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Dark forest hypothesis

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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.[1] 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,[2] although the concept predates the novel.[3][4][5]


There is no known reliable or reproducible evidence that aliens have visited or attempted to contact Earth.[6][7] 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.[8] 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.[9][10]

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.[11]

The name of the hypothesis derives from Liu Cixin’s 2008 novel The Dark Forest,[12] as in a "dark forest" filled with "armed hunter(s) stalking through the trees like ghosts".[13][14] 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 Liu's concerns about alien contact.[2]

Relationship to other proposed Fermi paradox solutions[edit]

The Berserker hypothesis, also known as the deadly probes scenario, proposes self-reproducing machines seek to destroy organic life.[3]: 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.[9]

Game theory[edit]

The dark forest hypothesis is a special case of the "sequential and incomplete information game" in game theory.[15][10][16]

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.[17] In the case of this particular game, the only win condition is continued survival.[9] An additional constraint in the special case of the "dark forest" is the scarcity of vital resources.[10] 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.[15]

Science fiction versions[edit]

In addition to Fred Saberhagen's Berserker novels,[3]: 112  variations of these ideas have been used in other science fiction stories. These include Alistair Reynolds’ Revelation Space 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.[18] 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."[4]

The term "dark forest" was coined for the idea in 2008 by science fiction author Liu Cixin in his novel The Dark Forest.[19][2] 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":[20][9]

  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."[20]
  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 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.[20][9]

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".[21] In other words, dark-forest-fearing civilizations are benign, civilizations that would reveal themselves are evil, and hunters are enforcers and protectors.


  1. ^ Paradis, Justine (18 February 2022). "Outside/In[box]: What is the Dark Forest Theory?". New Hampshire Public Radio. Retrieved 18 October 2022.
  2. ^ a b c Yu, C. (1 January 2015). "The Dark Forest Rule: One Solution to the Fermi Paradox". Journal of the British Interplanetary Society. 68: 142–144. Bibcode:2015JBIS...68..142Y. ISSN 0007-084X. Retrieved 18 October 2022.
  3. ^ a b c Webb, Stephen (2002). If the universe is teeming with aliens ... where is everybody? : fifty solutions to the Fermi paradox and the problem of extraterrestrial life. New York: Copernicus Books in association with Praxis Pub. doi:10.1007/b97464. ISBN 0387955011. Retrieved 24 March 2024.
  4. ^ a b Stanway, Elizabeth (3 June 2023). "The Dark Forest". University of Warwick. Retrieved 25 March 2024.
  5. ^ Dyer, Dyer; O'Dowd, Matt (14 March 2024). "Dark Forest: Should We NOT Contact Aliens?". PBS Space Time. Retrieved 26 March 2024 – via YouTube.
  6. ^ Tingay, Steven (19 May 2022). "Is there evidence aliens have visited Earth? Here's what's come out of US congress hearings on 'unidentified aerial phenomena'". The Conversation. Retrieved 27 October 2022.
  7. ^ Kolbert, Elizabeth (14 January 2021). "Have We Already Been Visited by Aliens?". The New Yorker. Retrieved 27 October 2022.
  8. ^ Papagiannis, Michael D. (1978). "Are We All Alone, or could They be in the Asteroid Belt". Quarterly Journal of the Royal Astronomical Society. 19: 277. Bibcode:1978QJRAS..19..277P. Archived from the original on 7 January 2019. Retrieved 27 October 2022.
  9. ^ a b c d e Williams, Matt (7 January 2021). "Beyond "Fermi's Paradox" XVI: What is the "Dark Forest" Hypothesis?". Universe Today. Retrieved 18 October 2022.
  10. ^ a b c Konior, Bogna (6 July 2020). "The Dark Forest: Theory of the Internet". BLOK Magazine. ISSN 2719-4973. Retrieved 18 October 2022.
  11. ^ Brin, G. D. (1 September 1983). "The Great Silence - the Controversy Concerning Extraterrestrial Intelligent Life". Quarterly Journal of the Royal Astronomical Society. 24: 283–309. Bibcode:1983QJRAS..24..283B. ISSN 0035-8738. Retrieved 18 October 2022.
  12. ^ "The Fermi Paradox". The Higgs Centre for Theoretical Physics. Retrieved 24 February 2024.
  13. ^ Liu, Cixin (2015). The Dark Forest (First ed.). New York: Tor Publishing Group. p. 484. ISBN 9780765377081.
  14. ^ Hsu, Jeremy (31 October 2015). "China's 'Dark Forest' Answer to 'Star Wars' Optimism". Discover. Archived from the original on 17 October 2022.
  15. ^ a b Yasser, Shehab (12 October 2020). "Aliens, The Fermi Paradox, And The Dark Forest Theory". Project Nash. Retrieved 18 October 2022.
  16. ^ Fudenberg, Drew; Tirole, Jean (April 1983). "Sequential Bargaining with Incomplete Information". The Review of Economic Studies. 50 (2): 221. doi:10.2307/2297414. JSTOR 2297414. Retrieved 18 October 2022.
  17. ^ Ausubel, Lawrence M.; Cramton, Peter; Deneckere, Raymond J. (2002). "Bargaining with incomplete information". In Aumann, Robert; Hart, Sergiu (eds.). Handbook of Game Theory with Economic Applications. Vol. 3. pp. 1897–1945. doi:10.1016/S1574-0005(02)03013-8. ISBN 978-0-444-89428-1.
  18. ^ Cramer, John (20 September 1987). "Self-Reproducing Machines From Another Planet : THE FORGE OF GOD by Greg Bear". Los Angeles Times. Retrieved 13 May 2024.
  19. ^ Kevra, Derek (11 October 2022). "Dark Forest theory: should we try to contact aliens?". FOX 2 Detroit. Retrieved 18 October 2022.
  20. ^ a b c Liu, Cixin (2015). "Chapter 1". The dark forest (First ed.). New York: Tor. ISBN 9780765386694.
  21. ^ Liu, Cixin; Liu, Ken; Liu, Cixin (2016). Death's end. Three-body trilogy (First ed.). New York: Tor. ISBN 978-0-7653-7710-4. OCLC 936360629.