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The Great Filter is one possible resolution of the Fermi paradox. It posits that in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there exists some particular barrier to development that makes detectable extraterrestrial life exceedingly rare. The concept originates in Robin Hanson's argument that the failure to find any
extraterrestrial Extraterrestrial refers to any object or being beyond ( extra-) the planet Earth ( terrestrial). It is derived from the Latin words ''extra'' ("outside", "outwards") and ''terrestris'' ("earthly", "of or relating to the Earth"). It may be abbrevia ...
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). 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. The main conclusion of this argument is that the easier it was for life to evolve to the present stage, 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?", written by economist Robin Hanson. The first version was written in August 1996 and the article . Hanson's formulation has received recognition in several published sources discussing the Fermi paradox and its implications.


Main argument


Fermi paradox

There is no reliable evidence that
aliens Alien primarily refers to: * Alien (law), a person in a country who is not a national of that country ** Enemy alien, the above in times of war * Extraterrestrial life, life which does not originate from Earth ** Specifically, intelligent extrate ...
have visited Earth; we have observed no intelligent
extraterrestrial life Extraterrestrial life, colloquially referred to as alien life, is life that may occur outside Earth and which did not originate on Earth. No extraterrestrial life has yet been conclusively detected, although efforts are underway. Such life might ...
with current technology, nor has SETI found any transmissions from other
civilizations A civilization (or civilisation) is any complex society characterized by the development of a state, social stratification, urbanization, and symbolic systems of communication beyond natural spoken language (namely, a writing system). Civ ...
. The Universe, apart from the Earth, seems "dead"; Hanson states:
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.
Life is expected to expand to fill all available niches. With technology such as
self-replicating spacecraft The concept of Self-replicating spacecraft, as envisioned by mathematician John von Neumann, has been described by futurists including physicist Michio Kaku and discussed across a wide breadth of hard science fiction novels and stories. Self-rep ...
, 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."


The Great Filter

With no evidence of intelligent life in places other than the 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 A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speak ...
(including organics and potentially habitable planets) # Reproductive
molecules A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
(e.g.
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
) # Simple ( prokaryotic) single-cell life # Complex ( eukaryotic) single-cell life # Sexual reproduction # Multi-cell life # Tool-using animals with intelligence # A civilization advancing toward the potential for a colonization explosion (where we are now) # 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 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 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). 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. 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
local environment Local may refer to: Geography and transportation * Local (train), a train serving local traffic demand * Local, Missouri, a community in the United States * Local government, a form of public administration, usually the lowest tier of administrat ...
) then an anthropically biased observer can
infer Inferences are steps in reasoning, moving from premises to logical consequences; etymologically, the word '' infer'' means to "carry forward". Inference is theoretically traditionally divided into deduction and induction, a distinction that in ...
nothing about the general probabilities from its ( 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 technosignatures 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. In a specific formulation named the "
Berserker hypothesis The berserker hypothesis, also known as the deadly probes scenario, is the idea that humans have not yet detected intelligent alien life in the universe because it has been systematically destroyed by a series of lethal Von Neumann probes. The hypo ...
", a filter exists between steps 8 and 9 in which each civilization is destroyed by a lethal Von Neumann probe created by a more advanced civilization.


Responses

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 Dirk Schulze-Makuch (born 1964) is a professor at the Center for Astronomy and Astrophysics at the Technical University Berlin, Germany and Adjunct Professor at the School of Earth and Environmental Sciences Washington State University, Pullman, ...
and William Bains, reviewing the history of life on Earth, including convergent evolution, concluded that transitions such as
oxygenic photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
, the 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. Astronomer
Seth Shostak Seth Shostak (born July 20, 1943) is an American astronomer and author, and is currently the senior astronomer for the SETI Institute. Shostak hosts SETI's weekly radio show/podcast ''Big Picture Science'', has played himself numerous times in TV ...
of the SETI Institute argues that one can postulate a galaxy filled with intelligent extraterrestrial civilizations that have failed to colonize the Earth. Perhaps the
alien Alien primarily refers to: * Alien (law), a person in a country who is not a national of that country ** Enemy alien, the above in times of war * Extraterrestrial life, life which does not originate from Earth ** Specifically, intelligent extrater ...
s lacked the intent and purpose to colonize or depleted their resources, or maybe the
galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
''is'' colonized but in a
heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
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? According to Shostak:
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 Sir Arthur Charles Clarke (16 December 191719 March 2008) was an English science-fiction writer, science writer, futurist, inventor, undersea explorer, and television series host. He co-wrote the screenplay for the 1968 film '' 2001: A Spac ...
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 spheres, so we can't really rule them out.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.


See also

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References


Further reading

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External links


The Great Filter – Are We Almost Past It? (1998), Robin Hanson

Why Alien Life Would be our Doom - The Great Filter (2018), Kurzgesagt – In a Nutshell
{{Portal bar, Society, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science Extraterrestrial life Open problems Fermi paradox 1996 introductions 1996 in science