The Dark Forest Theory: A Chilling Solution to Fermi’s Paradox

Gaurav Deshmukh
6 min readJan 4, 2019


This is not my theory. This has been detailed quite elegantly by Cixin Liu in his book “The Dark Forest” . It is the second part in the “Remembrance of Earth’s Past” trilogy which I highly recommend for some great sci-fi reading.

Famous science fiction author Arthur C Clarke quite neatly articulated humanity’s possible reaction(s) to the presence (or absence) of extraterrestrial beings in the universe.

Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.

-Arthur C Clarke

Why are these possibilities so terrifying though? It could be daunting to realize our position as the only extant living presence in the universe. While that would give us free reign to explore our solar system (and perhaps, even beyond) and colonize planets, the anomalous survival of only one civilization may be a terrifying indicator of near certain collapse of perhaps every civilization that has ever existed and tried to survive. It would indicate that the inanimate universe is inherently inhospitable to life.

However, if the signs of existence of other extraterrestrial civilizations were to be found, our apprehensions would be put to rest, right? Well, it would certainly mean that the universe is quite habitable and favorable for the survival and expansion of civilization. Our situation would nevertheless be similar to that of the early explorers of the earth. They weren’t sure of the shape of the earth, their relative position on the earth with respect to other landmasses and most importantly, they weren’t sure whether they were safe from potential raids and assaults carried out by hostile civilizations.

But as time progressed and civilizations came in contact and began trading, the apprehensions and hostilities subsided. Although wars still broke out (conflict is inevitable), there was less of a fear of the unknown. Today, international bodies like the United Nations exist to increase familiarity between nations and amicably resolve any disputes. If we were to juxtapose the exploration and progression of civilization on the Earth with the universe, we might in the future have an inter-galactic federation like the UN and a possibility of ever-lasting peace throughout the universe.

Except, this won’t ever happen according to the Dark Forest theory.

To arrive at the chilling conclusion of the Dark Forest theory, Cixin Liu puts forward a few axioms of “cosmic sociology”, a discipline that aims to study and predict the relations between cosmic civilizations. The axioms quoted directly from the book are as follows:

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

The first axiom is basically “survival of the fittest”. Civilizations will do anything and everything they can to survive because that is their primary drive.

The second axiom basically states that cosmic civilizations will have to fight for a finite number of resources that exist in the universe to expand. The exhaustibility of resources may seem inconceivable considering the vast expanse of space but as it is evident, most of the universe is “empty” and the exploitable resources are indeed limited. Unless civilizations find a way to harness dark energy, it is bound to stay that way.

Now, it might seem that these axioms can also be applied to civilizations or nations on earth. The conflict for survival and over resources between civilizations on earth is manifest since time immemorial. What then distinguishes our situation from that of the universe? Here Cixin Liu brings in two assumptions.

  1. Chains of Suspicion
  2. Technological Explosion

To demonstrate the meaning of the first assumption, first consider that all the extraterrestrial civilizations that exist can be grouped into two categories benevolent and malevolent. Malevolent civilizations are warmongers and will attack other civilizations at will. Benevolent civilizations are not warmongers they will not needlessly attack other civilizations unless they feel threatened.

Now, the concept can be explained through a decision tree as follows:

Decision Tree for a civilization considering an attack on a newly encountered civilization. M denotes Malevolent and B denotes Benevolent

A quick explanation for the tree. Imagine you are a benevolent civilization, you live and let live. You’ve recently encountered a new civilization and are considering whether to attack. If you find that the new civilization is malevolent, it would be prudent to attack and destroy them before they do the same to you. However, if they are benevolent it is not all sunshine and rainbows. You have to consider whether they think you are benevolent. If they think that you are malevolent, they are likely to attack you on the basis of the same logic that you applied before and so you must destroy them to protect yourself. And so on.

As it is evident, this decision tree can be extended infinitely. So either your decision ends at Yes, in which case you attack and destroy the civilization you have encountered or you enter an endless chain of suspicion with the other civilization (that is bound to end in Yes for either them or you).

Why is this problem not encountered on earth among nations? Because of two factorsthe relative speed of communication on earth and the cultural as well as biological similarities between the people. Any kind of misunderstanding or miscommunication can be resolved by quickly sending a response and with the advent of the internet today, communication is blazing fast. Also, since we belong to the same species, we kind of “understand” each other so the chains of suspicion terminate at some level.

However in space, communication may take years depending upon the distance between the civilizations. In addition, due to the cultural and special differences, it would be difficult to communicate intentions clearly even with a common language. This makes “chains of suspicion” a big problem in interstellar communication.

The second assumption is the technological explosion. Imagine that you find out that the newly encountered civilization is malevolent and that they must be destroyed. You also find out that they’re technologically vastly inferior as compared to you. You salivate at the prospect of destroying their swords and shields with your machine guns. However, there is a slight hitch. Their world is far away and it would take you nearly 150 years to reach it. You have hibernation technology so survival on the trip is a non-issue. So what’s the problem?

It is quite possible that when you reach the civilization 150 years later, they may have developed and mastered the use of laser cannons that blow your machine guns out of the water. But how? It is possible for a civilization to experience an era of rapid technological advancement such that the very face of the civilization changes. Think it is not possible? We are in one such technological explosion ourselves right now. Think about how fast we came along from ancient calculators to modern smartphones in a span of just about 150–200 years or so.

So based on the two assumptions, you cannot let them live (because of chain of suspicion) but you also cannot openly attack them (from a fear of technological explosion). What do you do then?

The result of this conundrum is the Dark Forest Theory. According to the theory, the universe is a dark forest and every civilization is a silent armed hunter who is treading very carefully without making any noise. The hunter cannot make his position known and if he does encounter anyonea predator, another hunter or even a harmless herbivorethe only option he has is to eliminate them.

Photo by Rosie Fraser on Unsplash

Fermi’s paradox is the dissonance between the high probability of existence of extraterrestrials and the lack of evidence pointing towards their existence. There are many innocuous solutions to the paradoxlack of technology to communicate or travel, intelligent civilizations being too far apart and so on. However, the Dark Forest theory is a truly terrifying explanation for the paradox.

It implies that every civilization is either waiting in radio silence on its planetary system or moving quietly and ruthlessly eliminating anyone that it comes across without broadcasting its presence. If you envisage that along with the eerie silence that pervades the universe, it makes for a terrifying picture.

And if the theory were indeed true, should we really be sending out signals hoping to contact and reach potential neighboring extraterrestrial civilizations?



Gaurav Deshmukh

I'm a PhD candidate in chemical engineering at Purdue University and I study catalysts using computer simulations. I write about physics, math, and programming.