“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 other life — another hunter, an angel or a demon, a delicate infant or a tottering old man, a fairy or a 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. It’s the explanation for the Fermi Paradox."

Cixin Liu, The Dark Forest

If the Drake equation is right that there are many advance civilization in the universe, why haven't we heard from them? Cixin Liu's answer in the second book of this Three Body Problem gives a compelling and chilling answer: The Dark Forest Theory. The argument is built on two axioms:

1. Survival is the primary need of civilization
2. Civilization continuously grows and expands, but the total matter in the universe remains constant.

Civilizations can be grouped into either one of two categories: benevolent or malicious.

benevolent - a benevolent civilization will not attack another civilization unprovoked

malicious - a malicious civilization will take initiative to attack and eradicate another civilization

One last concept: chains of suspicion. That is, what makes intergalactic civilizations unique from those on earth is the vast distance between them that prevents efficient communication or understanding.

Putting these together into a scenario: If civilization A learns of civilization B, but civilization B does not know about A, A has three actions:

1. Establish contact
2. Do nothing
3. Destroy Civilization B

What should A do?

According to Dark Forest Theory the only viable action is to annihilate B because establishing contact makes B aware of A, and doing nothing allows B to progress to a technological state where it will have be able to destroy A. This thought experiment of interstellar game theory predicts that the optimal strategy for civilizations in a dark forest universe is to avoid making its presence known and destroy those that do expose themselves.

Let's put together a model that can map out a simulation of how a dark forest universe might play out. Our variables are:

U = The number universes that support life

c = the fraction of civilizations that develop technology capable of establishing contact with other civilizations

d = the fraction of civilizations that develop technology capable of destroying other civilizations

t = the length of time required from a civilization to move from between civilization types

r = the fraction of civilizations which will try to make contact with other civilizations

m = the fraction of civilizations which are malicious, i.e. when learning of a civilization will destroy it.