Fermi's Paradox

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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 scientific, philosophical and cultural importance. By a simple analysis of observation selection effects, 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 field 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 classification of the numerous solutions proposed in an already huge literature on the subject. Finally, we consider the ramifications 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 justification to optimism regarding our current and near-future SETI efforts.

Key words. Astrobiology – Extraterrestrial intelligence – Galaxy: evolution – His- tory and philosophy of astronomy

If you do not expect the unexpected, you will not find it; for it is hard to be sought out and 1. INTRODUCTION: WHERE difficult. 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 scientific 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 defined, 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 specific 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 difference 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 scientific 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 scientific research. of faith, considering the volume of space comprising Copernicanism and gradualism are somewhat more our planetary system1). It is sufficient to consider specific 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, • 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 fierce 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. Lineweaver (2001; Lineweaver, Fenner and Finally, the role of non-exclusivity (or ”hard- Gibson 2004) show that Earth-like planets be- ness” in some of the literature) assumption needs to gan forming more than 9 Gyr ago, and that be elucidated. Non-exclusivity (following Brin 1983) their median age is hti = (6.4 ± 0.7) × 109 is simply a principle of causal parsimony applied to yrs, significantly more than Earth’s age. This the set of hypotheses for resolving FP: we should pre- means that the age difference fer those hypotheses which involve a smaller number 9 of local causes. FP is eminently not resolved by pos- hti − t⊕ = (1.9 ± 0.7) × 10 years, (3) tulating that a single old civilization self-destructs in a nuclear holocaust. FP is resolved by hypothesizing is large in comparison with the Fermi-Hart that all civilizations self-destruct soon after develop- timescale in (1). This also means that not only ing nuclear weapons, but the major weakness of such the oldest ones, but a large majority of habit- a solution is obvious: it requires many local causes able planets are much older than Earth. The acting independently in uniform to achieve the de- significance of this result cannot be overstated, sired explanatory end. In other words, such solu- since it clearly shows that the naive natural- tion is exclusive (or ”soft”). As long as we have any ist, gradualist and Copernican view must be choice, we should prefer non-exclusive (or ”hard”) wrong, since it implies that millions of plan- solutions, i.e., those which rely on small number of ets in the Milky Way are inhabited by Gyr-old independent causes. For instance, the hypothesis, we supercivilizations in clear contrast with obser- shall discuss in more detail below, that a γ-ray burst vations. can cause mass extinction over a large portion of the • Confirmation of the rapid origination of life Galaxy and thus arrest evolution toward advanced on early Earth (e.g. Mojzsis et al. 1996); this technological society, is quite non-exclusive. rapidity, in turn, offers a strong probabilistic support to the idea of many planets in the 2. RECENT DEVELOPMENTS Milky Way inhabited by at least the simplest lifeforms (Lineweaver and Davis 2002). Fermi’s Paradox has become significantly • Discovery of extremophiles and the general re- more serious, even disturbing, of late. This is due to sistance of simple lifeforms to much more se- several independent lines of scientific and technolog- vere environmental stresses than it had been ical advances occurring during the last two decades: thought possible earlier (Cavicchioli 2002). • The discovery of nearly 350 extrasolar planets These include representatives of all three great so far, on an almost weekly basis (for regu- domains of terrestrial life (Bacteria, Archaea, lar updates see http://exoplanet.eu/). Al- and Eukarya), showing that the number and though most of them are ”hot Jupiters” and variety of cosmic habitats for life are probably not suitable for life as we know it (some of much larger than conventionally imagined. their satellites could still be habitable, how- • Our improved understanding in molecular bi- ever; cf. Williams et al. 1997), many other ology and biochemistry leading to heightened exoworlds are reported to be parts of systems confidence in the theories of naturalistic origin with stable circumstellar habitable zones (No- of life or biogenesis (Lahav et al. 2001, Ehren- ble et al. 2002, Asghari et al. 2004, Beaug´e freund et al. 2002, Bada 2004). The same can et al. 2005). It seems that only the selec- be said, to a lesser degree, for our understand- tion effects and capacity of present-day in- ing of the origin of intelligence and technolog- struments stand between us and the discov- ical civilization – which we shall henceforth label noogenesis – (e.g. Chernavskii 2000).

3Note that this does not mean that our locations in these spaces are random. The latter statement is obviously wrong, since a random location in configuration space is practically certain to be in the intergalactic space, which fills 99.99...% of the volume of the universe. This is a sort of a long-standing confusion and the reason why Copernicanism is most fruitfully used in conjuction with some expression of the observational selection effects, usually misleadingly known as the anthropic principle; for detailed treatment see Bostrom 2002. 3 M. M. CIRKOVI´ C´

• Exponential growth of the technological civ- above shows, parenthetically, that quite widespread ilization on Earth, especially manifested (especially in popular press) notion that there is through Moore’s Law and other advances in nothing new or interesting happening in SETI stud- information technologies (see, for instance, ies is deeply wrong. Schaller 1997, Bostrom 2000). This is closely In the rest of this review, we survey the al- related to the issue of astroengineering: the ready voluminous literature dealing with Fermi’s energy limitations will soon cease to constrain Paradox, with an eye on the classification scheme human activities, just as memory limitations which could help in understanding many hypothe- constrain our computations less than they ses posed in this regard. FP is fundamentally inter- once did. We have no reason to expect the twined with so many different disciplines and areas development of technological civilization else- of human knowledge, that it is difficult to give more where to avoid this basic trend. than a very brief sketch in the present format. It • Improved understanding of the feasibility of should be noted straight at the beginning that it is interstellar travel in both the classical sense not entirely surprising that several scientific hypothe- (e.g. Andrews 2003), and in the more efficient ses resolving FP have been formulated, in a qualita- form of sending inscribed matter packages tive manner, in the recreational context of a piece of over interstellar distances (Rose and Wright SF art; astrobiology is perhaps uniquely positioned 2004). The latter result is particularly im- to exert such influence upon human minds of vari- portant since it shows that, contrary to the ous bents. After all, much of the scientific interest conventional skeptical wisdom, it makes good in questions of life beyond Earth in the 20. century sense to send (presumably extremely minia- was generated by works such as Herbert G. Wells’ turized) interstellar probes even if only for the War of the Worlds, Sir Arthur Clarke’s 2001: Space sake of communication. Odyssey, or Sir Fred Hoyle’s The Black Cloud. • Theoretical grounding for various astroengi- In Fig. 1, we schematically present a ver- neering/macroengineering projects (Badescu sion of FP based upon the scenario of Tipler (1980), 1995, Badescu and Cathcart 2000, 2006, Ko- using self-replicating, von Neumann probes which, rycansky et al. 2001, McInnes 2002) po- once launched, use local resources in visited plane- tentially detectable over interstellar distances. tary systems to create copies of themselves. It is clear Especially important in this respect is the that the exponential expansion characteristic for this mode of colonization leads to the lowest values for possible combination of astroengineering and the Fermi-Hart timescales (1). It is important to un- computation projects of advanced civiliza- derstand, however, that FP is aggravated with von tions, like those envisaged by Sandberg (1999). Neumann probes, but it is not really dependent on • Our improved understanding of extragalactic them. FP would still present a formidable challenge universe has brought a wealth of information if at some stage it could be shown that interstellar about other galaxies, many of them similar von Neumann probes are unfeasible, impractical or to the Milky Way, while not a single civiliza- unacceptable for other reasons (possibly due to the tion of Kardashev’s (1964) Type III has been danger they will pose to their creators, as speculated found, in spite of the huge volume of space by some authors; see the ”deadly probes” hypothesis surveyed (Annis 1999b). in Section 7). Although admittedly uneven and partially Two further general comments are in order. conjectural, this list of advances and developments (I) Although it is clear that philosophical issues are (entirely unknown at the time of Tsiolkovsky’s and unavoidable in discussing the question of life and in- Fermi’s original remarks and even Viewing’s, Hart’s telligence elsewhere in the universe, there is a well- and Tipler’s later re-issues) testifies that Fermi’s delineated part of philosophical baggage which we paradox is not only still with us more than 75 shall leave at the entrance. Part of it is the mislead- years after Tsiolkovsky and more half a century af- ing insistence on the definitional issues. The precise ter Fermi, but that it is more puzzling and disturbing definition of both life and intelligence in general is than ever.4 In addition, we have witnessed substan- impossible at present, as accepted by almost all biol- tial research leading to a decrease in confidence in ogists and cognitive scientists. This, however, hardly the so-called Carter’s (1983) ”anthropic” argument, prevents any of them in their daily research activi- the other mainstay of SETI scepticism (Wilson 1994, ties. There is no discernible reason why we should Livio 1999, Cirkovi´cet´ al. 2009). All this is accom- take a different approach in astrobiology and SETI panied by an increased public interest in astrobiol- studies and insist on the higher level of formal pre- cision in those fields. Intuitive concepts of life and ogy and related issues (Des Marais and Walter 1999, intelligence are developed enough to enable fruitful Ward and Brownlee 2000, 2002, Webb 2002, Grin- research in these fields, in the same manner as the spoon 2003, Cohen and Stewart 2002, Dick 2003, intuitive concept of life enables research in the ter- Chyba and Hand 2005, Michaud 2007). The list

4One is tempted to add another item of a completely different sort to the list: The empirical fact that we have survived more than sixty years since the invention of the first true weapon of mass destruction gives us at least a vague Bayesian argument countering the ideas—prevailing at the time of Fermi’s original lunch—that technological civilizations tend to destroy them- selves as soon as they discover nuclear power. This is not to contest that the bigger of part of the road toward safety for humankind is still in front of us; see, e.g. Bostrom and Cirkovi´c(2008).´ 4 FERMI’S PARADOX – THE LAST CHALLENGE FOR COPERNICANISM?

It is clear, for instance, that the Darwinian evolution on Earth brought about at best a few in- telligent species5 and only one with technological ca- pacities for engaging in SETI and similar large-scale cosmic activities. In these cases, the precise defi- nition of intelligent species (much less a conscious one; see the disturbing comments of Jaynes 1990 and Raup 1992, showing that consciousness is in any case much less than what is colloquially presumed) is un- necessary; while the awareness that this might be radically different in the SETI context is desirable, we need to proceed along the same, broadly oper- ationalist lines. For this reason, we shall use the terms ”extraterrestrial intelligence”, ”intelligent be- ings”, etc. in their non-technical or vernacular mean- ing, roughly as placeholders for beings we are inter- ested in meaningfully communicating with. (II) A useful way of thinking about FP is by analogy with Olbers’ paradox in classical cosmol- ogy, which has been first elucidated by Alm´ar(1992). Both intentional signals and unintentional manifes- tations of advanced technological civilizations in FP are analogous to the light of distant stars which we would expect, on the basis of wide spatiotemporal assumptions, to flood us, terrestrial observers. That this is not happening points to some flaw in either the reasoning or the assumptions. We know now (e.g. Wesson et al. 1987) that Olbers’ paradox is resolved mainly by the fact that the stellar population of the universe is of finite age: the light simply has not had enough time to establish thermodynamical equilib- rium with the cold and empty interstellar (intergalac- tic) space. Contrary to a popular opinion – occasion- ally found even in astronomy textbooks – Hubble ex- pansion actually is almost negligible, minor effect in resolving Olbers’ paradox. FP can, in principle, also be resolved by the finite age of the stellar popula- tion (and hypothetical extraterrestrial civilizations), which would correspond to the ”rare Earth” class of hypotheses (see Section 6 below). However, FP is significantly less constrained and thus allows for Fig. 1. Fermi’s paradox in a model with slow von additional classes of explanation, as will be eluci- Neumann probes, giving a typically low Fermi-Hart dated below. But this analogy strengthens the gen- timescale for the colonization of the Milky Way. The eral analogy which exists between the current imma- relevant timescales are also shown. ture and vigorous stage of astrobiology and the state in which physical cosmology has been in 1920s and restrial biology and other life sciences; or, even more 1930s (Kragh 1996, 2007, Dick 1996, 2003). prominently and dramatically, the intuitive concept of number has enabled immensely fruitful research 3. WHAT’S PAST IS PROLOGUE in mathematics for millennia before the advent of set theory as the axiomatic foundation for modern mathematics finally enabled completely general and It has been noticed as early as the Byurakan formal definition of number (by personalities such conference (Sagan 1973) that the search for extrater- as Frege, Russell, G¨odel,Turing, Church, Kleene, restrial intelligence and the issue of the future of in- and Post; e.g. Hatcher 1982, Penrose 1989). His- telligence here, on Earth, are closely linked. If we tory of science also teaches us that formalization accept Copernicanism, than within reasonable tem- of paradigms (including precise definitions) occurs poral and physical constrains, we expect the status only at later stages of mature disciplines (Butterfield of evolution on Earth to reflect the Galactic average 1962, Kragh 1996) and there is no reason to doubt for given age of our habitat. This is exactly the ratio- that astrobiology will conform to the same general nale for the assumption (widely used in the orthodox picture. SETI; e.g. Shklovskii and Sagan 1966, Tarter 2001,

5The status of intelligence of marine mammals is still unclear (e.g. Browne 2004), while we still do not know whether undoubt- edly intelligent neanderthals were truly separate species, distinct from Homo sapiens (e.g. Hawks and Wolpoff 2001). 5 M. M. CIRKOVI´ C´

Duric and Field 2003) that most of the members of Exactly this form of ”mirroring” of whatever the hypothetical ”Galactic Club” of communicating provides the solution to Fermi’s paradox is the rea- civilizations are significantly older from ours.6 This son why some of the researchers interested in the applies to the future as well – the status of extrater- future of humanity are expressing their hopes that restrial biospheres older than the Earth reflects, on the Earth is unique in the Galaxy, at least in terms the average, the future status of the terrestrial bio- of evolving intelligent beings (e.g., Hanson 1998a, sphere. This reflects a deeper tension at the very Bostrom 2008). This would correspond to those so- heart of FP: belief in unlimited progress coupled with lutions of FP rejecting Copernicanism (see Section the Copernican assumption, lead to either contradic- 6 below), which these authors consider a lesser evil. tion or bleak prospects for our future. However, such a form of pessimism is not mandatory This is especially pertinent and disturbing in – we can have both optimism toward SETI and op- view of Fermi’s paradox. The fact that we observe timism about humanity’s future. This forms one of no supercivilizations (of Kardashev’s Type III, for the motivation for developing some of the neocatas- example) in the Milky Way in spite of plentiful time trophic solutions to FP (Section 7) which avoid this for their emergence is prima facie easiest to explain tension. by postulating the vanishing probability or impos- sibility of their existence in general. An obvious consequence is that for humanity or its descendants 4. NATURALISM AND CONTINUITY the transformation into a supercivilization is either overwhelmingly unlikely or flatly impossible. But The successes of science since the so-called the cut goes deeper both ways – if, as some disen- ”Scientific Revolution” of the 17. century (cele- chanted SETI pioneers (in particular Iosif Shklovskii brated, among other things, in the International Year and Sebastian von Hoerner; see, e.g. von Hoerner of Astronomy 2009, as 400 years since Galileo’s in- 1978 and comments in Lem 1977) argued, the rea- vention of the telescope and consequent revolution- son beyond absence of extraterrestrial signals is the ary discoveries) have led to a worldview which could prevalent self-destruction of each individual extrater- be called naturalistic, since it assumes the absence restrial civilization, for instance, through nuclear an- of supernatural forces and influences on the phenom- nihilation soon after the discovery of nuclear energy, ena science is dealing with (Kuhn 1957, Butterfield that would mean that humanity is also overwhelm- 1962). Here, as in the case of intelligence, we are us- ingly likely to self-destruct in a nuclear holocaust. ing rough, non-technical definition which is entirely If natural hazards (in form of, for example, impacts sufficient for meaningful discussion.7 by comets and asteroids or supervolcanic eruptions; One of the central issues of astrobiology is to cf. Rampino 2002) are the main culprits beyond the what extent we can talk about biogenesis (and, by ex- absence of extraterrestrials – automatically implying tension, noogenesis) in naturalistic terms. This issue that they are, on the average, more frequent than has been investigated in depth by Fry (1995, 2000), inferred from the terrestrial history thus far, which who showed that a necessary ingredient in any sci- might be a consequence of the anthropic bias (cf. entific account of biogenesis is so-called continuity Bostrom 2002, Cirkovi´c2007)´ – then we, humans, thesis: ”the assumption that there is no unbridge- have statistically bleak prospects when faced with able gap between inorganic matter and living sys- similar natural catastrophes. And the same applies tems, and that under suitable physical conditions the to whatever causative agent causes the contact cross- emergence of life is highly probable.” Adherence to section to be extremely small; for instance, if intelli- the continuity thesis, as Fry demonstrates, is a pre- gent communities remain bound to their home plan- condition for scientific study of the origin of life; con- ets in a form of cultural and technological stasis due trariwise, the views that biogenesis is a ”happy acci- to imposition of global totalitarianism which, pro- dent” or ”almost miracle” are essentially creationist, vided technological means already clearly envisioned i.e., unscientific. The classification suggested below (Caplan 2008), could permanently arrest progress, relies on this analysis of the continuity thesis and in this would mean that our own prospects of avoiding part on its extension to noogenesis.8 such hellish fate are negligible. In that sense, the The continuity thesis has been supported by astrobiological history of the Milky Way is a Shake- many distinguished scientist throughout history, but spearian prologue to study of the future of humanity. none did more to promote it than the great British

6The magnitude of the age difference has been, however, constantly underestimated even before the results of Lineweaver cited above became available. The orthodox SETI literature does not discuss the age differences of the order of Gyr, which is indicative of the optimistic bias on part of the authors. 7It might be interesting to note that Alfred Russell Wallace, co-discoverer of natural selection with Darwin, has in several regards been a precursor to the contemporary astrobiology and in particular to study of FP. Beside speculating on the life on Mars in a separate treatise, in his fascinating book Man’s Place in the Universe (Wallace 1903), preceding even Tsiolkovsky’s formulation of FP for about three decades, he argued that naturalism cannot account for the fine-tuned structure of the universe. That was perhaps the last attempt of large-scale denial of naturalism. 8Whether such an extension is legitimate, remains an open question, too difficult to be tackled here. We mention in passing that at least one of the proposed solutions discussed below – the adaptationist hypothesis of Raup (1992) and Schroeder (2002) – explicitly denies this generalization. 6 FERMI’S PARADOX – THE LAST CHALLENGE FOR COPERNICANISM?

Fig. 2. The proposed high-level classification of the solutions to FP. In an extremely simplified form, the respective replies to Fermi’s question Where is everybody? by proponents of solipsist, ”Rare Earth” and (neo)catastrophic hypotheses are ”They are here”, ”They do not exist”, and ”They have been prevented from coming yet”. Only a small subset of proposed hypotheses is shown as examples in each category. polymath John B. S. Haldane (1892-1964). In both which are—more or less successfully—”derived from his research writings in biology, mathematics, astron- nature” lose their relevance. Other important guide- omy, etc., and in philosophical essays (especially Hal- lines must be derived which will encompass the vast dane 1972 [1927]), he insisted on the continuity be- realm of possibilities stemming from the concept of tween physical (in particular cosmological), chemi- postbiological evolution. cal, biological and even cultural evolutions. Haldane was a co-author of the famous Oparin-Haldane the- ory of biogenesis, which emphasized law-like aspects 5. SOLIPSIST SOLUTIONS of the process. This was in complete accordance with his philosophical and methodological principles, and enabled him to be put down foundations of what is The label refers to a classic 1983 paper of today often called future studies as well (Clark 1968; Sagan and Newman criticizing Tipler’s (1980, 1981) Adams 2000). skepticism toward SETI studies based on Fermi’s An important novelty in comparison to the Paradox (FP) and strengthened by the idea of col- previous SETI reviews is the necessity of taking onization via von Neumann probes. Here, however, into account hitherto unrecognized possibilities, es- we would like to investigate solipsist solutions to FP pecially the Haldanian notion of postbiological evo- in a different - and closer to the usual - meaning. lution, prompted by Moore’s Law and great strides Solipsist solutions reject the premise of FP, made in the cognitive sciences. For instance, the namely that there are no extraterrestrial civilizations great historian of science Steven J. Dick (2003) co- either on Earth or detectable through our observa- gently writes: tions in the Solar System and the Milky Way thus far. On the contrary, they usually suggest that ex- But if there is a flaw in the logic of the traterrestrials are or have been present in our vicin- Fermi paradox and extraterrestrials are a nat- ity, but that the reasons for their apparent absence ural outcome of cosmic evolution, then cul- lie more with our observations and their limitations tural evolution may have resulted in a post- than with the real state-of-affairs. biological universe in which machines are the Of course, this has been for so long the predominant intelligence. This is more than province of lunatic fringe of science (either in older mere conjecture; it is a recognition of the fact forms of occultism or more modern guise of ”ufol- that cultural evolution - the final frontier of the Drake Equation - needs to be taken into ogy”) but to neglect some of these ideas for that account no less than the astronomical and bi- reason is giving the quacks too much power. In- ological components of cosmic evolution. [em- stead, we need to consider all the alternatives, and phasis in the original] these clearly form well-defined, albeit often provably wrong or undeveloped ideas. Hypotheses in this class It is easy to understand the necessity of re- serve another important role: they remind us of the defining SETI studies in general and our view of magnitude of the challenge posed by FP to our naive Fermi’s Paradox in particular in this context. For worldview – and they should be evaluated in this example, postbiological evolution makes those be- light. Some of the solipsist hypotheses discussed at havioral and social traits like territoriality or ex- least half-seriously in the literature are the following pansion drive (to fill the available ecological niche) (listed in rough order from less to more viable ones):

7 M. M. CIRKOVI´ C´

• Those who believe UFOs are of extraterres- vations do not represent reality, but a form trial intelligent origin quite clearly do not have of illusion, created by an advanced technologi- any problem with FP (e.g. Hynek 1972; for cal civilization capable of manipulating matter a succinct historical review see Chapter 6 of and energy on interstellar or Galactic scales. Dick 1996). The weight of evidence obviously For a fictional description of this scenario, see tells otherwise. Reynolds (2004). • As far as it can be formulated as a hypoth- • The Simulation hypothesis of Bostrom esis, traditional views of special creation (2003), although motivated by entirely differ- of Earth and humanity belong to this class. ent reasons and formulated in a way which The most valiant attempt in this direction seemingly has nothing to do with FP, offers a has been made, as already mentioned, by Al- framework in which FP can be naturally ex- fred Russel Wallace (1903), who argued for plained. Bostrom offers a Bayesian argument the key role of ”cosmic mind” in the grand why we might rationally think we live in a scheme of things and on the basis of teleo- computer simulation of an advanced techno- logical (mis)interpretation of the then fash- logical civilization inhabiting the ”real” uni- ionable model of the universe similar to the verse. This kind of argument has a long classical Kapteyn universe. As discussed in philosophical tradition, going back at least to detail by Crow (1999), such views were occa- Descartes’ celebrated second Meditation dis- sionally dressed in garb of the traditional the- cussing the level of confidence we should have ology (especially of Christian provenance), but about our empirical knowledge (for an inter- the association is neither logically nor histori- esting recent review, see Smart 2004). Novel cally necessary (see also Dick 2000, 2003). To- points in Bostrom’s presentation are invok- day, this way of looking at the problem of life ing Moore’s Law for suggesting that we might and intelligence beyond Earth is abandoned be technologically closer to the required level in most mainstream theologies (William Lane of computing sophistication than we usually Craig, personal communication).9 think, as well as adding a Bayesian condi- • The Zoo hypothesis of Ball (1973) and the tioning on the number (or sufficiently gener- related Interdict hypothesis of Fogg (1987) alized ”cost” in resources) of such ”ancestor- simulations” as he dubs them. It is trivial to suggest that there is a uniform cultural pol- see how FP is answered under this hypothe- icy of advanced extraterrestrial civilization to sis: extraterrestrial civilizations are likely to avoid any form of contact (including having be simply beyond the scope of the simulation a visible manifestations) with the newcom- in the same manner as, for example, present- ers to the ”Galactic Club”. The reasons be- day simulation of the internal structure of Sun hind such a behavior may be those of ethics, neglect the existence of other stars in the uni- prudence or practicality (Deardorff 1987). In verse. each case, these do not really offer testable predictions (if the extraterrestrial civilizations It is difficult to objectively assess the value of are sufficiently powerful, as suggested by the solipsist hypotheses as solutions to FP. Most of them age difference in 3), for which they have been are either untestable in principle like the eponymous criticized by Sagan, Webb and others. As a metaphysical doctrine, or testable only in limit of consequence, a ”leaky” interdict scenario is very long temporal and spatial scales, so that they occasionally invoked to connect with the al- do not belong to the realm of science, convention- leged extraterrestrial origin of UFOs (Dear- ally understood. In other words, they violate a sort of ”naive” realism which underlies practically entire dorff 1986), which is clearly problematic. scientific endeavor. Their proponents are likely to re- • Directed panspermia of Crick and Orgel tort that the issue is sufficiently distinct from other (1973) suggests that Earth has indeed been scientific problems to justify greater divergence of visited in a distant past with very obvious epistemological attitudes but this is rather hard to consequence – namely the existence of life on justify when one could still pay a smaller price. For Earth! Those two famous biochemists pro- instance, one could choose to abandon Copernican- posed – partly tongue-in-cheek, but partly to ism, like the Rare Earth theorists (Section 6), or one point out the real problems with the then the- ories of biogenesis – that our planet has been might abandon gradualism (which has been discred- intentionally seeded with microorganism origi- ited in geo- and planetary sciences anyway) and end nating elsewhere. In other words, we are aliens up with a sort of neocatastrophic hypothesis (Section ourselves! This motive has been extensively 7). used in fiction (e.g. Lovecraft 2005 [1931]). It Some of them, but not all, violate the non- is very hard to see how we could ever hope to exclusivity requirement as well; this is, for instance, test the hypothesis of directed panspermia, in obvious in Zoo, Interdict or Planetarium scenarios, particular its intentional element. since they presume a large-scale cultural uniformity. • The Planetarium hypothesis of Baxter This is not the case, however, with the Simulation (2000) suggests that our astronomical obser- hypothesis, since the simulated reality is likely to be

9Special creation, however, possesses some methodological similarities with the ”rare Earth” hypotheses as well; see Section 6 below. 8 FERMI’S PARADOX – THE LAST CHALLENGE FOR COPERNICANISM? clearly designed and spatially and temporally lim- biological evolution on other Solar System bodies. ited. Directed panspermia has some additional prob- Finally, solipsist hypotheses need not worry about lems – notably the absence of any further manifes- evolutionary contingency or generic probabilities of tations of our ”parent civilization”, in spite of its biogenesis or noogenesis, unlike the other contenders. immense age. If they became extinct in the mean- Jumping ahead, a clearly non-exclusive solu- time, what did happen with other seeded planets? tion to FP obeying all methodological desiderata has The Copernican reasoning suggests that we should not, in general, been found thus far. Even the most expect evolution to occur faster at some places than objective, mathematical studies, such as the one of on Earth (and, of course, slower at other sites as well) Newman and Sagan, were compelled to, somewhat – where are our interstellar siblings, then? resignedly, conclude that ”[i]t is curious that the so- Observation selection effects are important in- lution to the problem ’Where are they?’ depends gredient in at least some of these hypotheses. The powerfully on the politics and ethics of advanced so- directed panspermia could, for instance, be linked cieties” (Newman and Sagan 1981, p. 320). There with a curious puzzle posed recently by Olum (2004), is something deeply unsatisfactory about this sort of which also helps illustrate intriguing interplay be- answer. It is especially disappointing to encounter it tween modern cosmology and astrobiology. Starting after a lot of mathematical analysis by the same au- from the assumption of an infinite universe (following thors, and keeping in mind by now more than half a from the inflationary paradigm), Olum conjectures century of sustained and often carefully planned and that there are civilizations much larger than ours executed SETI efforts.10 This circumstance, as well (which currently consists of about 1010 observers). as occasional (sub)cultural and even political appeal, Spatial extent and amount of resources at disposal explains why solipsist hypotheses are likely to reap- of such large civilizations would lead, in principle, pear from time to time in the future. to much larger number of observers (for example, 1019 observers in a Kardashev Type III civilization). 6. ”RARE EARTH” SOLUTIONS Now, even if 99% of all existing civilizations are small ones similar to our own, anthropic reasoning sug- This class of hypotheses is based upon the cel- gests that the overwhelming probabilistic prediction ebrated book Rare Earth by Peter Ward and Donald is that we live in a large civilization. Since this Brownlee, whose appearance in 2000 heralded birth prediction is spectacularly unsuccessful on empiri- of the new astrobiological paradigm. They have ex- cal grounds; with a probability of such failure being pounded a view that while simple microbial life is about 10−8, something is clearly wrong here. Olum probably ubiquitous throughout the Galaxy, com- offers a dozen or so hypothetical solutions to this al- plex biospheres, like the terrestrial one, are very rare leged conflict of the anthropic reasoning with cosmol- due to the exceptional combination of many distinct ogy, one of them being the possibility that we are in- requirements. These ingredients of the Rare Earth deed part of a large civilization without being aware hypothesis (henceforth REH) are well-known to of that fact. Directed panspermia hypothesis can be even a casual student of astrobiology: regarded as operationalization of that option. There are several systematic deficiencies in Olum’s conclu- • Circumstellar habitable zone: a habitable ´ planet needs to be in the very narrow inter- sions (Ho and Monton 2005, Cirkovi´c2006), but in val of distances from the parent star. any case the very fact that some form of the prin- • ”Rare Moon”: having a large moon to stabi- ciple of indifference and the counting of observers is lize the planetary axis is crucial for the long- used in this discussion shows how closely the theory term climate stability. of observation selection effects (cf. Bostrom 2002) is • ”Rare Jupiter”: having a giant planet tied with the issues at the very heart of FP. (”Jupiter”) at right distance to deflect much We mention the solipsist hypotheses mostly of the incoming cometary and asteroidal ma- for the sake of logical completeness, since they are terial enables sufficiently low level of impact in any case the council of despair. If and when all catastrophes. other avenues of research are exhausted, we could • ”Rare elements”: Radioactive r-elements (es- always turn toward these hypotheses. Still, this nei- pecially U and Th) need to be present in the ther means that they are all of equal value nor it should mislead us into thinking that they are neces- planetary interior in sufficient amount to en- sarily improbable for the reason of desperation alone. able plate tectonics and functioning of the Bostrom’s simulation hypothesis might, indeed, be carbon-silicate cycle. quite probable, given some additional assumptions • ”Rare Cambrian-explosion analogs”: the evo- related to the increase in our computing power and lution of complex metazoans requires excep- decrease of information-processing cost. Directed tional physical, chemical and geological con- panspermia could, in principle, get a strong boost if, ditions for episodes of sudden diversification for instance, the efforts of NASA and other human and expansion of life. agencies aimed at preventing planetary contamina- Each of these requirements is prima facie un- tion (e.g. Rummel 2001, Grinspoon 2003), turn out likely, so that their combination is bound to be in- to be unsuccessful, thus unintentionally setting off credibly rare and probably unique in the Milky Way.

10Therefore, it is not surprising to notice the Conway Morris (2003), as a leading proponent of ”intelligent design” in science is at least honest in admitting that in such picture it could be that naturalism will have to be abandoned after all. 9 M. M. CIRKOVI´ C´

In addition, Ward and Brownlee break new grounds an opponent can argue that supernatural origin of with pointing the importance of hitherto downplayed life is clearly more plausible hypothesis! Namely, factors, like the importance of plate tectonics, iner- even a fervent atheist and naturalist could not ra- tial interchange events, or ”Snowball Earth” episodes tionally claim that her probability of being wrong of global glaciation for the development of complex on this metaphysical issue is indeed smaller than life. In many ways, REH has become somewhat of a 10−100, knowing what we know on the fallibility of default position in many astrobiological circles, and – human cognition. According to the dominant rules since it predicts the absence of rationale for SETI – a of inference, we would have been forced to accept mainstay of SETI scepticism. Thus, its challenge to the creationist position, if no other hypothesis were Copernicanism has been largely accepted (although, present (Hoyle and Wickramasinghe 1999)! Now, as argued below, there are lower prices to be paid on REH in strict sense avoids this problem by postulat- the market of ideas) as sound in the mainstream as- ing ubiquitous simple life (actually implying a high trobiology. Particular Rare Earth hypotheses (inso- probability of biogenesis ceteris paribus). However, far as we may treat them as separate) are difficult to if the continuity thesis applies further along ”Hal- assess lacking first-hand knowledge of other Earth- dane’s ladder” – specifically, to origin of complex like planets, but some of the difficulties have been metazoans and to noogenesis – an analogous argu- exposed in the literature thus far. ment is perfectly applicable to REH. Obviously, this For instance, the famous argument about necessitates further research in evolutionary biology, Jupiter being the optimal ”shield” of Earth from cognitive sciences and philosophy. cometary bombardment has been brought into ques- There are other hypotheses for resolving FP tion by recent work of Horner and Jones (2008, which violate Copernicanism. The idea of Wesson 2009) who use numerical simulation to show that the (1990) that it is cosmology which limits the contact off-handed conclusion that Jupiter acts as a shield between civilizations in the universe also belongs to against bombardment of inner Solar System plan- this category. It implies that the density of civiliza- ets is unsupported. Moreover, they conclude ”that tions is so low that only a few are located within our such planets often actually increase the impact flux cosmological horizon. However, this is just begging greatly over that which would be expected were a the question, since such extreme low density of in- giant planet not present.” If results of Horner and habited sites – less than 1 Gpc−1, say – is not only Jones withstand the test of time and further research, un-Copernican, but clearly requires some additional it is hard to imagine a more detrimental result for the explanatory mechanism. It may consist in biologi- entire Rare Earth paradigm. cal contingency or rarity of the Cambrian-explosion This example highlights the major problem analogs or any number of other instances invoked by with REH. In supposing how the state-of-affairs the proponents of REH, but it is clearly necessary. could be different, Rare Earth theorists assume sim- On the other hand, no further explanation is ple, linear change, not taking into account self- necessary for the adaptationist version of REH, organizing nature of the relevant physical systems. which in this case could truly be dubbed ”rare The example of Jupiter is again instructive, since mind” hypothesis. It has been hinted at by Raup asking about the fate of Earth in the absence of (1992), but developed in more detail in the novel Jupiter is self-contradictory: Earth is a part of the Permanence by the Canadian author Karl Schroeder complex system which includes Jupiter as a major (2002). A detailed discussion of this particular so- component, so there are no guarantees that Earth lution of FP is given in Cirkovi´c(2005).´ This in- would have existed at all if Jupiter were not present. triguing hypothesis uses the prevailing adaptationist Even if it existed, we would have to account for many mode of explanation in evolutionary biology to argue other differences between that particular counterfac- that conscious tool-making and civilization-building tual situation and the actual one, so the question to are ephemeral adaptive traits, like any other in the what degree is justified to call such a body ”Earth” living world. Adaptive traits are bound to disap- would be very pertinent. pear once the environment changes sufficiently for Another important methodological problem any selective advantage which existed previously to for the ”rare Earth” hypotheses is that at least in disappear. In the long run, the intelligence is bound some respects they are equivalent to the doctrines to disappear, as its selective advantage is temporally openly violating naturalism, e.g., creationism. This limited by ever-changing physical and ecological con- similarity in style rather than in substance has been ditions. The outcome of the cultural evolution in lim- most forcefully elaborated by Fry (1995), as men- its of very long timescales is a reversion to the direct, tioned above. If one concludes that the probability of non-technological adaptation – similar to the sugges- biogenesis – even under favorable physical and chem- tion of Raup that animals on other planets may have ical preconditions – astronomically small, say 10−100, evolved, by natural selection, the ability to commu- but one still professes that it was completely natu- nicate by radio waves (and, by analogy, at least some ral event,11 than a curious situation arises in which of the other traits we usually think about as possi-

11Even smaller probabilities have been occasionally cited in the literature. Thus, Eigen (1992) cites the probability of random assembly of a polymer with a thousand nucleotides corresponding to a single gene as 1 part in 10602. This sort of ”superas- tronomical” numbers have led Hoyle and Wickramasinghe (1981, 1999) to invoke either an eternal universe – in contradiction with cosmology – or a creative agency. The (in)famous metaphor of random assembly of ”Boeing 747” out of junkyard, cited by Sir Fred Hoyle, nicely expresses this sort of desperation, which has, luckily enough, been overcome in the modern theories of biogenesis. 10 FERMI’S PARADOX – THE LAST CHALLENGE FOR COPERNICANISM? ble only within the conscious civilization). This form • Classical nuclear self-destruction hypoth- of downgrading the role of consciousness – present in esis was, perhaps more obvious during the many circles of the contemporary philosophy of mind Cold War era (cf. von Hoerner 1978) – but and cognitive science – is beautifully exposed in the ephemeral cultural changes in our recent his- controversial book of Julian Jaynes (1990).12 tory should not really modify prior probability There are many difficulties with the adapta- for this dramatic possibility. Problem with the tionist hypothesis. For instance, its insistence on exclusive nature of such a hypothesis – consid- adaptationism at all times is a form of inductivist ering the fact that social and political devel- fallacy. As in earlier times inductivists argued that opments on habitable planets throughout the it is natural to assume a meta-rule of inference along Galaxy are quite unlikely to be correlated – the lines of ”the future will resemble the past”, thus are obvious. there is a creeping prejudice that the present and fu- • Self-destruction options have multiplied ture modes of evolution need to be the same as those in the meantime, since the spectrum of leading to the present epoch. This is a consequence potentially destructive technologies in hu- of the present-day idolatry of adaptation: almost re- man history have recently broadened. This flex and non-thinking assumption that any evolution now includes misuse of biotechnology (includ- has to be adaptationist (e.g., Dennett 1995; for a crit- ing bioterrorism), and is likely to soon in- icism, see Ahouse 1998). In spite of such fashionable clude misuse of nanotechnology, artificial in- views like evolutionary psychology/behavioral ecol- telligence, or geoengineering (see reviews in ogy/sociobiology, there is no reason to believe that Bostrom and Cirkovi´c´ 2008, Cirkovi´c´ and all complex living systems evolve according to the Cathcart 2004). If most of technological soci- rules of functionalist natural selection, and not, for eties in the Galaxy self-destructs through any instance, in a Lamarckian, orthogenetic or saltation- of these – or other conceivable – means, this ist manner. Besides, even if all Gyr-old civilizations would be an explanation for the ”Great Si- are now extinct, what about their astroengineering lence”. Quite clearly, the same qualms about traces and manifestations? For a detailed review of exclusivity apply as above. further problematic issues with this intriguing hy- • Ecological holocaust: Solar System and pothesis, see Cirkovi´c,Dragi´cevi´cand´ Beri´c-Bjedov surrounding parts of GHZ belong to a ”post- (2005). colonization wasteland”, a bubble created by rapid expansion and exhaustion of local re- sources on the part of early advanced tech- 7. (NEO)CATASTROPHIC SOLUTIONS nological civilizations (Stull 1979, Finney and Jones 1985). Since colonization front is likely to be spherically symmetric (or axially sym- This is the most heterogeneous group, contain- metric when the vertical boundaries of the ing both some oldest speculations on the topic and Galactic disk are reached), they will tend to the newest ones. Before we review some of the main leave vast inner area exhausted. If the pa- contenders, it is important to emphasize that the pre- rameters describing the rates of expansion and fix ”neo” is used almost reflexively with this mode of natural renewal of resources are in a particu- thinking for historical reasons. The defeat of ”clas- lar range of values, it is possible that younger sical”, 19th century catastrophism of figures such as civilizations will find themselves in a This hy- Cuvier, Orbigny, de Beaumont, Agassiz or Sedgwick pothesis has been recently revived in numer- in the grand battle with the gradualism of Charles ical models of Hanson (1998b), showing that Lyell and his pupils (including Charles Darwin) im- in some cases fairly plausible initial conditions posed a lasting stigma on views which were per- will lead to ”burning of the cosmic commons”, ceived as beloging to this tradition of thought. This i.e. catastrophic depletion of usable resources has clearly impeded the development of geosciences in a large volume of space. This is rather con- (see historical reviews in Raup 1991, Huggett 1997, troversial as a solution to FP since, apart Palmer 2003). In addition, the association of catas- from some fine-tuning, it still does not answer trophism with the pseudo-scientific (although often the essential question: where did the ”precur- thought-provoking!) views of Immanuel Velikovsky sors” go and why we do not perceive their has brought an additional layer of suspicion upon immensely old astro-engineering signatures? the label itself (for a review of the Velikovskian con- They have either become extinct (thus beg- troversy, see Bauer 1984). Thus, the resurgence of ging the question and requiring another layer catastrophism after 1980 and the discovery of Al- of explanation) or changed into something else varez and collaborators that an asteroidal/cometary (see the Transcedence item below). How- impact was the physical cause of the extinction of ever, this hypothesis is non-exclusive (since ammonites, dinosaurs and other species at the Cre- the volume of space within the ancient colo- taceous/Tertiary boundary 65 Myr ago (Alvarez et nization front is large) and it does make some al. 1980) is often referred to as neocatastrophism. well-defined predictions as far as renewal of resources and the traces of possible previous

12A particularly thought-provoking section (pp. 36-41) of the first chapter of Jaynes’ disturbing book is entitled ”Consciousness Not Necessary for Thinking”. 11 M. M. CIRKOVI´ C´

cycle of their depletion in the Solar vicinity are within the temporal window of a ”phase are concerned. transition” – from essentially dead place, the • Natural hazards: The risk of cometary/as- Galaxy will be filled with intelligent life on a teroidal bombardment (Clube and Napier timescale similar to tFH . 1984, 1990, Chyba 1997), supervolcanism • Deadly probes hypothesis: A particu- (Rampino 2002), nearby supernovae (Terry larly disturbing version of the Tipler’s (1980, and Tucker 1968, Gehrels et al. 2003) or 1981) reductio ad absurdum scenario presumes some other, more exotic catastrophic pro- that self-replicating von Neumann probes are cess (Clarke 1981) might be in general much not peaceful explorers or economically-minded higher than we infer from the recent history of colonizers, but intentionally or accidentally Earth. These natural hazards are much like- created destructive weapons. This might oc- lier to break the evolutionary chain leading cur either due to malevolent creators (which to the emergence of intelligent observers, so in that case had to be the first or one of the we should not wonder why we do not perceive first technological civilizations in the Galaxy, manifestations of older Galactic communities. close to the Lineweaver limit) or through a For instance, one well-studied case is the sys- random dysfunction (”mutation”) in a partic- tem of the famous nearby Sun-like star Tau ular self-replicating probe which has passed to Ceti which contains both planets and a mas- its ”offspring”. In both cases, it seems that sive debris disk, analogous to the Solar Sys- the originators of the probes have vanished or tem Kuiper belt. Modeling of Tau Ceti’s dust are in hiding, while the Galaxy is completely disk observations indicate, however, that the different (and more hostile) ecological system mass of the colliding bodies up to 10 kilome- than it is usually assumed. Depending on the ters in size may total around 1.2 M⊕, com- unknown mode of operation of destructive von pared with 0.1 M⊕ Earth-masses estimated Neumann probes, they might be homing on to be in the Solar System’s Edgeworth-Kuiper the sources of coherent radio emission (indi- Belt (Greaves et al. 2004). It is only reason- cating a young civilization to be eliminated) or able to conjecture that any hypothetical ter- might be automatically sweeping the Galaxy restrial planet of this extrasolar planetary sys- in search for such adversaries. Brin (1983) tem is subjected to much more severe impact concludes that this one of only two hypothe- stress than Earth has been during the course ses which maintain wholesale agreement with of its geological and biological history.13 both observation and non-exclusivity. In the • Phase-transition hypotheses (Annis realm of fiction, this hypothesis has been topic 1999a, Cirkovi´c2004b,´ Cirkovi´cand´ Vukoti´c of novels by Fred Saberhagen (1998), Gregory 2008) offer a plausible astrophysical scenario Benford (1977, 1983) and Alastair Reynolds for a delay in the emergence of intelligent ob- (2002). servers and their technological civilizations • ”Freedom is slavery”: If all civilizations, based on the notion of a global regulation instead of self-destruction, slip into perma- mechanism. Such a mechanism could oc- nent totalitarianism (perhaps in order to avoid casionally reset astrobiological ”clocks” all self-destruction or other global catastrophic over GHZ and in a sense re-synchronize them. risks; see Caplan 2008), this could also dra- This is is a prototype disequilibrium as- matically decrease the contact cross-section. trobiological hypothesis: there is no Fermi’s Orwellian state is quite disinterested in the paradox, since the relevant timescale is the external universe; even if it were willing to time elapsed since the last ”reset” of astrobi- communicate, its paranoid nature would have ological clocks and this can be substantially made any opportunity for contact orders of smaller than the age of the Milky Way or magnitude more difficult. For a gruesomely the age difference in (3). Annis suggests that dramatic description of this possibility see Fi- gamma-ray bursts (henceforth GRBs), whose asco (Lem 1987). On the other hand, it is con- cosmological and extremely energetic nature ceivable that at least some totalitarian states is now increasingly understood (e.g. M´esz´aros would actually engage in aggressive interstel- 2002, Woosley and Bloom 2006) serve as such lar expansion, even if through releasing the catastrophic reset events when they occur in deadly probes sketched above. Here, as else- our home Galaxy. Astrobiological significance where, we might have a case for synergy of of GRBs has recently been subject of much different FP solutions. research (Thorsett 1995, Scalo and Wheeler • Transcedence hypothesis: Advanced tech- 2002, Thomas et al. 2005, 2008, Galante and nological civilizations have neither destroyed Horvath 2007). The discussion of other con- themselves nor spread through the Galaxy, but have transformed themselves into ”some- ceivable regulation mechanisms is given by thing else”, not recognizable as a civilization Vukoti´cand Cirkovi´c(2007,´ 2008). In gen- and certainly not viable as a SETI target. His- eral, this hypothesis leads to the situation torically, this has been the first solution to FP, schematically envisioned in Fig. 3: where we

13For a good recent introduction to the complex topic of the relationship between catastrophes and habitability, see Hanslmeier (2009). 12 FERMI’S PARADOX – THE LAST CHALLENGE FOR COPERNICANISM?

offered by Konstantin Tsiolkovsky who posed phase-transition – fare much better here. In some the paradox in the first place. Tsiolkovsky, cases, it is still impossible to estimate how tightly under the influence of his teacher, N. F. Fe- correlated some of the postulated events might be; dorov and other Russian cosmists, concluded this applies in particular to the transcendence-type that the only reason why we do not perceive scenarios, where the extent and the nature of ”Sin- manifestations of much older civilizations is gularity” remains a mystery.14 their evolving into a form of ”superreason” Among the non-exclusive hypotheses, the with near-godly powers and, presumably, in- phase-transition model is in advantage in comparison conceivable interests (Tsiolskovsky 1933; see to the ”deadly probes”, since we understand possi- also Lytkin et al. 1995, Lipunov 1997); the ble dynamics of the global regulation mechanisms. ideas of Tsiolkovsky have some similarities Moreover, global catastrophic events affecting large with the Zoo hypothesis of Ball (1973), dis- parts of GHZ will tend to reset many local astro- cussed above. Today, it is often formulated in biological clocks nearly simultaneously, thus signifi- term of ”technological Singularity”, the con- cantly decreasing the probability of existence of ex- cept envisioned by Stanislaw Ulam and I. J. tremely old civilizations, in accordance with Annis’ Good, and popularized in 1990s by mathe- scenario. In both of these hypotheses, however, it matician and author Vernon Vinge (e.g. Vinge is possible that pockets of old (in effective, astrobio- 1986, 1991, 1993, Kurzweil 2005). Smart’s logical terms) habitable sites remain, either through (2007) concept of the ”Universal Transcen- purely stochasic nature of lethal regulation mecha- sion” is a variation of this idea. nisms, or through dysfunctional mode of operation of destructive von Neumann probes. As the Cold War cultural pessimism retreated, Predictions of these two hypotheses and their neocatastrophic hypotheses obtained a strong boost ramifications for the ongoing SETI projects cannot from the resurgence of catastrophism in Earth and differ more dramatically. While the ”deadly probes” planetary science, as well as in astrobiology. Follow- scenario is particularly bleak and offers no significant ing the seminal work of Alvarez et al. (1980), we prospect for SETI, punctuation of the astrobiological have become aware that global catastrophes played evolution of the Milky Way with large-scale catas- very significant role in the evolution of terrestrial bio- trophes affecting significant fraction of GHZ will, sphere (e.g. Jablonski 1986, Raup 1991, Courtillot somewhat counterintuitively, have the net effect of 1999, Erwin 2006). Moreover, some of the actual strengthening the rationale for our present-day SETI catastrophes whose traces are seen in the terrestrial efforts. Namely, as the secular evolution of the reg- record are of astrophysical origin, emphasizing the ulation mechanisms leads to the increase in the av- new paradigm according to which the Solar System is erage astrobiological complexity (Fig. 3), we might an open system, strongly interacting with its Galac- expect that more and more civilizations enter the tic environment (e.g. Clube and Napier 1990, Leitch ”contact window” and join efforts in expansion to- and Vasisht 1998, Shaviv 2002, Melott et al. 2004, wards Kardashev’s Type III status. Pavlov et al. 2005, Gies and Helsel 2005). This neocatastrophist tendency is present in the mod- 8. OTHER SOLUTIONS ern research on biogenesis (e.g. Raup and Valen- tine 1983, Maher and Stevenson 1988), and even A small number of hypotheses have been pro- posed which do not fall easily into one of the broad in the debates on evolution of humanity (Rampino categories described above. Although the total vari- and Self 1992, Ambrose 1998, Bostrom and Cirkovi´c´ ation of approaches to FP is already stupendous, it 2008), bul all its ramifications have not yet been elu- is remarkable how small number of ideas escapes the cidated in any detail. The major feature of these general philosophical categories discussed. solutions is the abandonment of the classical gradu- For instance, Landis (1998) and Kinouchi alist dogma that ”the present is key to the past” and (2001) have investigated the dynamics of interstellar acknowledgement that sudden, punctuated changes colonization which, under some particular assump- present a major ingredient in shaping both Earth’s tions, can leave large bubbles of empty space sur- and Milky Way’s astrobiological history (or ”land- rounded by colonized regions. This phenomenon is scape”; cf. Vukoti´cand Cirkovi´c2008).´ in the context of condensed-matter physics known as Intuitively, it seems clear that any form of persistence. An obvious weakness of this hypothesis catastrophic events affecting planetary biospheres in is that it still implies cultural uniformity regarding the Milky Way will reduce the hypothetical extrater- the dynamical parameters of colonization, which vi- restrial civilizations’ ages and thus reduce the tension olates the non-exclusivity requirement. In addition, inherent in FP. If such events are spatially and tem- we would expect to detect either extraterrestrial sig- porally uncorrelated – as in the ”mandatory” nu- nals coming from outside of the local non-colonized clear self-destruction hypothesis or the totalitarian bubble, or to detect manifestations of Gyr-older tech- scenario – such an explanation is obviously low on nological societies even in the absence of the direct the non-exclusivity scale. In contrast, hypotheses presence of extraterrestrials in the Solar System or with correlated events – such as ”deadly probes” or in its vicinity.

14Consequently, it is impossible to state confidently whether the transcendence hypotheses resolve FP, i.e., what additional assumptions are necessary for this rather vague concept to be a viable solution. On the other hand, obvious – and rather dramatic – importance of this scenario for future studies remains a strong motivation for further research. 13 M. M. CIRKOVI´ C´

Fig. 3. Very simplified scheme of the phase-transition hypotheses (from Cirkovi´cand´ Vukoti´c2008): an appropriately defined astrobiological complexity will tend to increase with time, but the increase will not become monotonous until a particular epoch is reached.

Similar approach has been favored in nu- most efficiently. This solution modestly violates non- merical simulations of Bjork (2007), although the exclusivity requirement, depending on how univer- timescales obtained in his model are quite short in sally valid is the assumption of resource-optimization comparison with (3) even with his explicit rejection as the major motivator of advanced extraterrestrial of self-reproducing probes, thus being more in line societies. Not surprisingly, some of these ideas have been with the older calculations of Hart (1975), Jones prefigured in a loose form within the discourse of (1976, 1981) and Newman and Sagan (1981). Bjork science fiction. Karl Schroeder in Permanence not concludes, rather too optimistically, that FP could only formulated the above-mentioned adaptationist be resolved by through statement that ”[w]e have answer to Fermi’s question, but also envisaged the not yet been contacted by any extraterrestrial civi- entire Galaxy-wide ecosystem based on brown dwarfs lizations simple because they have not yet had the (and the halo population in general) and a low- time to find us.” In view of the timescale (3) it is temperature environment (Schroeder 2002). Most clearly wrong as long as we do not postulate some strikingly, the idea of advanced technological civ- additional reason for the delay in starting the Galac- ilization inhabiting the outer fringes of the Milky tic exploration. Way has been suggested—though without the ther- The approach of Cirkovi´cand´ Bradbury (2006; modynamical rationale—by Vernon Vinge in A Fire see also Cirkovi´c2008)´ offers an alternative solution upon the Deep (Vinge 1991). Vinge vividly envisages based on the assumption that most or all advanced ”Zone boundaries” separating dead and low-tech en- technological societies will tend to optimize their re- vironments from the truly advanced societies inhab- source utilization to an extreme degree. It could be iting regions at the boundary of the disk and high shown that such optimization will ultimately be lim- above the Galactic plane. This is roughly analogous ited by the temperature of the interstellar space – to the low- temperature regions Cirkovi´cand´ Brad- and that temperature decreases with increased galac- bury (2006) outlined as the most probable Galactic tocentric distance in the Milky Way (towards the technological zone. ideal case of the CMB temperature of about 2.7 K It has been claimed in the classical SETI liter- achievable only in the intergalactic space). The log- ature that the interstellar migrations will be forced ical conclusion is that most of the advanced tech- by the natural course of stellar evolution (Zucker- nological species (which will be most likely post- man 1985). However, even this ”attenuated” ex- biological, consisting of intelligent machines or up- pansionism – delayed by on the order of 109 years loaded minds; cf. Dick 2003) will migrate towards – is actually unnecessary, since naturally occurring the outer rim of the Galaxy, far from the star- thermonuclear fusion in stars is extremely inefficient formation regions, supernovae and other energetic energy source, converting less than 1% of the total astrophysical events in order to process information stellar mass into potentially useable energy. Much

14 FERMI’S PARADOX – THE LAST CHALLENGE FOR COPERNICANISM? deeper (by at least an order of magnitude) reservoir neocatastrophic solutions – gradualism) should give of useful energy is contained in the gravitational field us pause.15 This testifies on the toughness and in- of a stellar remnant (white dwarf, neutron star or herent complexity of the puzzle. In accordance with black hole), even without already envisaged stellar the strong position of REH in contemporary astro- engineering (Criswell 1985, Beech 2008). Highly op- biology, our analysis shows that we should interpret timized civilization will be able to prolong utilization it as a challenge to Copernicanism. In the view of of its astrophysically local resources to truly cosmo- the present author, by far the lowest price if paid logical timescales. The consequences for our conven- through abandoning of gradualism, which is anyway tional (that is, predominantly empire-state) view of undermined by the contemporary developments in advanced societies have been encapsulated in an in- geosciences, evolutionary biology and astronomy. teresting paper by Beech (1990): Gradualism, parenthetically, has not shone as a brilliant guiding principle in astrophysics and cos- [A] star can only burn hydrogen for a finite mology. It is well-known, for instance, how the time, and it is probably safe to suppose that strictly gradualist (and from many points of view a civilisation capable of engineering the condi- tion of their parent star is also capable of initi- methodologically superior) steady-state theory of the ating a programme of interstellar exploration. universe of Bondi and Gold (1948), as well as Hoyle, Should they embark on such a programme of has after the ”great controversy” of 1950s and early exploration it is suggested that they will do so, 1960s succumbed to the rival evolutionary models, however, by choice rather than by neces- now known as the standard (”Big Bang”) cosmol- sitated practicality. [emphasis M. M. C.]´ ogy (Kragh 1996). Balashov (1994) has especially In brief, the often-quoted clich´ethat life fills stressed this aspect of the controversy by showing all available niches is clearly non sequitur in the rel- how deeply justified was the introduction—by the evant context; thus, interstellar colonial expansion Big Bang cosmologists—of events and epochs never should not be a default hypothesis, which it sadly seen or experienced. Similar arguments are applica- is in most SETI-related and far-future-related dis- ble in the nascent discipline of astrobiology, which courses thus far. might be considered to be in an analogous state to- The sustainability solution of Haqq-Misra day as cosmology was half a century ago (Cirkovi´c´ and Baum (2009) is related to the compact, highly- 2004a). efficient model of advanced extraterrestrial civiliza- This leads us to the practical issue of ram- tion postulated in Parkinson (2004), Cirkovi´cand´ ifications of various hypotheses sketched above for Bradbury (2006), Smart (2007), and Cirkovi´c(2008).´ practical SETI activities. While solipsist hypotheses Haqq-Misra and Baum envision a situation in which have nothing substantial to offer in this regard, Rare large-scale interstellar expansion is infeasible due to Earth hypotheses obviate the very need for practical sustainability costs (and perhaps dysgenic factors, SETI efforts. In the best case, we could expect to similar to the ones in Schroeder’s adaptationist hy- find archaeological traces of vanished Galactic civi- pothesis), so that the prevailing model would be lizations (as per adaptationist hypothesis). In con- a compact, ”city-state” sophisticated technological trast, most neocatastrophic options offer support for civilization, possibly slowly expanding, but at rates SETI optimism, since their proponents expect prac- tically all extraterrestrial societies to be roughly of negligible in comparison to the expansion in either 16 Newman-Sagan-Bjork (no self-replicating probes) or the same effective age as ours, and to be our com- Tipler (with self-replicating probes) regimes. Parkin- petitors for the Fermi-Hart-Tiplerian colonization of the Milky Way. The price to be paid for bringing son’s (2004) containment scenario offers a differ- the arguments of ”optimists” and ”pessimists” into ent rationale for predominance of the ”city-states” accord is, obviously, the assumption that we are liv- over the ”interstellar empires”, resulting in the same ing in a rather special epoch in Galactic history—i.e. observed dearth of interstellar empires. These hy- the epoch of phase transition. In any case, it is clear potheses meet with the same criticisms based on (i) that our choice of hypotheses for resolving FP needs the non-exclusivity and (ii) the lack of astroengineer- to impact our SETI efforts in a most direct way. ing detection signatures considered above. A related issue too complex to enter here in more detail is the inadequacy of most of the ortho- 9. INSTEAD OF CONCLUSIONS: A dox SETI projects thus far. Radio listening for in- PUZZLE FOR THE 3. MILLENNIUM? tentional messages has been a trademark of orthodox SETI since the time of ”founding fathers” (Drake, The very fact that each wide class of an- Morrison, Sagan, etc.) and it has demonstrated quite swers to FP requires abandoning one of the a strong resilience to dramatic changes in other fields great methodological assumptions of mod- of learning in the past four decades. Several issues ern science (solipsist solutions reject naive real- touched upon in this review strongly indicate that ism, ”rare Earth” solutions reject Copernicanism and the conventional SETI (Tarter 2001, Duric and Field

15We have assumed naturalism throughout in accordance with the proclaimed goal of investigating to which degree FP remains unresolved. 16A qualification ”effective” is required here since in the case of arrested development (e.g., under the totalitarianism scenario, the age of civilization is almost irrelevant for its capacity for cosmic colonization. 15 M. M. CIRKOVI´ C´

2003, and references therein), as exemplified by debt is owed to Zoran Zivkovi´c,whoˇ as a publisher, the historical OZMA Project, as well its current translator, critic and author of fiction has, during counterparts such as META, ARGUS, Phoenix, the last three decades, done more than anyone else SERENDIP/Southern SERENDIP—and notably to make me interested in the topic of extraterrestrial those conveyed by NASA and the SETI Institute— life and intelligence. Special thanks are due to Duˇsan are fundamentally flawed. Some of the alternatives Indji´cand Srdjan Samurovi´cfor kind help with the have existed for quite a long time, starting with the illustrations and general support. This is also an seminal paper by Dyson (1960) and elaborated in opportunity to thank KoBSON Consortium of Ser- ´ bian libraries, which at last enabled overcoming of Dyson (1966) and Cirkovi´cand Bradbury (2006). the gap in obtaining the scientific literature during What we can dub the Dysonian approach to SETI the tragic 1990s. The author acknowledges support puts the emphasis on search for extraterrestrial tech- of the Ministry of Science and Technological Devel- nological manifestations and artifacts. Even if they opment of the Republic of Serbia through the project are not actively communicating with us, that does ON146012. not imply that we cannot detect their astro- en- gineering activities. Unless advanced technological communities have taken great lengths to hide or dis- REFERENCES guise their IR detection signatures, the terrestrial observers should still be able to observe them at those wavelengths and those should be distinguish- Adams, M. B.: 2000, J. Hist. 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FERMIJEV PARADOKS – POSLEDNjI IZAZOV ZA KOPERNIKANIZAM?

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]

UDK 52–37 Pregledni rad po pozivu

U ovom qlanku pravimo pregled Fermi- nog qlanka u literaturi koji je napisao G. jevog paradoksa (ili paradoksa ”Velike ti- Dejvid Brin - generisao je mnoge domixlja- xine”), ne samo najstarijeg i po mnogo qemu te rasprave i ingeniozne hipoteze. Ovde analiziramo preutne pretpostavke ugraene kljuqnog problema potrage za vanzemaljskom u razliqite odgovore na ovu zagonetku i inteligencijom (SETI), ve i zagonetke sa predlaemo novu klasifikaciju brojnih re- dubokim opxte nauqnim, filozofskim i kul- xenja koja se pojavljuju u ve ogromnoj li- turnim znaqajem. Jednostavnom primenom teraturi na ovu temu. Konaqno razmatramo analize posmatraqkih selekcionih efekata, posledice razliqitih klasa hipoteza na prak- korektno rexenje Fermijevog paradoksa nam tiqne SETI projekte. Donekle paradoksalno, gotovo izvesno govori nexto o budunosti izgleda da (neo)katastrofiqke hipoteze daju, qoveqanstva. Ovaj problem, star ve vixe kad se sve uzme u obzir, najvixe razloga za od tri qetvrti veka - a proxlo je vixe od optimizam u pogledu naxih sadaxnjih SETI qetvrt veka od poslednjeg kljuqnog pregled- poduhvata i onih u bliskoj budunosti.

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