Argh. I left this blog post half done for a bit, and then Phil Plait went and did a similarly themed one, which is doubtless much better. I almost threw this away, but what the hell. My next post on this subject, though, will attempt to answer the question he didn't: ways to make an alien invasion actually plausible. So stay tuned!
'Yet across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.'
HG Wells, The War of the Worlds
There’s one thing that makes HG Wells’ War of the Worlds make sense, and it’s the thing that no longer makes any sense at all: the invaders come from Mars. In the closing years of the 19th century, it was still possible to imagine that there were aliens on the red planet, even if it looked rather arid. These days, we know that ‘arid’ doesn’t even begin to describe the Martian environment. Words like ‘freezing bloody cold’, ‘absence of oxygen’ and ‘no fun whatsoever’ give you a much fuller picture. Unless the little green men have been hiding really well, there’s no chance of alien invasion from anywhere nearby.
Which doesn’t stop us imagining it even for a second. Because, after all, alien invasions sell books, discs, downloads and cinema tickets in their millions. So we've jumped to assuming that invasions come from other star systems, even though there's a major problem with that: it's one hell of a long journey. A trip from Mars might take a few months, but getting between the stars can take much longer. We regard our 'stellar neighbourhood' as a sphere about 50 light years in radius - that's fifty years of travel at actual lightspeed. Which may not even be possible. And 50 LY is a piddling distance compared to the rest of the galaxy, which is 100,000 LY in diameter. It could take thousands of years to get here. Even if the aliens have amazingly fast FTL that gets them from A to B in only a few years, it's still a long journey in extremely harsh conditions.
So the end goal had better be worth it. There has to be something here that's worth mounting an invasion. I wonder what that might be...?
Life needs water – or at least, all the life we’ve seen so far needs it. Virtually everywhere that water exists on Earth, life does too. It’s both the easiest medium to live in, and the perfect solvent in which the chemistry of life can happen. It’s no coincidence that life on Earth began in puddles and seas and oceans; and just because we live on dry land, it doesn’t mean we really escaped the sea. We just brought it with us. Most land creatures are just bags of water that have to keep their H20 supply constantly topped up in order to survive, making water a precious, finite resource for life, a resource that’s rapidly running out in some areas.
But that’s just on Earth. If you’re able to travel through space, water suddenly becomes less of a problem, because it turns out to be extremely common. In fact, we got most of our water from space in the first place, during the early days of the solar system, when the Earth was constantly bombarded not only by asteroids, but by comets. You see, comets are mostly made of water ice. And there’s a lot of them out there. There may well also be a thing called the Oort cloud on the edge of the solar system that consists of untold numbers of the things – and if that exists for our system, it may do for others as well.
If you can fly through space as far as Earth, you can easily process some comets for your water needs. Unless, of course, you didn’t fly through space to get here, but that’s another issue entirely.
Minerals have the same issue as water, because they also come in handy clumps floating around in space. This time we call them asteroids. And they’re easier to get at than minerals on Earth, because they aren’t at the bottom of an annoying gravity well that forces you to use lots of energy to escape.
And there really shouldn’t be any difficulty in finding the specific minerals you need. Everything in the solar system was formed from the same cloud of gas and dust, which clumped together over many millions of years to form asteroids, comets, moons and planets. The Earth is the result of many of these ‘planetesimals’ falling in on each other; so anything useful you can find in the rocks on Earth should be present in the asteroid belt, which is really just a collection of debris that managed to escape becoming a planet. In fact, some things may be even easier to get hold of. The Earth was molten for a long time after it formed, and the heaviest stuff (mostly iron) sank to the centre. In an asteroid, this won’t be the case – they should be even richer in heavy elements than the outer shell of the Earth (which is all we have access to).
And if it’s like this in our solar system, it’s going to be like this elsewhere – oh, maybe the ratio of gas giants to rocky planets will be different, or they’ll be in different places, or there’s a different balance of minerals – but there should still be more than enough floating around out there to satisfy the needs of resource-hungry aliens.
The exception to this would be the ‘hegemonising swarm’, as Iain M Banks puts it, AKA Von Neumann devices. Self-replicating machines that exist only to make copies of themselves wouldn’t target us specifically, but if we happen to be in their path, they won’t say no to gobbling up everything they can get their manipulators on. The main objection to the possibility of our planet being disassembled by hordes of robots is a variation on the simple question posed by Enrico Fermi: given the age of the galaxy and the number of times such waves of invasions could have been launched, why hasn't this happened already?
Now it gets a bit trickier, because while raw materials are pretty easy to find and likely to be generic in every solar system, life might not be. Evolution throws up all kinds of weirdness, and the plants and animals we see on Earth are just the tip of the iceberg for what’s possible. And maybe, just maybe, there’s something in the terran biota they want. For example…
There are problems for any alien species looking to live off the land: their digestive systems would have to be able to cope. There’s all kinds of ways they could be poisoned or given terrible indigestion or just find that our flesh passes right through them virtually untouched. For example: amino acids. These are the building blocks of proteins, which are in turn the building blocks for much of our bodies. Amino acids come in two types: ‘left-handed’ and ‘right handed’, essentially mirror images of each other. Every lifeform on our planet uses left-handed amino acids, and onlyleft-handed amino acids. There’s no particular reason why this should be the case – right-handed ones can do exactly the same things – it’s just that our earliest ancestors randomly decided to be lefties. If our invaders are built of the right-handed versions, they'd be unable to get much sustenance out of us because their whole body chemistry is based on dealing with the wrong kind of amino acid. And that’s just one of many potential problems. They’d better hope they grabbed some lunch before they set out.
The notion that aliens have come all the way here to make babies is patently absurd. They almost certainly won't be able to mate with us. They probably haven't even got the right, um, equipment. And even if they did, there are all the other problems of having a completely alien biological heritage to worry about. The fact that this concept keeps rearing it's nasty little head in science fiction is probably more to do with our own psychological makeup; we have an unpleasant history of kidnapping women from other tribes.
But maybe they're not bothered by reproduction itself. Maybe they're just perverts who want to do horrible things to alien species because they get turned on by the fact that we're ugly and weird and strange. To them, mating with us might be seen as a form of bestiality: disgusting to the vast majority, but compelling to a few people. This, oddly enough, makes more sense than most reasons for invasion, simply because it's irrational; an irrational desire could override the common sense objections to hurtling halfway across the galaxy and doing something incredibly stupid and destructive. But it's hard to see this being a very common desire, so it's (hopefully) very unlikely. After all, they're just as likely to be turned on by sea urchins as us.
Drugs have the same problems of physiology as anything else an alien might want to ingest. But many of the drugs we use on a regular basis happened by accident. Caffeine, for example, was evolved as a defence against insect pests, who tend to find it poisonous. We, being somewhat larger, only experience milder effects which we consider pleasurable (most of the time). There’s a whole heap of compounds being produced in nature which might turn out to have unexpected effects on alien life. Of course, you’d think that a civilisation advanced enough to travel between the stars would be able to synthesise their own drugs, but still…
Another historically vital resource for which humans launched invasions of foreign lands. In fact, slaves were a prime spoil of war all the way into recent centuries; it’s only very recently that we’ve stopped mounting raids on each other to steal people. Because, after all, much of the brute labour that humans used to do is now done by machines. Which brings up the main problem for this as a reason for alien invasion: if they’re technically advanced, manual labour shouldn’t be an issue. Or mental labour, for that matter. If the aliens need us as slaves, then there's something very, very wrong with their technology and/or society.
This makes perhaps the most sense of all – maybe the aliens just want to learn about us for the sake of learning itself. Ethical ones would keep themselves secret and just observe, but unethical ones might well consider it interesting to either use our planet as a resource for lab animals to harvest, or just conquer the place and breed us as required. It’s a long way to travel just to get some test subjects, but this is a more likely – and terrifying – scenario than many of the others, mainly because the difference between us and them is no longer a barrier to them interacting with us. Instead, the differences between our species becomes the reason why they take an interest in the first place.But before you panic, make some note of why such a species would come to us in the first place. If they want to learn about us, they're more likely to study us in our own environment than they are to take us out of it and conduct pointless acts of vivisection. The more we learn how to do science, the more we realise that we have to be careful to prevent our own presence contaminating the data we gather; there's no point in studying the feeding patterns of ants if those ants pick up a new diet because the researchers keep leaving rubbish in the forest. If aliens have to conquer us before they start studying us, there's vastly less that they can learn, and vastly less chance that they'd make the journey in the first place.
Maybe the aliens don’t have enough room at home. Maybe they just need more living space, and all that stands in their way are some pesky natives who aren’t using the planet properly anyway. In fact, they’re overusing it and they’ll kill it off before long. Surely it’s the aliens’ manifest destiny to take the planet and colonise it properly?
This, of course, presupposes that travel between the stars is easier than just building somewhere else to live. There’s a lot of asteroids in any given solar system, and if you have the technology it shouldn’t be too difficult to throw together a habitat that biological lifeforms could thrive in. Maybe you're just hollowing out the asteroid to form a glorified but comfortable cave, or maybe you're getting a bit more ambitious (see image).Of course, this presupposes that the aliens are actually biological, and aren’t already adapted to living in space – sure, it’s a harsh environment, but solar energy is free and all the necessary minerals are readily accessible, if you happen to be a machine with the right equipment. And if you’re capable of getting between the stars, that’s one of the things you may well be.
Of course, there’s an easy argument to take it the other way: if space is crowded and living space is jealously guarded, then there’s a good reason to come here, get rid of the natives and set up shop. But if there were such enormous pressures on aliens to find new spaces to live, you’d think they’d have done it already. It’s not impossible that colonists should come calling one day; just very unlikely.
There’s no reason why aliens should like us, and many reasons why they shouldn’t. We pollute our environment, we drive other species to extinction, and we fill the cosmos with broadcasts of reality TV shows. Surely we deserve euthanasia for that crime alone.
As well as killing us off for the crimes we’ve already committed, aliens may also decide to just wipe us out to prevent us ever challenging them in the future. A pre-emptive strike to stop us polluting the spaceways might be justified by projections of our likely behaviour once we get hold of FTL technology. Even a quick glance at human history will show that we’re liable to go invading other worlds if given half a chance, so why give us that chance?
If aliens believe they were made in the image of god, they may take exception to us running around, pretending to be intelligent while looking like anything but their conception of a decent, god-fearing species. Or they might simply consider that we have no souls and are therefore animal pests that need to be exterminated. Or if they think we do have souls, they might come here with the express intent of converting us to their religion, whatever that might be. You might think that aliens capable of travelling to distant stars would be more rational, but there’s no guarantee of that. And the more irrational they are, the more likely they are to do something that has no material benefit to them (or us).
Or maybe they just like killing and genocide. The life of a spacefaring species would be long, and eventually they’re likely to get bored. If you’ve tried everything else that life has to offer, maybe you’ll turn to killing and destruction just to get a thrill. Or maybe it's the aliens' children who are looking to waste time in a particularly nasty way. Of course, that's pretty much what's happening on our planet - except that we're turning more and more to virtual thrills that have little or no impact upon reality. Aliens would likely have even better ways to distract themselves, making this a little less likely than it first appears. Hopefully.
Maybe they blunder into our solar system without checking it first to see if anything’s there, and there’s some awful misunderstanding when they interpret our radar signals as a precursor to an attack. Or maybe they deposit some kind of highly virulent plague without meaning to. Or maybe they don’t even notice us at all and kill us off on their way through via excessive radiation from their interspace drive wotsits. Or maybe they're planet sized and wreck our solar system by shifting all the orbits so that the Earth either freezes, drifts into the sun, or gets whacked by Mars.
Any alien invasion has to get past the biggest barrier between them and us: space. So far as we know, doing that would present a vast, possibly insurmountable cost that makes any invasion very unlikely.
And even if they could get here, why would they bother? Their technology is likely superior to ours. Their biology is almost certainly incompatible with ours. There's little or no resources here that they can't get elsewhere, and much more easily. Pretty much any sensible reason to invade is incredibly unlikely, leaving only the irrational, random, unethical and accidental reasons, which would be rare in the first place, and even rarer given the vast gulfs of space that lie between stars. You can overcome this problem by assuming that the galaxy is crowded with life and therefore the distance to the nearest civilisations are small enough that they could actually get here - but if that's the case, you still have to answer to Enrico Fermi and his paradox.
So based upon what we know now, the probability is low. The next time I look at this, I'll go through all the things that could make alien invasion a real possibility, which means we may have to move beyond what we know. Realism and the laws of physics might have to be abandoned, but then, after all, science is a work in progress, and it's still possible that we've been very, very wrong about the universe...