In this paper, Sagan & Newman respond to the faulty conclusions derived from the solipsist worldview and offer their response to the 1980 Tipler paper entitled “Extraterrestrial Intelligent Beings do not Exist.” In that paper, Tipler argues that the probability of the emergence of another intelligent civilization in our galaxy is so low as to have not possibly have occurred, and thus we are the only such example in the Milky Way. He justifies this assertion with a variety of lines of reasoning. Firstly, that the problem of interstellar colonization can be reduced to two components, namely: a) the development of rocket propulsion technology to the modern level, and b) the development of a universal assembler or von Neumann machine. Both of these he presumes are feats that ought to appear early on in the list of accomplishments of a mature technological society. Consequently, upon mastery of these technologies, the timescale for galactic colonization would only be some 300 million years, a value that is short relative to the age of the galaxy. Secondly, he uses Drake’s Equation to suggest that the probability of the development of spacefaring civilizations is something like 10^-11 based on biological arguments, and since there are something like 10^11 stars in the galaxy, it is reasonable to think that there may only be one. In response to this, Sagan & Newman point out that the inevitability and reproduction fidelity of von Neumann machines cannot be guaranteed. It must be either that the builders can guide the proliferation of the von Neumann machines or they cannot; in the former case, they could program them to obey a non-interventionist code and avoid systems where life has emerged, and in the latter their exponentiation will continue unchecked until they the dominate the matter of the galaxy.
Sagan & Newman also revisit a calculation performed relating galactic colonization timescale to the historical colonization time across North America. This paper and their previous work taken as a unit are valuable because they represent one of the earliest attempts to model the timescale for galactic colonization. Unfortunately, the glaring flaw in their simulation was to treat the stars in the galaxy as static entities and simply allow a bubble to expand outward from a localized region, similar to molecular diffusion. This ignores the dynamical shearing that occurs with a differentially rotating galactic plane, allowing the separations between the stars to vary, accelerating the mixing and hence shortening colonization time. A new model accounting for this feature would be a valuable step towards a more realistic estimate of t_col.
Nonetheless, they remain steadfastly neutral regarding the ETI question and denounce the conclusions of the solipsist viewpoint by insisting on an experimental radio search.