View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Directory of Open Access Journals Serb. Astron. J. } 178 (2009), 1 - 20 UDC 52{37 DOI: 10.2298/SAJ0978001C Invited review FERMI'S PARADOX { THE LAST CHALLENGE FOR COPERNICANISM? M. M. Cirkovi¶c¶ 1;2 1Astronomical Observatory, Volgina 7, 11060 Belgrade 38, Serbia 2Department of Physics, University of Novi Sad, Trg Dositeja Obradovi¶ca4, 21000 Novi Sad, Serbia E{mail: [email protected] (Received: May 23, 2009; Accepted: May 23, 2009) SUMMARY: We review Fermi's paradox (or the "Great Silence" problem), not only arguably the oldest and crucial problem for the Search for ExtraTerrestrial Intelligence (SETI), but also a conundrum of profound scienti¯c, philosophical and cultural importance. By a simple analysis of observation selection e®ects, the correct resolution of Fermi's paradox is certain to tell us something about the future of humanity. Already more than three quarters of century old puzzle { and a quarter of century since the last major review paper in the ¯eld by G. David Brin { has generated many ingenious discussions and hypotheses. We analyze the often tacit methodological assumptions built in various answers to this puzzle and attempt a new classi¯cation of the numerous solutions proposed in an already huge literature on the subject. Finally, we consider the rami¯cations of various classes of hypotheses for the practical SETI projects. Somewhat paradoxically, it seems that the class of (neo)catastrophic hypotheses gives, on the balance, the strongest justi¯cation to optimism regarding our current and near-future SETI e®orts. Key words. Astrobiology { Extraterrestrial intelligence { Galaxy: evolution { His- tory and philosophy of astronomy If you do not expect the unexpected, you will not ¯nd it; for it is hard to be sought out and 1. INTRODUCTION: WHERE di±cult. IS EVERYBODY? Heraclitus of Ephesus (cca. 500 BC) Fermi's paradox (henceforth FP) presents ar- guably the biggest challenge for any practical SETI How many kingdoms know us not! acitivity as well as the least understood of "grand Blaise Pascal, Thoughts (cca. 1660) questions" posed in the history of science. As is well-known and established by the research of Jones (1985), the key argument follows a lunchtime remark What's past is prologue... of the great physicist, Enriko Fermi: "Where is ev- William Shakespeare, erybody?" First discussed in print by the Russian The Tempest, II, 1 (1610-11) space science pioneer Konstantin Eduardovich Tsi- olkovsky, and in the last decades elaborated in detail 1 M. M. CIRKOVI¶ C¶ by Viewing, Hart, Tipler and others (for detailed interstellar distances (Dyson 1960, Sagan and Walker reviews see Brin 1983, Webb 2002), the argument 1966, Freitas 1985, Harris 1986, 2002, Zubrin 1995, presents a formidable challenge for any theoretical Timofeev et al. 2000, Arnold 2005). In words of the framework assuming naturalistic origin of life and great writer and philosopher Stanislaw Lem, who au- intelligence. As such, this should worry not only a thored some of the deepest thoughts on this topic, small group of SETI enthusiasts, but challenges some Fermi's paradox is equivalent to the "absence of cos- of the deepest philosophical and cultural foundations mic miracles" or the Silentium Universi ("cosmic si- of the modern civilization. It is hard to conceive a lence"; Lem 1977, 1984). Following the classic review scienti¯c problem more pregnant and richer in mean- by Brin (1983), we may introduce "contact cross- ing and connection with the other "big questions" of section" as a measure of the probability of contact science throughout the ages. In addition, it presents { by analogy with introduction of cross-sections in a wonderful opportunity for public outreach, popu- atomic and particle physics { and reformulate FP as larization and promotion of astronomy, evolutionary the question why this cross-section in the Milky Way biology, and related sciences. at present is so small in comparison to what could Tsiolkovsky, Fermi, Viewing, Hart, and their be naively expected. followers argue on the basis of two premises: Schematically, Fermi's paradox can be repre- (i) the absence of extraterrestrials in the Solar sented as System ("Fact A" of Hart 1975), and (ii) the fact that they have had, ceteris spatiotemporal scales of the Galaxy + the absence paribus, more than enough time in the history of of detected extraterrestrial civilizations (+ additional Galaxy to visit, either in person or through their assumptions) ! paradoxical conclusion. conventional or self-replicating probes. Here, under spatiotemporal scales we include our Characteristic time for colonization of the Galaxy, understanding of the age of the Galaxy, the Solar according to these investigators, is what we shall call System and the ages (incompletely known) of other the Fermi-Hart timescale (Hart 1975, Tipler 1980): planetary systems in the Milky Way. The additional 6 8 assumptions can be further explicated as tFH = 10 ¡ 10 years; (1) additional assumptions = "naive realism" + nat- making the fact that the Solar System is (obviously) uralism + Copernicanism + gradualism + non- not colonized hard to explain, if not for the total exclusivity. absence of extraterrestrial cultures. It is enough for our purposes to content that this timescale is well- These assumptions are quite heterogeneous. By de¯ned, albeit not precisely known due to our ig- "naive realism" we denote the working philosophy norance on the possibilities and modes of interstellar of most of science (as well as everyday life), imply- travel. For comparison, the accepted age of the Earth ing that there is a material world out there, com- as an object of roughly present-day mass is (All`egre posed of objects that occupy space and have prop- et al. 1995) erties such as size, mass, shape, texture, smell, taste and colour.2 These properties are usually perceived correctly and obey the laws of physics. In the speci¯c 9 t© = (4:46 § 0:02) £ 10 years: (2) case of FP, the basic premise following from naive re- alism is that there are, indeed, no traces of extrater- The drastic di®erence between the timescales in (1) restrial intelligent presence detected either directly and (2) is one of the ways of formulating Fermi's or indirectly ("Fact A" of Hart 1975). We shall dis- paradox. In the next section, we shall see that there cuss below some of the hypotheses for resolving FP is still more serious numerical discrepancy in play, which directly violate this realist view; an extreme when we account for the distribution of ages of ter- example { but powerfully present in pop-culture { of restrial planets in the Milky Way. such naively anti-realist standpoint is a view that, Even more generally, we need not consider the contrary to scienti¯c consensus, some humans are in direct physical contact between an extraterrestrial contact with extraterrestrial visitors and are conspir- civilization and Earth or the Solar System (insofar ing with them (e.g. Barkun 2003). Naive realism and as we do not perceive evidence of extraterrestrial vis- naturalism (Section 4 below) are methodological as- its in the Solar System; however, this is still an act sumptions, usually used in any scienti¯c research. of faith, considering the volume of space comprising Copernicanism and gradualism are somewhat more our planetary system1). It is su±cient to consider speci¯c tenets, stemming more from our experiences a weaker requirement: namely that no extraterres- in the history of physical science than from the gen- trial civilizations are detectable by any means from eral epistemology. Copernicanism (often called the Earth at present. This includes the detectability of Principle of Mediocrity) in narrow sense tells us that astroengineering or macroengineering projects over there is nothing special about the Earth or the So- 1In view of this circumstance, it is occasionally suggested that we also need a Search for ExtraTerrestrial Artifacts (SETA) programs as well (Freitas and Valdes 1980, Arkhipov 1996, 1997). Although we neglect this possibility in the further consid- erations in this text it worth noticing that this is a special case of a more generally understood unorthodox SETI programs which we consider in the concluding section. 2Philosophical literature often calls this view direct realism or common sense realism. 2 FERMI'S PARADOX { THE LAST CHALLENGE FOR COPERNICANISM? lar System or our Galaxy within large sets of sim- ery of Earth-like extrasolar planets, envisioned ilar objects throughout the universe. In somewhat by the new generation of orbital observatories. broader sense, it indicates that there is nothing par- In addition, this relative wealth of planets de- ticularly special about us as observers: our temporal cisively disproves old cosmogonic hypotheses or spatial location, or our location in other abstract regarding the formation of the Solar System spaces of physical, chemical, biological, etc., param- as a rare and essentially non-repeatable oc- eters are typical or close to typical.3 Gradualism, currence, which have been occasionally used on the other hand, is often expressed as the motto to support skepticism on issues of extraterres- that "the present is key to the past" (with corollary trial life and intelligence. that "the past is key to the future"). This paradigm, ² Improved understanding of the details of emerging from geological science in the 19th cen- chemical and dynamical structure of the tury with the work of Charles Lyell { and expanding, Milky Way and its Galactic Habitable Zone through Lyell's most famous pupil, Darwin, into life (GHZ; Gonzalez et al. 2001, Pe~na-Cabrera sciences - - has been subject of the ¯erce criticism in and Durand-Manterola 2004, Gonzalez 2005). the last quarter of century or so. We shall return to In particular, the important calculations of this issue in Section 7.