A Hard Look at SETI and the Drake Equation

by Paul Gilster on October 20, 2005

The famous Drake Equation was developed as a way to estimate how many technological civilizations might exist and thereby be targets for SETI research. Conceived in 1961 as astronomer Frank Drake worked at the National Radio Astronomy Observatory (Green Bank, WV), the equation exists in a variety of forms depending on which authors you consult (see, for example, this SETI Institute discussion of the equation). But all variants draw on the same idea: to study extraterrestrial civilizations, you must consider such factors as:

  • the mean rate of star formation in the Galaxy;
  • the fraction of stars that can support life;
  • the fraction of stars that have planetary systems;
  • the number of planets per system with conditions suitable for life;
  • the number of planets where life does originate and evolve;
  • the fraction of planets where intelligent life forms develop;
  • the fraction of planets where intelligent life develops technology;
  • and a final, crucial measure:

  • the mean lifetime of a technological civilization
  • The Drake Equation has had such currency as a way of estimating extraterrestrial civilizations that it deserves further scrutiny in light of current work, which is just what it receives in Milan Ćirković’s 2004 paper “The Temporal Aspect of the Drake Equation and SETI (Astrobiology Vol. 4 No. 2, pp. 225-231. Ćirković (Astronomical Observatory of Belgrade) is rapidly becoming a preeminent analyst of issues involving SETI and the Fermi Paradox — we looked at his recent JBIS study “Permanence — An Adaptationist Solution to Fermi’s Paradox?” in an earlier post. In the paper in question, he examines the Drake Equation from the standpoint of its unwarranted assumptions and lack of temporal structure; the equation, Ćirković argues, fails to take into account evolutionary processes that profoundly alter any conclusions we might draw from it.

    These issues are important because they affect the kind of time scales we might expect a sustained SETI effort to need before it actually detects either extraterrestrial signals or artifacts of an alien civilization. We’ve come a long way from the easy optimism of the 1970s, when some believed there might be as many as a billion technological civilizations in the Milky Way. In fact, a school of what Ćirković calls ‘contact pessimists’ (think Frank Tipler, for example, or Michael Hart) has arisen, one pointing out that self-reproducing von Neumann probes could visit all solar systems in the galaxy within a period that is only a minute fraction of the age of the galaxy itself. Can we counter such pessimism with reasons for continuing the search?

    We know from recent work by Charles Lineweaver and others that Earth-like planets around other stars in the galactic habitable zone should be, on average, 1.8 billion years older than our own planet. And this is only an average; Ćirković points out that there are likely to be inhabitable worlds in the galaxy as much as 3 billion years older than the Earth. The small number of oldest and most advanced societies is likely to dominate any galactic civilization, meaning we might expect to encounter civilizations significantly older than the 1.8 billion year average, cultures with whom communication seems unlikely. From the paper:

    “Remember that 1 Gyr ago the appearance of even the simplest animals on Earth lay in the distant future [Ediacaran fauna -- a kind of fuse on the famous Cambrian Explosion -- is now being dated at 'only' 565-543 Myr before the present...] Thus, the set of the civilizations interesting from the point of view of SETI is not open in the temporal sense, but instead forms a ‘communications window,’ which begins at the moment the required technology is developed…and is terminated either through extinction of the civilization or through its passing into the realm of ‘supercivilizations’ unreachable by our primitive SETI means…”

    Thus one shortcoming of the Drake Equation is that we would need to add a term to it corresponding to the duration of this ‘communications window,’ and establishing its ratio to the larger value of the lifetime of a technological civilization. The net effect of this would surely be to reduce the number of civilizations we would be likely to encounter. But there is a corresponding sense of optimism, as the author notes: “Fortunately (from the SETI point of view) this is not the only evolutionary bias hidden in the Drake equation.” And some of these biases may actually work in our favor.

    Ćirković’s paper is rich enough that I want to return to it tomorrow to discuss what these biases are and how they affect the result. The Lineweaver reference above appears in an earlier post here, but for those who need it, “An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect” ran in Icarus Vol. 151, No. 2 (2001), pp. 307-313 (available here in PDF form), and was followed by a paper in collaboration with Yeshe Fenner and Brad Gibson titled “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way.” The latter ran in Science Vol. 303 (2004), pp. 59-62, and is also available as a PDF online.

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