Fermi's paradox (also known as the 'Great Silence' problem) consists in the tension between (1) naturalistic origin of life and intelligence, as well as astrophysical sizes and ages of our Galaxy and (2) the absence of extraterrestrials in the Solar System, or any other traces of extraterrestrial intelligent activities in the universe.10 In particular, the lack of macroengineering (or astroengineering) activities observable from interstellar distances tells us that it is not the case that life evolves on a significant fraction of Earth-like planets and proceeds to develop advanced technology, using it to colonize the universe or perform astroengineering feats in ways that would have been detected with our current instrumentation. The characteristic time for colonization of the Galaxy, according to Fermi's argument, is 106-108 years, making the fact that the Solar System is not colonized hard to explain, if not for the absence of extraterrestrial cultures. There must be (at least) one Great Filter - an evolutionary step that is extremely improbable - somewhere on the line between Earth-like planet and colonizing-in-detectable-ways civilization (Hanson, 1999). If the Great Filter is not in our past, we must fear it in our (near) future. Maybe almost every civilization that develops a certain level of technology causes its own extinction.
10 It would be more appropriate to call it the Tsiolkovsky-Fermi-Viewing-Hart-Tipler Paradox (for more history, see Brin, 1983; Kuiper and Brin, 1989; Webb, 2002, and references therein). We shall use the locution 'Fermi's Paradox' for the sake of brevity, and with full respect for the contributions of the other authors.
Fermi's paradox has become significantly more serious, even disturbing of late. This is due to several independent lines of scientific and technological advances occurring during the last two decades:
• The discovery of nearly 250 extrasolar planets so far, on an almost weekly basis (for regular updates see http://www.obspm.fr/planets). Although most of them are 'hot Jupiters' and not suitable for life as we know it (some of their satellites could still be habitable, however; see Williams et al., 1997), many other exoworlds are reported to be parts of systems with stable circumstellar habitable zones (Asghari et al., 2004; Beauge et al., 2005; Noble et al., 2002). It seems that only the selection effects and capacity of present-day instruments stand between us and the discovery of Earth-like extrasolar planets, envisioned by the new generation of orbital observatories.
• Improved understanding of the details of chemical and dynamical structure of the Milky Way and its GHZ. In particular, the already mentioned calculations of Lineweaver (2001; Lineweaver et al., 2004) on the histories of Earth-like planet formation show their median age as 6.4± 0.7 Gyr, significantly larger than the Earth's age.
• Confirmation of the relatively rapid origination of life on early Earth (e.g., Mojzsis et al., 1996); this rapidity, in turn, offers weak probabilistic support to the idea of many planets in the Milky Way inhabited by at least simple life forms (Lineweaver and Davis, 2002).
• Discovery of extremophiles and the general resistance of simple life forms to much more severe environmental stresses than it was hitherto thought possible (Cavicchioli, 2002). These include representatives of all three great domains of terrestrial life (Bacteria, Archaea, and Eukarya), showing that the number and variety of cosmic habitats for life are probably much larger than conventionally imagined.
• Our improved understanding in molecular biology and biochemistry leading to heightened confidence in the theories of naturalistic origin of life (Bada, 2004; Ehrenfreund et al., 2002; Lahav et al., 2001). The same can be said, to a lesser degree, for our understanding of the origin or intelligence and technological civilization (e.g., Chernavskii, 2000).
• Exponential growth of the technological civilization on Earth, especially manifested through Moore's Law and other advances in information technologies (see, for instance, Bostrom, 2000; Schaller, 1997).
• Improved understanding of the feasibility of interstellar travel in both the classical sense (e.g., Andrews, 2003) and in the more efficient form oa sending inscribed matter packages over interstellar distances (Rose and Wright, 2004).
Theoretical grounding for various astroengineermg/macroengineering projects (Badescu and Cathcart, 2000, 2006; Korycansky et al., 2001) potentially detectable over interstellar distances. Especially important in this respect is the possible synergistic combination of astroengineering and computation projects of advanced civilizations, like those envisaged by Sandberg (1999).
Although admittedly uneven and partially conjectural, this list of advances and developments (entirely unknown at the time of Tsiolkovsky's and Fermi's original remarks and even Viewing's, Hart's and Tipler's later re-issues) testifies that Fermi's paradox is not only still with us more than half a century later, but that it is more puzzling and disturbing than ever.
There is a tendency to interpret Fermi's paradox as an argument against contemporary Search for Extra-Terrestrial Intelligence (SETI) projects (e.g., Tipler, 1980). However, this is wrong, since the argument is at best inconclusive - there are many solutions which retain both the observed 'Great Silence' and the rationale for engaging in vigorous SETI research (Gould 1987; Webb, 2002). Furthermore, it is possible that the question is wrongly posed; in an important recent paper, the distinguished historian of science Steven}. Dick argued that there is a tension between SETI, as conventionally understood, and prospects following exponential growth of technology as perceived in recent times on Earth (Dick, 2003, p. 66):
«[I]f there is a flaw in the logic of the Fermi paradox and extraterrestrials are a natural outcome of cosmic evolution, then cultural evolution may have resulted in a postbiological universe in which machines are the predominant intelligence. This is more than mere conjecture; it is a recognition of the fact that cultural evolution -
the final frontier of the Drake Equation - needs to be taken into account no less than the astronomical and biological components of cosmic evolution, [emphasis in the original]»
It is easy to understand the necessity of redefining SETI studies in general and our view of Fermi's paradox in particular in this context. For example, post-biological evolution makes those behavioural and social traits like territoriality or expansion drive (to fill the available ecological niche) which are - more or less successfully - 'derived from nature', lose their relevance. Other important guidelines must be derived which will encompass the vast realm Possibilities stemming from the concept of post-biological evolution. In addition, we have witnessed substantial research leading to a decrease in confidence in the so-called Carter's (1983) 'anthropic' argument, the other mainstay of SETI scepticism (Cirkovic et al., 2007; Livio, 1999; Wilson, 1994). this is accompanied by an increased public interest in astrobiology and related issues (such as Cohen and Stewart, 2002; Grinspoon, 2003; Ward and Brownlee, 2000). 6.4.1 Fermi's paradox and GCRs
Faced with the aggravated situation vis-a-vis Fermi's paradox the solution is usually sought in either (1) some version of the 'rare Earth' hypothesis (i.e., the picture which emphasizes inherent uniqueness of evolution on our planet and hence uniqueness of human intelligence and technological civilization in the Galactic context), or (2) 'neo-catastrophic' explanations (ranging from the classical 'mandatory self-destruction' explanation, championed for instance by disenchanted SETI pioneers from the Cold War epoch like Sebastian von Hoerner or Iosif Shklovsky, to the modern emphasis on mass extinctions in the history of life and the role of catastrophic impacts, gamma-ray bursts, and similar dramatic events). Both these broad classes of hypotheses are unsatisfactory on several counts: for instance, the 'rare Earth' hypotheses reject the usual Copernican assumption (the Earth is a typical member of the planetary set), and neo-catastrophic explanations usually fail to pass the non-exclusivity requirement11 (but see Cirkovic, 2004, 2006). None of these is a clear, straightforward solution. It is quite possible that a 'patchwork solution', comprised of a combination of suggested and other solutions, remains our best option for solving this deep astrobiological problem. This motivates the continuation of the search for plausible explanations of Fermi's paradox. It should be emphasized that even the founders ofrare Earth' picture readily admit that simple life forms are ubiquitous throughout the universe (Ward and Brownlee, 2000). It is clear that with the explosive development of astrobiological techniques, very soon we shall be able to directly test this default conjecture.
On the other hand, neo-catastrophic explanations pose important dilemmas related to GCRs - if the 'astrobiological clock' is quasiperiodically reset by exogenous events (like Galactic gamma-ray bursts; Annis, 1999; Cirkovic, 2004, 2006), how dangerous is it to be living at present? Seemingly paradoxically, our prospects are quite bright under this hypothesis, since (1) the frequency of forcing events decreases in time and (2) exogenous forcing implies 'astrobiological phase transition' - namely that we are currently located in the temporal window enabling emergence and expansion of intelligence throughout the Galaxy. This would give a strong justification to our present and future SETI projects (Cirkovic, 2003). Moreover, this class of solutions of Fermi's paradox does not suffer from usual problems like assuming something about arguably nebulous extraterrestrial sociology in contrast to solutions such as the classical 'Zoo' or 'Interdict' hypotheses (Ball, 1973; Fogg, 1987).
11 The requirement that any process preventing formation of a large and detectable interstellar civilization operates over large spatial (millions of habitable planets in the Milky Way) and temporal (billions of years of the Milky Way history) scales. For more details, see Brin (1983).
Somewhat related to this issue is Olum's anthropic argument dealing with the recognition that, if large interstellar civilizations are physically possible, they should, in an infinite universe strongly suggested by modern cosmology, predominate in the total tally of observers (Olum, 2004). As shown by Cirkovic (2006), neo-catastrophic solution based on the GRB-forcing of astrobiological timescales can successfully resolve this problem which, as many other problems in astrobiology, including Carter's argument, is based upon implicit acceptance of insidious gradualist assumptions. In particular, while in the equilibrium state most of observers would indeed belong to large (in an appropriately loose sense) civilizations, it is quite reasonable to assume that such an equilibrium has not been established yet. On the contrary, we are located in the phase-transition epoch, in which all civilizations are experiencing rapid growth and complexincation. Again, neo-catastrophic scenarios offer a reasonable hope for the future of humanity, in agreement with all our empirical evidence.
The relevance of some of particular GCRs discussed in this book to Fermi's paradox has been repeatedly addressed in recent years (e.g., Chapter 10, this volume; Rampino, 2002). It seems that the promising way for future investigations is formulation of joint 'risk function' describing all (both local and correlated) risks facing a habitable planet; such a multi-component function will act as a constraint to the emergence of intelligence and in conjunction with the planetary formation rates, this should give us specific predictions on the number and spatiotemporal distribution of SETI targets.
6.4.2 Risks following from the presence of extraterrestrial intelligence
A particular GCR not covered elsewhere in this book is the one of which humans have been at least vaguely aware since 1898 and the publication of H .G. Wells' The War of the Worlds (Wells, 1898) - conflict with hostile extraterrestrial intelligent beings. The famous Orson Welles radio broadcast for Halloween on 30 October 1938 just reiterated the presence of this threat in the mind of humanity. The phenomenon of the mass hysteria displayed on that occasion has proved a goldmine for psychologists and social scientists (e.g., Bulgatz, 1992; Cantril, 1947) and the lessons are still with us. However, we need to recognize that analysing various social and psychological reactions to such bizarre events could induce disconfirmation bias (see Chapter 5 on cognitive biases in this volume) in the rational consideration of the probability, no matter how minuscule, of this and related risks.
The probability of this kind of GCR obviously depends on how frequent extraterrestrial life is in our astrophysical environment. As discussed in the preceding section, opinions wildly differ on this issue.12 Apart from a couple of 'exotic' hypotheses ('Zoo', 'Interdict', but also the simulation hypotheses below), most researchers would agree that the average distance between planets inhabited by technological civilizations in the Milky Way is at least of the order of 102 parsecs.13 This directs us to the second relevant issue for this particular threat: apart from the frequency of extraterrestrial intelligence (which is a necessary, but not sufficient condition for this GCR), the reality of the risk depends on the following:
1. the feasibility of conflict over huge interstellar distances
2. the magnitude of threat such a conflict would present for humanity in the sense of general definition of GCRs, and
3. motivation and willingness of intelligent communities to engage in this form of conflict.
Item (1) seems doubtful, to say the least, if the currently known laws of physics hold without exception; in particular, the velocity limit ensures that such conflict would necessary take place over timescales measured by at least centuries and more probably millennia or longer (compare the timescales of wars between terrestrial nations with the transportation timescales on Earth!). The limitations of computability in chaotic systems would obviate the detailed strategic thinking and planning on such long timescales even for superintelligences employed by the combatants. In addition, the nature of clumpy astronomical distribution of matter and resources, which are tightly clustered around the central star(s) of planetary systems, ensures that a takeover of an inhabited and industrialized planetary system would be possible only in the case of large technological asymmetry between the parties in the conflict. We have seen, in the discussion of Fermi's paradox that, given observable absence of astroengineering activities, such an asymmetry seems unlikely. This means, among other things, that even if we encounter hostile extraterrestrials, the conflict need not jeopardize the existence of human (or post-human) civilization in the Solar System and elsewhere. Finally, factor (3) is even more unlikely and not only for noble, but at present hardly conceivable, ethical reasons. If we take seriously the lessons of sociobiology that suggest that historical human warfare is part of the 'Darwinian baggage' inherited by human cultures, an obvious consequence is that, with the transition to a post-biological phase of our evolution, any such archaic impulses will be obviated. Per analogiam, this will apply to other intelligent communities in the Milky Way. On the other hand, the resources of even our close astronomical environment are so vast, as is the space of efficiency-improving technologies, that no real ecological pressures can arise to prompt imperial-style expansion and massive colonization over interstellar distances. Even if such unlikely pressures arise, it seems clear that the capacities of seizing defended resources would always (lacking the already mentioned excessive technological asymmetry) be far less cost-effective than expansion into the empty parts of the Milky Way and the wider universe.
12 In the pioneering paper on GCRs/existential risks Bostrom (2002b) has put this risk in the 'whimpers' column, meaning that it is an exceedingly slow and temporally protracted possibility (and the one assigned low probability anyway). Such a conclusion depends on the specific assumptions about extraterrestrial life and intelligence, as well as on the particular model of future humanity and thus is of rather narrow value. We would like to generalize that treatment here while pointing out that still further generalization is desirable.
13 Parsec, or paralactic second is a standard unit in astronomy: 1 pc = 3.086 x 1016 m. One parsec is, for instance, the average distance between stars in the solar neighbourhood.
There is one particular exception to this generally optimistic view on the (im)possibilities of interstellar warfare which can be worrying: the so-called 'deadly probes' scenario for explaining Fermi's paradox (e.g., Brin, 1983; for fictional treatments of this idea see Benford, 1984; Schroeder, 2002). If the first or one of the first sets of self-replicating von Neumann probes to be released by Galactic civilizations was either programmed to destroy other civilizations or mutated to the same effect (see Benford, 1981), this would explain the 'Great Silence' by another non-exclusive risk. In the words of Brin (1983) '[i]ts logic is compellingly self-consistent'. The 'deadly probes' scenario seems to be particularly disturbing in conjunction with the basic theme of this book, since it shares some of the features of conventional technological optimism vis-a-vis the future of humanity: capacity of making self-replicating probes, AI, advanced spaceflight propulsion, probably also nanotechnology.
It is unfortunate that the 'deadly probes' scenario has not to date been numerically modelled. If is to be hoped that future astrobiological and SETI research will explore these possibilities in the more serious and quantitative manner. In the same time, our astronomical SETI efforts, especially those aimed at the discovery of astroengineering projects (Freitas, 1985; Cirkovic and Bradbury, 2006) should be intensified. The discovery of any such project or artefact (see Arnold, 2005) could, in fact, gives us strong probabilistic argument against the 'deadly probes' risk and thus be of long-term assuring comfort.14
A related, but distinct, set of threats follows from the possible inadvertent activities of extraterrestrial civilizations which can bring the destruction to humanity. A clear example of such activities are quantum field theory-related risks (see Chapter 16, this volume), especially the vacuum decay triggering.
14 A version of the 'deadly probes' scenario is a purely informatics concept of Moravec (1988), where the computer viruses roam the Galaxy using whatever physical carrier available and replicating at the expense of resources of any receiving civilization. This, however, hinges on the obviously Umited capacity to pack sufficiently sophisticated self-replicating algorithm in the bit-string of size small enough to be received non-deformed often enough - which raises some interesting issues from the point of view of algorithmic information theory (e.g., Chaitin, 1977). It seems almost certain that the rapidly occurring improvements in information security will be able to clear this possible threat in check.
A 'new vacuum' bubble produced anywhere in the visible universe - say by powerful alien particle accelerators - would expand at the speed of light, possibly encompassing the Earth and humanity at some point. Clearly, such an event could, in principle, have happened somewhere within our cosmological horizon long ago, the expanding bubble not yet having reached our planet. Fortunately, at least with a set of rather uncontroversial assumptions, the reasoning of Tegmark and Bostrom explained in Section 6.2.4 above applies to this class of events, and the relevant probabilities can be rather tightly constrained by using additional astrobiological information. The conclusion is optimistic since it gives a very small probability that humanity will be destroyed in this manner in the next billion years.
Continue reading here: The Simulation Argument
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