I've moved!

The apocalypse never happened, dammit. So I went to a parallel universe at neverenoughworlds.blogspot.com. In fact, if you hang around here much longer, you'll be sucked through the vortex, too...

BOGOF, the lot of you!

A whole month of happy writing has gone by and I've not written a word on this here blog. Oh, well, I'll likely have a bit more of a chance soon, given that I'm 90K into the new novel, and can't possibly make it stretch much further than 115K.

Anyway! I'll be taking part in another one of these wondrous Win With Ebooks promotions again this month; this time, the deal is that you buy one of the books on the list and get another one for free! Also there are Amazon gift cards and suchlike to be given away. Blogging peeps can get more chances to win by mentioning us. More details at winwithebooks.com!

It all runs between March 15th-18th, and the participating books have all been reduced to 99c (or stirling/euro equivalent) for the duration of the whole shebang. So it's a good time to grab a book anyway.

And should you wish to enter the happy competition, say hello to my friend the Rafflecopter!



a Rafflecopter giveaway

Win a Kindle Fire. No, seriously!

So I'm taking part in this crazy fun thing called Win With eBooks. Here's the basic idea: twenty lovely, wonderful ebooks are getting knocked down to 99c from Feb 12th to 15th. If you can provide proof of purchase, or do something else to help us out (reviews, tweets, FB likes - check the Rafflecopter widget thing, either here on on winwithebooks.com!), you get to enter the giveaway (but you get more of a chance to win if you buy the books). Even if you don't win that lovely Kindle Fire, there are still $200 of Amazon vouchers to be had.

IT'S THAT SIMPLE.

(also very handy if you're British/Australian/Martian and can't get a Kindle Fire in the normal way! Amazon UK/DE/IT/FR/ES purchases will count, BTW, and the books should all be discounted on all Amazon sites to a 99c equivalent)

a Rafflecopter giveaway Disclaimer: Does not apply to paperback edition. Does not apply to parallel world editions. Does not apply if world ends before the completion date of the competition. A Rafflecopter is not a substitute for a fully armed Blackhawk but is nevertheless quite awesome in its own way. Grilled cheese is not, technically a meal. Aliens exist and find you very amusing. This disclaimer is a figment of your imagination.

Progress. Terrifying progress.

Five days off work, five thousand words a day to write, twenty-five thousand word target for the five days... done! Which, I think, leaves me at two fifths of the way through the first draft. Five universes have to be travelled through, and I'm barely on the verge of getting out of the second one.

Meanwhile, the story required a certain amount of unpleasant torture and it ended up disturbing me as I wrote it. Why is it so easy to invent horrible, horrible things? Why is it such a simple process to go one step further into hideousness?

(or am I just a sick puppy?)

Meanwhile, The Last Man on Earth Club will be cheaper soon.

(yay!)

Briefly.

(yoy.)

More info as soon as my hideous internet connection allows this blog post writing thing to take less than half an hour each time :-(

Can't talk. Working!

Five days off work, all in a row! Yippee!

So I'm finally getting some time to work on the new novel. My target is 5,000 words per day - which I managed today even after being sociable with old friends I bumped into (twice!) and spending an hour on a planning session over lunch when I realised I'd used a plot device too early and had to come up with another one.

Image comes from a film demonstrating the Magic
Bullet plugin, which is awesome.

(luckily, plot devices are on sale at all good branches of PC World)

Apologies to the old friends I was sociable with if I seemed preoccupied; I've got worlds to kill. Yay!

Building an Alien Invasion Part 1

It’s the most basic terror that science fiction can provide – not only are there aliens out there, but they want what’s ours and will stop at nothing to get it. Alien Invasion stories are a staple of the genre, but they’re often undermined by the failure of the writers to do some basic preparation before they begin – such as understanding the laws of physics. Or biology. Or economics. Or common sense…

So I’m going to go over a few of the major problems and suggest solutions writers could use when building an invasion – solutions that are less about an excuse for cool CGI, and more about writing a compelling and surprising story.


PROBLEM ONE: THE DISTANCE

Voyager 1. Top speed: 0.0056% lightspeed.

The first major problem is that it’s an unimaginable distance to even the nearest stars. At our present level of technology, it’s taken us 34 years just to get a probe out to the edge of our solar system – 0.18% of a light year. The nearest star is 4.2 light years away. It’s not a journey we’re likely to be making soon. Alien invaders will have to overcome exactly the same problem, probably magnified because they’re unlikely to come from the very nearest star. So here’s some solutions…

SCIENCE-Y SOLUTIONS

• Very Fast Ships
• Generation Ships/Hibernation
• Time Perception
• Long Distance Communications

FICTION-Y SOLUTIONS

• Faster Than Light Travel
• Instantaneous Travel
• Invaders from Another Dimension
• Invaders from Another Time
• Energy Lifeforms

SCIENCE-Y SOLUTIONS

Very Fast Ships

Project Orion: Nuuuukes iiiin Spaaaace!

Travel time to the nearby stars would take many thousands of years at present speeds, but a decent amount of technological advance might bring that down to a hundred years or less – something doable in a human (or alien) lifetime. It’s still a very long journey if you’re launching an invasion, though – if you assume that an invasion has to have some kind of political support back home, is that going to last for decades? Would radio signals reach the invasion fleet halfway to their goal, recalling them because their homeworld has had a massive social revolution? And of course, we’re still only talking about the nearest stars, a mere handful of light years away and unlikely to be inhabited, so longer journeys from the vast bulk of the galaxy might not be worth considering.

Once you reach truly fast speeds, though, another factor comes into play which makes the journey a little more possible: relativistic time dilation. As anyone who’s read Einstein’s papers (or, more likely, Joe Haldeman’s The Forever War) will know, time travels differently depending on how fast you’re moving. A ship travelling at a significant percentage of lightspeed might take five hundred years to make a journey, but the people on board might only perceive it as five years. The invaders might be fresh and ready for the fight, but everyone they ever knew at home will probably be dead. Maybe they've had to abandon their own world – which actually works quite well if they’re coming to stay and have no interest in returning home.

Generation ships/Hibernation

Asteroid Ship. A nice place to bring up ten generations
of your descendants before they fall upon the enemy.

On a really long journey (far beyond human or alien lifespans), there are two obvious ways to keep the inhabitants going: either they don’t age in some kind of suspended animation, or they do grow old and die, but their descendants keep running the ship and will be the ones to conduct the invasion at the end of the journey. If they’re in suspended animation, then they’ll likely be up and ready for the invasion, unless there have been some problems along the way like losing all their personalities in a computer failure, which could create extremely confused and unpredictable invaders (as in Douglas Adam’s Mostly Harmless). If the invaders have come on a generation ship, they may have tired of the whole purpose of the journey somewhere along the way – or they may have become religious fanatics even more insanely dedicated to wiping us out. Indeed, they might not even have started out as invaders in the first place.

Essentially, the point is: no matter what you do to alleviate the effects of a long journey, humans (and aliens) might not be the same once they reach their destination, leading to a certain unpredictability that can make things interesting.

Time Perception

Minimum travel time, end to end: 100,000 years.
Enough time to get really good at Scrabble.

But why restrict ourselves to invasions from beings quite so similar to us? Humans have evolved (and are still evolving) to meet the demands of our environment. Might not space travelling aliens adapt to the requirements of a spacefaring life? Including, most importantly, the vast gulfs of time that space journeys require?

These kinds of aliens are likely to be vastly more advanced than us, and may not be biological at all, meaning they may have vastly greater control of not only their bodies but their minds. If there’s any kind of life to be had in space, then adjusting time perception so that they can experience long journeys in the blink of an eye will be a vital adaptation. Or maybe they are simply capable of enduring the boredom without going mad or changing into something completely different along the way.

Of course, this level of advancement will put them so far beyond us that they might not have any interest in planets, let alone the antlike beings on the surface of a little blue-green world. Or they may see us much as we look at a farm on our world: something to be managed lest some dangerous growth gets out of control. They might decide that our whole biosphere needs pruning, or that we simply need to be removed and placed somewhere else. It all depends on their perspective, and the trick here is figuring out exactly how a vast alien intelligence will think. A difficult choice, but perhaps a refreshing one.

Long Distance Communications

Very Large Array: we're hoping the aliens don't use cable.

Of course, there’s one thing we do that already travels at the maximum speed of the universe: radio. Our signals have travelled more than 100 lightyears out into the galaxy, and (though very faint) will continue to travel on, possibly alerting aliens to our presence, and, as time goes on, providing a vast wealth of information about us.

What if their ‘invasion’ is one not of military might, but of information? We can’t expect them to link to the internet and hack our systems directly if they’re transmitting at the same speed as us, but human systems can be hacked in other ways: by ideas. Carl Sagan’s Contact shows what happens when a benevolent elder race sends us a package of information to give us a technological boost. Sagan, however, was an optimist; what might a malevolent alien race send us?

The first temptation is to say reality TV, but (if I’m serious for a moment), the aliens wouldn’t be able to respond adequately to our own culture without being laughed at – they’d be years, decades or even centuries out of date. So they have to send something that can destabilise our world without much reference to our own transmissions. But that can be surprisingly easy; on our own world, we’ve seen stone age societies disrupted by the cargoes transported to distant Pacific islands during World War 2. If similarly advanced cargoes of information were dumped on us, these too could disrupt our world – maybe causing wars, maybe making us dependent on their transmissions for continued survival, maybe lying to us about what to expect from galactic society so we’re unprepared for the real invasion. Ideas themselves can be very dangerous sometimes…


FICTION-Y SOLUTIONS

Faster Than Light Travel

How to make FTL boring.

The main purpose of FTL in fiction is to turn interstellar travel into an analogue of either sea or air travel on Earth – something we can understand and cope with. Planets then become distant lands across the sea rather than strange unknown worlds, and it seems possible to walk around on any of them without instantly dying from the lack of breathable atmosphere. In other words, it gives us a way to ignore how different alien worlds, species and cultures are likely to be; alien invaders become more like foreigners from our own world - weird but understandable.

That’s fine if you just want a way to get the aliens from A to B and don’t much care how they do it. But it can be a bit dull just to wave a wand and say ‘hyperspace’ or ‘warp drive’ or whatever: that's essentially magic. FTL should have a cost, and it's the cost that can make it interesting. One obvious example is the folding of space in Dune, where interstellar travel is accomplished with the help of the drug Spice, which makes the navigator prescient and thus able to operate the hideously complex systems that actually fold space; the enormous value of Spice then drives interstellar conflict. Or another example, from Iain M Banks' Culture series: FTL travel has long become routine to the point where no-one particularly thinks about it, but there's still a basic rule: the size of the engine you use has a direct effect on the speed a ship can travel. This then becomes a critical point in the plot of Excession as one particular ship races to prevent two invasions threatening the Culture at the same time. Or what if the systems that travel between the stars are somehow biological, and the invaders have either harnessed them somehow, or perhaps are living within them? Maybe such a lifeform would want to pause in our solar system to feed – most likely on something manageable like an asteroid or a comet, but this could then present an opportunity for the aliens living on them to make forays against us.

So when you're building an FTL system, bear in mind a simple principle: nothing in this universe comes for free. Sometimes it might seem as though it does, but there are always hidden costs somewhere...

Instantaneous Travel

Also a bit dull.

Another way to avoid the problem of distance altogether is literally to avoid the problem of distance altogether - by connecting two parts of the universe with a portal that lets you travel from one to another with minimal effort. Maybe you're piloting a ship through a wormhole, or perhaps simply walking between two rooms that actually exist in different star systems - either way, you're making life easy for the aliens.

Much of how an invasion using this system would go really depends on how you set up the rules of how the system works. Can invaders just turn up in the middle of London with no warning, or must they engineer a wormhole that manifests somewhere in the solar system and forces them to use some kind of basic space travel as well? If the former, then some of the reasons for invasion that are usually absurd suddenly become viable – a need for planets to colonise, for example. Because if you can travel from planet to planet without the trouble of going into space, then you’d probably wouldn’t bother with space travel at all.

And this leads to more interesting possibilities. It could mean a vastly skewed technological approach, especially if these portals turn out to be somehow natural in origin and societies using them have been able to develop with no concern for actual space travel; we could see steampunk-style invaders, or ones whose technology relies exclusively on genetically altered creatures, or any kind of strangeness (or maybe just cheap copies of Egyptian warriors).

And, of course, this kind of interstellar travel should still have a cost. If not an economic cost, then maybe a social one; for example, when two societies on Earth get to know each other, their cultures soon start to leak across the divide, and a good deal is often lost. In the end, maybe an alien race could turn us into copies of them simply by having a more advanced culture and being willing to share. Not your typical alien invasion, but a very effective one in the long run.

Invaders from Another Dimension

More interesting, but only because of the beard.
(and the potential for some very wrong slashfic)

But why should the invaders come from another star? There’s another situation that could lead to alien invasion – an incursion from another universe entirely. Parallel universes are a common enough trope in science fiction, usually intended as a way of exploring an alternative version of our own world. But if the technology exists, and those worlds aren’t able to travel to distant stars, then the temptation for an invasion in search of living space, minerals, water, and maybe even slaves becomes very possible. An interesting idea would be to show a culture similar to ancient Rome stumbling across portals to our world, then launching raids against us for all the things ancient peoples would have wanted: loot, people to enslave, and then weapons and technology once they realised we were more advanced.

Or, of course, there’s nothing to stop this other world being rather more alien than that. Nor is it necessary to make it an actual alternate earth; it could be another kind of universe altogether – maybe some kind of hell dimension where the demons have decided it’s time to spread their empire of pain to our world. Or maybe even an invasion from heaven because God decided we needed to be taught a lesson.

And all this is only for universes that are side by side and equally real. You might also consider invasions from universes that are created within ours as simulations, where the inhabitants we created break loose and try to claim some kind of place in a more real world. The opposite is also possible: if it turns out that our universe is only a simulation within some greater universe, the what’s to stop people from the world above just simulating an invasion happening to us, from any direction they care to imagine? Or entering our world themselves and playing with us as though we were toys?

Once you start playing with other realities, virtually anything becomes possible – the main trick is to find something interesting and then follow through on the implications without just using it as a gimmick.

Invaders from Another Time

Aaaand right back to the dullness.

It gets even stranger once you allow for time travel, because then you could have invaders coming from the past or future of our own world – our ancestors or descendants. The first thing you have to decide is how time travel works: can time be rewritten, or can’t it? If it can, then that offers an instant reason for travelling back into time: the invaders are from the future and want to make some change in the history of the world. Most of the time we see stories about individual travellers or small groups trying to accomplish a change, but why not have an entire civilisation facing extinction in some future time come back to try and make it right – at our expense? And what if they make mistakes and found reinforcements coming back in time, each from a different, changed future, leaving to another conflict among the various groups of time travellers? This could get very messy, and very interesting.

If time cannot be rewritten, then time travel can still be useful. Firstly because you could always be going forwards in time – maybe we’re being invaded by beings who existed here millions of years ago, or maybe it’s an enemy from our own past coming forwards in time, with no way of returning home and a desperate need to conquer us in order to survive. Secondly, you can always invoke the many worlds hypothesis, in which a journey to another time means you end up in a parallel universe that’s exactly the same as ours was at that moment, allowing access to the past without any way of affecting the present. But why they would come to our time when they could skip back a few thousand years and find a world much easier to conquer? Why not hop back to the Dark Ages and rebuild Rome, or supplant the ruling dynasty of China (as so many invaders did)? There’s nothing to stop this being a historical story as well as a science fiction one, and that could make it even more interesting.

Energy Lifeforms

Less dull, but twinkly.

Finally, here’s a pretty weird idea for a way to get between the stars: become the only thing we know of that makes this journey on a constant basis – light itself. If we have aliens that are composed entirely of energy, and especially if they can transform themselves from one kind of energy to another, then they could take advantage not only of lightspeed, but also a time dilation effect that would make the journey seem almost instantaneous. Quite why they’d bother invading is another question entirely, but they’d certainly be a bugger to fight.

(I did actually find a reason why such a race might invade and exterminate humanity in The Last Man on Earth Club, but I won’t spoil it here).

IN PART TWO...

We'll look at some reasons for alien invasion that make sense. Unless the invasion starts first. In which case it's already making sense.

Why Would Anyone Want to Invade Earth?

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!

Slightly more realistic than Tom Cruise.

'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

Nothing to see here. Move along.

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 19^th 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 for a moment. 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...?

Comet Tempel (actual NASA image). Basically a great
big slushie. NASA didn't say what flavour.

Resources

Water

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.

Very bad example of sending miners to an asteroid.

Minerals

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?

The Inhabitants
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…

How did I just know this image would
exist somewhere?

Food

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 only left-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.

Reproduction

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.

Pink Sea Urchin. NSFW.
(Honestly. Not kidding.)

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. 

Seriously, I could have used images from Futurama to
illustrate this whole thing.

Drugs

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…

Okay, so maybe some aliens need slaves.
Especially slaves with opposable thumbs.

Slaves

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.

No, it won't be these guys. Not unless they're so incompetent
that they let people post pictures of them on their blogs.

Study

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.

Colonisation

Believe it or not, this may be easier than
finding another planet to live on.

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.

Extermination
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.

Tarkin later said in his statement that Alderaan was
"looking at him in a funny way".

Pre-emptive Strike
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?

The Space Pope says that
scaliness is next to godliness.

Religion
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).

I mean, like, THORIUM BOMBS, man.

Fun
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.

Travelling faster than light means you can't see what's in
front of your spaceship until you've hit it... 

Accidental
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. 

YOU WILL ANSWER TO PROFESSOR FERMI FOR THIS.

Conclusions

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...