1990 00 00 QJR. Astronomica, V 31 - Cosmology, Extraterrestrial Intelligence, and a Resolution of the Fermi-Hart Paradox - Wesson

Magazine Overview

This document is a scientific article titled "Cosmology, Extraterrestrial Intelligence, and a Resolution of the Fermi-Hart Paradox," authored by Paul S. Wesson and published in the Quarterly Journal of the Royal Astronomical Society (Q. Jl R. astr. Soc.), Volume 31, Issue 2, in 1990. The article was received on October 27, 1989, and in its original form on April 3, 1989. Wesson is affiliated with the Department of Physics at the University of Waterloo, Ontario, Canada.

Summary of the Article

The article addresses the Fermi-Hart paradox, which questions why, if extraterrestrial civilizations are likely to exist, we have no evidence of them. Wesson considers cosmological constraints on receiving signals from such civilizations. He reviews evidence suggesting that life in the Universe may be sparse. A key argument is that the nearest civilization could be beyond our cosmological (particle) horizon, making communication impossible. This "cosmological resolution" implies that extraterrestrials may exist but cannot be contacted.

Introduction

The introduction highlights the significance of the absence of extraterrestrials on Earth for the search for other intelligent beings. It references Enrico Fermi's famous question about the whereabouts of extraterrestrials and Michael Hart's 1975 study suggesting Earth might be the only advanced civilization in the Galaxy. The paradox arises because a technologically advanced race could colonize the Milky Way quickly, yet no evidence is seen. Furthermore, the existence of life on Earth suggests it should exist elsewhere, but no intelligent signals have been detected. The paradox is taken seriously because even with a tiny probability of life evolving, an infinite universe would imply numerous extraterrestrials. The article aims to provide a resolution by evaluating astrophysical and biophysical data.

Cosmological Constraints on Receiving Signals from Extraterrestrials

Attempts to detect extraterrestrial signals have focused on stars, but the article argues for looking at galaxies. It presents an evolutionary argument: if many extraterrestrial races exist and can signal over distances of 100 light-years, some should be advanced enough to signal across the Galaxy (10^5 light-years). The argument extends to other galaxies, noting that civilizations in the Local Group are only 10 times further away, and those in distant galaxies could communicate over intergalactic distances (10^7 light-years). Despite this, most researchers focus on signals from our own galaxy. The article posits that numerically, looking for signals from galaxies is as sensible as from stars, especially since a single galaxy might contain numerous civilizations.

The core of the argument lies in the finite age of the Universe, even if it is spatially infinite. The lookback time to observe distant objects is limited by the speed of light and the age of the Universe (approximately 15 x 10^9 years). This creates imaginary surfaces, or horizons, that delimit our view. The particle horizon, at about 16 x 10^9 light-years, represents the furthest we can see. If extraterrestrial civilizations exist only around stars in normal galaxies, we cannot hope to receive signals from further than about 15 x 10^9 light-years. Even for hypothetical life forms in primeval gas, the limit is about 16 x 10^9 light-years. This establishes a fundamental cosmological limit for detecting life using photons.

The article quantifies this by estimating that within the horizon distance of 16 x 10^9 light-years, there are about 10^10 galaxies. If each galaxy has 10^10 habitable planets, the total number of potential signal sources is around 10^20. For us to receive signals, the probability of life evolving on a habitable planet must be greater than 1 in 10^20.

Biophysical Probability of Life's Evolution

This section shifts to biophysics, discussing the difficulty in quantifying the probability of life's origin. While the existence of life on Earth might suggest it's common, this is questioned as potentially anthropocentric. The critical step from abiological molecules to a one-celled organism is not well understood. The organization of life through information-carrying molecules like nucleic acids remains a major challenge. Studies by Argyle (1977) and Hart (1982) are cited. Argyle's model suggests that random interactions on a prebiotic Earth would produce very little information, far less than required for even a simple virus, let alone a bacterium. Even with 10^9 Earth-like planets in the Milky Way, the potential for random information generation is limited. For 10^20 habitable planets within our cosmological horizon, the potential information content is only 261 bits.

Hart (1982) considered Earth-like planets and concluded that uncertainties in astrophysical parameters are overshadowed by the uncertainty in the biophysical probability of life's evolution. He calculated the probability of forming a single strand of DNA by random interactions over 10^10 years to be only 1 in 10^30, and for a viable organism, potentially as low as 1 in 10^3000. Even with optimistic estimates, life should not exist on other habitable planets within the observable Universe than Earth.

Similar conclusions are drawn by other researchers, suggesting that based on chance statistics and standard cosmology, life should exist only on Earth within the observable Universe.

Conclusion and Discussion

The article concludes that if extraterrestrial intelligent civilizations exist, we could receive signals from stars and galaxies, lending a cosmological perspective to the Fermi-Hart paradox. The proposed resolution combines astrophysical work (Ellis & Brundrit, Gott) and biophysical work (Argyle, Hart). The core idea is that we can only receive signals from a region of the Universe bounded by the particle horizon, and if life evolves by chance chemical reactions, it is so sparse that no planet within this horizon, other than Earth, is expected to harbor it.

This conclusion, that we are alone in the observable Universe, may be unpalatable but is presented as a result of optimistic assumptions throughout the calculation. To avoid this conclusion, one might speculate about faster-than-light communication or alternative cosmologies that modify the horizon concept. However, the article emphasizes that these are speculative. A more realistic biophysical approach suggests that if life originates in a non-random manner, the probability of its evolution could be higher, potentially leading to other intelligences within the observable Universe. However, this would reintroduce the Fermi-Hart paradox if the probability is too high, leading to a densely populated Universe.

The main objective was to resolve the paradox without speculation. If the cosmological horizon argument is correct, we are alone in the part of the Universe we can see and communicate with. However, if the Universe is spatially infinite and the probability of life's evolution is finite, then extraterrestrials must exist somewhere beyond our horizon.

Recurring Themes and Editorial Stance

The recurring theme is the Fermi-Hart paradox and its potential resolution through cosmological and biophysical constraints. The article takes a scientific and analytical stance, aiming to provide a quantitative, non-speculative explanation. It critically examines assumptions about the prevalence of life and the detectability of extraterrestrial signals, grounding its arguments in established cosmological models and biophysical probabilities, even while acknowledging the limitations and uncertainties in these fields.