Carl Sagan and William Newman, in their 1983 paper, excoriate the solipsistic view of extraterrestrial intelligence (ETI) championed by Frank Tipler in his 1980 papers and seek to persuade the reader intelligent life exists outside Earth. The Tipler series argued that humanity was the only intelligent species in the Universe. Tipler surmised that if intelligent life were capable of interstellar travel and colonization, there should be evidence. Tipler presented a reduction ad absurdum argument (see Gray 2015) in that, for the conclusion to be valid, every statement going into the argument must true, even those not necessarily known to be true. One of the first concepts addressed in the rebuttal is that of the “von Neumann machines”. von Neumann machines are self-replicating probes (vaguely reminiscent of Bracewell probes) which Tipler argued an advanced civilization would release. For Tipler, the lack of von Neumann machines indicated the lack of ETI. Sagan and Newman saw no problem with the lack of probes and instead reworked the argument presented by Tipler. Due to the programming adherence of self-replicating automata, the Galaxy or even entire parts of the Universe could be reworked. Sagan and Newman argued that a lack of such changes was in line with Tipler’s argument against SETI, but they went further and speculated any advanced civilization would fear these machines as they could ultimately destroy their creators in their inherent need to replicate.
After dismissing the potential dangers of exponentiating automata, the rebuttal focuses on the feasibility of galactic colonization as one of the “more ‘conventional’ scenarios of biological and mechanical beings”. Sagan and Newman map microscopic models of colonization to macroscopic scales with a nonrandom dispersal model that behaved as a Markov process for long emigration time scales and a nonlinear partial differential equation for shorter time scales. The assumptions going into this were that (i) colonization required the large populations of a society and not a colony, (ii) colonies would be established in worlds devoid of life, and (iii) colonization would be focused near the civilization. These assumptions, while perhaps sound from an anthropocentric perspective, must hold true for the remainder of the calculation to be valid. The authors claim the selected model “describes all but the mathematically most catastrophic situations, and different colonization scenarios are reduced to the selection of numerical parameters”. These parameters were rejected by Tipler with his own approximation of a colonization velocity that would result in colonization of Earth within 1000 years. The authors argue that Tipler’s assumptions were wrong and instead the rate should be adopted for values after the 1700s. Finally, Sagan and Newman address the lack of colonization might be a result of the short lifetime of advanced civilizations, particularly if they aggressively colonize, as they could self-destruct with of weapons of mass destruction. On the opposite extreme, civilizations that reach a quasi-permanence may be inherently peaceful and naturally avert colonization.
To this blogger, Sagan and Newman present a logical argument with a foundation of fiction bolstered by knowledge of human history. This does not detract from the efficacy of the message within the paper. If anything, the proclamation that “the Milky Way Galaxy is teeming with civilizations as far beyond our level of advance as we are beyond the ants, and paying us about as much attention as we pay to the ants” provides significant support to the search for ETI (SETI). While there is little scientific merit in this argument, the political repercussions are of great consideration. It is no surprise this article was released around the time NASA terminated initial funding for a SETI programme (see Garber, 1999). Sagan and Newman form a strong logical basis to dismiss the pessimism of Tipler while providing order of magnitude calculations that would warrant the funding of another SETI programme. Soon after this publication, Sagan talked one of the staunchest ETI doubters, Senator Proxmire, to convince him of the scientific merit and potential existence of life elsewhere. While a scientific critique of this paper falls flat, the sound logic and calculations suggesting intelligent life could exist without galactic hegemony would certainly rebuff ETI deniers and potentially foment future inquiries into SETI projects.