On the Detectability of Intelligent Civilizations in the Galaxy

In this paper we argue for the possibility that even in the event of a Galaxy teeming with extraterrestrial intelligence (ETI) the probability of receiving recognizable signals from the ETIs may be very low. There are two majors factors that may limit our ability to detect other civilizations. (i) Evolutionary mismatches may cause difficulties analogous to humans attempting to communicate with lower primates. (ii) Independent evolutionary paths resulting from differing planetary/stellar environments may result in life whose cognitive processes and consequent perceptions of the universe are very different from ours. Interpreting signals from such civilizations may prove a very difficult or even futile task. Even on Earth, an example of a cognitive mismatch is that between humans and dolphins, where evolution in very different environments has led to difficulty in establishing communication between these two species. The main effect of the second factor is to limit communication while the effect of the first is to constrain what communication is possible to a ”window of opportunity”, a finite period of time, τω, when communication may be possible before diverging evolution makes it impossible. For example, if the number of ETIs in the Galaxy is one million and if τω < 10 years, the average separation of ”contactable” civilizations, 〈r〉 > 5 × 10 light years so that one star in 10 harbors such a civilization. If the above arguments are correct we reach the following conclusions. • The absence of detected signals does not translate into an absence of ETI’s. • Targeting individual stars in the search for ETI has a low probability of success. • The use of radio signals is of limited value because with such large separations between ”contactable” civilizations interstellar scintillation strongly limits the propagation of radio signals. Similarly, optical communication would be hindered by interstellar extinction. • Possible alternatives to current searches for narrow band signals include listening for modulated broadband signals, searching for narrow-band signals in the microwave/FIR spectrum and searching for evidence of artificially processed environments. All such searches would need to be ”all sky” to have a reasonable chance of success.