Great Filter

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The Great Filter is the 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. [1] [2] The Great Filter is one possible resolution of the Fermi paradox.

Contents

The concept originates in Robin Hanson's argument that the failure to find any 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). [3] 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. [1] [4] 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 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

There is no reliable evidence that 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: [1]

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. [5] 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." [1]

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:

  1. The right star system (including organics and potentially habitable planets)
  2. Reproductive molecules (e.g. RNA)
  3. Simple (prokaryotic) single-cell life
  4. Complex (eukaryotic) single-cell life
  5. Sexual reproduction
  6. Multi-cell life
  7. Tool-using animals with intelligence
  8. A civilization advancing toward the potential for a colonization explosion (where we are now)
  9. 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). [6] 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. [4]

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) then an anthropically biased observer can infer 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. [7]

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 probe created by a more advanced civilization. [8]

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 and William Bains, reviewing the history of life on Earth, including convergent evolution, concluded that transitions such as oxygenic photosynthesis, 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. [9]

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". 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? [10] 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. [10] [11]

See also

Related Research Articles

<span class="mw-page-title-main">Drake equation</span> Estimate of extraterrestrial civilizations

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.

<span class="mw-page-title-main">Extraterrestrial life</span> Life that did not originate on Earth

Extraterrestrial life, alien life, or colloquially simply aliens is life which does not originate from Earth. No extraterrestrial life has yet been conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more advanced than humanity. The Drake equation speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology.

<span class="mw-page-title-main">Fermi paradox</span> Lack of evidence that aliens exist

The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."

The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other planets.

<span class="mw-page-title-main">Kardashev scale</span> Measure of a civilizations evolution

The Kardashev scale is a method of measuring a civilization's level of technological advancement based on the amount of energy it is capable of using. The measure was proposed by Soviet astronomer Nikolai Kardashev (1932–2019) in 1964 and was named after him.

Extraterrestrial intelligence, or non-human intelligence, refers to hypothetical intelligent extraterrestrial life. No such life has ever been proven to exist in the Solar System except for humans on Earth, and its existence on other star systems is still speculative. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged when the Copernican Revolution demonstrated that the Earth was a planet revolving around the Sun, and other planets were, conversely, other worlds. The question of whether other inhabited planets or moons exist was a natural consequence of this new understanding. It has become one of the most speculative questions in science and is a central theme of science fiction and popular culture.

The concept of self-replicating spacecraft, as envisioned by mathematician John von Neumann, has been described by futurists and has been discussed across a wide breadth of hard science fiction novels and stories. Self-replicating probes are sometimes referred to as von Neumann probes. Self-replicating spacecraft would in some ways either mimic or echo the features of living organisms or viruses.

<span class="mw-page-title-main">Rare Earth hypothesis</span> Hypothesis that complex extraterrestrial life is improbable and extremely rare

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth required an improbable combination of astrophysical and geological events and circumstances.

In astrobiology, Uniformity of Motive is an essential assumption of the Zoo Hypothesis explanation to Fermi’s Paradox. The Zoo Hypothesis states that alien civilizations refrain from contacting Earth, so as to not interfere in natural evolution and cultural development, or to minimize risk for themselves. Certain technological, political or ethical thresholds and standards could be implemented for other civilizations to pass, after which contact would be established.

The zoo hypothesis speculates on the assumed behavior and existence of technologically advanced extraterrestrial life and the reasons they refrain from contacting Earth. It is one of many theoretical explanations for the Fermi paradox. The hypothesis states that alien life intentionally avoids communication with Earth to allow for natural evolution and sociocultural development, and avoiding interplanetary contamination, similar to people observing animals at a zoo. The hypothesis seeks to explain the apparent absence of extraterrestrial life despite its generally accepted plausibility and hence the reasonable expectation of its existence. A variant on the zoo hypothesis suggested by the former MIT Haystack Observatory scientist John Allen Ball is the "laboratory" hypothesis, in which humanity is being subjected to experiments, with Earth serving as a giant laboratory.

<span class="mw-page-title-main">Neocatastrophism</span> Hypothesis for lack of detected aliens

Neocatastrophism is the hypothesis 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. It is one of several proposed solutions to the Fermi paradox since it provides a mechanism which would have delayed the advent of intelligent beings in local galaxies near Earth.

The planetarium hypothesis, conceived in 2001 by Stephen Baxter, attempts to provide a solution to the Fermi paradox by holding that our astronomical observations represent an illusion, created by a Type III civilization capable of manipulating matter and energy on galactic scales. He postulates that we do not see evidence of extraterrestrial life because the universe has been engineered so that it appears empty of other life.

The cultural impact of extraterrestrial contact is the corpus of changes to terrestrial science, technology, religion, politics, and ecosystems resulting from contact with an extraterrestrial civilization. This concept is closely related to the search for extraterrestrial intelligence (SETI), which attempts to locate intelligent life as opposed to analyzing the implications of contact with that life.

<span class="mw-page-title-main">Technosignature</span> Property that provides scientific evidence for the presence of technology

Technosignature or technomarker is any measurable property or effect that provides scientific evidence of past or present technology. Technosignatures are analogous to biosignatures, which signal the presence of life, whether intelligent or not. Some authors prefer to exclude radio transmissions from the definition, but such restrictive usage is not widespread. Jill Tarter has proposed that the search for extraterrestrial intelligence (SETI) be renamed "the search for technosignatures". Various types of technosignatures, such as radiation leakage from megascale astroengineering installations such as Dyson spheres, the light from an extraterrestrial ecumenopolis, or Shkadov thrusters with the power to alter the orbits of stars around the Galactic Center, may be detectable with hypertelescopes. Some examples of technosignatures are described in Paul Davies's 2010 book The Eerie Silence, although the terms "technosignature" and "technomarker" do not appear in the book.

The aestivation hypothesis is a hypothesized solution to the Fermi paradox conceived in 2017 by Anders Sandberg, Stuart Armstrong and Milan M. Ćirković. The hypothesis, published on 27 April 2017, suggests advanced alien civilizations may be storing energy and aestivating, until the universe cools to better make use of the stored energy to perform tasks.

<span class="mw-page-title-main">Outline of extraterrestrial life</span> Overview of and topical guide to extraterrestrial life

The following outline is provided as an overview of and topical guide to extraterrestrial life:

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. In this framing, it is presumed 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, as in a "dark forest" filled with "armed hunter(s) stalking through the trees like ghosts". The conjecture was first proposed in Liu Cixin’s 2008 novel The Dark Forest, arguing that 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 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 hypothesis is named after the Berserker series of novels (1963–2005) written by Fred Saberhagen.

The Hart–Tipler conjecture is the idea that an absence of detectable Von Neumann probes is contrapositive evidence that no intelligent life exists outside of the Solar System. This idea was first proposed in opposition to the Drake equation in a 1975 paper by Michael H. Hart titled "Explanation for the Absence of Extraterrestrials on Earth". Assuming that the probes traveled at 1/10 the speed of light and that no time was lost in building new ships upon arriving at the destination, Hart surmised that a wave of Von Neumann probes could cross the galaxy in approximately 650,000 years, a comparatively minimal span of time relative to the estimated age of the universe at 13.7 billion years. Hart’s argument was extended by cosmologist Frank Tipler in his 1981 paper entitled ‘Extraterrestrial intelligent beings do not exist’.

The firstborn hypothesis is a proposed solution to the Fermi paradox which states that no extraterrestrial intelligent life has been discovered because humanity is the first form of intelligent life in the universe.

References

  1. 1 2 3 4 Hanson, Robin (1998). "The Great Filter – Are We Almost Past It?". Archived from the original on 2010-05-07.
  2. ↑ Overbye, Dennis (August 3, 2015). "The Flip Side of Optimism About Life on Other Planets". New York Times . Archived from the original on September 19, 2019. Retrieved October 29, 2015.
  3. ↑ 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?"
  4. 1 2 Bostrom, Nick (May–June 2008). "Where Are They? Why I hope the search for extraterrestrial life finds nothing" (PDF). Technology Review . Massachusetts Institute of Technology: 72–77. Archived (PDF) from the original on 2019-12-24. Retrieved 2008-06-19.
  5. ↑ Papagiannis, Michael D. (1978). "1978QJRAS..19..277P Page 277". Quarterly Journal of the Royal Astronomical Society. 19: 277. Bibcode:1978QJRAS..19..277P. Archived from the original on 2022-11-08. Retrieved 2022-11-08.
  6. ↑ Baum, Seth (8 February 2010). "Is Humanity Doomed? Insights from Astrobiology". Sustainability. 2 (2): 591–603. Bibcode:2010Sust....2..591B. doi: 10.3390/su2020591 .
  7. ↑ Haqq-Misra, Jacob; Ravi Kumar Kopparapu; Schwieterman, Edward (2020). "Observational Constraints on the Great Filter". Astrobiology. 20 (5): 572–579. arXiv: 2002.08776 . Bibcode:2020AsBio..20..572H. doi:10.1089/ast.2019.2154. PMID   32364797. S2CID   211204863.
  8. ↑ Williams, Matt (23 August 2020). "Beyond "Fermi's Paradox" VI: What is the Berserker Hypothesis?". Universe Today . Archived from the original on 18 October 2022. Retrieved 18 October 2022.
  9. ↑ Schulze-Makuch, Dirk; Bains, William (2017). The Cosmic Zoo: Complex Life on Many Worlds. Springer. pp. 201–206. ISBN   978-3-319-62045-9. Archived from the original on 2023-03-31. Retrieved 2020-02-26.
  10. 1 2 Pethokoukis, James M. (2003-11-04). "Keeping His Eyes on the Skies". U.S. News & World Report . Archived from the original on 2013-06-19.
  11. ↑ 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.

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