The Dark Forest hypothesis is absurd
It's fun sci-fi but it doesn't make a lot of sense.
WARNING: This post contains major spoilers for Cixin Liu’s The Three Body Problem, The Dark Forest, and Death’s End.
As you know by now, I’m a fan of Cixin Liu’s sci-fi series Remembrance of Earth’s Past. The writing is excellent, the characters are original and deep, the plots are gripping, and the concepts are highly imaginative. The first two books chronicle humankind’s encounters with an alien civilization, the Trisolarans, who almost destroy Earth. In the second book, Earth is saved when a sociology professor, Luo Ji, realizes that the Universe functions according to a principle called the Dark Forest Hypothesis, and uses this knowledge to ward away the attackers.
Basically, the Dark Forest Hypothesis, as articulated in Liu’s novel, is that it’s rational to destroy any alien civilization you come across:
The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds another life—another hunter, angel, or a demon, a delicate infant to tottering old man, a fairy or demigod—there’s only one thing he can do: open fire and eliminate them. In this forest, hell is other people. An eternal threat that any life that exposes its own existence will be swiftly wiped out. This is the picture of cosmic civilization.
Why are we all trying to wipe each other out? Because the Universe is a struggle between civilizations over scarce resources:
Suppose a vast number of civilizations distributed throughout the universe…
Suppose that survival is the primary need of a civilization.
Suppose that civilizations continuously expand over time, but the total matter in the universe remains constant.
The only logical conclusion from the acceptance of these axioms, is that any intelligent life in the universe will be pitted against all other life in the struggle for survival.
Luo Ji saves Earth by threatening to expose the invading Trisolarans’ existence to other civilizations, which will result in their own inevitable destruction. The Trisolarans, faced with mutually assured destruction, have no choice but to surrender and give humans their advanced technologies.
Liu’s Dark Forest Hypothesis is similar in some regards to ideas suggested by an earlier generation of sci-fi authors, including David Brin, to explain our failure to find evidence of other sentient species in the cosmos. But there is a crucial difference. Brin and his contemporaries posited that alien species throughout the galaxy are hiding, out of fear of being destroyed by one or more potential interstellar genocidaires. That makes a lot of sense. Liu, by contrast, posits that being an interstellar genocidaire is the rational course of action for any sentient species. It’s the difference between saying “You should be afraid of Space Hitler,” and saying “It’s rational for you to become Space Hitler.”
Since the book came out, the hypothesis described above has fascinated a number of people around the world, including in America. Some have even used it as an analogy for the internet, or cryptocurrency. But as fascinating as the Dark Forest idea is, it makes little sense. There’s no reason the Universe should work like it does in Liu’s book.
Now, an important note here: I am not criticizing Liu’s books! All science fiction has an element of fantasy in it. It’s fun to read about a universe where things work differently than our own. And the Dark Forest hypothesis is certainly a fascinating and engaging piece of fantasy. But a piece of fantasy it remains. It’s scientifically suspect, the game theory doesn’t make sense, and it doesn’t fit with what we observe in the real world.
The forest isn’t actually very dark
Although the sociologist Luo Ji claims to derive the behavior of cosmic society from just three axioms, the behavior of the aliens in the story relies on other, unstated assumptions. Most crucially, it relies on a technological assumption — the idea that cosmic civilizations are naturally hidden from each other. The dark forest has to be dark for it all to work. That’s why in the book, it’s so crucial for civilizations not to reveal their position on the cosmic chessboard; if they simply keep quiet, there’s a good chance they won’t be found.
In reality, this is a dubious assumption. The fact is that although the night sky looks black, it’s actually pretty transparent. Other than a little bit of interstellar dust, there’s not much standing between any two alien worlds.
And this means that in the real world, you don’t necessarily have to wait for aliens to send you radio messages in order to detect them. You can just build a space telescope and look for them. Already, we humans are starting to observe many of the alien worlds out there in the cosmos. As telescopes get bigger and better, we’ll be able to see with greater and greater resolution.
Now, that doesn’t mean we'll ever be able to see the look on aliens’ faces. By my rough back-of-the-envelope calculation, to get a 1cm resolution on a world 10 light-years away would require an interferometer the size of the orbit of Saturn. That isn’t happening. But if aliens have built big structures, or machines that give off a lot of energy, we might be able to see them someday.
Remember that the Trisolarans in Liu’s books are advanced enough to send multidimensional sentient particles called “sophons” to Earth to block humans from developing specific technologies. Building a big telescope probably isn’t beyond their fictional capabilities. Even in the real world, we’re starting to think about how to use the sun’s gravitational lensing to increase our resolution to the point where we could see mountains and oceans on the surfaces of exoplanets. Even if telescopes don't work or are too dangerous to deploy, civilizations can always use deep space probes.
If the galaxy’s most advanced civilizations can see you, they can kill you. So if Liu’s hypothesis is right, it makes sense for the galaxy’s top civilizations to just look at every star, see if there’s a civilization there, and wipe it out if so. The fact that bumbling, low-tech humans are still managing to hide from the galaxy’s most powerful aliens (until one human contacts them deliberately) suggests that this isn’t happening. Liu seems to have assumed away the existence of very powerful telescopes.
On the other hand, it might be that fear of attack motivates most galactic civilizations to hide, by not constructing any megastructures, powerful spaceships, or anything else that prying alien eyes could see. But if everyone is out there hiding their capabilities and biding their time so as not to get detected, who is there to do the galactic extermination?
That brings us to the second reason Dark Forest theory doesn’t make much sense: it doesn’t seem like a robust game-theoretic equilibrium.