Dark Forest Hypothesis

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.

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. This runs counter to the knowledge that the universe is filled with a very large number of planets, some of which likely hold conditions hospitable for life. Life typically expands until it fills all available niches. These contradictory facts form the basis for the Fermi paradox, of which the dark forest hypothesis is one proposed solution.

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". Since the intentions of any newly contacted civilisation can never be known with certainty, meaning that, 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 Brin's concerns about alien contact.

The Berserker hypothesis, also known as the deadly probes scenario, proposes self-reproducing machines 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 (e.g. due to resource scarcity) only sent to star systems that show signs of intelligent life.

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.

In addition to Fred Saberhagen's Berserker novels,^: 112 variations of these ideas have been used in other science fiction stories. These include Alistair Reynolds’ Redemption Ark series, Gregory Benford and Larry Niven in Starship, and Greg Bear in The Forge of God.

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":

1. "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."
2. Suppose that survival is the primary need of a civilization.
3. 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, Ye says, is that any intelligent life in the universe will be pitted against all other life in the struggle for survival.