As a matter of course, any astronomer has an intellectual understanding that Carl Sagan was the face of popular astronomy for many years. But as someone who was born the year before his death, I never got to experience that era first-hand. Now, reading his writing, I’m struck by the gorgeous simplicity of his arguments and the philosophical grace of his writing, and I understand more readily the “legacy” (if you will) of his name.
All of this to say: I absolutely adore the first paragraph of his 1982 paper with William Newman, “The Solipsist Approach to Extraterrestrial Intelligence”. I wrote a similar paragraph to kick off my undergraduate honor’s thesis, but this was the poetic distillation of the argument that I was aiming toward (and definitely didn’t reach).
The main argument of the paper is a contradiction to Tipler’s “solipsistic” argument that if there were other intelligent beings in the universe, we would’ve seen von Neumann machines in our own solar system; we haven’t so there aren’t. Sagan and Newman correct some of the more optimistic values in Tipler’s order of magnitude calculation. This originally made me very happy, as I felt rather uncomfortable with Tipler’s isotropically expanding, quickly replicating, unsupervised von Neumann machine argument in the paper we read for Tuesday.
The last part of the paper is a more philosophical argument than a mathematical one. I had never actually considered the “intrinsic instability of societies devoted to an aggressive galactic imperialism”. I always found that the “they blew themselves up” solution to the Fermi paradox felt too convenient: did they ALL blow themselves up??? But considering Sagan and Newman’s reasoning, I actually changed my mind about that line of argument. They contend that aggressive, imperialistic societies, like those of Colonial European times, don’t actually survive to become galactic powers because of the deadly combination of infighting + nuclear weapons. It is possible to survive such a stage, but only if you’re a group that is “pre-adapted to live with other groups in mutual respect” through (essentially) Darwinian means. So the ones that remain will be respectful and probably a little more hesitant to run around randomly colonizing or unleashing hostile von Neumann probes on the galaxy. It’s an interesting idea, though it would probably be dangerous idea to apply as a blanket statement.
It ends, in Sagan-like fashion, with a caution that the question of ETI can never be resolved without an actual observational program / search, and no philosophical arguments or hypothetical calculations can be a substitute for that real, quantitative effort.