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.U ovom radu se iznose argumenti u prilog mogucnosti da je, cak i u slucaju da nasa Galaksija vrvi od vanzemaljskih inteligencija (VZI), verovatnoca prijema prepoznatljivih signala vestackog porekla veoma niska. Postoje dva glavna faktora koja ogranicavaju nasu sposobnost da detektujemo druge civilizacije. (i) Evolutivne razlike mogu uzrokovati teskoce analogne onima koje postoje u ljudskim pokusajima komunikacije sa nizim primatima. (ii) Nezavisni evolutivni putevi koji proizlaze iz razlicitih planetarnih/zvezdanih okruzenja mogu proizvesti VZI ciji su kognitivni procesi i proizlazeca percepcija univerzuma veoma razliciti od nasih. Interpretiranje signala takvih civilizacija moze se pokazati veoma teskim, ako ne i nemogucim zadatkom. Cak i na Zemlji, primer takvih kognitivnih razlika postoji izmedju ljudi i delfina, gde evolucija u veoma razlicitim okruzenjima dovodi do teskoca u uspostavljanju komunikacije izmedju dve vrste. Glavni efekat drugog faktora jeste sustinsko ogranicenje komunikacije, dok je efekat prvog ogranicenje one komunikacije koja je moguca na "prozor", tj. konacan period vremena ?? u kome komunikacija moze biti moguca pre nego sto je divergentna evolucija ucini nemogucom. Na primer, ako je broj VZI u nasoj Galaksiji jedan milion i ako ?? < 10 6 godina, prosecno rastojanje civilizacija koje bi mogle da komuniciraju je (r) > 5 ? 10 3 svetlosnih godina, tako da tek jedna zvezda u 10 10 predstavlja staniste takve civilizacije. Ako su gornji argumenti korektni, dolazimo do sledecih zakljucaka: Odsustvo detektovanih signala ne znaci i odsustvo VZI. Posmatranje pojedinacnih zvezda u SETI projektima ima veoma malu verovatnocu uspeha. Koriscenje radio signala je od ogranicene vrednosti, zato sto na tako velikim rastojanjima izmedju civilizacija koje mogu da komuniciraju medjuzvezdana scintilacija snazno ogranicava sirenje radio signala. Slicno, opticka komunikacija je ometena medjuzvezdanom ekstinkcijom. Moguce alternative sadasnjim potragama za signalima uskog opsega ukljucuju osluskivanje modulisanih signala velikog opsega, potraga za signalima uskog opsega u mikrotalasnom/FIR delu spektra i potraga za tragovima vestacki procesiranih astrofizickih okruzenja. Sve takve potrage trebalo bi da se vrse preko citavog neba da bi imale razumne izglede na uspeh.