On the detectability of intelligent civilizations in the galaxy

Abstract

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, ??, 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 ?? < 10 6 years, the average separation of "contactable" civilizations, (r) > 5 ? 10 3 light years so that one star in 10 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.

Fermi's paradox: The last challenge for copernicanism?

“…If we accept Copernicanism, than within reasonable temporal and physical constrains, we expect the status of evolution on Earth to reflect the Galactic average for given age of our habitat. This is exactly the rationale for the assumption (widely used in the orthodox SETI; e.g., Shklovskii and Sagan 1966, Tarter 2001, Duric and Field 2003 that most of the members of the hypothetical "Galactic Club" of communicating civilizations are significantly older than ours. 6 This applies to the future as well -the status of extraterrestrial biospheres older than the Earth reflects, on the average, the future status of the terrestrial biosphere.…”

“…) and it has demonstrated quite a strong resilience to dramatic changes in other fields of learning over the past four decades. Several issues touched upon in this review strongly indicate that the conventional SETI (Tarter 2001, Duric andField 2003, andreferences therein), as exemplified by the historical OZMA Project, as well its current counterparts such as META, ARGUS, Phoenix, SERENDIP/Southern SERENDIP -and notably those conveyed by NASA and the SETI Institute -are fundamentally flawed. Some of the alternatives have existed for quite a long time, starting with the seminal paper by Dyson (1960) and elaborated in Dyson (1966) and Ćirković and Bradbury (2006).…”

Astrobiological Phase Transition: Towards Resolution of Fermi’s Paradox

“…There is something deeply unsatisfactory about their answer. It is especially disappointing to encounter it after a lot of displayed mathematical analysis by the authors, and keeping in mind by now about half a century of sustained and often carefully planned and executed SETI efforts (e.g., Duric and Field 2003). In view of this council of despair, it is not surprising to notice both the disenchantment of some SETI pioneers (Shklovsky, von Hoerner) and the obvious eagerness with which they endorsed the pessimistic, mandatory self-destruction hypothesis.…”

Broadcasting but not receiving: density dependence considerations for SETI signals

“…In [8] Duric and Field also calculate the average density of civilizations based on a time available for intelligible contact and come up with an average distance between CCs quantitatively similar to equation 13 where . If 2 t ⩽ L then reciprocal contact is likely on average assuming a CC immediately detects a signal on arrival and immediately sends a response back which arrives before the first CC dies out.…”