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Page 1: EXTRATERRESTRIAL INTELLIGENCE AND DOOMSDAY: A …saj.matf.bg.ac.rs/166/pdf/01-11.pdf · it is very small for any realistic prior. In other words, ... like Carter’s anthropic argument

Serb. Astron. J. 166 (2003), 1 - 11 UDC 52–37Original scientific paper

EXTRATERRESTRIAL INTELLIGENCE AND DOOMSDAY:A CRITICAL ASSESSMENT OF THE NO-OUTSIDER REQUIREMENT

M. M. Cirkovic1 and V. Milosevic-Zdjelar2

1Astronomical Observatory, Volgina 7, 11160 Belgrade 74, Serbia and Montenegro2Physics Department, University of Winnipeg, Winnipeg MB, R3B 2E9, Canada

(Received: January 23, 2003; Accepted: March 14, 2003)

SUMMARY: The so-called Doomsday Argument has attracted a lot of attentionin recent years. One of the suggested ways to make this argument ineffective isthe so-called No-Outsider requirement, indicating that the presence of other intel-ligent observers (’aliens’) beside humans invalidates the reasoning leading to theapocalyptic conclusion. Obviously, this argumentation bears relevance not only tothe effectiveness of the Doomsday Argument, but also to the issues like the rel-ativization of the reference class in anthropic reasoning, transhumanism and theSETI theory. Hereby we criticically investigate the No-Outsider Requirement andconclude that it is either fallacious or irrelevant.

Key words. extraterrestrial inteligence – history and philosophy of astronomy

1. INTRODUCTION: ANTHROPICREASONING AND THEDOOMSDAY ARGUMENT

The Doomsday argument (henceforth DA)was first formulated some fifteen years ago whenBrandon Carter found a hitherto unnoticed conse-quence of the weak anthropic principle. Carter didnot publish his finding, but the idea was taken up andfurther developed by John Leslie (e.g. 1992, 1993,1996) and Richard Gott (1993, 1994), who had dis-covered the argument independently. Roughly, theDoomsday argument reasons from our temporal posi-tion according to a principle directly analogous to theone followed by the other applications of anthropicreasoning from the expected typicality of our posi-tion in the multiverse or our spatial position withina universe.

The core idea can be expressed through thefollowing urn ball experiment. Assume that two largeurns are put in front of you, and you know that one

of them contains ten balls and the other a million,but you are ignorant as to which is which. The ballsin each urn are numbered 1, 2, 3, 4 ... etc. Now youtake a ball at random from the left urn, and it showsthe number 7. Clearly, this is a strong indicationthat that urn contains only ten balls. If originallythe odds were fifty-fifty (identically-looking urns), anapplication of Bayes’ theorem gives you the posteriorprobability that the left urn is the one with only tenballs as Ppost (n=10) = 0.99999. But now considerthe case where instead of the urns you have two possi-ble models of humanity, and instead of balls you havehuman individuals, ranked according to birth order.One model suggests that human race will soon goextinct (or at least that the number of individualswill be greatly reduced), and as a consequence, thetotal number of humans that will have ever existedis about 1011. The other model indicates that hu-manity will colonize other planets, spread throughthe Galaxy, and continue its existence for many fu-ture millennia; consequently, we can take the num-ber of humans in this model to be of the order of,

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M.M. CIRKOVIC and V. MILOSEVIC-ZDJELAR

say, 1018. As a matter of fact, you happen to findthat your rank is about sixty billion. According toCarter and Leslie, we should reason in the same wayas we did with the urn balls. To have a rank of sixtybillion or so is much more likely if only 100 billionpersons will ever have lived than if there will be 1018

persons. Therefore, by Bayes’ theorem, you shouldupdate your beliefs about mankind’s prospects andrealize that an impending doomsday is much moreprobable than you have previously thought.

Leslie, in particular, has written on the topicat great length and has clarified the Bayesian struc-ture of the argument and defended it against a rangeof philosophical objections and misconceptions. Inrecent years there has been a surge of interest in theDA; e.g. Bostrom (1999, 2000, 2001, 2002a); Kopf,Krtous and Page (1994); Olum (2002); Bostrom andCirkovic (2003). However, most of the publicationshave been in journals of philosophy, and the discus-sion has so far not connected closely to parallel de-bates among cosmologists and astrobiologists.

This state of affairs is unsatisfactory, not onlybecause the DA is concerned with the same subjectmatter (the place and future prospects of intelligentlife in the universe) but also because the Dooms-day argument is an application of the very sameidea of observer randomness that is an integral partof anthropic reasoning in cosmology (Leslie 1989b;Bostrom 2002a). The Doomsday argument is there-fore not an optional additional consideration that canbe brought to bear on the issue, but a direct con-sequence of an assumption already made in discus-sions about how the world-ensemble theories (such asmany versions of inflationary cosmology) might ex-plain the appearance of fine-tuning in the observablecosmos. It is a consistency requirement, there-fore, that the Doomsday argument is to be taken intoaccount in any application of anthropic reasoning. Inparticular, it is so, when the subjects of our investi-gation are actual prospects of survival of humanityin the framework of realistic cosmological models.The latter topic has generated an enormous inter-est among astronomers and philosophers alike in lastseveral decades, as a part of the nascent discipline ofphysical eschatology (e.g. Adams and Laughlin 1997,1999; Tipler 1986; Cirkovic and Bostrom 2000; Oppy2002).

2. A CHEAP SOLUTION:NO-OUTSIDER REQUIREMENT

A rather simple solution to the DA problemhas been proposed by Bostrom (2002a), under thename of no-outsider requirement (henceforth NOR):

The second reason for the doomsayer notto grant a probability shift in the aboveexample is that the no-outsider require-ment is not satisfied. The no-outsiderrequirement states that in applying SSAthere must be no outsiders – beings thatare ignored in the reasoning that really be-long in the reference class. Applying SSAin the presence of outsiders will yield erro-neous conclusions in many cases.

Bostrom extensively uses NOR in dealing with theobjections to DA, but also in an attempt to showthat DA reasoning is formally valid, but should notworry us seriously, since in the real world none of theassumptions behind it is satisfied.

What justification does Bostrom offer forNOR? Although not explicitly obvious, we can findsome arguments in the following analyses:

Consider first the original applicationof DA (to the survival of the humanspecies). Suppose you were certain thatthere is extraterrestrial intelligent life, andthat you know that there are a million”small” civilizations that will have con-tained 200 billion persons each and a mil-lion ”large” civilizations that will have con-tained 200 trillion persons each. Supposeyou know that the human species is oneof these civilizations but you don’t knowwhether it is small or large.

To calculate the probability that doomwill strike soon (i.e. that the humanspecies is ”Small”) we can proceed in threesteps:

Step 1. Estimate the empirical priorPr(Small), i.e. how likely it seems thatgerm warfare etc. will put an end to ourspecies before it gets large. At this stageyou don’t take into account any form of theDoomsday argument or anthropic reason-ing.

Step 2. Now take account of the fact thatmost people find themselves in large civi-lizations. Let H be the proposition ”I ama human.” And define the new probabilityfunction Pr∗( . ) = Pr( . |H) obtained byconditioning on H . By Bayes’ theorem,

Pr∗(Small) = Pr(Small|H) =

=Pr(H |Small) × Pr(Small)

Pr(H).

A similar expression holds for ¬ Small.Assuming you can regard yourself a ran-dom sample from the set of all persons, wehave

Pr(H |Small) =

=200 billion

(200 billion + 200 trillion)× 1 million, and

Pr(H |¬ Small) =

=200 trillion

(200 billion + 200 trillion) × 1 million.

(If we calculate Pr∗(Small) we find thatit is very small for any realistic prior. Inother words, at this stage in the calcula-tion, it looks as if the human species is verylikely long-lasting.)

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Step 3. Finally we take account of DA. LetE be the proposition that you find yourself”early”, i.e. that you are among the first200 billion persons in your species. Condi-tioning on this evidence, we get the poste-rior probability function Pr∗∗ ( . ) =Pr∗( . | E ). So

Pr∗∗(Small) = Pr∗(Small|E) =

=Pr∗(E|Small) × Pr∗(Small)

Pr∗(E).

Note that Pr∗(E | Small) = 1 and Pr∗(E|¬Small) = 1/1000. By substituting backinto the above expressions it is then easyto verify that

Pr∗∗(Small)Pr∗∗(¬ Small)

=Pr(Small)

Pr(¬ Small).

We thus see that we get back theempirical probabilities we started from.The Doomsday argument (in Step 3) onlyserved to cancel the effect which we tookinto account in Step 2, namely that youwere more likely to turn out to be inthe human species given that the humanspecies is one of the large rather thanone of the small civilizations. This showsthat if we assume we know that there areboth ”large” and ”small” extraterrestrialcivilizations – the precise numbers in theabove example don’t matter – then theright probabilities are the ones given by thenaıve empirical prior.

Status of NOR is highly uncertain, since it looksmore than almost anything else in science and phi-losophy like a non sequitur. At first glance, the exis-tence of aliens should not have anything to do withthe fate of humanity. However, the issue here is notthe fate of humanity as such, but rather the formalvalidity of reasoning leading to DA. Thus, NOR hassome chances of success. It is our goal in this paperto show that these chances are rather slim, and thathere the first glance is the correct one.

We shall try to reach this goal along two mainlines. First, we shall attempt to investigate the otherpossible sources of NOR and to demonstrate theirnon sequitur or fallacious nature. We shall demon-strate that NOR is truly closely related to severalconjectures and arguments appearing in the litera-ture, like Carter’s anthropic argument or rather con-troversial Self- Indication Assumption (henceforthSIA), which are sources of much confusion in an-thropic reasoning. In addition, there seems to besome sort of wishful thinking, and partly sociologi-cal pressure leading to some of the reasoning behindNOR.

Second, we wish to point out that NOR fails totake into account very important cosmological facts,notably the existence of cosmological horizons,

as well as new data on the size and geometry of theuniverse. When these are understood properly, theygive us superb reasons for rejection of NOR, at leastin its unrestricted form given above. Jumping some-what ahead of the discussion, we may wish to stateour opinion that there are only two real issues here:(i) whether there are ETIs within our cosmologicalhorizon, and (ii) whether it is the nature of our do-main such that the reference class may be definedat the superhorizon scale or not. We may wish toconsider two cases here: the universe with only vi-sual horizon, and that with both visual and eventhorizons.1 At least in the case of a constant eventhorizon we intuitively seek to restrict NOR to those”outsiders” located within our horizon. However,Bostrom (private communication) does not acceptsuch restriction, and his assessment is repeated, al-beit in critical terms, by Olum (2002). Unrestrictedin this way, NOR does not tell us anything useful orinteresting, since we may be certain that Ntotal = ∞,but the problem is that the restriction is not unique.We shall return to this point in the discourse below.

Finally, we comment upon the relationship ofDA and NOR with one of the most interesting andfascinating enterprises of modern age, which is SETI(Search for ExtraTerrestrial Intelligence). We sug-gest that, in the spirit of Gott’s original argument,DA gives us a consistency argument for explanationof the so-called Fermi’s paradox, which plays a cru-cial role in theoretical SETI studies. This, of course,is antithetical to the spirit of NOR, and presents afurther reason for its rejection. To paraphrase Ein-stein’s well-known judgement on Bohm-de Brogliequantum theory in a letter to Born (e.g. Bell 1987,p. 91), this solution to a DA problem ”seems toocheap”.

3. MARS ATTACKS?

Let us consider a counterfactual world–ratherclose to the actual one in terms of Lewis’ theory ofcounterfactuals (e.g. Lewis 1986)–in which Mars isinhabited by a sophisticated technological civiliza-tion in the manner of H. G. Wells. As is well-known,Martians in Wells’ story are slowly declining in num-bers due to the long-term climatic change on theirnative planet. Suppose that their current popula-tion is rather small, say 106 or so, much smaller thanthe current human population. Would we have takenNOR seriously in such a world? Could we argue thatDA reasoning is invalid because a couple of millionof Martians live several dozens million miles awayfrom Earth? Hardly. Instead, while pondering issueof past histories and relationship to future, we couldchoose one of the two major options. We could (i)consider the existence of Martians as non sequitur(which is certainly a usual, intuitive reaction), theirelevant but admissible piece of information, some-thing similar to the fact that we observe the gravi-

1Of course, there may be a combination of the two, like in the models with decaying cosmological constant.

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tational microlensing optical depth towards LargeMagellanic Cloud to be several times 10−7. Other-wise, we could (ii) simply add terrestrial and Martianpopulations at all times, and then draw our Bayesian(i.e. ”doomslike”) conclusions. In the particularcase, small Martian population would not essentiallychange the conclusions of the doomsyear, as can beeasily checked by elementary calculation.2 In bothoptions (i) and (ii) NOR is refuted, although in dif-ferent manners.

In a sense, the option (i) corresponds to thenegative, and option (ii) to the affirmative answer tothe question ”Could I have been born a Martian?”3

Since the range of answers to this central questionis thus exhausted, it seems that we have reasons toreject NOR in this particular counterfactual world.However, even in the case of accepting option (ii), itis quite legitimate to add the information on whetheryou are an Earthling or a Martian to the informationof one’s own existence in a particular point in his-tory of intelligent beings native to the Solar system.This is something which lies outside of the scope ofthe conventional DA, but presents its generalizationrather than a special case. Now, it looks plausible toassume that this additional information should actu-ally strengthen one’s belief in the impending doom.It is perhaps easiest to see why when you think of thevery meaning of ”doom” in such circumstances. Incontradistinction to the ”single intelligent race” case,now the doomslike consequences encompass both thegeneral case of destroying intelligent beings in the en-tire Solar system (due to the explosion of a nearbyγ-ray burst, for instance), and the particular caseof destruction of such beings only on human homeplanet (due to a collision with an asteroid, for in-stance).

In total, the existence of such a declining Mar-tian population would rather have strengthened ourbelief in impeding doom–something completely con-trary to the spirit of NOR. Now, we may return fromthat counterfactual world to the actual one, in whichthere are no Martians (or at least no multicelled, in-telligent ones), and try to perceive the impact of thisabsence on our probabilistic calculations. It seemsutterly unreasonable to ascribe to possible extrater-restrials living parsecs or kiloparsecs away what wehave denied our neighbours, the Martians in the ex-ample above. If the distance plays no significant role,it seems that we are left with very few reasons to ac-cept NOR.

4. POSSIBLE SOURCES OF NOR

In this Section we analyze possible sources ofcredence in NOR. We shall see that these are eitherinconclusive or irrelevant, or just reflections of wish-ful thinking. Of course, here we exclude the basicsuch source or motivation, that is, the desire to avoidaccepting at face value the unpleasant conclusionsof DA. We have seen, in the previous Section, thatthe bona fide attempt to overcome DA with NOR ishighly counterintuitive, and based upon very dubi-ous premises. If NOR is to be taken seriously, someindependent support has to be found; here we inves-tigate possible sources of such support.

4.1. Computationalism

The essence of this widespread opinion is thatcognition (as the major aspect of intelligent ob-servers) is computation, along with sufficiently com-plicated algorithm (for a review, see for instanceShapiro 1995). It is usually claimed to be onlya ”working hypothesis”, but in practice it is stan-dardly taken much more seriously, especially in cir-cles of cognitive and computer scientists, and bysome physicists and cosmologists as well (e.g. Tipler1994; Deutsch 1997).

Now, computationalism is related to NOR,since it is commonly regarded as an explanationof why all intelligent observers should be included inthe reference class. This is particularly strong argu-ment in various forms of transhumanist ideas (e.g.Drexler 1987; Moravec 1988). However, it does seemto beg to be questioned, since it presupposes a prop-erty of objects never empirically investigated, andon top of that asserts that the same property (notproven to be coherent or even sensible, at that!) en-ables us to classify these objects and put them in thesame reference class. There does seem to be a strongcircular element in this way of reasoning.

In other words, the application of computa-tionalism to the anthropic selection effect problemis doubly dubious. First of all, human minds maynot be the computable after all (as many quite re-spectable researchers have suggested; e.g. Penrose1989), and it seems that this explanatory task hasclear priority over the solving of anthropic puzzleslike DA. Secondly, computationalist dogma indicatesthat a construction of true, human-level or higher ar-tificial intelligence (AI) is a matter of near future.4

2This is certainly so if we suppose that (1) Martian population is, like terrestrial, finite overall to avoid problems with infinities(we shall return to this later), and (2) Martian population in the past has not been so high as to cause what might be called an”inverse” Doomsday Argument, that is our posterior probability of living thus late in the overall history of intelligent beingsin the Solar system is diminished.

3Note that the true answer to this question crucially hinges upon the definition of ”I”, which is one of the most long-standingand difficult problems in the entire history of philosophy. We cannot enter into this dispute here.

4Parenthetically, let us recall that the father of the modern computers, Alan Turing, predicted that we were to have thinkingmachines well before the year 2000!

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Future AIs could, arguably, play the same role in ageneralized form of NOR as aliens in Bostrom’s ver-sion quoted above. The fact that AIs come later inhistory than humans is irrelevant, since some alienscould as well be younger than ourselves somewherein the Galaxy. On the other hand, is it not an inclu-sion of AIs in the reference class in the DA reasoningmanifestly absurd? If AIs entirely replace humans atsome point in the future, this would in fact provethe validity of DA-based prediction, not falsify it.If humanity persists in parallel with AI, should wenot be normally applying Bayesian reasoning (withpossibly slightly modified priors) all along?

In brief, it seems somewhat ironic thatBostrom, correctly, criticizes Korb and Oliver in stat-ing that (Bostrom 2002)

Korb and Oliver’s application of theDoomsday argument form to individual lifespans presupposes a specific solution to theproblem of the reference class. This is theproblem, remember, of determining whatclass of entities from which one should con-sider oneself a random sample...

But at the same time NOR does something similar:presupposes, in a wide and rather vague way, theproperties of an all-inclusive reference class.

4.2. Carter’s Argument

The argument due to astrophysicist BrandonCarter (godfather of all ”anthropic principles”) pur-ports to indicate extreme scarcity of life and intel-ligence in the universe. This argument has usuallybeen taken very seriously by proponents of the an-thropic principles and anthropic thinking in general(e.g. Barrow and Tipler 1986; Bostrom 2002a; Tipler1994). Since it is not widely known, especially amongphysicists and astronomers, we shall summarize ithere.

If characteristic astrophysical (τ∗–say the stel-lar evolution) and biological (τl–say the eucaryotesevolution) timescales are truly uncorrelated, life ingeneral, and intelligent life in particular, forms atrandom epochs with respect to the characteristictimescales of its astrophysical environment (notably,the main-sequence lifetime of the considered star). Inthe Solar system, τ∗ ≈ τl, within the factor of two.However, in general, it should be either τl >> τ∗ orτ∗ >> τl. In the latter case, however, it is difficultto understand why the very first inhabited planetarysystem (that is, the Solar System) exhibits τ∗ ≈ τl

behaviour, since we would then expect that life (andintelligence) arose on Earth, and probably at otherplaces in the Solar System, much earlier than they infact did. This gives us probabilistic reason to believethat τl >> τ∗ (in which case the anthropic selec-tion effects explains very well why we do perceivethe τ∗ ≈ τl case in the Solar System). Thus, theextraterrestrial life and intelligence have to be veryrare, which is the reason why we have not observedthem yet.

If the number of aliens within our cosmolog-ical horizon iz zero or very small, it will not influ-

ence any statistical considerations, and DA will beas valid (or invalid) as it was apart from consideringany aliens. However, as we shall discuss later, themention of horizons (and their heterogeneous nature)is not made anywhere in the statement of NOR. Thisleads to a strange situation that, since the universeis most probably open, containing infinite number ofgalaxies, even extremely small chance of emergenceof life and intelligence will lead to the infinite num-ber of alien beings and civilizations. Thus, Carter’sargument is essentially ineffective as a remedy to theDA problem. However, it does give a sort of psycho-logical advantage to proponents of NOR, who effec-tively postpone the neccesity to deal with any alienobservers till some far, far future (see, for instance,Bostrom 2002b).

This said, it’s worth mentioning that Carter’sargument is inherently on rather shaky legs. Severalcriticisms of it appeared in the literature, two mostimportant being those of Wilson (1994) and Livio(1999). There is a significant interest in deepeningtheir criticisms by including other possible sorts ofcorrelations between the two timescales, thus un-dermining the crucial assumption of the argument(Cirkovic and Dragicevic, in preparation).

4.3. ”Finitism”

The realization that there are most probablyinfinitely many galaxies in the universe, slowly dif-fused from the narrow circle of theoretical cosmolo-gists to the wider, including philosophical, audience.In particular the philosophical community (includingreferees!) shows strong resistance to the idea of infi-nite universe implied by Einstein’s general relativityfor density smaller than or equal to the critical den-sity. Infinity of the universes bears many strangeconceptual consequences, some of them pointed inthe study of Ellis and Brundrit (1979). But, there isno indication that these consequences (like existenceof exact duplicates of all humans that ever existed)are leading into incoherencies or contradictions.

Fortunately, we are spared the empirical ver-ification of these bizarre features by the presence ofcosmological horizons (e.g. Ellis and Rothman 1993).However, there are two possible kinds of horizonsin cosmology: particle horizons and event horizons.All models ever thought to be realistic, including thestandard Big Bang cosmologies, possess particle hori-zons; roughly speaking, it is equivalent to saying thatlight from objects more distant than some value sim-ply did not have time to reach us since the Big Bang.If this were the only type of horizon in the real world,we would have a chance to eventually meet our dou-bles and empirically verify some of the strange fea-tures of an infinite universe at some future epoch.However, the things are not so simple. According tothe new results confirming existence of a large posi-tive cosmological constant, there are not only parti-cle horizons, but the event horizons as well, i.e. thosesurfaces which prevent communication between twoco-moving observers at all times (e.g. Cirkovic andBostrom 2000).

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The formulation of NOR given by Bostromcontains no mention of horizons of any type. Thus, itis immediately liable to the criticism that it enablesa form of causality violation in that some (albeit onlystatistical) influence propagates instantaneously (seealso Olum 2002).

4.4. ”Galactic Club” and liberalism

In our opinion, a further impetus for NORmay be obtained from the early SETI optimism of1960s, carried by ideas of Drake, Sagan, Shklovskii,Bracewell, and others. As is well-known, the enthusi-asm receeded in the meantime, but some of its mainideas deeply underlie the contemporary philosophi-cal and scientific thought and even popular culture.We find Tipler’s (1981, esp. Appendix II) criticismof some of the motivations behind SETI highly jus-tified.5 NOR is the other side of the coin of thesame desire for immortality, known since Gilgamesh,which Tipler finds (and rightly ridicules) in Drakeand some other contact-optimists. By assuming themaximally comprehensible reference class, NOR in-stills the idea that whenever and wherever we en-counter extraterrestrials, they will be able to solveor help solving our own problems. Moreover, it sug-gests that even their very existence anywhere inthe universe is, in a sense, solution to the DA prob-lem; and although people may profess to disbelievethe existence of ETIs, on a psychological level the im-pression of vastness of the universe certainly makesone less worry about DA if one accepts NOR. Thissolution is unsound, simply speaking, since it prej-udicates the solution to the reference-class problem,the most important open problem in the field of an-thropic reasoning.

Really problematic aspect of this attitude con-sists in its profound political consequences here onEarth. It’s clear that there are people, in liberal andleftist circles, who would certainly find such ideasabout ”cosmic club” or even ”cosmic brotherhood”a natural extension of the fight for freedom and uni-versality of human rights among human observerson this planet. However, the extension is more thandubitable, and leads to a sort of anthropocentric car-icature, nonsensical in the same manner as its oppo-site picture of ”alien invaders” or ”buggy-eyed mon-sters”, well-known from (bad) SF movies and litera-ture. Attempting to ”understand” possible alien be-ings as ”human beings with strange physical shapeand/or skin color” are in reality attempts to bothignore the problem and to insult both reason andimagination, since the tacit assumption here is that(i) to be a ”human being” is something universallyexalted by definition, and (ii) we cannot really com-prehend something or somebody that is profoundlydifferent from ourselves. Ironically and indicativelyenough, the attitude (i) has been most loudly pro-claimed in XX century by communist ideologues, likeMaxim Gorky, i.e. intellectual collaborators in thegreatest mass murder in human history, and inde-

scribable sufferings of hundreds of million of thosesame human beings.

Among other things, it is said from time totime that AI studies will enable us to investigatean ”alien” (in the limited sense of non-human) in-telligent being soon enough. As such, the idea per-meates literature in transhumanism, futurology andeven cosmology (paradigmatical example of the lat-ter is Tipler’s Omega-point theory). We find it to beanother instance of rather wishful thinking on be-half of people proposing optimistic, melioristic viewsof the universe.

4.5 Self-Indication Assumption

Finally, there is a sensitive issue of the so-called Self-Indication Assumption (henceforth SIA).Roughly speaking, the idea is that an observeris more likely to observe anything (i.e. to findhim/herself alive) if there is a large collection of ob-servers compared to the case of a small collectionof observers. This assumption has, surprisingly, notbeen precisely defined in the papers proposing ordefending it, but Bostrom (2000)–who criticized it–proposed a name Self-Indication Assumption, as wellas the definition we may use for a start:

SIA: Given the fact that you exist, youshould (other things equal) favor hypothe-ses according to which many observers ex-ist over hypotheses on which few observersexist.

Other locutions used in the literature (e.g. ”one ismore likely to find oneself in the long-lived race”,Olum 2002) are equivalent to this. The fact thatSIA exactly compensates for the DA-inducing prob-ability shift has been demonstrated by Kopf et al.(1994), and it remains beyond doubt. However, theother merits and demerits of SIA have remained anopen question; it has been criticised by Leslie (1996)and Bostrom (1999, 2000).

Again, ironically enough (the subject seemspermeated with irony), the thought experimentBostrom proposes as a justification for NOR–the onewith a million ”small” and a million ”large” civiliza-tions in the universe–is actually invoked by the pro-ponents of SIA, notably Olum (2002). The fact thatSIA cancels the DA has been known since the workof Kopf et al. (1994), but the possibility that some-thing rather vague like NOR can cancel it, certainlysounds presumptuous!

5. INFINITY OF OBSERVERS?

In an open universe, the total number of in-telligent observers is infinite; in the closed case, it isnecessarily finite. How reasonable is, then, that a

5Although, of course, we do not endorse other features of Tipler’s view, notably his reliance on Carter’s argument (see above),or his pseudo-theological conclusions.

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tiny change in the cosmological density fraction Ω =1 ± ε, which represents the boundary between thetwo cases (open vs. closed universe) and which man-ifests itself on the timescales of ∼1010 years, maycause the drastic change in the epistemic probabilityof us being extinct on the timescale of ∼101 years?Not very much, arguably.

If humankind is not entirely miraculous, thereis a finite probability density for a technological civi-lization to arise at each point of spacetime. By virtueof the cosmological principle (large-scale homogene-ity and isotropy of matter and spacetime) all theseprobability densities, when averaged over a largeenough volume, must be the same. In an infiniteuniverse, this immediately means that the numberof technological civilizations is infinite. However, theinfinite number of civilizations entails multiple copiesof every civilization, including multiple copies of our-selves as observers (Ellis and Brundrit 1979). Thus,all relevant reference classes are infinite, and allauxiliary anthropic assumptions become vacuous: inthe same sense as it seems fallacious to derive prob-ability shifts from any finite rank in an infinite set ofobservers (DA), it seems that there is no predictivepower in SIA if the set of observers is (regardless ofthe actual theory used) infinite. How do we com-pare various probabilistic theories if we know thatthere is an actual infinity of observers in any case?Bostrom (2002a) correctly notes that infinities causemany problems in this area of statistical reasoning(Pascal’s Wager, etc.), but he leaves no prescriptionwhat to do when cosmology forces such a situationon us.

(And even if we do not trust the cosmolog-ical principle, it is enough to accept much weakerassumption that variations between spatial regionsare not too large to prevent intelligence from arisingin entire, but infinitesimal fraction of all spacetime.Since intelligent communities/civilizations are dis-crete entities, the relevant densities cannot asymp-totically approach zero, and therefore any sum overinfinitely many regions must diverge.)

6. DA vs. SETI: FALLACITYOF GOTT’S ARGUMENT

Obviously, the validity (or otherwise) of NORbears great relevance to one of the most interestingprojects ever conceived in the history of humankind:the search for extraterretrial intelligence. In any con-sideration regarding the probable evolution and fateof intelligent observers and their communities, we arenecessarily limited by the absence of other such com-munities known to us. This circumstance, togetherwith our knowledge of the relatively large size both

in space and in time of (at least in principle) inhabit-able universe, leads to the problem of ”Great Silence”(Brin 1983), sometimes dubbed the Fermi’s ”para-dox” (although it is certainly misleading to speak ofit as a paradox in a methodological sense) or ”the as-trosociological problem” (Kardashev and Strelnitskij1988; Lipunov 1997). Namely, we expect to perceivetraces of activities of advanced civilizations in ourpast light cone, since the age of the Galaxy is sig-nificantly larger than the age of Earth and the Solarsystem.6 Since we have not perceived such traces,this puts a strong empirical constraint on the char-acter and distribution of intelligent communities inthe Galaxy.

Proponents of DA sometimes claim that it of-fers a solution to the puzzle. Gott (1996) thus writes:

Another point... is that you find your-selves born on the home planet where yourspecies originated. If that is not special,then a reasonable fraction of all intelligentobservers must still be on their home plan-ets. This answers Fermi’s question: Whereare they? (Answer: a significant fractionare still at home just like us.) This is suf-ficient to explain why we have not beencolonized.

Similar to Gott’s idea is the one hinted at by Leslie,but immediately rejected by himself (Leslie 1996, p.192):

Why... do we see no sings of extraterres-trial intelligent beings? Might there bevery many technologically advanced civi-lizations in space-time as a whole, but onlyvery few, and those ones unusually small,at the early time at which I am living?Just conceivably this scenario is correct.But an observer in a technological civi-lization would be far less likely to be inan early period, a period when such civ-ilizations were small and few, than in alater one when they were huge and many.I therefore have grounds for thinking thescenario wrong. This could mean that mytechnological civilization wasn’t among thevery earliest: many others had developedpreviously without making their presenceknown to us humans, probably becausethey quickly became extinct. Alternativelyit could mean that only a very few tech-nological civilizations will ever have devel-oped, in the entire history of the universe.

In our oppinion, Gott’s argument (and Lesli’shint) is fallacious for the following reasons. First ofall, it actually leaves out the key ingredient of DA,that is the Doomsday itself–the reasoning which ap-plies to humanity should apply to other intelligentspecies as well (it is not so obvious, and therefore

6This conclusion holds even if we consider the ”effective” age of the Galaxy defined as the period of time for which the Galacticchemical evolution maintained conditions favorable to appearance of life. As discussed recently by Livio (1999), such condi-tions arose only a billion years or so after the epoch of galaxy formation. Therefore, we conclude that an effective age is still∼ 1010 yrs. By comparison, estimates of the timescale necessary for colonizing the entire galaxy through self-replicating vonNeumann probes is, depending on the exact model, between 107 and 108 yrs.

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NOR has been formulated by Bostrom and others;more on this below). It is improbable (although suchcontrived scenarios are imaginable) that the pop-ulation of a particular advanced intelligent speciesdecreases in the same time as that species ex-pands and investigates many planetary systems inthe galaxy. It is reasonable to conclude that most ofspecies simply do not reach the stage at which thedivision between the population on the home planetand colonizing population is significant.

In addition, a simple analogy with the colo-nization of Earth’s surface shows the fallacity of thisargument. Let us imagine an observer living in pre-Columbian America or on some Polynesian island,and consider his opinions about civilizations arisingon different parts of the Earth’s surface (effectively”other worlds”). His analogue of the Fermi’s para-dox will be as well-defined as our deliberations onthe SETI questions. Notably, he might conclude thatother landmasses are uninhabited, since he perceivesno traces of such visitors, and since there had beenenough time for advanced societies to arise there.We know, a posteriori, that different civilizations didarise on various parts of Earth’s landmass, and thatsome of them eventually reached all inhabited partsof our planet. But in each case, by far the largestpart of the population of colonizing civilizations sim-ply stayed at home (if at all aware of the coloniz-ing endevoar!). Per analogiam, lacking any externalknowledge whatsoever about the motivations and ca-pacities of advanced extraterrestrial civilizations, wemay judge that the case of human expansion is atypical one, and therefore invoke the very Coperni-can assumption to undermine Gott’s ”explanation”of Fermi’s paradox.

The problem with such an explanation, in ad-dition, is that it in principle violates some basic ideasabout the nature of systems sufficiently complex tobe called intelligent. In agreement with the weakreductionist assumption that all systems, includingintelligent and conscious ones, are subject to the lawsof thermodynamics, this means that all observers arebound to consume free energy and produce entropyfor any, even the simplest acts of consciousness, likethe computation.7 In principle, a sufficiently uniformemergence of conscious communities throughout theuniverse would result in the existence of such com-munities of age comparable to that of the universe it-self. This seems strongly precluded by our astronom-ical knowledge, as Fermi and other have noted, evenwithout the actual physical presence of extraterres-trials in our midst. Conceptual nature of this argu-ment is emphasized since its proponents can alwaysclaim that this uniform rate is very low, such that

its characteristic timescale is still large compared tothe present age of the universe (and the structure in-side it), and thus the emergence and disappearanceof conscious communities did not have enough timeto reach an equilibrium state. In other words, theCopernican assumption about emergence of life canyet be saved, but at a large price: it is claimed thatintelligent life is ”almost” miraculous, since for var-ious reasons the preconditions for it are only veryrarely satisfied (e.g. Ward and Brownlee 2000).

However, this chain of reasoning still requiresa particular well-defined ingredient in the history ofeach civilization: its disapperance from the scene.As one may note, if the basic conclusion of DA iscorrect, this may as well be solution to the Fermi’sparadox a fortiori. Among various apocalyptic sce-narios considered in some detail in the book of Leslie(1996), it is worth noticing that even some of thenon-catastrophic ones, like the ”transcedence” of anykind, possibly resulting in a collective conscious-ness, would also act to reduce the efficient cross-section for interaction of the civilization with ex-ternal cosmos, therefore producing the empiricallyestablished ”Great Silence” (e.g. Brin 1983). Thiscircumstantial argument does lend some support toserious study of DA and its possible physical re-alizations. One thing seems rather certain: thedanger of extinction is the heaviest at some earlypoints in the history of each intelligent community,since once this community has learnt to use re-sources beyond its home planet, and possibly be-yond its home planetary system, the risks becomeconsiderably smaller, even if still non-zero.8 Thisis the picture of ”Great Filter” of Hanson (1998)(http://hanson.gmu.edu/greatfilter.html). DA agreeswith the sharp nature of this filter.

But is DA effective on wider, galactic scale atall? As we have seen, Bostrom (2000) claims thatDA should always apply only to the entirereference class. In his interpretation, it effectivelyrules out DA if there are extraterrestrial intelligentbeings of plausible characteristics (more precisely,approximately uniform distribution of longevities oftheir societies). This requirement has been previ-ously argued against by Leslie (1996), and recentlyby Olum (2002). Especially significant is Olum’scriticism which is based on physical causality, stat-ing that it is senseless to claim that beings whichexist outside of our causally connected region (andin Bostrom’s version, most of extraterrestrials wouldactually be located outside of our particle horizon)could change our probability priors. Olum’s studypurports to support SIA, stating that one’s exis-

7While, of course, we do not claim that computation is sufficient condition for intelligence (as proponent of the so-called strongAI conjecture would), but it is reasonable to have it as a necessary condition. This leaves open the option recently stronglydefended by Penrose (1989, 1994), that consciousness necessarily includes non-computable elements.

8Possibility of truly cosmic catastrophes, like the decay of false vacuum (if we are unhappy enough to live in one) or tearingapart the fabric of spacetime due to emergence of naked singularities (Barrow and Tipler 1986), will still present some risk for acommunity of any age. Still, however, even the most pessimistic estimates give such dangers orders of magnitude smaller priorprobabilities than more local challenges, like the nuclear war, extinction due to ecological disruptions or asteroidal/cometaryimpacts. The latter will be considered obsolete (as the sources of extinction and not, of course, as great dangers to be avoided!)once first self-sufficient extra-planetary communities are established, in human case when we establish colonies on Moon orMars, or even in the Earth’s orbit (Bostrom 2002b).

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tence a priori indicates existence of a large number ofobservers. Although Bostrom explicitly rejects SIA(which is indeed based on very suspicious–to say theleast–argumentation), it is interesting and indicativethat in justifying the no-outsider requirement he usesexactly the SIA-like argument:

The Doomsday argument... only served tocancel the effect... that you were morelikely to turn out to be in the humanspecies given that the human species is oneof the large rather than the one of the smallcivilizations. This shows that if we as-sume we know that there are both ”large”and ”small” extraterrestrial civilizations–the precise numbers in the above exampledon’t matter–then the right probabilitiesare the ones given by the naive empiricalprior. Only if there are no ”outsiders” (ex-traterrestrial civilizations) does DA workas intended.

To the present authors, this argumentationseems rather poorly founded. Apart from correctOlum’s criticism, one may always ask what indepen-dent reason one has to put this or that extraterres-trial civilization in the reference class. This does notseem like something that can be established a priori.Rather, except explicitly metaphysical assumptionthat intelligence is somehow so strictly specified thatone cannot make a misidentification of it (very badform of epistemic anthropocentrism), there seems tobe no clear physical reason for such a belief.9 Afterall, if it is so easy to solve the DA problem in thismanner, why not just admit that there are other in-telligent beings on Earth, like dolphins and whales,and we have changed our empirical prior, and ev-erybody is happy! In addition, the idea that thedistribution of longevities will be so uniform as inthe Bostrom’s example (million ”small” and million”large” civilizations) flatly contradicts astrophysicalrealities of any civilization’s environment. For in-stance, one may claim that resources necessary forany form of information processing (characterizingany intelligent being and any community of such be-ings) are ”quantized”, since the natural steps in de-velopment of any society include its home planet,home planetary system and a number of surroundingplanetary systems, its home galaxy, galaxy group,etc. The risks of extinction are expected to vary ininverse proportion to the spatial volume ”technolo-

gized” by a community, and therefore the relativeincrease in probability of surviving past some fixedtime t by acquiring (say) resources of a single planetis very different for ”small” and ”large” civilizations(and any intermediate case as well).

The underlying idea we would like to suggest isthat DA without the no-outsider requirementoffers a consistency argument for the most reason-able solution of the Fermi’s paradox. Such solutionsuggests that intelligent species are genericallytruly frequent in the universe, but the num-ber of those able to survive the ”Great Fil-ter” is very, very small. In other words, oura priori confidence in the individualistic interpreta-tion of Final Anthropic Hypothesis (FAH)10 is sig-nificantly decreased when we are faced with both DAand Fermi’s paradox. This problem does not degen-erate in a vicious circle in the realistic case–as occursin the steady-state cosmological model, for instance–because the age of the universe is finite, and there-fore, it is plausible that the technologization, whichbehaves essentially as diffusion processes in statisti-cal physics, has not yet reached the ”equilibrium”state.

DA, as Leslie has repeatedly emphasized, isnot something ”mystical” or ”fateful”, but just a rea-son for rational reassessment of probabilities of ex-tinction through physical and specific (albeit not nec-essarily known at present) causes. Let us suppose fora moment that the true average risk of extinction ofa young (i.e. not-yet-space-faring) civilization in theGalaxy through cometary/asteroidal bombardmentis thousand times larger than our empirical assess-ment on the basis of Earth’s past in the Solar systemsuggests. In that case, although still not doubtingnatural causes of our existence, we may be rather cer-tain that the number of other societies in the Galaxyis very small, so small indeed that we may well bethe only one galactic society on the verge of seri-ous space travelling capacity. This is in accordancewith the Fermi’s observation, and also in agreementwith the DA conclusion as far as the other intelli-gent species share some common properties with us(basically the nature of their intelligence is such thatthe theory of observational selection effects appliesmutatis mutandis). The anthropic selection effectobviates the need for answering the question why isit exactly us (i.e. homo sapiens) who are ”chosen”in this manner.

9This is not to say that such reason is unimaginable or even unreasonable. This oecumenical assumption can be supported,for instance, by establishing the clear link between the properties of intelligence and consciousness and cosmological initialconditions fixed at the Big Bang singularity (or appropriately generalized spacetime boundary). In fact, one of the present

authors (M. M. C.) has recently proposed, with collaborators, exactly one similar scheme, based on the possible resolution

of the quantum measurement puzzle (Dugic, Cirkovic and Rakovic 2002). However, any such idea has still a long way to gobefore being accepted on physical, rather than metaphysical grounds.

10We refer to the following conjecture: Once intelligent information processing comes into existence, it will never die out(Cirkovic and Bostrom 2000). FAH is susceptible to various differing interpretations. In particular, we need to distinguishbetween the following two meanings: (1) There is at least one intelligent race in the universe that will continue to existindefinitely. For the sake of brevity, we shall call this interpretation individualistic. (2) Any particular intelligent race mighteventually die out, but intelligent life as a whole will exist indefinitely. This interpretation may be termed holistic. Statement1. logically implies statement 2., but not vice versa.

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7. CONCLUSIONS: NO SALVATIONFROM LITTLE GREEN MEN

The interplay between cosmology and philoso-phy has–since Keppler and Galileo–often showed newand in many cases unexpected insights. However,this success story should not lull us into a belief thatit is easy to solve important problems by simply sail-ing on–sometimes murky–waters of philosophical ar-guments. This is the case with the No-Outsider Re-quirement.

It is worth noticing that recently Bostrom haspartially revised his stance on NOR and DA. Now,his claim is (Bostrom 2002a; several private commu-nications) that if we had good direct evidence of thestatistical distribution of the longevity of extraterres-trial civilizations (e.g. if we knew that half of themwere long-lasting and half of them short-lived, as inthe example quoted above) then this would diffusethe DA. This presents a significant and bold move toavoid the most serious of the problems facing NORas described in this essay. In Bostrom’s current view,seemingly, NOR teaches that one cannot in generalarbitrarily choose to omit observers from one’s ref-erence class. One should note that this, however,requires an additional information (the one which isvery difficult to come by, ultimately relying on astro-biology) in order to judge whether we should, afterall, heed the doomsayer’s warnings or not.

The claim that NOR should hold in the partic-ular case of DA as exposed by Gott and Leslie, bearssome emotional charge as well, since it is the samefeeling of cosmic loneliness which frightened Pascaland in modern times motivated both serious SETI(and, one suspects, AI as well) efforts and variousUFO cults and enterprises. However, this should bespelled out explicitly: the thought that aliens can,even by their very presence, save us from doomsday(which, as both Leslie’s book and the recent studyby Bostrom (2002b) indicate, is in most scenarios aconsequence of our own actions) tells us more about’growing pains’ of humanity’s cosmic infancy thanabout the universe, its inhabitants or its probabilis-tic aspects.

In one very specific sense NOR deserves a jus-tification, but it is ethical, not probabilistic. If oneday our SETI efforts are crowned with success and wediscover an advanced extraterrestrial society (or atleast incontrovertible proofs of its existence), we shallobtain the final proof that life and intelligence arenot flukes, but a ubiqutous and persistent phenom-ena of nature. This, in turn, might bolster our self-confidence in the face of great challenges of the fu-ture, like colonization of space or building a truly sta-ble and humane society. If–as seems quite plausible–the greatest source of doomsday risks are human ac-tions, then this situation might alleviate some of theperils. But this understanding of NOR seems remotefrom its original proposed role in Bayesian reasoning.

To summarize, the reference class objection ofKorb and Oliver should be taken as a serious possi-bility of correct relativization of the reference class(and thus offers a ray of hope in solving of this dif-

ficult problem in the field of anthropic reasoning).Before the situation in respect to the reference classproblem is investigated in more detail (which cer-tainly lies beyond the scope of the present study), itis more reasonable to conclude that any extraterres-trial intelligent observers do not properly belong tothe reference class of relevance for DA. Accordingly,we conclude that arguments against DA should besought on different sides, and that aliens cannot–apart from the case of actual contact and physicalinteraction–influence either our fate, or our calcula-tions about it.

Acknowledgements – It is a pleasure to thank NickBostrom for great inspiration and kind support. Oneof the authors (MMC) has been partially supportedby the Ministry of Science, Technology and Develop-ment of Serbia through the projects No. 1196, ”As-trophysical Spectroscopy of Extragalactic Objects”and No. 1468, ”Structure and Kinematics of theMilky Way.”

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VANZEMALjSKA INTELIGENCIJA I SUDNjI DAN:KRITIQKO ISPITIVANjE ZAHTEVA ZA ODSUSTVOM TUINACA

M. M. irkovi1 i V. Miloxevi-Zdjelar2

1Astronomska opservatorija, Volgina 7, 11160 Beograd 74, Srbija i Crna Gora2Physics Department, University of Winnipeg, Winnipeg MB, R3B 2E9, Canada

UDK 52–37Originalni nauqni rad

Takozvani ”Argument Sudnjeg dana”privukao je dosta panje u nauqnim i filo-zofskim krugovima poslednjih godina. Jedanod naqina predloenih da se neprihvatljiv za-kljuqak ovog argumenta neutralixe jeste tzv.”zahtev za odsustvom tuinaca” koji sugeri-xe da prisustvo drugih inteligentnih pos-matraqa (”tuinaca”) obesmixljava rezono-vanje koje vodi ka apokaliptiqkom zakljuqku.

Oqigledno, ova argumentacija je relevantnane samo za efikasnost argumenta Sudnjeg dana,ve i za pitanja kao xto su referentna klasau antropiqkom rasuivanju, transhumanizami teorija SETI projekata. U ovom radu mikritiqki preispitujemo zahtev za odsustvomtuinaca i zakljuqujemo da je on ili pogrexanili irelevantan.

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