours, and they seem to require roughly numerical parity to defeat human soldiers. That implies that the aliens have transported enough soldiers to earth to rival all of the world’s militaries — maybe ten million soldiers. This strains the limits of plausibility. Interstellar flight may be so slow and expensive that no civilizations ever have the resources to pursue it. Transporting ten million beings to invade one planet that isn’t home to any unique resource is certainly not cost effective.
Of course, analyzing the plausibility of a movie with the word “battle” in the title probably isn’t a great use of anyone’s time. Still, it is encouraging that it is difficult to imagine circumstances where even radically alien extraterrestrials would have an incentive to invade Earth. Alien invasion movies are fun, but they aren’t intellectually challenging. Something tells me this isn’t a problem.