# Dark forest hypothesis

The dark forest hypothesis is the conjecture that many alien civilizations exist throughout the universe, but they are both silent and hostile, maintaining their undetectability for fear of being destroyed by another hostile and undetected civilization. It is one of many possible explanations of the Fermi paradox, which contrasts the lack of contact with alien life with the potential for such contact. The hypothesis derives its name from Liu Cixin's 2008 novel The Dark Forest, although the concept predates the novel.

## Background

There is no known reliable or reproducible evidence that aliens have visited or attempted to contact Earth. No transmissions and no firm evidence of intelligent extraterrestrial life have been detected or observed.$^{:2}$ This runs counter to the general observations, that:
* The universe is filled with a very large number of planets, where the probability is that some present conditions hospitable for life;$^{:336}$ and
* Terrestrial life is observed to expand until it fills all niches suited to it.
These contradictory facts form the basis for the Fermi paradox.

## Concept

The "dark forest" hypothesis presumes that any space-faring civilization would view any other intelligent life as an inevitable threat and thus destroy any nascent life that makes itself known. As a result, the electromagnetic spectrum would be relatively quiet, without evidence of any intelligent alien life.
A similar hypothesis, under the name "deadly probes", was described by astronomer and author David Brin in his 1983 summary of the arguments for and against the Fermi paradox.
The name of the hypothesis derives from Liu Cixin’s 2008 novel The Dark Forest, as in a "dark forest" filled with "armed hunter(s) stalking through the trees like ghosts". According to the dark forest hypothesis, since the intentions of any newly contacted civilisation can never be known with certainty, then if one is encountered, it is best to shoot first and ask questions later, in order to avoid the potential extinction of one’s own species. The novel provides a detailed investigation of Liu's concerns about alien contact.

## Relationship to other proposed Fermi paradox solutions

The Berserker hypothesis, also known as the deadly probes scenario, proposes self-reproducing machines that would seek to destroy organic life.$^{:112}$ The name derives from short stories by Fred Saberhagen written in the 1960s. The dark forest hypothesis is distinct from the Berserker hypothesis in that under the former, many alien civilizations could still exist provided they keep silent. The former can be viewed as a special case of the latter, if the deadly probes are only sent to star systems that show signs of intelligent life (e.g. due to resource scarcity).

## Game theory

The dark forest hypothesis is a special case of the "sequential and incomplete information game" in game theory.
In game theory, a "sequential and incomplete information game" is one in which all players act in sequence, one after the other, and none are aware of all available information. In the case of this particular game, the only win condition is continued survival. An additional constraint in the special case of the "dark forest" is the scarcity of vital resources. The "dark forest" can be considered an extensive-form game with each "player" possessing the following possible actions: destroy another civilization known to the player; broadcast and alert other civilizations of one's existence; or do nothing.

## Science fiction versions

In addition to Fred Saberhagen's Berserker novels,$^{:112}$ variations of these ideas have been used in other science fiction stories. In 1987, science fiction author Greg Bear explored this concept that he called a "vicious jungle" in his novel The Forge of God. In The Forge of God, humanity is likened to a baby crying in a hostile forest: "There once was an infant lost in the woods, crying its heart out, wondering why no one answered, drawing down the wolves." One of the characters explains, "We've been sitting in our tree chirping like foolish birds for over a century now, wondering why no other birds answered. The galactic skies are full of hawks, that's why. Planetisms that don't know enough to keep quiet, get eaten."
The term "dark forest" was coined for the idea in 2008 by science fiction author Liu Cixin in his novel The Dark Forest. In Liu Cixin's novel, the dark forest hypothesis is introduced by the character Ye Wenjie, while visiting her daughter's grave. She introduces three key axioms to a new field she describes as "cosmic sociology":
* "Suppose a vast number of civilizations distributed throughout the universe, on the order of the number of observable stars. Lots and lots of them. Those civilizations make up the body of a cosmic society. Cosmic sociology is the study of the nature of this super-society."
* 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 as well as two other considerations, "chain of suspicion" and "technological explosion", according to the character Ye was talking to, is that any civilization that revealed itself will be considered as an imminent existential threat by at least some of the other civilizations, among which some will then proceed to destroy the civilization that makes itself known.
In the third book of the trilogy, the perspective of the hunters in The Dark Forest is further illustrated through an alien character called Singer, who thinks that intelligent life that does not fear the dark forest would "expand and attack without fear". In other words, dark-forest-fearing civilizations are benign, civilizations that would reveal themselves are evil, and hunters are enforcers and protectors.