The Great Filter
Encyclopedia
The Great Filter, in the context of the Fermi paradox
, is whatever prevents "dead matter" from giving rise, in time, to "expanding lasting life". The concept originates in Robin Hanson
's argument that the failure to find any extraterrestrial
civilizations in the observable universe
implies the possibility 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 actually observed (currently just one: human
). This probability threshold, which could lie behind us or in front of us, might work as a barrier to the evolution of intelligent life, or as a high probability of self-destruction. The main counter-intuitive conclusion of this observation is that the easier it was for life to evolve to our stage, the bleaker our future chances 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 was last updated on September 15, 1998. Since that time, Hanson's formulation has received recognition in several published sources discussing the Fermi paradox and its implications.
with current technology nor has SETI
found any transmissions from other civilizations. The universe, apart from the Earth, seems "dead"; Hanson states:
According to the Great Filter hypothesis at least one of these steps - if the list were complete - must be improbable. If it's not an early step (i.e. in our past), then the implication is that the improbable step lies in our future and our 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 race. 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 thing that appears likely to keep us from step 9 is some sort of catastrophe
. 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 1–7 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
) then an anthropically biased observer can infer
nothing about the general probabilities from its (pre-determined
) surroundings.
argument "They do exist, but we see no evidence". Additional arguments include it is too expensive to spread physically throughout the galaxy, they tend to experience a technological singularity, Earth is purposely isolated, it is dangerous to communicate and hence civilizations actively hide, and many others.
As one example, astronomer Seth Shostak
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 aliens 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". But, absence of evidence is not 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:
Fermi paradox
The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations....
, is whatever prevents "dead matter" from giving rise, in time, to "expanding lasting life". The concept originates in Robin Hanson
Robin Hanson
Robin D. Hanson is an associate professor of economics at George Mason University and a research associate at the Future of Humanity Institute of Oxford University. He is known as an expert on idea futures, markets and was involved in the creation of the Foresight Exchange and DARPA's FutureMAP...
's argument that the failure to find any extraterrestrial
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...
civilizations in the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...
implies the possibility 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 actually observed (currently just one: human
Human
Humans are the only living species in the Homo genus...
). This probability threshold, which could lie behind us or in front of us, might work as a barrier to the evolution of intelligent life, or as a high probability of self-destruction. The main counter-intuitive conclusion of this observation is that the easier it was for life to evolve to our stage, the bleaker our future chances 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
Robin Hanson
Robin D. Hanson is an associate professor of economics at George Mason University and a research associate at the Future of Humanity Institute of Oxford University. He is known as an expert on idea futures, markets and was involved in the creation of the Foresight Exchange and DARPA's FutureMAP...
. The first version was written in August 1996 and the article was last updated on September 15, 1998. Since that time, Hanson's formulation has received recognition in several published sources discussing the Fermi paradox and its implications.
Fermi paradox
There is no evidence aliens have visited earth and we have observed no intelligent extraterrestrial lifeExtraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...
with current technology nor has SETI
SETI
The search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
found any transmissions from other civilizations. The universe, apart from the Earth, seems "dead"; Hanson states:
Life is expected to expand to fill all available niches. With technology such as self-replicating spacecraft, these niches would include neighbouring 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."
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.
The Great Filter
With no evidence of intelligent life other than ourselves, 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 organicsOrganic chemistryOrganic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
and potentially habitable planetPlanetA planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s) - Reproductive molecules (e.g., RNARNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
) - Simple (prokaryotic) single-cell life
- Complex (archaeaArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
tic and eukaryotic) single-cell life - Sexual reproductionSexual reproductionSexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
- Multi-cell life
- Tool-using animals with big brainsIntelligenceIntelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....
- 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's not an early step (i.e. in our past), then the implication is that the improbable step lies in our future and our 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 race. However, none appear to have made it to step 9, or the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
would be full of colonies. So perhaps step 9 is the unlikely one, and the only thing that appears likely to keep us from step 9 is some sort of catastrophe
Risks to civilization, humans and planet Earth
Various existential risks could threaten humankind as a whole, have adverse consequences for the course of human civilization, or even cause the end of planet Earth.-Types of risks:...
. 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 1–7 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
Realization (probability)
In probability and statistics, a realization, or observed value, of a random variable is the value that is actually observed . The random variable itself should be thought of as the process how the observation comes about...
) then an anthropically biased observer can infer
Statistical hypothesis testing
A statistical hypothesis test is a method of making decisions using data, whether from a controlled experiment or an observational study . In statistics, a result is called statistically significant if it is unlikely to have occurred by chance alone, according to a pre-determined threshold...
nothing about the general probabilities from its (pre-determined
A priori (statistics)
In statistics, a priori knowledge is prior knowledge about a population, rather than that estimated by recent observation. It is common in Bayesian inference to make inferences conditional upon this knowledge, and the integration of a priori knowledge is the central difference between the Bayesian...
) surroundings.
Counterargument
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 scenarios are part of the Fermi paradoxFermi paradox
The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations....
argument "They do exist, but we see no evidence". Additional arguments include it is too expensive to spread physically throughout the galaxy, they tend to experience a technological singularity, Earth is purposely isolated, it is dangerous to communicate and hence civilizations actively hide, and many others.
As one example, astronomer Seth Shostak
Seth Shostak
Seth Shostak is an American astronomer. He grew up in Arlington, VA and earned his physics degree from Princeton University and a Ph.D...
of the SETI Institute
SETI Institute
The SETI Institute is a not-for-profit organization whose mission is to “explore, understand and explain the origin, nature and prevalence of life in the universe”. SETI stands for the "search for extraterrestrial intelligence". One program is the use of both radio and optical telescopes to search...
argues that one can postulate a galaxy filled with intelligent extraterrestrial civilizations that have failed to colonize the Earth. Perhaps the aliens 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". But, absence of evidence is not evidence of absence. The absence of extraterrestrial megascale engineering
Megascale engineering
Megascale engineering is a form of exploratory engineering concerned with the construction of structures on an enormous scale. Typically these structures are at least 1,000 kilometers in length—in other words, at least 1 megameter, hence the name...
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 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.
See also
- Black swan theoryBlack swan theoryThe black swan theory or theory of black swan events is a metaphor that encapsulates the concept that The event is a surprise and has a major impact...
- Doomsday argumentDoomsday argumentThe Doomsday argument is a probabilistic argument that claims to predict the number of future members of the human species given only an estimate of the total number of humans born so far...
- Drake equationDrake equationThe Drake equation is an equation used to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the Search for ExtraTerrestrial Intelligence...
- Final anthropic principle
- Goldilocks PrincipleGoldilocks PrincipleThe Goldilocks principle states that something must fall within certain margins, as opposed to reaching extremes. It is used, for example, in the Rare Earth hypothesis to state that a planet must neither be too far away from, nor too close to the sun to support life. Either extreme would result in...
- Inverse gambler's fallacyInverse gambler's fallacyThe inverse gambler's fallacy, named by philosopher Ian Hacking, is a formal fallacy of Bayesian inference which is similar to the better known gambler's fallacy. It is the fallacy of concluding, on the basis of an unlikely outcome of a random process, that the process is likely to have occurred...
- Kardashev scaleKardashev scaleThe Kardashev scale is a method of measuring an advanced civilization's level of technological advancement. The scale is only theoretical and in terms of an actual civilization highly speculative; however, it puts energy consumption of an entire civilization in a cosmic perspective. It was first...
- NeocatastrophismNeocatastrophismNeocatastrophism is the theory that life-exterminating events such as gamma-ray bursts have acted as a galactic regulation mechanism in the Milky Way upon the emergence of complex life in its habitable zone...
- Principle of mediocrity
- Rare Earth hypothesisRare Earth hypothesisIn planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...
- Selection biasSelection biasSelection bias is a statistical bias in which there is an error in choosing the individuals or groups to take part in a scientific study. It is sometimes referred to as the selection effect. The term "selection bias" most often refers to the distortion of a statistical analysis, resulting from the...