Treason doth never prosper: what’s the reason? Why if it prosper, none dare call it treason.
Sir John Harrington (1561 – 1612)
Different types of global catastrophic risks (GCRs) are studied in various chapters of this book by direct analysis. In doing so, researchers benefit from a detailed understanding of the interplay of the underlying causal factors. However, the causal network is often excessively complex and difficult or impossible to disentangle. Here, we would like to consider limitations and theoretical constraints on the risk assessments which are provided by the general properties of the world in which we live, as well as its contingent history. There are only a few of these constraints, but they are important because they do not rely on making a lot of guesses about the details of future technological and social developments. The most important of these are observation selection effects.
Physicists, astronomers, and biologists have been familiar with the observational selection effect for a long time, some aspects of them (e.g., Malmquist bias in astronomy1 or Signor-Lipps effect in paleontology2) being the subject of detailed mathematical modelling. In particular, cosmology is fundamentally incomplete without taking into account the necessary ‘anthropic bias’: the conditions we observe in fundamental physics, as well as in the universe at large, seem atypical when judged against what one would expect as ‘natural’ according to our best theories, and require an explanation compatible with our existence as intelligent observers at this particular epoch in the history of the universe. In contrast, the observation selection effects are still often overlookedin philosophy and epistemology, and practically completely ignoredin risk analysis, since they usually do not apply to conventional categories of risk (such as those usedin insurance modelling). Recently, Bostrom (2002a) laid foundations for a detailed theory of observation selection effects, which has applications for both philosophy and several scientific areas including cosmology, evolution theory, thermodynamics, traffic analysis, game theory problems involving imperfect recall, astrobiology, andquantum physics. The theory of observation selection effects can tell us what we should expect to observe, given some hypothesis about the distribution of observers in the world. By comparing such predictions to our actual observations, we get probabilistic evidence for or against various hypotheses. 3
Many conclusions pertaining to GCRs can be reached by taking into account observation selection effects. For instance, people often erroneously claim that we should not worry too much about existential disasters, since none has happened in the last thousand or even million years. This fallacy needs to be dispelled. Similarly, the conclusion that we are endangered primarily by our own activities and their consequences can be seen most clearly only after we filter out selection effects from our estimates.
In the rest of this chapter, we shall consider several applications of the anthropic reasoning to evaluation of our future prospects: first the anthropic overconfidence argument stemming from the past-future asymmetry in presence of intelligence observers (Section 6.2) and then the (in) famous Doomsday Argument (DA; Section 6.3). We proceed with Fermi’s paradox and some specific risks related to the concept of extraterrestrial intelligence (Section 6.4) and give a brief overview of the Simulation Argument in connection with GCRs in Section 6.5 before we pass on to concluding remarks.
One important selection effect in the study of GCRs arises from the breakdown of the temporal symmetry between past and future catastrophes when our existence at the present epoch and the necessary conditions for it are taken into account. In particular, some of the predictions derived from past records are unreliable due to observation selection, thus introducing an essential qualification to the general and often uncritically accepted gradualist principle that ‘the past is a key to the future’. This resulting anthropic overconfidence bias is operative in a wide range of catastrophic events, and leads to potentially dangerous underestimates of the corresponding risk probabilities. After we demonstrate the effect on a toy model applied to a single catastrophic event situation in Section 6.2.1, we shall develop the argument in more detail in Section 6.2.2, while considering its applicability conditions for various types of GCRs in Section 6.2.3. Finally, we show that with the help of additional astrobiological information, we may do even better and constrain the probabilities of some very specific exogenous risks in Section 6.2.4. 4
Fig.6.1 A schematic presentation of the single-event toy model. The evidence E consists of our present-day existence.
Consider the simplest case of a single very destructive global catastrophe, for instance, a worse-than-Toba super-volcanic eruption (see Chapter 10, this volume). The evidence we take into account in a Bayesian manner is the fact of our existence at the present epoch; this, in turn, implies the existence of a complicated web of evolutionary processes upon which our emergence is contingent; we shall neglect this complication in the present binary toy model and shall return to it in the next subsection. The situation is schematically shown in Fig. 6.1. The a priori probability of catastrophe is P and the probability of human extinction (or a sufficiently strong perturbation leading to divergence of evolutionary pathways from the morphological subspace containing humans) upon the catastrophic event is Q . We shall suppose that the two probabilities are (1) constant, (2) adequately normalized, and (3) applicable to a particular well-defined interval of past time. Event B2 is the occurrence of the catastrophe, and by E we denote the evidence of our present existence.
The direct application of the Bayes formula for expressing conditional probabilities in form
using our notation, yields the a posteriori probability as
By simple algebraic manipulation, we can show that
that is, we tend to underestimate the true catastrophic risk. It is intuitively clear why: the symmetry between past and future is broken by the existence of an evolutionary process leading to our emergence as observers at this particular epoch in time. We can expect a large catastrophe tomorrow, but we cannot – even without any empirical knowledge – expect to find traces of a large catastrophe that occurred yesterday, since it would have pre-empted our existence today.
Note that
Very destructive events completely destroy predictability! An obvious consequence is that absolutely destructive events, which humanity has no chance of surviving at all (Q = 0), completely annihilate our confidence in predicting from past occurrences. This almost trivial conclusion is not, however, widely appreciated.
The issue at hand is the possibility of vacuum phase transition (see Chapter 16, this volume). This is an example par excellence of the Q = 0 event: its ecological consequences are such that the extinction not only of humanity but also of the terrestrial biosphere is certain. 5 However, the anthropic bias was noticed neither by Hut and Rees (1983), nor by many of subsequent papers citing it. Instead, these authors suggested that the idea of high-energy experiments triggering vacuum phase transition can be rejected by comparison with the high-energy events occurring in nature. Since the energies of particle collisions taking place, for instance, in interactions between cosmic rays and the Earth’s atmosphere or the solid mass of the Moon are still orders of magnitude higher than those achievable in human laboratories in the near future, and with plausible general assumptions on the scaling of the relevant reaction cross-sections with energy, Hut and Rees concluded that in view of the fact that the Earth (and the Moon) survived the cosmic-ray bombardment for about 4.5 Gyr, we are safe for the foreseeable future. In other words, their argument consists of the claim that the absence of catastrophic event of this type in our past light cone gives us the information that the probability P (or its rate per unit time p) has to be so extremely small, that any fractional increase caused by human activities (like building and operating of a new particle collider) is insignificant. If, for example, p is 10−50 per year, then its doubling or even a 1000-fold increase by deliberate human activities is arguably unimportant. Thus, we can feel safe with respect to the future on the basis of our observations about the past. As we have seen, there is a hole in this argument: all observers everywhere will always find that no such disaster has occurred in their backward light cone – and this is true whether such disasters are common or rare.
In order to predict the future from records of the past, scientists use a wide variety of methods with one common feature: the construction of an empirical distribution function of events of a specified type (e.g., extraterrestrial impacts, supernova/gamma-burst explosions, or super-volcanic eruptions). In view of the Bayesian nature of our approach, we can dub this distribution function the a posteriori distribution function. Of course, deriving such function from observed traces is often difficult and fraught with uncertainty. For instance, constructing the distribution function of asteroidal/cometary impactors from the sample of impact craters discovered on Earth (Earth Impact Database, 2005) requires making physical assumptions, rarely uncontroversial, about physical parameters of impactors such as their density, as well as astronomical (velocity distribution of Earth-crossing objects) and geological (the response of continental or oceanic crust to a violent impact, formation conditions of impact glasses) input.
However, the a posteriori distribution is not the end of the story. What we are interested in is the ‘real’ distribution of chances of events (or their causes), which is ‘given by Nature’ but not necessarily completely revealed by the historical record. This underlying objective characteristic of a system can be called its a priori distribution function. It reflects the (evolving) state of the system considered without reference to incidental spatio-temporal specifics. Notably, the a priori distribution function describes the stochastic properties of a chance-generating system in nature rather than the contingent outcomes of that generator in the particular history of a particular place (in this case planet Earth). The relationship between a priori and a posteriori distribution functions for several natural catastrophic hazards is shown in a simplified manner in Table 6.1. Only a priori distribution is useful for predicting the future, since it is not constrained by observation selection effects.
Table 6.1 Examples of Natural Hazards Potentially Comprising GCRs and Two Types of Their Distribution Functions
Fig. 6.2 A sketch of the common procedure for deriving predictions about the future from the past records. This applies to quite benign events as well as to GCRs, but only in the latter case do we need to apply the correction symbolically shown in dashed-line box. Steps framed by dashed line are – surprisingly enough – usually not performed in the standard risk analysis; they are, however, necessary in order to obtain unbiased estimates of the magnitude of natural GCRs.
The key insight is that the inference to the inherent (a priori) distribution function from the reconstructed empirical (a posteriori) distribution must take account of an observation selection effect (Fig 6.2). Catastrophic events exceeding some severity threshold eliminate all observers and are hence unobservable. Some types of catastrophes may also make the existence of observers on a planet impossible in a subsequent interval of time, the size of which might be correlated with the magnitude of the catastrophe. Because of this observation selection effect, the events reflected in our historical record are not sampled from the full events space but rather from just the part of the events space that lies beneath the ‘anthropic compatibility boundary’ drawn on the time-severity diagram for each type of catastrophe. This biased sampling effect must be taken into account when we seek to infer the objective chance distribution from the observed empirical distribution of events. Amazingly, it is usually not taken into account in most of the real analyses, perhaps ‘on naive ergodic grounds’. 6
This observation selection effect is in addition to what we might call ‘classical’ selection effects applicable to any sort of event (e.g., removal of traces of events in the distant part by erosion and other instances of the natural entropy increase; see Woo, 1999). Even after these classical selection effects have been taken into account in the construction of an empirical (a posteriori) distribution, the observation selection effects remain to be corrected for in order to derive the a priori distribution function.
It seems obvious that the reasoning sketched above applies to GCRs of natural origin since, with one partial exception there is no unambiguous way of treating major anthropogenic hazards (like the global nuclear war or misuse of biotechnology) statistically. This is a necessary, but not yet sufficient, condition for the application of this argument. In order to establish the latter, we need natural catastrophic phenomena which are
• sufficiently destructive (at least in a part of the severity spectrum)
• sufficiently random (in the epistemic sense) and
• leaving traces in the terrestrial (or in general local) record allowing statistical inference.
There are many conceivable threats satisfying these broad desiderata. Some examples mentioned in the literature comprise the following:
1. asteroidal/cometary impacts (severity gauged by the Turin scale or the impact crater size)
2. super-volcanism episodes (severity gauged by the so-called volcanic explosivity index (VEI) or a similar measure)
3. Supernovae/gamma-ray bursts (severity gauged by the distance/ intrinsic power)
4. superstrong Solar flares (severity gauged by the spectrum/intrinsic power of electromagnetic and corpuscular emissions).
The crucial point here is to have events sufficiently influencing our past, but without too much information which can be obtained externally to the terrestrial biosphere. Thus, there are differences between kinds of catastrophic events in this regard. For instance, the impact history of the Solar System (or at least the part where the Earth is located) is, in theory, easier to be obtained for the Moon, where erosion is orders of magnitude weaker than on Earth. In practice, in the current debates about the rates of cometary impacts, it is precisely the terrestrial cratering rates that are used as an argument for or against existence of a large dark impactor population (see Napier, 2006; Chapter 11 in this volume), thus offering a good model on which the anthropic bias can, at least potentially, be tested. In addition to the impact craters, there is a host of other traces one attempts to find in field work which contribute to the building of the empirical distribution function of impacts, notably searching for chemical anomalies or shocked glasses (e.g., Schultz et al., 2004).
Supernovae/gamma-ray bursts distribution frequencies are also inferred (albeit much less confidently!) from observations of distant regions, notably external galaxies similar to the Milky Way. On one hand, finding local traces of such events in the form of geochemical anomalies (Dreschhoff and Laird, 2006) is excessively difficult and still very uncertain. This external evidence decreases the importance of the Bayesian probability shift. On the other hand, the destructive capacities of such events have been known and discussed for quite some time (see Chapter 12, this volume; Hunt, 1978; Ruderman, 1974; Schindewolf, 1962), and have been particularly enhanced recently by successful explanation of hitherto mysterious gamma-ray bursts as explosions occurring in distant galaxies (Scalo and Wheeler, 2002). The possibility of such cosmic explosions causing a biotic crisis and possibly even a mass extinction episode has returned with a vengeance (Dar et al., 1998; Melott et al., 2004).
Super-volcanic episodes (see Chapter 10, this volume) – both explosive pyroclastic and non-explosive basaltic eruptions of longer duration – are perhaps the best example of global terrestrial catastrophes (which is the rationale for choosing it in the toy model above). They are interesting for two additional recently discovered reasons: (1) Super-volcanism creating Siberian basaltic traps almost certainly triggered the end-Permian mass extinction (251.4 ± 0.7 Myr before present), killing up to 96% of the terrestrial non-bacterial species (e.g., Benton, 2003; White, 2002). Thus, its global destructive potential is today beyond doubt. (2) Super-volcanism is perhaps the single almost-realized existential catastrophe: the Toba super-eruption probably reduced human population to approximately 1000 individuals, nearly causing the extinction of humanity (Ambrose, 1998; Rampino and Self, 1992). In that light, we would do very well to consider seriously this threat which, ironically in views of historically well-known calamities like destructions of Santorini, Pompeii, or Tambora, has become an object of concern only very recently (e.g., McGuire, 2002; Roscoe, 2001).
As we have seen, one frequently cited argument in the debate on GCRs, the one of Hut and Rees (1983), actually demonstrates how misleading (but comforting!) conclusions about risk probabilities can be reached when anthropic overconfidence bias is not taken into account.
The bias affecting the conclusions of Hut and Rees (1983) can be at least partially corrected by using the additional information coming from astrobiology, which has been recently done by Tegmark and Bostrom (2005). Astrobiology is the nascent and explosively developing discipline that deals with three canonical questions: How does life begin and develop? Does life exist elsewhere in the universe? What is the future of life on Earth and in space? One of the most interesting of many astrobiological results of recent years has been the study by Lineweaver (2001), showing that the Earth-like planets around other stars in the Galactic Habitable Zone (GHZ; Gonzalez et al., 2001) are, on average, 1.8 ± 0.9 Gyr older than our planet (see also the extension of this study by Lineweaver et al., 2004). His calculations are based on the tempo of chemical enrichment as the basic precondition for the existence of terrestrial planets. Moreover, Lineweaver’s results enable constructing a planetary age distribution, which can be used to constrain the rate of particularly destructive catastrophes, like the vacuum decay or a strangelet catastrophe.
The central idea of the Tegmark and Bostrom study is that planetary age distribution, as compared to the Earth’s age, bounds the rate for many doomsday scenarios. If catastrophes that permanently destroy or sterilize a cosmic neighbourhood were very frequent, then almost all intelligent observers would arise much earlier than we did, since the Earth is a latecomer within the habitable planet set. Using the Lineweaver data on planetary formation rates, it is possible to calculate the distribution of birth rates for intelligent species under different assumptions about the rate of sterilization by catastrophic events. Combining this with the information about our own temporal location enables the rather optimistic conclusion that the cosmic (permanent) sterilization rate is at the most of order of one per 109 years.
How about catastrophes that do not permanently sterilize a cosmic neighbourhood (preventing habitable planets from surviving and forming in that neighbourhood)? Most catastrophes are obviously in this category. Is biological evolution on the other habitable planets in the Milky Way influenced more or less by catastrophes when compared to the Earth? We cannot easily say, because the stronger the catastrophic stress is (the larger analogue of our probability 1 – Q is on average), the less useful information can we extract about the proximity – or else – of our particular historical experience to what is generally to be expected. However, future astrobiological studies could help us to resolve this conundrum. Some data already exist. For instance, one recently well-studied case is the system of the famous nearby Sun-like star Tau Ceti which contains both planets and a massive debris disc, analogous to the Solar System Kuiper belt. Modelling of Tau Ceti’s dust disc observations indicate, however, that the mass of the colliding bodies up to 10 km in size may total around 1.2 Earth-masses, compared with 0.1 Earth-masses estimated to be in the Solar System’s Edgeworth-Kuiper Belt (Greaves et al., 2004). Thus, Tau Ceti’s dust disc may have around 10 times more cometary and asteroidal material than is currently found in the Solar System – in spite of the fact that Tau Ceti seems to be about twice as old as the Sun (and it is conventionally expected the amount of such material to decrease with time). Why the Tau Ceti System would have a more massive cometary disc than the Solar System is not fully understood, but it is reasonable to conjecture that any hypothetical terrestrial planet of this extrasolar planetary system has been subjected to much more severe impact stress than the Earth has been during the course of its geological and biological history. 7
The Doomsday Argument (DA) is an anthropic argument purporting to show that we have systematically underestimated the probability that humankind will become extinct relatively soon. Originated by the astrophysicist Brandon Carter and developed at length by the philosopher John Leslie, 8 DA purports to show that we have neglected to fully take into account the indexical information residing in the fact about when in the history of the human species we exist. Leslie (1996) – in what can be considered the first serious study of GCRs facing humanity and their philosophical aspects – gives a substantial weight to DA, arguing that it prompts immediate re-evaluation of probabilities of extinction obtained through direct analysis of particular risks and their causal mechanisms.
The core idea of DA can be expressed through the following thought experiment. Place two large urns in front of you, one of which you know contains 10 balls, the other a million, but you do not know which is which. The balls in each urn are numbered 1, 2, 3, 4, … Now take one ball at random from the left urn; it shows the number 7. This clearly is a strong indication that the left urn contains only 10 balls. If the odds originally were 50:50 (identically looking urns), an application of Bayes’ theorem gives the posterior probability that the left urn is the one with only 10 balls as Ppost (n = 10) = 0.99999. Now consider the case where instead of two urns you have two possible models of humanity’s future, and instead of balls you have human individuals, ranked according to birth order. One model suggests that the human race will soon become extinct (or at least that the number of individuals will be greatly reduced), and as a consequence the total number of humans that ever will have existed is about 100 billion. Even the vociferous optimists would not put the prior probability of such a development excessively low – certainly not lower than the probability of the largest certified natural disaster (so-called ‘asteroid test’) of about 10−8 per year. The other model indicates that humans will colonize other planets, spread through the Galaxy, and continue to exist for many future millennia; we consequently can take the number of humans in this model to be of the order of, say, 1018. As a matter of fact, you happen to find that your rank is about 60 billion. According to Carter and Leslie, we should reason in the same way as we did with the urn balls. That you should have a rank of 60 billion is much more likely if only 100 billion humans ever will have lived than if the number was 1018. Therefore, by Bayes’ theorem, you should update your beliefs about mankind’s prospects and realize that an impending doomsday is much more probable than you thought previously. 9
Its underlying idea is formalized by Bostrom (1999, 2002a) as the Self-sampling Assumption (SSA):
SSA: One should reason as if one were a random sample from the set of all observers in one’s reference class.
In effect, it tells us that there is no structural difference between doing statistics with urn balls and doing it with intelligent observers. SSA has several seemingly paradoxical consequences, which are readily admitted by its supporters; for a detailed discussion, see Bostrom (2001). In particular, the reference class problem (’what counts as an observer?’) has been plaguing the entire field of anthropic reasoning. A possible response to it is an improved version of SSA, ‘Strong SSA’ (SSSA):
SSSA: One should reason as if one’s present observer-moment were a random sample from the set of all observer-moments in its reference class.
It can be shown that by taking more indexical information into account than SSA does (SSA considers only information about which observer you are, but you also have information about, for example, which temporal part of this observer = observer-moment you are at the current moment), it is possible to relativize your reference class so that it may contain different observers at different times, depending partly on your epistemic situation on the occasion. SSA, therefore, describes the correct way of assigning probabilities only in certain special cases; and revisiting the existing arguments for SSA, we find that this is all they establish. In particular, DA is inconclusive. It is shown to depend on particular assumptions about the part of one’s subjective prior probability distribution that has to do with indexical information – assumptions that one is free to reject, and indeed, arguably, ought to reject in light of their strongly counterintuitive consequences. Thus, applying the argument to our actual case may be a mistake; at least, a serious methodological criticism could be made of such an inference.
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 colonizedhard 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.
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; Beaugé 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 of 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 of sending inscribed matter packages over interstellar distances (Rose and Wright, 2004).
• Theoretical grounding for various astroengineering/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 J. 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 nïche) which are – more or less successfully – ‘derived from nature’, lose their relevance. Other important guidelines must be derived which will encompass the vast realm of 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 (Ćirković et al., 2007; Livio, 1999; Wilson, 1994). All 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).
Faced with the aggravated situation vis-à-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 Ćirković, 2004, 2006). None of these is a clear, straight forward 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 of ‘rare 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; Ćirković, 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 (Ćirković, 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).
Somewhat relatedto 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 Ćirković (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 complexification. 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.
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 Fermis 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.
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-à-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; Ćirković 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. 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.
A particular speculative application of the theory of observation selection leads to the so-called Simulation Argument of Bostrom (2003). If we accept the possibility that a future advanced human (post-human) civilization might have the technological capability of running ‘ancestor-simulations’ – computer simulations of people like our historical predecessors sufficiently detailed for the simulated people to be conscious – we run into an interesting consequence illuminated by Bostrom (2003). Starting from a rather simple reasoning for the fraction of observers living in simulations (fsim )
Bostrom reaches the intriguing conclusion that this commits one to the belief that either (1) we are living in the simulation, or (2) we are almost certain never to reach the post-human stage, or (3) almost all post-human civilizations lack individuals who run significant numbers of ancestor-simulations, that is, computer-emulations of the sort of human-like creatures from which they evolved. Disjunct (3) looks at first glance most promising, but it should be clear that it suggests a quite uniform or monolithic social organization of the future, which could be hallmark of totalitarianism, a GCR in its own right (see Chapter 22, this volume). The conclusion of the Simulation Argument appears to be a pessimistic one, for it narrows down quite substantially the range of positive future scenarios that are tenable in light of the empirical information we now have. The Simulation Argument increases the probability that we are living in a simulation (which may in many subtle ways affect our estimates of how likely various outcomes are) and it decreases the probability that the post-human world would contain lots of free individuals who have large computational resources and human-like motives. But how does it threaten us right now?
In a nutshell, the simulation risk lies in disjunct (1), which implies the possibility that the simulation we inhabit could be shut down. As Bostrom (2002b, P. 7) writes: ‘While to some it may seem frivolous to list such a radical or “philosophical” hypothesis next the concrete threat of nuclear holocaust, we must seek to base these evaluations on reasons rather than untutored intuition.’ Until a refutation appears of the argument presented in Bostrom (2003), it would be intellectually dishonest to neglect to mention simulation-shutdown as a potential extinction mode. Until a refutation appears of the argument presented in Bostrom (2003), it would intellectually dishonest to neglect to mention simulation-shutdown as a potential extinction mode.
A decision to terminate our simulation, taken on the part of the post-human director (under which we shall subsume any relevant agency), may be prompted by our actions or by any number of exogenous factors. Such exogenous factors may include generic properties of such ancestor-simulations such as fixed temporal window or fixed amount of allocated computational resources, or emergent issues such as a realization of a GCR in the director’s world. Since we cannot know much about these hypothetical possibilities, let us pick one that is rather straight forward to illustrate how a risk could emerge: the energy cost of running an ancestor-simulation.
From the human experience thus far, especially in sciences such as physics and astronomy, the cost of running large simulations may be very high, though it is still dominated by the capital cost of computer processors and human personnel, not the energy cost. However, as the hardware becomes cheaper and more powerful and the simulating tasks more complex, we may expect that at some point in future the energy cost will become dominant. Computers necessarily dissipate energy as heat, as shown in classical studies of Landauer (1961) and Brillouin (1962) with the finite minimum amount of heat dissipation required per processing of 1 bit of information. 15 Since the simulation of complex human society will require processing a huge amount of information, the accompanying energy cost is necessarily huge. This could imply that the running of ancestor-simulations is, even in advanced technological societies, expensive and/or subject to strict regulation. This makes the scenario in which a simulation runs until it dissipates a fixed amount of energy allocated in advance (similar to the way supercomputer or telescope resources are allocated for today’s research) more plausible. Under this assumption, the simulation must necessarily either end abruptly or enter a prolonged phase of gradual simplification and asymptotic dying-out. In the best possible case, the simulation is allocated a fixed faction of energy resources of the director’s civilization. In such case, it is, in principle, possible to have a simulation of indefinite duration, linked only to the (much more remote) options for ending of the director’s world. On the other hand, our activities may make the simulation shorter by increasing the complexity of present entities and thus increasing the running cost of the simulation. 16
Identifying and correcting for observation selection biases plays the same role as correcting for other biases in risk analysis, evaluation, and mitigation: we need to know the underlying mechanisms of risk very precisely in order to make any progress towards making humanity safe. Although we are not very well prepared for any of the emergencies discussed in various chapters of this book, a general tendency easily emerges: some steps in mitigation have been made only for risks that are rationally sufficiently understood in as objective manner as possible: pandemics, nuclear warfare, impacts, and so on. We have shown that the observation selection acts to decrease the perceived probability of future risks in several wide classes, giving us a false sense of security.
The main lesson is that we should be careful not to use the fact that life on Earth has survived up to this day and that our humanoid ancestors did not go extinct in some sudden disaster to infer that the Earth-bound life and humanoid ancestors are highly resilient. Even if on the vast majority of Earth-like planets life goes extinct before intelligent life forms evolve, we should still expect to find ourselves on one of the exceptional planets that were lucky enough to escape devastation. In particular, the case of Tau Ceti offers a glimpse of what situation in many other places throughout the universe may be like. With regard to some existential risks, our past success provides no ground for expecting success in the future.
Acknowledgement
I thank Rafaela Hildebrand, Cosma R. Shalizi, Alexei Turchin, Slobodan Popović, Zoran Knežević, Nikola Božić, Momˇilo Jovanović, Robert J. Bradbury, Danica Ćirković, Zoran Živković and Irena Diklić for useful discussion and comments.
Bostrom, N. (2002). Anthropic Bias: Observation Selection Effects (New York: Routledge). A comprehensive summary of the theory of observation selection effects with some effective examples and a chapter specifically devoted to the Doomsday Argument.
Bostrom, N. (2003). Are you living in a computer simulation? Philosophical Quarterly, 53, 243–255. Quite accessible original presentation of the Simulation Argument.
Grinspoon, D. (2003). Lonely Planets: The Natural Philosophy of Alien Life (New York: HarperCollins). The most comprehensive and lively written treatment of various issues related to extraterrestrial life and intelligence, as well as excellent introduction into astrobiology. Contains a popular-level discussion of physical, chemical, and biological preconditions for observership.
Webb, S. (2002). Where Is Everybody? Fifty Solutions to the Fermi’s Paradox (New York: Copernicus). The most accessible and comprehensive introduction into the various proposed solutions of Fermi’s paradox.
Ambrose, S.H. (1998). Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans. J. Human Evol., 34, 623–651.
Andrews, D.G. (2003). Interstellar Transportation using Today’s Physics. AIAA Paper 2003–4691. Report to 39th Joint Propulsion Conference &Exhibit.
Annis, J. (1999). An astrophysical explanation for the great silence. J. Brit. Interplan. Soc., 52, 19–22.
Arnold, L.F.A. (2005). Transit lightcurve signatures of artificial objects. Astrophys. J., 627, 534–539.
Asghari, N. et al. (2004). Stability of terrestrial planets in the habitable zone of Gl 777 A, HD 72659, Gl 614, 47 UMa and HD 4208. Astron. Astrophys., 426, 353–365.
Bada, J.L. (2004). How life began on Earth: a status report. Earth Planet. Sci. Lett., 226, 1–15.
Badescu, V. and Cathcart, R.B. (2000). Stellar engines for Kardashev’s type II civilisations. J. Brit. Interplan. Soc., 53, 297–306.
Badescu, V. and Cathcart, R.B. (2006). Use of class A and class C stellar engines to control Sun’s movement in the galaxy. Acta Astronautica, 58, 119–129.
Balashov, Yu. (1991). Resource Letter AP-1: The Anthropic Principle. Am. J. Phys., 59, 1069–1076.
Ball, J.A. (1973). The Zoo hypothesis. Icarus, 19, 347–349.
Barrow, J.D. and Tipler, F.J. (1986). The Anthropic Cosmological Principle (New York: Oxford University Press).
Baxter, S. (2000). The planetarium hypothesis: a resolution of the Fermi paradox. J. Brit. Interplan. Soc., 54, 210–216.
Beaugé, C., Callegari, N., Ferraz-Mello, S., and Michtchenko, T.A. (2005). Resonance and stability of extra-solar planetary systems. In Knež zević, Z. and Milani, A. (eds.), Dynamics of Populations of Planetary Systems. Proceedings of the IAU Colloquium No. 197, pp. 3–18 (Cambridge: Cambridge University Press).
Benford, G. (1981). Extraterrestrial intelligence? Quarterly J. Royal Astron. Soc., 22, 217.
Benford, G. (1984). Across the Sea of Suns (New York: Simon &Schuster).
Benton, M.J. (2003). When Life Nearly Died: The Greatest Mass Extinction of All Time (London: Thames and Hudson).
Bostrom, N. (1999). The Doomsday Argument is alive and kicking. Mind, 108, 539–550.
Bostrom, N. (2000). When machines outsmart humans. Futures, 35, 759–764.
Bostrom, N. (2001). The Doomsday Argument, Adam &Eve, UN++ and Quantum Joe. Synthese, 127, 359.
Bostrom, N. (2002a). Anthropic Bias: Observation Selection Effects in Science and Philosophy (New York: Routledge).
Bostrom, N. (2002b). Existential risks. J. Evol. Technol., http://www.jetpress.org/volume9 /risks.html
Bostrom, N. (2003). Are you living in a computer simulation? Philos. Quarterly, 53, 243–255.
Brillouin, L. (1962). Science and Information Theory (New York: Academic Press).
Brin, G.D. (1983). The Great Silence – the controversy concerning extraterrestrial intelligent life. Quarterly J. Royal Astron. Soc., 24, 283.
Bulgatz, J. (1992). Ponzi Schemes, Invaders from Mars and More Extraordinary Popular Delusions and the Madness of Crowds (New York: Harmony Books).
Cantril, H. (1947). The Invasion from Mars: A Study in the Psychology of Panic (Princeton, NJ: Princeton University Press).
Carter, B. (1983). The anthropic principle and its implications for biological evolution. Philos. Trans. Royal Soc. London A, 310, 347–363.
Caves, C. (2000). Predicting future duration from present age: a critical assessment. Contemp. Phys., 41, 143.
Cavicchioli, R. (2002). Extremophiles and the search for extraterrestrial life. Astrobiology, 2, 281 -292.
Chaitin, G.J. (1977). Algorithmic information theory. IBM J. Res. Develop., 21, 350.
Chernavskii, D.S. (2000). The origin of life and thinking from the viewpoint of modern physics. Physics-Uspekhi, 43, 151–176.
Ćirković, M.M. (2003). On the importance of SETI for transhumanism. J. Evol. Technol., 13. http://www.jetpress.org/volume13/cirkovic.html
Ćirković, M.M. (2004). On the temporal aspect of the Drake Equation and SETI. Astrobiology, 4, 225–231.
Ćirković, M.M. (2006). Too early? On the apparent conflict of astrobiology and cosmology. Biol. Philos., 21, 369–379.
Ćirković, M.M. (2007). Evolutionary catastrophes and the Goldilocks problem. Int. J. Astrobiol. 6, 325–329.
Ćirković, M.M. and Bradbury, R.J. (2006). Galactic gradients, postbiological evolution and the apparent failure of SETI. New Astronomy, 11, 628–639.
Ćirković, M.M., Dragićević, I., and Vukotic, B. (2008). Galactic punctuated equilibrium: how to undermine Carter’s anthropic argument in astrobiology. Astrobiology, in press.
Cohen, J. and Stewart, I. (2002). What Does a Martian Look Like? (Hoboken, NJ: John Wiley &Sons).
Dar, A., Laor, A., and Shaviv, N.J. (1998). Life extinctions by cosmic ray jets. Phys. Rev. Lett., 80, 5813–5816.
Dick, S.J. (2003) Cultural evolution, the postbiological universe and SETI. Int. J. Astrobiol., 2, 65–74.
Dreschhoff, G.A.M. and Laird, C.M. (2006). Evidence for a stratigraphic record of supernovae in polar ice. Adv. Space Res., 38, 1307–1311.
Earth Impact Database. (2005). http://www.unb.ca/passc/ImpactDatabase/
Egan, G. (2002). Schild’s Ladder (HarperCollins, New York).
Ehrenfreund, P. et al. (2002). Astrophysical and astrochemical insights into the origin of life. Rep. Prog. Phys., 65, 1427–1487.
Fogg, M.J. (1987). Temporal aspects of the interaction among the First Galactic Civilizations: The ‘Interdict Hypothesis’. Icarus, 69, 370–384.
Freitas, R.A. Jr (1985). Observable characteristics of extraterrestrial technological civilizations. J. Brit. Interplanet. Soc., 38, 106–112.
Gonzalez, G., Brownlee, D., and Ward, P. (2001). The Galactic Habitable Zone: galactic chemical evolution. Icarus, 152, 185–200.
Gott, J.R. (1993). Implications of the Copernican principle for our future prospects. Nature, 363, 315–319.
Gould, S.J. (1987). SETI and the Wisdom of Casey Stengel. In The Flamingo’s Smile: Reflections in Natural History, pp. 403–413 (New York: W. W. Norton &Company).
Greaves, J.S., Wyatt, M.C., Holland, W.S., and Dent, W.R.F. (2004). The debris disc around τ Ceti: a massive analogue to the Kuiper Belt. MNRAS, 351, L54-L58.
Grinspoon, D. (2003). Lonely Planets: The Natural Philosophy of Alien Life (New York: HarperCollins).
Hanson, R. (1999). Great Filter. Preprint at http://hanson.berkeley.edu/greatfilter.html
Hanson, R. (2001). How to live in a simulation. J. Evol. Technol., 7, http://www.jetpress.org/volume7/simulation.html
Hunt, G.E. (1978). Possible climatic and biological impact of nearby supernovae. Nature, 271, 430–431.
Hut, P. and Rees, M.J. (1983). How stable is our vacuum? Nature, 302, 508–509.
Korycansky, D.G., Laughlin, G., and Adams, F.C. (2001). Astronomical engineering: a strategy for modifying planetary orbits. Astrophys. Space Sci., 275, 349–366.
Kuiper, T.B.H. and Brin, G.D. (1989). Resource Letter ETC-1: Extraterrestrial civilization. Am. J. Phys., 57, 12–18.
Lahav, N., Nir, S., and Elitzur, A.C. (2001). The emergence of life on Earth. Prog. Biophys. Mol. Biol., 75, 75–120.
Landauer, R. (1961). Irreversibility and heat generation in the computing process. IBM J. Res. Develop., 5, 183–191.
Leslie, J. (1989), Risking the World’s End. Bull. Canadian Nucl. Soc., May, 10–15. Leslie, J. (1996). The End of the World: The Ethics and Science of Human Extinction (London: Routledge).
Lineweaver, C.H. (2001). An estimate of the age distribution of terrestrial planets in the Universe: quantifying metallicity as a selection effect. Icarus, 151, 307–313.
Lineweaver, C.H. and Davis, T.M. (2002). Does the rapid appearance of life on earth suggest that life is common in the Universe? Astrobiology, 2, 293–304.
Lineweaver, C.H., Fenner, Y., and Gibson, B.K. (2004). The Galactic Habitable Zone and the age distribution of complex life in the Milky Way. Science, 303, 59–62.
Livio, M. (1999). How rare are extraterrestrial civilizations, and when did they emerge? Astrophys. J., 511, 429–431.
McGuire, B. (2002). A Guide to the End of the World: Everything You Never Wanted to Know (Oxford: Oxford University Press).
Melott, A.L., Lieberman, B.S., Laird, C.M., Martin, L.D., Medvedev, M.V., Thomas, B.C., Cannizzo, J.K., Gehrels, N., and Jackman, C.H. (2004). Did a gamma-ray burst initiate the late Ordovician mass extinction? Int. J. Astrobiol., 3, 55–61.
Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P., and Friend, C.R.L. (1996). Evidence for life on Earth before 3800 million years ago. Nature, 384, 55–59.
Moravec, H.P. (1988). Mind Children: The Future of Robot and Human Intelligence (Cambridge, MA: Harvard University Press).
Napier, W.M. (2006). Evidence for cometary bombardment episodes. MNRAS, 366, 977–982.
Noble, M., Musielak, Z.E., and Cuntz, M. (2002). Orbital stability of terrestrial planets inside the Habitable Zones of extrasolar planetary systems. Astrophys. J., 572, 1024–1030.
Olum, K. (2002). The doomsday argument and the number of possible observers. Philos. Quarterly, 52, 164–184.
Olum, K. (2004). Conflict between anthropic reasoning and observation. Analysis, 64, 1–8.
Rampino, M.R. (2002). Supereruptions as a threat to civilizations on earth-like planets. Icarus, 156, 562–569.
Rampino, M.R. and Self, S. (1992). Volcanic winter and accelerated glaciation following the Toba super-eruption. Nature, 359, 50–52.
Roscoe, H.K. (2001). The risk of large volcanic eruptions and the impact of this risk on future ozone depletion. Nat. Haz., 23, 231–246.
Rose, C. and Wright, G. (2004). Inscribed matter as an energy-efficient means of communication with an extraterrestrial civilization. Nature, 431, 47–49.
Ruderman, M.A. (1974). Possible consequences of nearby supernova explosions for atmospheric ozone and terrestrial life. Science, 184, 1079–1081.
Sandberg, A. (1999). The physics of information processing superobjects: daily life among the Jupiter brains. J. Evol. Tech., 5, http://www.jetpress.org/volume5/Brains2.pdf
Scalo, J. andWheeler, J.C. (2002). Astrophysical and astrobiological implications of gamma-ray burst properties. Astrophys. J., 566, 723–737.
Schaller, R.R. (1997). Moore’s law: past, present, and future. IEEE Spectrum, June, 53–59.
Schindewolf, O. (1962). Neokatastrophismus? Deutsch Geologische Gesellschaf Zeitschrift Jahrgang, 114, 430–445.
Schroeder, K. (2002). Permanence (New York: Tor Books).
Schultz, P.H. et al. (2004). The Quaternary impact record from the Pampas, Argentina. Earth Planetary Sci. Lett., 219, 221–238.
Tipler, F.J. (1980). Extraterrestrial intelligent beings do not exist. Quarterly J. Royal. Astron. Soc., 21, 267–281.
Ward, P.D. and Brownlee, D. (2000). Rare Earth: Why Complex Life Is Uncommon in the Universe (New York: Springer).
Webb, S. (2002). Where Is Everybody? Fifty Solutions to the Fermi’s Paradox (New York: Copernicus).
Wells, H.G. (1898). The War of the Worlds (London: Heinemann).
White, R.V. (2002). Earth’s biggest ‘whodunnit’: unravelling the clues in the case of the end-Permian mass extinction. Philos. Trans. Royal Soc. Lond. A, 360, 2963–2985.
Williams, D.M., Kasting, J.F., and Wade, R.A. (1997). Habitable moons around extrasolar giant planets. Nature, 385, 234–236.
Wilson, P.A. (1994). Carter on Anthropic Principle Predictions. Brit. J. Phil. Sci., 45, 241–253.
Woo, G. (1999). The Mathematics of Natural Catastrophes (Singapore: World Scientific).