1975 00 00 Q.J.R. Astronomical Society- V 16, An Explanation for the Absence of Extraterrestrials on Earth - Michael H. Hart

Magazine Overview

This document is an article titled "An Explanation for the Absence of Extraterrestrials on Earth" by Michael H. Hart, published in the "Q. Jl R. astr. Soc." (Quarterly Journal of the Royal Astronomical Society), Volume 16, Issue 2, pages 128-135, in 1975. The article was received on December 6, 1974. Michael H. Hart is affiliated with the Advanced Study Program at the National Center for Atmospheric Research in Boulder, Colorado, which is sponsored by the National Science Foundation.

Main Argument: The Fermi Paradox

The article addresses the question of whether intelligent beings exist elsewhere in the Galaxy, a question frequently posed by laymen. Hart identifies a crucial empirical fact, which he terms 'Fact A': "There are no intelligent beings from outer space on Earth now." He argues that this fact is of great importance and should not be disregarded, despite scientists' tendency to focus on measurement-based data. Hart proposes that the most plausible explanation for Fact A is the hypothesis that there are no other advanced civilizations in our Galaxy. He suggests that if such civilizations existed, they would have developed space travel and explored/colonized the Galaxy, similar to humanity's exploration of Earth. Since they are not present on Earth, they likely do not exist.

Rejection of Alternative Explanations

Hart systematically examines and rejects several categories of alternative explanations for Fact A:

1. Physical Explanations

• These explanations posit that space travel is infeasible due to physical, astronomical, biological, or engineering difficulties. While acknowledging the immense distances and travel times involved in interstellar journeys (e.g., a trip to Sirius at 10% the speed of light taking 88 years), Hart argues these are not insurmountable. Potential solutions include:
• Suspended Animation/Cryogenics: Voyagers could be put into a form of suspended animation or frozen to slow metabolism or preserve them for the long journey. Future biological advancements or non-warm-blooded alien species could make this feasible.
• Longer Lifespans: Extraterrestrials might have lifespans of thousands of years, making a 200-year voyage a mere interlude.
• Speculative Methods: This includes relativistic time-dilation effects or using robots to man the spaceship, with frozen zygotes for colonization upon arrival.
• Multi-Generational Voyages: Planning voyages to take more than one generation, with adequate ship size and social structure, is presented as a practical solution.

Regarding energy requirements, Hart calculates that while significant, they are not extreme if nuclear energy is used. He estimates that a spaceship traveling at 0.10c would need to carry about nine times its own weight in fuel. He also notes that hazards like cosmic rays, meteoroids, and weightlessness, once considered prohibitive, have been largely mitigated by successes like the Apollo and Skylab missions.

2. Sociological Explanations

• These explanations suggest that extraterrestrials have not arrived because they have chosen not to, due to lack of interest, motivation, or organization. Examples include:
• The Contemplation Hypothesis: Advanced civilizations focus on spiritual matters rather than space exploration.
• The Self-Destruction Hypothesis: Technologically advanced species destroy themselves shortly after discovering atomic energy.
• The Zoo Hypothesis: Earth is preserved as a wildlife preserve by advanced civilizations.

Hart finds these hypotheses insufficient because they fail to explain why *all* potential extraterrestrial civilizations would adhere to such a behavior across vast timescales and diverse evolutionary paths. He argues that sociological theories must be based on evidence, and the only evidence available (human history) shows a species that has explored and colonized its globe, not one that has exclusively focused on spiritual matters or self-destruction. Furthermore, he contends that no scientific theory predicting such universal extraterrestrial behavior is likely to be accepted by social scientists.

3. Temporal Explanations

This category suggests that extraterrestrials have not reached Earth simply because they have not had enough time. Hart estimates that if a civilization embarked on space exploration, it could traverse most of the Galaxy within 650,000 years (or double that if voyage time equals colonization time). Given the Galaxy's age of approximately 10^10 years, for this explanation to hold, it would require a remarkable coincidence: that the first species capable of interstellar travel arose very early, and the second species (humans) arose less than one 'time-unit' (defined as 2x10^9 years) later. Hart deems this highly unlikely.

4. Perhaps They Have Come

• This category includes hypotheses where visitors arrived but did not stay or are still present.
• Recent Visitors (within 5000 years): Proponents point to archaeological finds, but this fails to explain why Earth wasn't visited earlier. If they could visit earlier, a sociological explanation is needed, facing the same issues as above. If they visited as soon as they were able, it implies a remarkable coincidence with humanity's own space age.
• Ancient Visitors (e.g., 50 million years ago): This avoids temporal coincidence but still requires a sociological explanation for why no other civilizations visited in the intervening millennia. The hypothesis that most extraterrestrials only wished to visit, not colonize, is deemed inadequate, as it would require *every* civilization to refrain from colonization.
• The UFO Hypothesis: This suggests extraterrestrials are currently present but not acknowledged by astronomers. Hart dismisses this as a denial of Fact A rather than an explanation, noting that few astronomers believe it.

Conclusions and Discussion

Hart challenges the optimistic view that intelligent life is common in the Galaxy, which is based on assumptions about planetary systems, Earth-like planets, life development, and intelligent evolution. He finds the idea of thousands of scattered advanced civilizations implausible in light of Fact A. While one or two civilizations might have destroyed themselves, it's unlikely *all* would have. He suggests that humanity might encounter a few advanced civilizations that chose not to engage in interstellar travel, but their number would be small, possibly zero.

The corollary conclusions are that an extensive search for radio messages is probably a waste of time and money, and that human descendants will likely occupy most habitable planets in the Galaxy.

Hart acknowledges that the question of why intelligent life evolved on Earth first remains open, suggesting future research in biochemistry, planetary formation, and atmospheric evolution might provide answers. However, Fact A provides strong evidence that Earth is the first civilization in the Galaxy to achieve interstellar travel, even if the reason for this priority is unknown.

Recurring Themes and Editorial Stance

The article's central theme is the Fermi Paradox, framed as "An Explanation for the Absence of Extraterrestrials on Earth." Hart's stance is rigorously scientific and skeptical of optimistic assumptions about extraterrestrial intelligence. He prioritizes empirical evidence (Fact A) and logical deduction, systematically dismantling less parsimonious or empirically unsupported hypotheses. The article advocates for a scientific approach, emphasizing the need for testable explanations and cautioning against wishful thinking in astronomical speculation. The editorial stance of the journal, publishing such a detailed and critical analysis, suggests an openness to rigorous scientific inquiry into fundamental questions about life in the universe.