1983 00 00 QJR. Astronomical, V 24 - The Great Silence, the Controversy Concerning Extraterrestrial Intelligent Life - Glen David Brin

Magazine Overview

This document is an article titled "The 'Great Silence': the Controversy Concerning Extraterrestrial Intelligent Life" by Glen David Brin, published in the Quarterly Journal of the Royal Astronomical Society (Q. JI R. astr. Soc.), Volume 24, No. 3, in 1983. The article was received on September 27, 1982.

Summary

The article addresses the apparent paradox of the "Great Silence" – the lack of evidence for extraterrestrial civilizations despite the contention that interstellar exploration and settlement are possible at non-relativistic speeds. The author suggests that current models are simplistic and that inadequate exploration of the problem's parameters contributes to the difficulty. The debate has moved from abstract speculation to public policy, evidenced by the excision of federal funds for SETI (Search for Extra-Terrestrial Intelligence) in the US Congress, with Senator William Proxmire citing scientists who believe advanced extraterrestrial technological species cannot exist.

The author notes that the subject is data-poor and prone to biased extrapolations, impacting mankind's ultimate destiny. Prominent scientists are drawn into a debate characterized by ill-matched arguments. Philosophical battle lines are drawn between 'Contact Optimists,' who believe life and intelligence are common, and proponents of the 'Uniqueness Hypothesis,' who suggest Earth is unique in its technical civilization.

Those denying the existence of extraterrestrial intelligent species (ETIS) often base their arguments on the possibility of space habitats and interstellar spaceflight, calculating that even one prior technological species could have colonized the Galaxy within approximately 108 years. John von Neumann's concept of self-reproducing robots is cited as a mechanism for such pervasive colonization.

Introduction

The possibility of intelligent life beyond Earth has entered public policy debate. The article highlights the reduction of federal funds for SETI in the US Congress and the opposing view that advanced extraterrestrial technological species cannot exist. The author emphasizes the lack of data and the tendency for biased extrapolations in this field, which is intertwined with humanity's ultimate destiny. The debate involves scientists like Ornstein, Crick, Orgel, Sagan, Hart, Tipler, Jones, and Bond, with differing views on the prevalence of life and intelligence.

The Quandary of the 'Great Silence'

Between the late 1950s and early 1970s, SETI discussions assumed that intelligent life evolved at isolated, random sites and that interstellar travel was virtually impossible, with contact occurring only via electromagnetic signals. Projects like OZMA, CYCLOPS, SETI, and CETI were based on this assumption. Despite nearly 20 years of SETI efforts yielding no results, supporters were not discouraged due to the limited resources and time spent. However, the 'islands' model is no longer widely accepted, leading to the "Mystery of the Great Silence."

• The 'Drake equation,' developed by Frank Drake, is presented as a central tool in xenological discussion, though it is now considered insufficient. The equation estimates E, the number of sites in the Galaxy where technological civilization has evolved and exists. The factors include:
• R: average galactic rate of star production
• f_g: fraction of stars that are 'good' stable dwarf stars with planets
• n_e: number of 'candidate' planets per system with life-supporting conditions
• f_i: fraction of such planets where life arises
• f_c: fraction of life-bearing planets where intelligence evolves
• L: average lifespan of such a technological culture

Disagreements arise over the values assigned to these factors. 'Contact Optimists' like Sagan suggest moderate variations in factors lead to a significant E, with L being the controlling factor. 'Uniqueness' advocates, like Jones, start with the presumption that E is very small, proposing minuscule values for f_i and f_c, suggesting life or intelligence is extremely unlikely.

The article points out the incompleteness of the Drake equation, as it does not account for the contact cross-section between ETIS and human society or the paradigm shift caused by the acceptance of non-relativistic interstellar spaceflight. A modified formulation, C = (1/N*) Σ A_j(n_j+1), is proposed to calculate the likelihood of verifiable contact, incorporating factors like the number of settled sites (n_j) and the 'contact cross-section' (A_j).

Philosophical Considerations

The SETI debate is unique in its interdisciplinary nature, combining biology, chemistry, sociology, and astrophysics with philosophical issues. The Cosmological Principle, or 'assumption of mediocrity,' suggests that Earth's time and place are not special. This principle was extended to the evolution of life and intelligence, implying their common occurrence. However, the 'Anthropic Principle' suggests that the universe might be structured such that observers witness a special place and time, necessary for observation itself.

Uniqueness advocates find no issue with the proposal that Earth's intelligent life is rare. The concept of 'equilibrium' is also discussed, with the assumption of a quasi-equilibrium state in scientific exploration. However, Sagan's suggestion that ETIS 'have not arrived yet' implies a state of profound disequilibrium. Tipler, Jones, and others suggest the Galaxy is moving from a state of emptiness to one of human expansion.

A 'Principle of Non-exclusiveness' is proposed, suggesting that diversity prevails unless a mechanism enforces conformity. The article emphasizes that observable facts are more persuasive than these principles.

The Factors Controlling Contact

The article aims to eliminate two factors, f_g (fraction of suitable planets) and n_e (number of suitable planets per system), from the SETI debate, stating there is general agreement that these are large numbers. Most uniqueness advocates accept this. The Solar System's angular momentum suggests planets are common, leading to a conservative estimate of f_g = 0.1. Similarly, uniqueness proponents generally do not dispute the SETI proposal that n_e is around one-tenth.

This leaves seven controversial factors. The article focuses on the incidence of life (f_i) and the evolution of intelligence (f_c). The origin of life is discussed, referencing the Miller-Urey experiments and the presence of organic compounds in interstellar clouds and meteorites. However, the leap from organic soup to self-replicating DNA-based structures is considered a long one, with statistical arguments suggesting low probabilities. Chandra Wickramasinghe's calculation of the probability of chance assembly of genes is cited as an example.

Attempts to derive Earthly biology from random organic reactions are deemed conservative. The 'levo v. dextro' argument regarding amino acid orientation is mentioned, suggesting that if Earth's all-one-way system is an accident, the odds against the same biochemistry on another planet are 2^20. The article also touches upon the delicate balance of Earth's environment and the possibility of fortuitous circumstances contributing to life's success.

Panspermia, the idea that life on Earth was seeded from elsewhere, is mentioned as fascinating but untestable. The article suggests that the 1.5 billion years after life appeared but before eukaryotic organisms evolved argues against outside intervention. The SETI forces assign f_i a value of 0.1-1.0, while uniqueness proponents are divided. New discoveries seem to support the idea that life, as we know it, is common.

The evolution of intelligence (f_c) is discussed, with a majority of uniqueness advocates accepting a large value for this factor. The concept of 'convergence,' where unrelated taxa independently evolve similar traits, is central to the argument that intelligence might be a common evolutionary outcome. Examples like the homeomorphic development of dolphins and tuna, and the independent evolution of lens eyes in vertebrates and cephalopods, are cited. Anti-convergence arguments suggest these similarities might be due to recapitulation of distant common ancestors. The article concludes that the paradigm is tipping in favor of convergence.

Recurring Themes and Editorial Stance

The recurring themes in this article are the "Great Silence" paradox, the limitations and applications of the Drake equation, the contrasting viewpoints of "Contact Optimists" and "Uniqueness Hypothesis" proponents, and the scientific and philosophical considerations surrounding the origin and evolution of life and intelligence in the universe. The author's stance appears to be analytical, aiming to break down the complex question of alien intelligence into manageable logical elements and to critically examine the assumptions and arguments put forth by various scientists in the field. The article advocates for a more rigorous and comprehensive approach to understanding the "Great Silence."

This issue of 'The 'Great Silence'' (No. 3, Vol. 24) from the Royal Astronomical Society, published in 1983, delves into the Fermi Paradox, exploring the profound silence observed in the universe regarding extraterrestrial civilizations. The central theme revolves around potential explanations for why, despite the vastness of space and the statistical probability of life, we have not encountered any evidence of alien intelligence.

The Nature of Intelligence and Convergence

The article begins by discussing the concept of convergent evolution, citing examples like the independent evolution of marsupial wolves in Australia and South America, and the similarities between flying squirrels and phalangers. It posits that intelligence itself might be a convergent trait, noting that both higher primates (apes) and cetaceans (dolphins) exhibit complex communication abilities and nurturing behaviors, suggesting they are approaching similar levels of 'threshold' intelligence. The text questions whether true intelligence is a natural stage of development, observable across diverse evolutionary paths.

It highlights that human intelligence is distinguished by its emphasis on extragenetic transmission of information and a less 'hard-wired' set of behaviors, requiring an extended childhood for programming 'software.' The comparison between humans and chimpanzees, biochemically very close, suggests that significant morphological differences can arise from regulatory shifts in gene expression. The idea of humans being 'neotenous' apes, retaining childlike curiosity, is explored as a potential evolutionary advantage.

Factors Affecting the Drake Equation and the 'Great Silence'

The core of the discussion focuses on factors within the Drake Equation that could explain the absence of detectable extraterrestrial civilizations. The article examines:

• 'fc' (The emergence of a technological culture): It's suggested that even if intelligent species evolve, they might not develop a technological culture capable of interstellar communication or travel. Scenarios include species focusing on speech and analytical ability without developing the 'vigorous imagination' that drives human technological advancement, or being limited by environmental factors like a lack of dry land. Intervention by transient space-farers is also mentioned as a factor that could influence 'fc'.

• 'L' (The homeworld lifespan of technological species): This factor is considered crucial. A common suggestion is that technological species face a 'crisis of survival' upon discovering technologies like nuclear power, leading to self-destruction. The article questions if this is a universal pattern, noting that humans currently face even odds of surviving the next hundred years. Other scenarios include resource exhaustion, leading to a decline into 'soft technologies or barbarism,' or 'Transcendence,' where species abandon technology as they outgrow it, akin to Arthur C. Clarke's 'Childhood's End.'

• 'v' (Star travel-factor): The possibility and implications of interstellar travel are thoroughly explored. While some argue interstellar travel is impossible, the author believes the case for slow interstellar travel is growing stronger. The article discusses models of galactic expansion, suggesting that even at sub-light speeds, a star-faring race could colonize the galaxy within tens of millions of years. The 'Deadly Probes Scenario' is presented as a compelling explanation: advanced civilizations might send self-replicating probes, but a xenophobic minority could program them with weapons, leading to the destruction of any newly detected civilizations. This scenario is consistent with all observed facts and doesn't require suppressing other Drake Equation factors.

• 'A' (Approach/avoidance cross-section): This factor addresses why advanced ETIS might choose not to contact us. Hypotheses include:
• Avoidance: Members of a 'galactic radio club' might avoid contacting 'beginners' to preserve the novices' unique future contributions to galactic culture. Knowledgeable ETIS might also be cautious about leaking electromagnetic radiation, selectively contacting others. The 'Deadly Probes' scenario also implies caution.
• Bad search strategies: We might be looking in the wrong way or on the wrong frequencies. Advanced species might use unknown, more efficient communication methods than radio, analogous to how New Guinea natives were unaware of radio waves around them.
• Quarantine: Earth might be intentionally isolated, either as a 'zoo,' to allow new sentience to nurture, or because humanity is considered dangerous. Extraterrestrials might even be in secret contact with governments.

Other Scenarios and Analogies

Further scenarios are explored:

• Macrolife: The abandonment of planet-dwelling as a lifestyle in favor of space habitats, leading to either the greedy exploitation of planets or the cherishing of 'Nursery Worlds.'
• 'Seniors only': Space-faring sophonts might 'graduate' to other interests, limiting expansion.
• 'Low rent': Earth might be inaccessible or undesirable. The possibility of FTL travel via 'wormholes' is discussed, suggesting that if usable black holes existed nearby, Earth would have already been settled, implying an Anthropic Principle at play.

Analogies are drawn from Earth's history, particularly the Polynesian colonization of Rapa Nui (Easter Island), to illustrate the potential for rapid expansion, resource competition, and self-destruction among technologically advanced species. The article notes that the Polynesian experience was often sanguinary, involving overpopulation and conflict.

Conclusion

The author expresses the opinion that proponents arguing for extremely small values of 'fi' (likely referring to the fraction of planets that develop life) and 'fi' (likely referring to the fraction of life that develops intelligence) have presented a weak case. However, this does not invalidate the basic conclusion that we might be alone in the Galaxy. The low value of 'C' (likely referring to the number of civilizations) in the Drake Equation is presented as a fact, and the article proceeds to examine other factors and explanations for the apparent absence of extraterrestrial civilizations.

Recurring Themes and Editorial Stance

The recurring theme is the multifaceted nature of the 'Great Silence' and the numerous, often speculative, hypotheses attempting to resolve it. The editorial stance appears to be one of critical examination, presenting various theories while acknowledging their speculative nature and the need for further data. The article emphasizes that while many explanations exist, few systematically prevent contact across immense timescales, suggesting that the 'non-exclusivity rule' (where even a few exceptions can lead to widespread presence) is a key challenge for many theories. The author seems to lean towards explanations that do not require the suppression of other Drake Equation factors, such as the 'Deadly Probes Scenario' or the limitations imposed by the lifespan of technological civilizations.

This issue of the journal features an article titled "THE 'GREAT SILENCE'" by G.D. Brin, discussing the apparent absence of extraterrestrial intelligence. The article delves into theoretical models of cosmic colonization and the potential reasons for the silence, exploring concepts such as the lifespan of technological civilizations, expansion spheres, and the 'Nursery World' hypothesis.

The 'Great Silence' and ETIS Expansion

The article begins by positing that planetary systems can support technological cultures for limited characteristic periods (L'). It introduces the concept of an Extraterrestrial Intelligence (ETIS) expanding from its home system at an average effective velocity (v), with a survival factor S(r) assigned to sites at distance r from the center. The number of surviving sites within a settled sphere is calculated using an integral equation. The author notes that the number of sites (n;) is sensitive to L', and if L' is small, the interior sites die off rapidly, with the outermost shell dominating. When considering the finite disc shape of the Galaxy, n; may rise linearly and then decline, akin to a grass fire burning out.

'Nursery Worlds' and Earth's Past

The text then shifts to the concept of a 'Nursery World' – a planet with an equable biosphere suitable for the long evolution of intelligence. It draws a parallel to Earth's history, specifically the Cretaceous-Tertiary Catastrophe approximately 65 million years ago, which caused a mass extinction of large land animals. The descendants of smaller creatures, like mice and lemurs, diversified to fill the ecological niches. The author speculates whether the extinction of a present technological race on Earth could be analogous to past events, suggesting that if 'mice' could rise once, they might do so again.

Suitable worlds, it is suggested, may undergo long, fallow periods before developing biological sophistication ripe for intelligence. The interval between the Cretaceous Catastrophe and the present is offered as a potential estimate for such recovery times. The article also considers cyclical waves of migration and colonization by spacefarers, where planets are settled, exploited, and then abandoned.

Ecological Impact of Colonization

Unless tenant sophonts leave large parts of their worlds fallow or engage in 'uplift' bio-engineering, their presence can prevent the emergence of indigenous sentient species. The cycle of producing intelligent species is likely delayed indefinitely by active technological settlement. When settlers vacate, the recovery time for another generation of tool-users to evolve depends on the extent of exploitation, which can severely impact the local biosphere. The article notes that Earth's technological civilization has simplified ecological networks, and 'higher' life forms are dependent on complex eco-networks.

The Cretaceous Catastrophe and ETIS

There is no clear evidence that Earth was ever settled by an ETIS. However, the Cretaceous Catastrophe is presented as an event with faint possibilities. This event featured the demise of dinosaurs and the extinction of many phytoplankton and marine micro-organisms. Thin layers of iridium-rich clay found at sediment levels associated with this period suggest a major meteorite impact that severely modified weather patterns, reduced insolation, and cut off photosynthesis for months. While the meteorite likely caused marine extinctions, it may have been the final blow for the dinosaurs, which were already in decline. The author suggests that the catastrophic conclusion of the age of dinosaurs could be consistent with the violent end of an epoch of settlement, perhaps a final war fought with asteroids by extraterrestrial settlers who cleaned the Earth's surface of higher forms, returning it to nursery status.

Alternative Explanations and Future Considerations

The article explores other major mass extinctions in Earth's sedimentary record, such as those at the end of the Devonian and the Permian-Triassic boundary, noting rough 10⁷-10⁸ year intervals that align with estimates for galaxy filling by space-travelling species. The ecological holocaust of the Cretaceous could be a local manifestation of a prior space-faring race's demise due to overpopulation and environmental degradation. This offers a potential mechanism for the 'Uniqueness' hypothesis, explaining why our region of space might be in a state of emptiness.

Looking to humanity's future, if we survive our current crises and initiate interstellar travel, we might encounter the remains of earlier epochs. The article warns that cultures exercising self-restraint and environmental sensitivity will be at a disadvantage against those who are rapacious. There may be zones where sensitive settlers preserve ecospheres, while others are exploitative. Jones's suggestion that inner worlds might be targets for colonial ventures from the outermost shell of failed settled spheres is discussed, but the recovery time required for a biosphere to regain attractiveness is a significant problem.

Table I: Causes of the Apparent Absence of Extraterrestrials

Table I provides a chart of suggested causes for the apparent absence of extraterrestrials, categorized into 'Solitude,' 'Quarantine,' 'Macrolife,' 'Seniors only,' and 'Low rent.' Each category lists specific hypotheses with their potential effects on factors in equations (1-4), such as the number of useful planets per star (n.), the percentage of life that develops (f₁), the percentage that gains intelligence (f₂), the percentage that attains technology (f₃), the span of isolated ETI (L), the characteristic ETI colony lifespan (L<0xE2><0x82><0x9C>), effective settlement expansion velocity (v), and approach/avoidance factor (A<0xE2><0x82><0x9B>). Hypotheses like 'Habitable planets are rare,' 'Inevitable' self-destruction,' 'Greedy breakup of 'nurseries',' and 'FTL selection' are presented with their potential positive or negative impacts.

Conclusion

The quandary of the Great Silence presents a struggle between optimistic excuses for the absence of neighbors and enthusiastic acceptance of humanity's isolation. The article attempts a morphological discussion of possibilities, breaking the subject into logical elements and presenting Table I as an overview of ideas and their effects on key factors. The author emphasizes that further work should consider experimental tests for this fundamental question. The survey demonstrates that the universe has many ways to be 'nasty,' and the hypotheses of 'Deadly Probes' and 'Ecological Holocaust' are presented as depressing but potentially consistent explanations. The article concludes by suggesting that elder species might have instilled a tradition of respect for life, and that the Great Silence might be akin to a child's nursery, where adults tread softly.

Recurring Themes and Editorial Stance

The recurring themes in this article are the Fermi Paradox (the 'Great Silence'), the potential limitations and lifespans of extraterrestrial civilizations, the concept of 'Nursery Worlds' and their vulnerability, and the impact of colonization on planetary biospheres. The author's stance appears to be one of cautious pessimism, presenting the 'Deadly Probes' and 'Ecological Holocaust' hypotheses as the most plausible, albeit disturbing, explanations for the lack of observed extraterrestrial intelligence. The article encourages further research and empirical testing of these fundamental questions about humanity's place in the universe.