The Drake Equation in its various forms has been tormenting us for decades, raising the question of how to adjust variables that range from astronomical (the abundance of terrestrial planets) to biological (the probability of life’s emergence) and even sociological (the average lifetime of a technological civilization). Wildly optimistic estimates of the number of technological civilizations in our galaxy are now giving way to more sober reflection. Now Reginald Smith (Bouchet-Franklin Institute, Rochester NY) offers up a new analysis looking at how likely radio contact is given a civilization’s lifetime, and how widely that civilization’s signals can be clearly received. The key question: What if there is a reasonable horizon for the detection of a signal from an extraterrestrial sender?

Signals and Their Lifetime

This is useful stuff, because contact depends not just upon the density of communicating civilizations (CC) but their average lifetime and the maximum detectable distance for their signals. A CC with a short lifetime is unlikely to be heard unless its neighborhood is crowded with other civilizations. But a longer living CC increases the necessary density — a culture producing a continuous signal over a span of a million years is obviously much more likely to be detected than one producing that signal over a single millennium. And if Fermi’s paradox continues to plague us, Smith would argue that perhaps it shouldn’t. From the paper:

What is most interesting about this analysis is that it demonstrates it can be possible for many CCs in the same galaxy to never contact one another. For example, even assuming the average CC has a lifetime of 1,000 years, ten times longer than Earth has been broadcasting, and has a signal horizon of 1,000 light-years, you need a minimum of over 300 CCs in the galactic neighborhood to reach a minimum density. For example, if there were only 200 CCs in our galactic neighborhood roughly meeting these parameters, probabilistically they will never be aware of each other. This finding can give pause to both those who predict no other CCs or those who predict a high number of CCs in our galactic neighborhood.

A pause for reflection is not a bad idea, given the flexibility of the values in the Drake Equation. Smith continues:

Arguing that the lack of contact signifies the lack of CCs may be tempered with the fact that if there is a signal horizon, even a galaxy replete with life may have relatively isolated CCs in the absence of interstellar travel or extremely power[ful] signals. On the other hand, high estimates of CCs in our galactic neighborhood does not guarantee that there will ever be contact between them, especially reciprocal.

From Radio to Artifact

Hold the density of technological cultures down to a certain level and contact is unlikely. Or is it? Note what this presupposes about the cultures we might expect to exist out there. Smith himself says that the constraints on contact can be circumvented by interstellar travel or automated beacons. And if we are willing to move into the realm of von Neumann probes and other self-replicating technologies, we have to deal with the possibility that a single ETI could blanket the galaxy with its sensors given enough time. Frank Tipler famously estimated the needed span to be about a million years, and argued from that that ETI does not exist.

Let’s assume for the sake of argument that there are no von Neumann probes. If this were the case, given that we believe a sufficiently advanced civilization ought to be able to build them, it seems to suggest that ETI would have universally chosen not to build them because, as Carl Sagan once speculated, such probes would constitute so potent a viral-like menace that no sane species would introduce them. The other possibility, far starker, is that no civilization can survive long enough to reach the stage of technology needed to produce them. For in terms of physics, a self-reproducing von Neumann probe appears to be feasible, assuming we or our hypothetical ETI develop the tools to the needed point without turning them on ourselves.

Statistical Approaches to Drake

I’m wandering far from Smith’s original point, which is to show that variables like short lifetimes or small maximum distances for communications could mask the presence of other technological cultures. We’re left to wrestle with the implications that follow from this. The paper is Smith, “Broadcasting but not receiving: density dependence considerations for SETI signals,” submitted to the International Journal of Astrobiology and available online. Also significant here is Claudio Maccone’s “The Statistical Drake Equation,” which was presented at the 2008 International Astronautical Congress held last year in Glasgow.

Maccone has re-worked the Drake values by taking what had been single value estimates and converting them into statistical form, with intriguing results. And while I don’t want to turn this into a bibliography, I do need to mention Duncan Forgan’s “A Numerical Testbed for Hypotheses of Extraterrestrial Life and Intelligence,” accepted by the International Journal of Astrobiology and available here. Working with statistical treatments of SETI’s key parameters is opening up new insight into their uses. Both the Maccone and Forgan papers have been on the back burner here for longer than I intended, an issue I hope to remedy soon.