黑暗森林假说

背景

目前，没有可靠或可重复的证据表明外星人曾访问过地球，人类也未在除地球以外的任何地方发现或观察到智能外星生命的传输或证据。这与宇宙充满大量行星并且其中一些行星很可能宜居的知识背道而驰。生命通常会扩张、发展，直到它填满所有可用的生境。这些相互矛盾的事实构成了费米悖论的基础，而黑暗森林假说是一种解释。

博弈论解读

黑暗森林假说是博弈论中“顺序且不完全信息博弈”的一个特例。

在博弈论中，“顺序且不完全信息博弈”是指所有参与者一个接一个依次轮流行动，但没有人知道所有可用信息的一种博弈。在这个特定的博弈中，唯一的获胜条件是继续生存。在“黑暗森林”这个特殊情形下，一个额外的限制是重要资源的稀缺性（有限性）。“黑暗森林”可以被认为是一个扩展形式的博弈，每个玩家（文明）拥有以下可能的行动：摧毁玩家已知的另一个文明；广播和提醒其他文明某个玩家的存在；或者什么都不做。

与费米悖论其它解说的关系

黑暗森林假说与狂战士假说的不同之处在于，如果外星文明保持沉默，他们仍然存在。如果“致命的狂战士探测器”（由于资源稀缺）仅发送到显示出智能生命迹象的恒星系统，则我们可以将黑暗森林假说视为狂战士假说的一个特殊例子。

The dark forest hypothesis is the idea that extraterrestrial civilizations may exist in abundance across the universe, but remain silent and hidden out of fear that revealing themselves would lead to destruction by a more technologically advanced and hostile civilization. It is one of several proposed explanations of the Fermi paradox, which contrasts the apparently high probability of extraterrestrial life with the lack of evidence for it. The hypothesis derives its name from Liu Cixin's 2008 novel The Dark Forest although similar concepts predate the work.

Relation to the Fermi paradox

There is currently no known reliable or reproducible evidence that extraterrestrial life forms have visited or attempted to contact Earth. Despite systematic searches, no transmission and no firm evidence of intelligent extraterrestrial life has been detected. This runs counter to the general observations that:

The universe is filled with a very large number of planets, some of which might possess conditions hospitable for life;

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 such as theirs as an inevitable threat and thus destroy any nascent life that makes itself known. As a result, the electromagnetic radiation surveys would not find evidence of 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 make a preemptive strike, 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. The name derives from short stories by Fred Saberhagen written in the 1960s, but the concept was introduced by John von Neumann in 1948 as part of his analysis of self-replicating machines. While the Berserker hypothesis is also about horrible outcomes from other civilizations, unlike the Dark Forest hypothesis, the probes or their actions would seem to be detectable.

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 players make decisions in sequence, with each player having limited knowledge about the others' choices and strategies. 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, 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 Luo Ji, while discussing with Ye Wenjie. She introduces three key axioms to a new field she describes as "cosmic sociology":