Last month, this New York Times article by Steven Johnson raised the alarming question: what if our efforts to find and communicate with other intelligent life in the Universe brings us to the attention of intellects vast and cool and unsympathetic who want to wipe us out? The author quoted Stephen Hawking, warning that the result of interstellar contact might resemble the conquest of the New World by the Spanish.
But there are worse things than a galleon-load of extraterrestrial Conquistadors dropping down out of the sky some afternoon. Instead of going to the bother of conquering the Earth, a sufficiently xenophobic and/or ruthless alien civilization might simply lob a few relativisitic planet-busting projectiles in our direction at 99.9 percent of the speed of light. A barrage of brick-sized warheads at that velocity would be an extinction-level event.
Scary stuff! Maybe we should try to hide. Maybe that's the reason we haven't yet detected any other civilizations: they're all either hiding or dead.
Except that I don't believe it, and this isn't just wishful thinking. Interstellar conquest or extermination are simply bad ideas. They're either literally impossible or so difficult and fraught with uncertainty that they might as well be impossible. (I am assuming real-world physics here, with the speed of light as an absolute speed limit.) I am not worried; let me explain why.
I'm going to consider the two cases — extermination and conquest — separately, although there's a lot of overlap among the reasons neither is practical. I will begin with interstellar conquest. There are three good reasons why nobody is likely to try conquest across interstellar distances.
I. Why? Seriously, why do it? What benefit would Earth get by sending out starships across tens or hundreds of light-years to conquer some other planet? If you're a Habsburg King of Spain, conquering Mexico makes perfect sense: you can ship home tons of gold, plant colonies which enrich the mother country through trade, and generally make out like a bandit.
But suppose Mexico was so far away that a galleon would have to sail for decades, if not centuries to reach it? And suppose sending a single galleon to Mexico would cost twice Spain's entire annual economic output? How keen on the project would King Charles be once you told him that?
If that's not enough to destroy His Most Catholic Majesty's enthusiasm for the conquering Mexico, suppose you told him that shipping anything back would cost just as much and require decades to build up the Mexican shipbuilding infrastructure? There is one bright spot for King Charles, though: it's not hard to send messages from Mexico to Spain, so you can mail back pictures, descriptions of the new land and its inhabitants, and maybe a few useful new ideas. But you could just as easily mail a letter to King Montezuma and ask him to do that, without those incredibly expensive galleons.
Conquering inhabited worlds across interstellar distances makes no economic sense. If the planet's technology is sufficiently primitive that a starship full of troops with AK-47s can overawe them and take over, then it's highly unlikely that world produces anything valuable enough to ship home. Certainly no non-biological substances are worth it; even diamonds, at $50,000 per gram, wouldn't be worth it — especially since one could probably find diamond deposits on uninhabited planets much nearer to Earth. (If anyone mentions "stealing our water" I'm going to drown him.) And if you like diamonds so much, developing ways to manufacture them would be a much more lucrative investment than sending starships out in search of some new Kimberley mine.
Bioproducts like heroin or truffles might be incredibly valuable, but the shipping cost is murder. It's far more practical to grab some of the plants or animals you want, take them home, and figure out how to raise them on your own planet.
And that's assuming conquest is easy. I doubt that conquering the Earth would be easy. Past experience shows that even trying to conquer smallish chunks of the Earth is very difficult. There simply isn't anything on Earth valuable enough to make us worth conquering, especially given the fact that there's seven billion humans armed with everything from pointed sticks to hydrogen bombs who might object.
Well, what about ideology? The aliens could be fanatical Marxists, or devotees of some hyper-evangelical religion, or just really insistent about which way the toilet paper roll goes. Ideology was a good enough reason for 180 million deaths on Earth in the 20th Century.1 We certainly can't assume any hypothetical aliens wouldn't be just as bloodthirsty in the name of their preferred cause.
But it still seems unlikely. The more bloody-minded the ideology, the shorter the shelf life. Of the regimes responsible for that horrifying 180 million death toll, only one is still around (the Chinese Communist Party) and it has transformed itself into a considerably more housebroken form. Obviously we can't tell if this is a "rule" of history (most "rules" of history are bunk anyway), and we certainly can't tell if it would apply to an alien civilization.
However, there is another reason to doubt the existence of hyper-ideological alien civilizations, and that's our old friend the Fermi Paradox. Radiotelescopes are considerably cheaper than starships, so one would expect that any fanatical alien ideologues would be flooding the Galaxy's airwaves with propaganda broadcasts. After all, if they can make converts via radio it frees up ships from the Holy People's Armada to go after the stubborn ones. But we don't see any of that; just silence.
So I won't rule out ideology, but I assign it a very low probability.
II. How? How do you conquer a planet across interstellar distances, anyway? If you're deliberately sending out an invasion, the planet's inhabitants have to be capable of giving you some indication that they exist — which in practice means at least a 1950s-era technology.
Assume that's what you're going after. Assume also that a world with substantial industrial technology is going to have a pretty big population; at least a billion. And while it would be convenient if some or all of them joined your side when you land, there's no way to find out before you launch the fleet. So you have to assume it will be a contested invasion. Finally, we assume that you actually want to conquer the planet — which means most of it must still be habitable when the conquest is over. (We'll cover simple destruction in the next post.)
Let's use the British Interplanetary Society's Daedalus spacecraft as our model starship, and assume arbitrarily good suspended animation for the crew and the soldiers of our invasion force. We will use the American plan for "Operation Olympic" as the model for that invasion force. Olympic was the invasion of Japan which never happened because of the timely deployment of the atomic bomb.
Operation Olympic would have involved 14 Army and Marine divisions, 15 air forces, and the aircraft from 42 carrier ships. That's about 300,000 soldiers, 10,000 airplanes, and at least 20,000 other vehicles. That force, with its supplies, would have a mass on the order of 1 million tons (and that assumes hyper-efficient power supplies for the planes and vehicles so there's no need to bring fuel).
The Daedalus starship was designed for a payload of 500 tons. So you'd need 2,000 of them to carry that invasion force. Each one would cost about 200 trillion dollars (or about twice the annual GDP of the planet). We're ignoring the cost of the actual military force. So building the ships for this invasion would require the entire economic output of a planet like the Earth for 4,000 years! That fanatical ideology has to maintain its grip on its home civilization for a period as long as recorded history just to get the invasion force built.
Oh, and since they have to detect the target civilization before they start building this armada, the target planet has 4,000 years to invest all of its economic output into technology research, industrial expansion, off-planet colonies, and massive defenses. They also get the multi-decade flight time of the armada to prepare to resist (because launching that fleet will be noticed across interstellar distances).
Let's just say the odds don't favor the invaders.
III. Time Lag. This is an important point and bears repeating. Interstellar travel takes a lot of time. The fleet will spend decades or centuries in transit. All of which means that when the invaders arrive, the target planet will be a very different place from when they were planning the operation years earlier.
I can't imagine what the reaction from any strategic planner would be if you asked him to prepare an invasion force against an enemy, but all data about the foe is at least a hundred years out of date. You don't know what their population numbers are, you don't know anything about their economy, you don't know the size of their military forces, you don't know anything about their weapons . . . it would be insanity.
Now, it is possible that the invaders are incredibly advanced and powerful, so that launching such an armada would be easy for them, and their weapons could overcome any opposition. Their species might be a Kardashev Type II civilization, able to command the entire mass and energy resources of an entire star system . . .
. . . except that such a civilization should be detectable across interstellar distances. Professor Fermi at the back of the classroom is holding up a sign: "WHERE ARE THEY?"
It seems pretty much impossible to conquer a planet across interstellar distances — at least, any planet which could possibly repay the effort of conquering it. If you want living space you can find worlds which aren't inhabited, or terraform lifeless worlds to suit yourself. If you want resources there's a vast number of lifeless bodies in the universe you can strip-mine.
So I'm not afraid of aliens coming to take over.
But what if they don't want to take us over? What if they just want to exterminate us? I'll get to that next time.
1I didn't even count World War I.
If you want to see stories about aliens who aren't conquering the Earth, check out my new ebook, Outlaws and Aliens!