Editing Great Filter

{{Short description|Hypothesis of barriers to forming interstellar civilizations}}
{{About|an implication of the Fermi paradox|the music album by Tub Ring|The Great Filter (album)}}
The '''Great Filter''' is an idea that, in the development of life from the earliest stages of [[abiogenesis]] to reaching the highest levels of development on the [[Kardashev scale]], there is a barrier to development that makes detectable extraterrestrial life exceedingly rare.<ref name="Hanson">{{cite web|last=Hanson |first=Robin |author-link=Robin Hanson |url=http://hanson.gmu.edu/greatfilter.html |title=The Great Filter &ndash; Are We Almost Past It? |date=1998 |archive-url=https://web.archive.org/web/20100507074729/http://hanson.gmu.edu/greatfilter.html |archive-date=2010-05-07 |url-status=dead }}</ref><ref name="NYT-20150803">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=The Flip Side of Optimism About Life on Other Planets |url=https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html |date=August 3, 2015 |work=[[New York Times]] |access-date=October 29, 2015 |archive-date=September 19, 2019 |archive-url=https://web.archive.org/web/20190919003259/https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html |url-status=live }}</ref> The Great Filter is one possible resolution of the [[Fermi paradox]]. The main conclusion of the Great Filter is that there is an inverse correlation between the probability that other life could evolve to the present stage in which humanity is, and the chances of humanity to survive in the future. 

The concept originates in [[Robin Hanson]]'s argument that the failure to find any [[extraterrestrial life|extraterrestrial]] civilizations in the [[observable universe]] implies that something is wrong with one or more of the arguments (from various scientific disciplines) that the appearance of advanced intelligent life is probable; this observation is conceptualized in terms of a "Great Filter" which acts to reduce the great number of sites where intelligent life might arise to the tiny number of intelligent species with advanced civilizations actually observed (currently just one: [[human]]).<ref>Hanson 1998: "No alien civilizations have substantially colonized our solar system or systems nearby. Thus among the billion trillion stars in our past universe, none has reached the level of technology and growth that we may soon reach. This one data point implies that a Great Filter stands between ordinary dead matter and advanced exploding lasting life. And the big question is: How far along this filter are we?"</ref> This probability threshold, which could lie in the past or following [[human extinction]], might work as a barrier to the evolution of intelligent life, or as a high probability of self-destruction.<ref name="Hanson" /><ref name="Bostrom">{{cite journal |last=Bostrom |first=Nick |author-link=Nick Bostrom |date=May–June 2008 |url=http://www.nickbostrom.com/extraterrestrial.pdf |title=Where Are They? Why I hope the search for extraterrestrial life finds nothing |journal=[[Technology Review]] |publisher=[[Massachusetts Institute of Technology]] |pages=72–77 |access-date=2008-06-19 |archive-date=2019-12-24 |archive-url=https://web.archive.org/web/20191224081953/https://www.nickbostrom.com/extraterrestrial.pdf |url-status=live }}</ref> The main conclusion of this argument is that the more probable it is that other life could evolve to the present stage in which humanity is, the bleaker the future chances of humanity probably are.

The idea was first proposed in an online essay titled "The Great Filter – Are We Almost Past It?". The first version was written in August 1996 and the article {{As of|1998|September|15|alt=was last updated on September 15, 1998}}. Hanson's formulation has received recognition in several published sources discussing the Fermi paradox and its implications.

== Main argument ==

===Fermi paradox===
{{Main|Fermi paradox}}
There is no reliable evidence that [[Extraterrestrial life|aliens]] have visited [[Earth]]; we have observed no [[intelligent]] [[extraterrestrial life]] with current technology, nor has [[SETI]] found any transmissions from other [[civilizations]]. The [[Universe]], apart from the Earth, seems "dead"; Hanson states:<ref name=Hanson/>
<blockquote>
Our planet and solar system, however, don't look substantially colonized by advanced competitive life from the stars, and neither does anything else we see. To the contrary, we have had great success at explaining the behavior of our planet and solar system, nearby stars, our galaxy, and even other galaxies, via simple "dead" physical processes, rather than the complex purposeful processes of advanced life.
</blockquote>  Life is expected to expand to fill all available niches.<ref>{{Cite journal |url=https://adsabs.harvard.edu/full/1978QJRAS..19..277P |title=1978QJRAS..19..277P Page 277 |journal=Quarterly Journal of the Royal Astronomical Society |bibcode=1978QJRAS..19..277P |last1=Papagiannis |first1=Michael D. |year=1978 |volume=19 |page=277 |access-date=2022-11-08 |archive-date=2022-11-08 |archive-url=https://web.archive.org/web/20221108140759/https://adsabs.harvard.edu/full/1978QJRAS..19..277P |url-status=live }}</ref> With technology such as [[self-replicating spacecraft]], these niches would include neighboring star systems and even, on longer time scales which are still small compared to the age of the universe, other galaxies.  Hanson notes, "If such advanced life had substantially colonized our planet, we would know it by now."<ref name="Hanson"/>

===The Great Filter===
With no evidence of intelligent life in places other than Earth, it appears that the process of starting with a star and ending with "advanced explosive lasting life" must be unlikely. This implies that at least one step in this process must be improbable.  Hanson's list, while incomplete, describes the following nine steps in an "evolutionary path" that results in the colonization of the observable universe:

# The right [[star system]] (including [[organic chemistry|organics]] and [[Drake equation#Average number of planets that might support life per star that has planets, ne|potentially habitable]] [[planet]]s)
# Reproductive [[molecules]] (e.g. [[RNA]])
# Simple ([[prokaryotic]]) [[Microorganisms|single-cell life]]
# Complex ([[eukaryotic]]) single-cell life
# [[Sexual reproduction]]
# [[Evolution of multicellularity|Multi-cell life]]
# Tool-using [[animal]]s with [[intelligence]]
# A civilization advancing toward the potential for a colonization explosion
# [[Space colonization#Beyond the Solar System|Colonization explosion]]

According to the Great Filter hypothesis, at least one of these steps—if the list were complete—must be improbable. If it is not an early step (i.e., in the past), then the implication is that the improbable step lies in the future and humanity's prospects of reaching step 9 (interstellar colonization) are still bleak.  If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none [[Fermi paradox#Alien life is already here, unacknowledged|appear to have]] made it to step 9, or the [[Milky Way]] would be full of colonies. So perhaps step 9 is the unlikely one, and the only things that appear likely to keep us from step 9 are [[Global catastrophic risk|some sort of catastrophe]], an underestimation of the impact of procrastination as technology increasingly unburdens existence,  or  resource exhaustion leading to the impossibility of making the step due to consumption of the available resources (for example highly constrained energy resources).<ref>{{Cite journal|title = Is Humanity Doomed? Insights from Astrobiology|last = Baum|first = Seth|date = 8 February 2010|journal = Sustainability|volume = 2|issue = 2|pages = 591–603|doi = 10.3390/su2020591|bibcode = 2010Sust....2..591B|doi-access = free}}</ref> So by this argument, finding [[multicellular]] life on Mars (provided it evolved independently) would be bad news, since it would imply steps 2–6 are easy, and hence only 1, 7, 8 or 9 (or some unknown step) could be the big problem.<ref name=Bostrom />

Although steps 1–8 have occurred on Earth, any one of these may be unlikely. If the first seven steps are necessary preconditions to calculating the likelihood (using the [[Realization (probability)|local environment]]) then an [[anthropic bias|anthropically biased]] observer can [[Statistical hypothesis testing|infer]] nothing about the general probabilities from its ([[prior probability|pre-determined]]) surroundings.

In a 2020 paper, Jacob Haqq-Misra, Ravi Kumar Kopparapu, and Edward Schwieterman argued that current and future telescopes searching for [[biosignatures]] in the ultraviolet to near-infrared wavelengths could place upper bounds on the fraction of planets in the galaxy that host life. Meanwhile, the evolution of telescopes that can detect [[technosignature]]s at mid-infrared wavelengths could provide insights into the Great Filter. They say that if planets with technosignatures are abundant, then this can increase confidence that the Great Filter is in the past. On the other hand, if finding that life is commonplace while technosignatures are absent, then this would increase the likelihood that the Great Filter lies in the future.<ref name="arXiv: 2002.08776">{{cite journal |arxiv=2002.08776|last1=Haqq-Misra|first1=Jacob|author2=Ravi Kumar Kopparapu|last3=Schwieterman|first3=Edward|title=Observational Constraints on the Great Filter|journal=Astrobiology|year=2020|volume=20|issue=5|pages=572–579|doi=10.1089/ast.2019.2154|pmid=32364797|bibcode=2020AsBio..20..572H|s2cid=211204863 }}</ref>

Recently, paleobiologist [[Olev Vinn]] has suggested that the great filter may exist between steps 8 and 9 due to inherited behavior patterns (IBP) that initially occur in all intelligent biological organisms. These IBPs are incompatible with conditions prevailing in technological civilizations and could inevitably lead to the self-destruction of civilization in multiple ways.<ref name=vinn2024>{{cite journal|last=Vinn|first=O.|date=2024|title=Potential incompatibility of inherited behavior patterns with civilization: Implications for Fermi paradox|journal=Science Progress|volume=107|issue=3|pages=1–6|doi=10.1177/00368504241272491|pmid=  39105260|s2cid=  |pmc=11307330}}</ref>

In a specific formulation named the "[[Berserker hypothesis]]", a filter exists between steps 8 and 9 in which each civilization is destroyed by a lethal [[Von Neumann probes|Von Neumann probe]] created by a more advanced civilization.<ref name="UniverseTodayBerserker2020">{{cite news |last1=Williams |first1=Matt |title=Beyond "Fermi's Paradox" VI: What is the Berserker Hypothesis? |url=https://www.universetoday.com/147396/beyond-fermis-paradox-vi-the-berserker-hypothesis/ |access-date=18 October 2022 |work=[[Universe Today]] |date=23 August 2020 |archive-date=18 October 2022 |archive-url=https://web.archive.org/web/20221018183819/https://www.universetoday.com/147396/beyond-fermis-paradox-vi-the-berserker-hypothesis/ |url-status=live }}</ref>

==Responses==
{{main|Fermi paradox#Hypothetical explanations for the paradox}}
There are many alternative scenarios that might allow for the evolution of intelligent life to occur multiple times without either catastrophic self-destruction or glaringly visible evidence. These are possible resolutions to the [[Fermi paradox]]: "They do exist, but we see no evidence". Other ideas include: it is too expensive to spread physically throughout the galaxy; Earth is purposely isolated; it is dangerous to communicate and hence civilizations actively hide, among others.

Astrobiologists [[Dirk Schulze-Makuch]] and William Bains, reviewing the history of life on Earth, including [[convergent evolution]], concluded that transitions such as [[oxygenic photosynthesis]], the [[eukaryote|eukaryotic cell]], [[multicellularity]], and [[tool]]-using [[intelligence]] are likely to occur on any Earth-like planet given enough time. They argue that the Great Filter may be [[abiogenesis]], the rise of technological human-level intelligence, or an inability to settle other worlds because of self-destruction or a lack of resources.<ref>{{cite book |last1=Schulze-Makuch |first1=Dirk |last2=Bains |first2=William |title=The Cosmic Zoo: Complex Life on Many Worlds |date=2017 |publisher=Springer |isbn=978-3-319-62045-9 |pages=201–206 |url=https://books.google.com/books?id=m7E_DwAAQBAJ |language=en |access-date=2020-02-26 |archive-date=2023-03-31 |archive-url=https://web.archive.org/web/20230331083015/https://books.google.com/books?id=m7E_DwAAQBAJ |url-status=live }}</ref>

Astronomer [[Seth Shostak]] of the [[SETI Institute]] argues that one can postulate a galaxy filled with intelligent  extraterrestrial civilizations that have failed to colonize [[Earth]].  Perhaps the [[Extraterrestrial life|alien]]s lacked the intent and purpose to colonize or depleted their resources, or maybe the [[galaxy]] ''is'' colonized but in a [[heterogeneous]] manner, or the Earth could be located in a "galactic backwater".  Although absence of evidence generally is only weak [[evidence of absence]], the absence of extraterrestrial [[megascale engineering]] projects, for example, might point to the Great Filter at work.  Does this mean that one of the steps leading to intelligent life is unlikely?<ref name="Shostak">{{cite news |last=Pethokoukis |first=James M. |date=2003-11-04 |title=Keeping His Eyes on the Skies |newspaper=[[U.S. News & World Report]] |url=https://www.usnews.com/usnews/tech/nextnews/archive/next031104.htm |url-status=dead |archive-url=https://web.archive.org/web/20130619025348/http://www.usnews.com/usnews/tech/nextnews/archive/next031104.htm |archive-date=2013-06-19 }}</ref>  According to Shostak:

<blockquote>This is, of course, a variant on the Fermi paradox: We don't see clues to widespread, large-scale engineering, and consequently we must conclude that we're alone.  But the possibly flawed assumption here is when we say that highly visible construction projects are an inevitable outcome of intelligence. It could be that it's the engineering of the small, rather than the large, that is inevitable. This follows from the laws of inertia (smaller machines are faster, and require less energy to function) as well as the speed of light (small computers have faster internal communication). It may be—and this is, of course, speculation—that advanced societies are building small technology and have little incentive or need to rearrange the stars in their neighborhoods, for instance. They may prefer to build nanobots instead. It should also be kept in mind that, as [[Arthur C. Clarke]] said, truly advanced engineering would look like magic to us—or be unrecognizable altogether. By the way, we've only just begun to search for things like [[Dyson sphere]]s, so we can't really rule them out.<ref name="Shostak"/><ref>Joseph Voros in "Macro-Perspectives Beyond the World System" (2007) points out that some researchers have attempted to search for energy signatures that could be traced to Dyson-like structures (shells, swarms, or spheres).  So far, none have been found.  See for example, Tilgner & Heinrichsen, "A Program to Search for Dyson Spheres with the Infrared Space Observatory", ''Acta Astronautica'' Vol. 42 (May–June, 1998), pp. 607–612; and Timofeev et al. "A search of the IRAS database for evidence of Dyson Spheres", ''Acta Astronautica'' Vol. 46, (June 2000), pp. 655–659.</ref></blockquote>

==See also==
{{div col|colwidth=30em}}
* {{annotated link|Black swan theory}}
* {{annotated link|Dark forest hypothesis}}
* {{annotated link|Doomsday argument}}
* {{annotated link|Drake equation}}
* {{annotated link|Anthropic principle}}
* {{annotated link|Global catastrophic risk}}
* {{annotated link|Goldilocks principle}}
* {{annotated link|Inverse gambler's fallacy}}
* {{annotated link|Kardashev scale}}
* {{annotated link|Neocatastrophism}}
* {{annotated link|mediocrity principle|Principle of mediocrity}}
* {{annotated link|Quiet and loud aliens}}
* {{annotated link|Rare Earth hypothesis}}
* {{annotated link|Selection bias}}
* {{annotated link|The Major Transitions in Evolution |''The Major Transitions in Evolution''}}
{{div col end}}

== References ==
{{reflist}}

==Further reading==
{{Library resources box}}

* {{cite journal |last=Bostrom |first=Nick |author-link=Nick Bostrom |date=Mar 2002 |title=Existential Risks: Analyzing Human Extinction Scenarios and Related Hazards |journal=[[Journal of Evolution and Technology]] |url=http://www.jetpress.org/volume9/risks.html |volume=9  }}
* {{cite journal |last=Ćirković |first=Milan M. |author2=Vesna Milosevic-Zdjelar |date=2003 |title=Extraterrestrial Intelligence and Doomsday: A Critical Assessment of the No-Outsider Requirement |url=http://saj.matf.bg.ac.rs/166/pdf/01-11.pdf |journal=Serbian Astronomical Journal |volume=166 |issue=166 |pages=1–11 |doi=10.2298/saj0366001c|bibcode=2003SerAJ.166....1C |doi-access=free }}
* {{cite journal |last=Ćirković |first=Milan M. |date=2004-08-27 |title=Permanence – An Adaptationist Solution to Fermi's Paradox? |journal=Journal of the British Interplanetary Society |volume=58 |pages=62 |arxiv=astro-ph/0408521 |bibcode=2005JBIS...58...62C }}
* {{cite journal |last=Ćirković |first=Milan M. |author2=Robert J. Bradbury |date=Jul 2006 |title=Galactic Gradients, Postbiological Evolution and the Apparent Failure of SETI |journal=[[New Astronomy (journal)|New Astronomy]] |volume=11 |issue=8 |pages=628–639|arxiv = astro-ph/0506110 |bibcode = 2006NewA...11..628C |doi = 10.1016/j.newast.2006.04.003 |s2cid=1540494 }}
* {{cite journal |last=Ćirković |first=Milan M. |date=Jul 2008 |title=Against the Empire |journal=[[Journal of the British Interplanetary Society]] |arxiv=0805.1821 |volume=61 |pages=246–254|bibcode = 2008JBIS...61..246C }}
* {{cite book |last=Ćirković |first=Milan M. |editor1=Nick Bostrom |editor2=Milan M. Ćirković|date=2008 |title=Global Catastrophic Risks |chapter=Observation selection effects and global catastrophic risks |publisher=[[Oxford University Press]]}}
* {{cite web |last=Dvorsky |first=George |author-link=George Dvorsky |date=2007-08-04 |title=The Fermi Paradox: Back with a vengeance |publisher=[[Sentient Developments]] |url=http://www.sentientdevelopments.com/2007/08/fermi-paradox-back-with-vengeance.html}}
* {{cite book |last=Hanlon |first=Michael |date=2008 |title=Eternity: Our Next Billion Years |publisher=[[Palgrave Macmillan]] |isbn=978-0-230-21931-1}}

==External links==
* [http://mason.gmu.edu/~rhanson/greatfilter.html The Great Filter – Are We Almost Past It? (1998), Robin Hanson]
* [https://www.youtube.com/watch?v=UjtOGPJ0URM Why Alien Life Would be our Doom - The Great Filter (2018), Kurzgesagt – In a Nutshell]

{{Extraterrestrial life}}
{{Future of Humanity Institute}}
{{Portal bar|Society|Astronomy|Stars|Spaceflight|Outer space|Solar System|Science}}

[[Category:Extraterrestrial life]]
[[Category:Open problems]]
[[Category:Fermi paradox]]
[[Category:1996 introductions]]
[[Category:1996 in science]]