In the Guest Comment in April (page 9), Frank Tipler argues that if extraterrestrial beings existed, our galaxy would be so full of their Von Neumann machine proxies that we could not have missed them. Tipler replaces Michael Hart's and James Trefil's humanoid colonizers with machines, to circumvent the costs of humanoid support and humanoid fragility. Francis Crick and Leslie Orgel replace them with cargoes of microorganisms in a revival of Svante Arrhenius' century‐old “Panspermia” hypothesis.
Extraterrestrial intelligence: the debate continues. A biologist looks at the numbers
Leonard Ornstein; Extraterrestrial intelligence: the debate continues. A biologist looks at the numbers. Physics Today 1 March 1982; 35 (3): 27–31. https://doi.org/10.1063/1.2914965
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