One of the commonest themes in science fiction movies is the alien invasion. Technologically-superior beings from another world descend upon Earth intent on conquering it, usually wreaking enormous damage before being defeated by the plucky hero who comes up with some clever idea that negates the vast power of the invaders' superior weaponry. Is this at all a credible scenario?
To begin with, the plucky hero's clever idea definitely is not. Sufficiently superior technology almost always wins. There's no one single clever idea that an American Indian or Australian aborigine could have come up with to permanently thwart the European conquest of those peoples' home continents. If invading aliens were advanced enough to get an armada of spaceships across interstellar distances in the first place, the imbalance of technology in their favor would be similarly unbeatable. Many of these scenarios involve the aliens' technology having a single critical failure point -- one single ship, machine, or individual on which everything depends, such that knocking out that one target defeats the whole invasion. It's unlikely that advanced aliens would design their weapons systems so stupidly.
The more interesting question, though, is why the aliens would be attacking us at all. An invasion implies at least some cost to the invader in the form of usage of expensive ships, trained crews, etc. What benefit would they be attempting to gain which would offset that cost? Those science fiction movies suggest a number of possibilities, but how plausible are they?
Stealing Earth's resources. This is one of the commonest ideas, and the most unrealistic. The concept reflects our own history of colonialism -- Europeans conquered other parts of the world to get resources Europe wanted. But the universe isn't like Earth a couple of centuries ago. Any natural resource available on Earth would likely be available on
many uninhabited planets where the aliens wouldn't need to fight for it. Interstellar distances are so vast that transport costs, in one form or another, would far exceed the value of whatever was being taken.
As an example,
the V series postulated aliens conquering Earth to seize all its
water, of which their own planet had a shortage. Just imagine the energy costs involved in lifting the whole mass of Earth's oceans into space against gravity, then pushing all that mass through a space warp or whatever they were using to get it home. Then, too, water is one of the most abundant substances in the universe. They could probably have found all they needed on some planet or moon in their own solar system, and not have had to fight recalcitrant natives for it.
The same is true of almost any imaginable resource. Anything aliens could find on Earth, they could find closer to home, probably in their own solar system, and with no local intelligent life to try to stop them from taking it.
Even if the needed resource were something peculiar to Earth -- say, an ingredient of our own biochemistry that was found nowhere else -- it would almost certainly be much cheaper to get a few samples and synthesize it
en masse at home.
Colonizing Earth. This is a little more plausible, but only if the environment on the aliens' home planet were almost identical to Earth's, which it probably wouldn't be. The gravity, the mix of gases in the atmosphere, the temperature range, the interaction of local biochemistry with theirs -- at least some of those things would probably be wrong for them, and would require a huge investment in terraforming to correct. A species with interstellar travel would have access to countless thousands of planets, and could likely find one better suited to their needs which didn't have its own technological civilization capable of making them fight for it -- and possibly ruining the planet in the process, since humans might well use nuclear weapons in the effort to fight off such an invasion.
And after all the investment of conquest and terraforming, all they'd have would be one marginally-habitable (to them) planet a long way from home. If they needed living space for a growing population, a
ringworld or smaller-scale (but still much bigger than a planet) artificial structures in their own solar system would probably make better sense from a cost-benefit viewpoint.
(There's a more fundamental reason why this scenario is unlikely, but I'll get to that in a minute.)
Destroying a potential future threat. They might fear that once we become advanced enough to cross interstellar space, we'd be a threat to them, so they'd better wipe us out now while they still can. Please. We're talking about beings who already have interstellar travel. When Europeans discovered Australia, they certainly didn't immediately start worrying that the aborigines would someday become so advanced that they could threaten Europe. The equivalent scenario with interstellar-spacefaring aliens is similarly absurd.
Enslaving humans. For what? If they're that advanced, they already have machines with artificial intelligence to do whatever they want, which will never rebel against them and don't require special food, special air, the effort of teaching them the aliens' language, etc.
Propagating a belief system. I'm not sure whether SF has ever actually used this scenario, but I'll throw it in because it has precedent in Earth's history. Some of the great conquests of history weren't motivated by economic gain or colonization, but to spread a religion or ideology (the Islamic conquests and the Crusades, for example). Could an alien invasion be motivated by something similar? Obviously we can't predict alien psychology, but it seems unlikely. Technological progress seems to be predicated on transcending irrational belief systems and embracing a scientific view of reality, and the more advanced the technology becomes, the more true this is. Any intelligent species that
can't cast off its irrational belief systems is probably permanently stuck on its home planet at a medieval level of technology, and thus won't ever visit us.
One could envisage a society in which a rational class of individuals builds and maintains advanced technology while a numerical majority (or ruling class) still under the sway of pre-modern thinking decides how that technology is used -- that describes the current United States to some extent -- but I can't see this being stable for very long. Either the rational class would win out and transform the society (or at least take over the ruling role), or the irrational elements would eventually cause the technology to stagnate and decline. And if there
is a species somewhere which has successfully combined religious fervor with advanced technology, they've probably wiped themselves out before getting very far into space (imagine if the Crusaders and their Muslim opponents, or the Catholics and Protestants during the Thirty Years War, had had ICBMs with fusion warheads).
In fact, any such advanced alien civilization would be radically different from how our invasion movies generally picture it. Less than a century from now -- likely far less -- our own species will fully develop mind-computer integration and achieve the
Technological Singularity, enabling our species to "migrate" from organic bodies and physical reality to consciousness running on
nanocircuitry-based supercomputers and inhabiting virtual reality -- allowing human intelligence and awareness to increase without limit, with every individual having access to a vastly richer sensory and sensual range of experience than the organic/physical world allows, and free from such organic constraints as disease, injury, aging, and involuntary death. I view this as the "maturing" of an intelligent species, its transition to adulthood.
Any species advanced enough to develop interstellar travel has almost certainly already achieved this. They'll already have reconfigured their own solar system to optimal for a post-Singularity civilization, by breaking up unnecessary planets (like Jupiter and Saturn, in our case) to obtain the material for enough nanocircuitry to "run" the minds of all the billions or trillions of individuals of their kind, along with all the virtual worlds and other projects which all those individuals might choose to create, and for a
Dyson swarm around the local sun to power it all. So what would such a civilization want or need from
us?
They won't be interested in stealing resources or in colonization. The only material resource needed by a civilization like that is computer processing power. If their needs grow beyond what can be provided by the amount of nanocircuitry they already have (unlikely, when they have a whole solar system's worth of matter to work with), then any kind of matter will do -- they'll just head out to the nearest neighboring systems full of dead worlds and start reconfiguring them the same way. They'll have no reason to target Earth, even if it's very similar to the planet they lived on when they were still fragile organic creatures. The idea that they might see us as a potential threat or as potential slaves is totally absurd, for reasons I'll assume are obvious.
I can imagine only one thing we have that they might possibly want -- one thing that they couldn't simply create for themselves. And that's our minds.
In a post-Singularity civilization, most individuals will probably occupy much of their time with the pursuit of new experiences -- creating new virtual worlds, interacting with other individuals and
their virtual worlds, absorbing imagined experiences created by those with a special talent for doing so (we already do this in a low-tech way by reading novels and watching movies). But every intelligent species will be the product of a different evolutionary process. Each species will have different experiences, different senses, different ways of thinking, different desires and passions and lusts. If there is "trade" between post-Singularity civilizations, it probably doesn't consist of the exchange of physical goods, but the exchange of recorded experiences and mental patterns, alien novelties to be absorbed, known, felt.
Even a species like us, not yet "mature", probably has psychological features unique to itself, even if somewhat pallid and limited by their standards. That's what they would want from us that they couldn't get elsewhere, because our thoughts and emotions and experiences would be different from theirs, novelties to them.
It would probably be quite easy for them to scan the brain of every human on Earth (maybe other higher animals as well) and record every thought, memory, emotion, desire, and perception of every one of our billions, to be absorbed at leisure back home by those among them who had an interest in primitive beings. They could likely do this without harming us at all, without us even being aware that it had been done.
In fact, this could have already happened.....any number of times.
