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KARDASHEV’S CLASSIFICATION AT 50+: 6 A FINE VEHICLE WITH ROOM FOR IMPROVEMENT 1 0 2 M. M. Círković1,2 n a 1Astronomical Observatory, Volgina 7, 11060 Belgrade 38, Serbia J 7 2Future of Humanity Institute, Faculty of Philosophy, University of Oxford, Suite 8, Littlegate House, 16/17 St Ebbe’s Street, Oxford, OX1 1PT, UK ] E–mail: [email protected] P E . h (Received: November 27, 2015;Accepted: November 27, 2015) p - o r t SUMMARY: We review the history and status of the famous classification of s extraterrestrial civilizations given by the great Russian astrophysicist Nikolai Se- a menovichKardashev,roughlyhalfacenturyafterithasbeenproposed. WhileKar- [ dashev’s classification(or Kardashev’s scale) has often been seen as oversimplified, andmultipleimprovements,refinements,andalternativestoithavebeensuggested, 1 it is still one of the major tools for serious theoretical investigationof SETI issues. v During these 50+ years, several attempts at modifying or reforming the classifica- 2 tion havebeen made; we review some of them here, together withpresenting some 1 ofthescenarioswhichpresent difficultiestothestandardversion. Recentresultsin 1 both theoretical and observational SETI studies, especially the Gˆ infrared survey 5 (2014-2015),havepersuasivelyshownthattheemphasisondetectabilityinherentin 0 Kardashev’sclassificationobtainsnewsignificanceandfreshness. Severalnewmove- mentsandconceptualframeworks,suchastheDysonianSETI, tallyextremelywell . 1 with these developments. So, the apparent simplicity of the classification is highly 0 deceptive: Kardashev’s work offers a wealth of still insufficiently studied method- 6 ological and epistemological ramifications and it remains, in both letter and spirit, perhaps the worthiest legacy of the SETI “founding fathers”. 1 : v Key words. astrobiology – extraterrestrial intelligence – history and philosophy of i astronomy X r a 1. INTRODUCTION: KARDASHEV’S LADDER By their fruits ye shall know them. Matthew, 7:16 One of the achievements of the early days of the Search for ExtraTerrestrial Intelligence (SETI), more than a half century ago now, was a practical Once out of nature I shall never take way of thinking how to classify potential search tar- My bodily form from any natural thing... gets by their impact on physical environment. The William Butler Yeats expressionofthiswasthefamousKardashev’sclassi- fication(or Kardashev’sscale)ofadvancedextrater- restrialsocieties, originallycontainingthree types of civilizationsdetectable,atleastinprinciple,through 1 M.M.CÍRKOVIC´ practical SETI activities:1 success in the entire endeavor are unimportant; it is the taxonomy that matters. Since hypothetical tar- [I]t will prove convenient to classify gets of any particular SETI programmeare likely to technologicallydevelopedcivilizationsin be wildly non-uniform – due to contingency of bio- three types: logical evolution, if nothing else (Gould 1989, Con- way Morris 2003, 2011) – it is only reasonable that I – technological level close to the level such taxonomic scheme is rather coarse-grained. In presently attained on the earth, with energy consumption at ≈ 4 × 1019 addition,itshouldtakeintoaccounttherealisticdis- cretedistributionofmatter–inessence,thefactthat erg/sec. the distribution of matter (both baryonic and non- II – a civilization capable of harness- baryonic) in the universe is distinctly clumpy and ing the energy radiated by its own star that the emergent complex configurations of mat- (for example, the stage of successful ter including living and intelligent and technolog- construction of a “Dyson sphere”...); ical systems need to follow that same clumpiness. energy consumption at ≈ 4 × 1033 Kardashev’s Types thus correspond to the most im- erg/sec. portant (from the point of view of any practical search) classes of objects we encounter in the uni- III – a civilization in possession of en- verse, namely planets, stars (with their planetary ergy on the scale of its own galaxy, systems) and galaxies. While minor deviations from with energy consumption at ≈ 4 × 1044 theprinciplethatlifeandintelligencefollowsthedis- erg/sec. tribution of matter are possible – and indeed desir- able – when considering advanced technological so- Inotherwords,andinmoreconventionalread- ing, we are dealing with the following basic types:2 cieties or taking into account the possibility of interstellar panspermia, these baseline celestial bodies Type 1: a civilization manipulating energy remain the foci of any observational and theoretical resources of its home planet. search. Type 2: a civilization manipulating energy Practicalneedfor ataxonomy inSETIstudies resources of its home star/planetary system. and the hierarchical distribution of matter are two Type 3: a civilization manipulating energy legs on which the significance of Kardashev’s scale resources of its home galaxy. rests; the other two are Copernicanism and univer- sality of evolution. Copernicanism (often called the Why is taxonomy important? Claude Lévi- Principle of Mediocrity, the Principle of Typicality, Strauss famously argued that “Darwin would not etc.) inthenarrowsensetellsusthatthereisnothing have been possible if he had not been preceded by special about the Earth or the Solar System or our Linnaeus.” Historicalexperienceinmanyotherfields Galaxy within large sets of similar objects through- of science (chemistry, particle physics, extragalactic outtheuniverse. Insomewhatbroadersense,itindi- astronomy) strongly confirms this dictum. The un- catesthatthereisnothingparticularlyspecialabout derlying idea is that the very act of formulating ex- us as observers: our temporal or spatial location, planatory hypotheses in any field is impossible to or our location in other abstract spaces of physical, perform without an appropriate taxonomical frame- chemical, biological, etc., parameters are typical or work. And the historical fact that Linnaeus held close to typical. Copernicanism did not only played viewsabout,say,biologicalspeciesandtheirorigina- an important role in the great scientific revolution tion and persistence which were diametrally opposed which coined the moniker, but continues to play a to what we regard as basic tenets of the Darwinian vital part in debates surrounding both classical and revolution does not influence Lévi-Strauss’ conclu- quantum cosmology, and in particular attempts to sion in the least. Linnaeus’ personal beliefs about apply various overarching theories of fundamental theoriginofspeciesandothertaxonswereunimpor- physics to an ensemble of universes, or the multi- tant; his taxonomy was the necessary,indeed magic, verse (e.g. Ellis et al. 2004, Page 2008, Linde and key to understanding. This could be immediately Vanchurin 2010). In the specific case of emerging applied to the SETI research as well: without prej- SETI theory, we have been witnessing attempts to udicating anything about the outcome of the SETI use Copernicanism in order to construct models of searchesorindeedourtheoreticalviewsontheemer- gence and frequency of extraterrestrial civilizations, thesetofhabitableplanetsintheGalaxy(Francket westillneedataxonomicalschemeindicativeofwhat al. 2007, Vukotić and Círković 2012, Hair and Hed- we might expect to find. Our personal beliefs about man2013)ortoconstraintheevolutionofintelligent the existence of SETI targets and the likelihood of 1Kardashev(1964),p. 219. IshalluseArabicnumeralsforKardashev’stypesthroughoutthisstudy,althoughitisahistorical fact that Kardashev, and indeed most subsequent authors, used Roman numerals. Apart from the latter being outdated in general, I have two justifications specific to the problem at hand: (i) Arabic numeration enables natural introduction of fractionalsubtypes,likeType2.5civilization,etc. whichwasalreadyaproblemforCarlSaganin1970s–seethequotebelow; and(ii)thereisnoRomannumeralforzero,whileitseemslogicallynaturaltointroduceType0civilization(anditsfractional successors)asthepre-technological stateofanyintelligentcommunity. MoreonthisinSection2. 2See Shklovskii and Sagan (1966) as the “urtext“ in this respect; Michaud (2007) or Bennett and Shostak (2011) for the prototypical “textbook” approachtotheclassification(andindeedmostSETIissues). 2 KARDASHEV’SCLASSIFICATIONAT50+ observers (Carter 2008, 2012, Gleiser 2010). Karda- lativeandmysticalauthorslikeTsiolkovskyorH.G. shev’s classification relies on Copernicanism for the Wells or Stapledon – has become part of the scien- underlying assumption that both the increase in en- tific discourse (for the historical accounts see Crow ergy consumption and the overall resources in the 1986, Dick 1996, Kragh 2004). In this context, the astrophysical environment are in general typical for emergence of Kardashev’s scale as a practical “rule the universe at large; some advanced exceptions to ofthumb”forquantifyingthiscentralissue–theim- this will be considered below. pact of intelligence on the physical universe – could be regarded as somewhat symbolical for new directions in thinking which came before their time and are only now reaching fruition. So,whyisKardashev’sclassificationstillofvi- tal importance to us after more than 50 years of (so farunsuccessful)SETIefforts? Answerstothisques- tion are multifold. Since 1995, we are in the period which is more and more often referred to as “astrobiological revolution” (e.g. Des Marais and Walter 1999, Grinspoon 2003, Gilmour and Sephton 2004, ChybaandHand2005). Rapidincreaseofourknowl- edge about the cosmic context of abiogenesis and evolution, as well as realization that there are nu- merouspotentialhabitatsforlifeintheGalaxy,have been accompaniedby the increase in both public in- terestandinstitutionalframework,includingnewre- searchdepartments,newpeer-reviewedjournals,etc. Entirelynewkeyconcepts,suchastheGalactic Hab- itableZone (henceforthGHZ,Lineweaver2001,Gon- zalez, Brownlee, and Ward 2001, Gonzalez 2005), have been introduced in this period, and the wider synergy between various fields of astronomical and life sciences has been achieved within this wide as- trobiologicalfront. And yet, much older SETI research (starting with Project OZMA in 1960, or in late 1950s with theworkofCocconiandMorrison1959)hasnotbeen entirelyandhappilyintegratedintotheemergingas- trobiological paradigm – for multiple reasons, some of which go way beyond the realm of science. On the purely cognitive level, the need for smooth in- Fig. 1. Academician Nikolay Semenovich Karda- tegration is obvious, since it follows the physicalist shev, the official photo. (Courtesy of the Russian and evolutionary foundations of all life sciences and Academy of Sciences.) technology. SETI studies cannot be anything but a particular research sector of the overall astrobiological effort; however, there have been unhealthy ten- The evolutionary character of the classifica- sionsonbothsides,fromthe highlevelofphilosoph- tion was rather obvious at the time of its origin, amidst all the great excitement and enthusiasm for ical approaches (best manifested in the rise of the SETIin 1960sand1970s. Kardashevandthe restof “rare Earth” hypothesis) down to the overt funding the “founding fathers” (Drake, Morrison,Bracewell, issues and controversies (Darling 2001, Ward 2005). Oliver,Cocconi,Papagiannis,Shklovsky,andSagan) Therefore, it is of much current methodological and clearly perceived SETI as a means for verifying the practical interest to seek those ideas and concepts assumptions about biological and cultural evolution whichcouldfacilitatethisintegrationandenablesta- they deemed “natural” or “typical” or “default”. bleandfruitfulinteractionofSETIwithothersectors Hence came a rather violent reaction of some crit- oftheastrobiologicalenterprise;thispragmaticargu- ics for perceived trampling on their hallowed turf of ment has been developed in more details in Círković inquiry, either biological (e.g. G. G. Simpson, E. (2012). Itis evenmoreimportantto emphasizesuch Mayr) or philosophical (e.g. N. Rescher, E. Mc- integrative concepts in an epoch in which SETI suf- Mullin, A. Kukla).3 This reaction could not, how- fers from serious perception and image problems. ever,turnthe wheel of historybackward: the role of Onesuchnovelandintegrativeconcepthassuggested mindandintelligenceinthe universeatlarge–what in Bradbury et al. (2011): wasforalongtime theprovinceofafew boldspecu- 3Simpson (1964), Rescher (1985), Mayr (1993), Kukla (2001). Simpson’s strong-worded attack on SETI is the prototype of thissortofcriticism. See,however, Círković(2014) forthepossibilityofmorefruitfulreinterpretationofSimpson’scriticism withtoday’shindsightandinthecontemporaryastrobiologicalcontext. 3 M.M.CÍRKOVIC´ Four strategies that characterize our ence to the 1010 galaxies existing in the supplemental approach, what we have portion of the universe accessible to ob- dubbed Dysonian SETI (...): servation. In any case, the deciding 1. The search for technological prod- word on this question is left to exper- ucts, artefacts and signatures of ad- imental verification. vanced technological civilizations. Weshalltrytoshow,however,thatitismuch 2. Thestudyofpostbiologicalandartifi- more than that – and that, with some quite natu- cially superintelligent evolutionary tra- ral refinements and fine-tunings, it can reasonably jectories, as wellas other relevant fields hope to guide our activities in the SETI domain, of future studies. both practical and theoretical, for quite some time 3. The expansion of admissible SETI to come (perhaps even to the centennial, if it is not target spectrum. too pretentious to speculate). 4. The further development and study The rest of this review is organized as fol- of astrobiology and the achievement of lows. In Section 2, I consider the relation of tighterinterdisciplinarycontactwithre- Kardashev’s scale and the notions of detectability, observation-selection effects, and astrobiological lated astrobiological subfields, including landscape, which can offer some new and provoca- magisteria like computer science, evo- tive perspectives on the place and role of Kardashev lutionary biology, etc. Types in our overall astrobiological research. While This leads to an entire new game. We are Section3isdevotedtorefinementssuchasfractional witnessing renaissance of the extragalactic SETI Types (notably Types 2.x, of relevance for practical searches, most notably the Gˆ infrared search for SETI),Section4 deals withextensions suchas Type Type 2.x/Type 3 civilizations (Wright et al. 2014a, 4. The theme of linking Kardashev’s scale with the b, Griffith et al. 2015) and the search for stellar- future of the universe and the future of humanity is reiterated in Section 5, where I suggest some strate- powered Type 3 civilizations by using the Tully- gies for undermining the applicability of the scale Fisher relation (pioneered by Annis 1999; for new to practical SETI searches, and on two scenarios attempts see Calissendorff 2013, Zackrisson et al. demonstrate how we could obtain wrong inferences 2015). Boththeseoriginalanddynamicalapproaches from naively sticking to the definition of individual sharethegroundingdirectlyinspiredbyKardashev’s Types. In the concluding section, the emphasis on classification and the Dysonian SETI. There is rea- detectability is reiterated in light of the preceding son to believe, therefore, that the extent of SETI analysis,someorthogonaldimensionsforquantifying activities will increase and diversify in the near fu- advanced technological societies are suggested, and ture,sothepresenttopicwillbecomemoreandmore several directions for further research are outlined. relevant in the years ahead. Table1is ofparticularimportanceasitpresentsthe While it was obvious at the time of its origin, maximal generalization of the scale obtained while subsequent use (and occasional misuse) of Karda- retainingtheorganizingprincipleintroducedbyKar- shev’s scale has obscured the key fact: it was meant dashev. to represent a practical guideline for what could be expected in the course of SETI searches, not a pro- 2. DETECTABILITY, SELECTION found theoretical insight into the nature of extrater- EFFECTS, AND THE restrialintelligence. In other words, a rule-of-thumb ASTROBIOLOGICAL LANDSCAPE goodmasonneed before starting workon any build- ing. As Kardashev modestly put it:4 If you do not expect the unexpected, you will not find it; for it is hard to be sought out and diffi- [W]e should like to note that the esti- cult. The pronouncement of Heraclitus of Ephesus mates arrived at here are unquestion- obtains a particular flavor in SETI studies, where ably of no more than a tentative na- proponents have often been labeled speculative fan- ture. But all of them bear witness to tasts or pseudoscientists. Kardashev’s classification thefact that, if terrestrialcivilization is oftenriskssimilarfate –its targetsaretoooftendis- not a unique phenomenon in the entire missed in a hand-waiving manner, without real un- universe, then the possibility of estab- derstanding. And yet, for the reasons given above, lishing contacts with other civilizations the needforsometaxonomicalformhasre-appeared by means of present-day radio physics time and again. capabilities is entirely realistic. At the We may start with limited and modest at- same time, it is very difficult to accept tempts at better elucidation and/or modification of the notion that, of all the 1011 stars the Kardashev’s scale, most notably by Carl Sagan, present in our Galaxy, only near the and recently by Robert Zubrin (Zubrin 1999). In sun has a civilization developed. It is 1973,Sagan wrote:5 still more difficult to extend this infer- 4Kardashev(1964), p. 221. 5Sagan[1973](2000),p. 234. The2000edition,producedbyJeromeAgel,withcontributionsbyFreemanDyson,AnnDruyan, and DavidMorrison,as well as fineillustrationsby Sagan’s oldfriendand collaborator Jon Lombergamong other artists, is averywelcometestimonyonthefreshnessandimportanceofSagan’sthought forthenewmillenium. 4 KARDASHEV’SCLASSIFICATIONAT50+ The energy gap between a Type I and nological civilization. Although it is often down- a Type II civilization, or between a played in the historical SETI discourse, detectabil- Type II and a Type III civilization is ity is clearly the most important parameter in both enormous—a factor of about ten billion theoreticalandpracticalSETIresearch(Tarter2001, in each instance. It seems useful, if the Duric andField 2003)–and the one whichis clearly matter is to be considered seriously, to very difficult to quantify in any detail. Kardashev’s have a finer degree of discrimination. scale gives a very crude, but still quite functional, I would suggest Type 1.0 as a civiliza- wayofquantifyingthepossibledetectability ofatech- tion using 1016 watts for insterstellar nological civilization. In other words, it gives us a communication; Type 1.1, 1017 watts; benchmark for gauging and comparing entities in Type 1.2, 1018 watts, and so on. Our an entirely new and previously unknown context; present civilization would be classed as benchmarks which are at least as useful as those in something like Type 0.7. computer science or risk analysis. This can be understood in the following manner. Morphology of any biosphere not including The equivalent formula suggested by Sagan technological civilization is entirely product of bi- would be ological evolution (presumably following upon pre- 1 E biotic chemical evolution). Therefore, such mor- n=1+ log , (1) 10 10(cid:18)1016 W(cid:19) phologies are located within the huge parameter space of biological evolutionary processes – the where n is the Kardashev type. It agrees within “library of Mendel” in Dennett’s (1995) famous an order of magnitude with Kardashev’s initial es- metaphor. With the advent of intelligent observers timates, and Type 2.0 corresponds to a civilization and their culture, including technology for modify- managing total energy emitted by the Sun (1 L⊙). ing physical and biotic environment, complexity in- This immediately gives us a hint that the en- creases tremendously and the corresponding param- ergyvaluesinKardashev’sscaleshouldnotbetaken eter space expands. Evolutionary trajectories now literally. Solar luminosity is convenient for us, since leadto manymore optionsand the number ofcorre- we are accustomed to it in myriad ways, not only in spondingmorphologiesexponentiallyincreasesinthe our astronomy, where L⊙ is a natural unit for stel- course of the cultural evolution. Navigating this gi- lar luminosities. But majority of stars in the Milky ganticparameterspaceinsearchforsomethingwhich Way are less luminous than our Sun, and the same couldbedetectedbyourmeagerSETIcapabilitiesis holds for majority of stars with potentially habit- completely hopeless unless we find some simple way ableplanets,eveninproperlyconservativeestimates of coarse-graining it; some crude way of interposing whichdo not considerM-dwarfsystems as habitable partitions and ordering this myriads of possibilities (e.g. Vukotić 2010). If we add M-dwarfs, according around a simple handle. This handle is power man- torecentratherliberalmodelsofhabitability(Heath agement/consumption and this task was fulfilled by et al. 1999, Tarter et al. 2007) and the conclusion Kardashev’s scale. thatalargefractionofthempossessEarth-likeplan- The cardinal virtue of Kardashev’s particular ets(Petiguraetal. 2013),thedifferencebetweenthe scheme–andonewhichhasnotbeenentirelyappre- median luminosity and L⊙ becomes profound. And ciated within the framework of the orthodox SETI so far, remaining an active challenge – is that it ex- recall that Kardashev’s scale is justified relative to actly speaks in the language of detectability. It sets the each individual extraterrestrial civilization – in asense, the value of aparticularmilestone is impor- the limits to what is achievable, within the known tant only insofar it indicates the level of complexity laws of physics, and it is exactly those limits which and the magnitude of capacities achieved locally. needtobe probed,especiallyinlightofthefactthat By Eq. (1), the humanity in 2012was of Kar- wenowknowthatsomeoftheEarth-likeplanetsare multipleGyrolderthantheEarth. Accordingtothe dashev’s type 0.72. Human energy resources are about 1.77 × 1013 W, roughly equal to the geother- resultsofLineweaver(2001),aswellasLineweaveret al. (2004), the median age of the Earth-like planets mal energy production in Earth’s interior (Shimizu is: etal. 2011)andarestillmuchsmallerthanthetotal ennuecrlegayrreenseerrgvyesoafvbaoitlahbfilessoionnoaunrdpfluasnioetn,iesstpaekceianlliyntiof τmed =(6.4±0.9)×109 yrs, (2) account. It is still much smaller than the total Solar which strongly suggests that Copernicanism is cor- irradiance integrated over Earth’s geometric cross- section at the upper atmosphere (about 1.74 × 1017 rect, at least regarding the ages of potential bio- spheres in the Galaxy (Lineweaver and Davis 2002, W). So, there is much more room for growth. On the other hand, the increase in power consumption Círković2009,2012). Inthesametime, thequestion of human civilization has been exponential, at least why we do not (yet) perceive any traces, manifes- during the lasttwo centuries,so any reasonablepro- tations, or other sorts of evidence of Gyr-older civi- jectionattimescalesnegligibleinastrophysicalterms lizations becomes particularly pertinent, since naive willleadusverysoontotheType1andsubsequently Copernicanism would suggest that the median age – barring a global catastrophe – to the 1.x status. oftechnologicalcivilizations iscorrespondinglylarger The key emphasis implied by Kardashev’s thanthecaseonEarth. And,asusuallyshownindis- scale – and the reason, I suspect, for its apparent cussions of Fermi’s paradox, the difference between longevity – is the focus on detectability of a tech- timescale in Eq. (2) and the age of our Earth and 5 M.M.CÍRKOVIC´ the Solar System is more than enough not only to the Milky Way thus far offer support to our work- colonizethe Galaxy,but presumablyalsoto createa ing conclusion that there was no Type 3 civilization Type 3 civilization. at any point in our Galaxy’s history. We shall see Obviously, there is no Type 3 civilization in later how this conclusion can be generalized and to the Milky Way at present. This basic empirical fact what important constraints in the parameter space isworthemphasizingforseveralreasons,notablybe- it points. causeitisonewayofformulatingthefamiliarFermi’s WhetherthereareType3civilizationsinsome paradox (we would naively expect one to form on oftheothergalaxieswithinourcosmologicalhorizon knowntemporalandspatialscales,oratleasttowit- remains unknown, but some preliminary indication nesssomeofthemanifestationsofitsemerging).6 In is that they are at best rare (Annis 1999). There addition,wehavegoodreasonsto assumethatthere is a large field for possible empirical studies there, has been no Type 3 civilization in the past of our in searching for possible outliers in the regularity Galaxy. We cannot be entirely certain on this, but of “natural” properties of external galaxies, like the considering the fact that we see no traces of Solar Tully-Fisher relation. However, this type of empiri- System being ever colonized by the galaxy-spanning calworkneedstobebasedonthefurthertheoretical insight and possible numerical modeling of what we large civilization, nor any artifacts or traces of such can reasonably expect Type 3 civilizations to look a civilization, we can exclude this possibility with like. Thishasnotbeendonesofar,forreasonsprob- reasonably high degree of confidence. This hinges ably having to do more with conservatism and bad crucially on the definition of Type 3 civilizations. public image of SETI studies in the scientific com- Smaller civilizations (of what I shall call Type 2.x, munity than with modesty. see Section 3 below) could certainly exist in Milky I shallarguebelow that evenif there is a law- Way’s past – whether any exist at present is an in- like regularity preventing the emergence of Type 3 teresting problem in SETI studies. civilizations in our past light cone, the concept it- But by both original Kardashev’s and the self is still quite fruitful. In particular it is due to modified Zubrin’s understanding, there is simply no the circumstance that such items in the classifica- possibility to reconcile the existence of the local (= tion obtain truly universal value in the cosmological MilkyWay-basedinthefurthertext)Type3civiliza- context, where timescales for propagation of signals tionwiththe astronomicaldata.7 So,this is another are comparable to both astrophysical and biologi- way of formulating Fermi’s paradox or the “Great cal evolutionary timescales. This circumstance links Silence” problem (Brin 1983, Círković 2009, Webb large-scalepropertiesofouruniverse–especiallyho- mogeneity as understood by the classical cosmolog- 2015). “Being stealthy” is at best in tension with ical principle of Eddington, Milne and other early the ascent along Kardashev’s ladder; Type 3 civi- cosmologists – with the concept of astrobiological lizations could arguably be detected over huge dis- evolution. And, of course, in astrobiology we wish tances, and only by making things extremely con- to obtain as universal (and “timeless”) perspective trivedcouldoneconceiveofa“stealthy”Type 3civ- as possible: if humanity is, in due course,to become ilization. Thesamepersistenceappliestoavanished a pangalactic civilizationon the timescale estimated Type 3 civilizationas well, orevenmore: since stay- byFermi, Hart,orTiplerinconnectionwith Fermi’s ingstealthywithsolargeenergyconsumptionwould paradox, it is useful to have such a scenario encom- require much intentional effort, it might well be the passed in a natural way into our classification. It case that untended artifacts or traces of activity of is exactly the concern for future astrobiologicalevo- a galaxy-spanning civilization would be detectable lution which prompts a generalization of the classi- for a long (cosmological) time after the civilization fication to include possible Type 4 civilizations, as itself goes extinct.8 The complete absence of such described in Section 3 below. artefacts or activities discovered by astronomers in 6Of course, there are some weird possibilities which should be mentioned for the sake of completeness. For instance, one of thehypotheses suggestedbyOlum(2004)toaccountforsomestrangeconsequences oftheanthropicreasoningisthatweare actuallypartofalargergalacticcivilizationora“lostcolony”,withoutbeingawareofthefact. 7Ifsomebodystilldoubtsthat,letusmentioninpassingjustahandfuloutofliterallyhundredsofpiecesofempiricalevidence forthat: thestar-formationrateintheMilkyWayissimilartothatinothernormallargespiralgalaxiesandtoourtheoretical understanding of the process (Mutch et al. 2011); stellar population colors (Strateva et al. 2001) or dynamical characteris- tics of its disk are completely in accordance with the models suggested for spiral galaxies per se (Kannappan et al. 2002). Even those wildly speculative suggestions about possible explanations of some astronomical anomalies by astroengineering (e.g. Beech 1990) are squarely local; it would not be an overstatement to claim that the (present-day) existence of Type 3 civilizationintheMilkyWayisatvariancewiththeentireedificeofcontemporarystellarandgalacticastronomy. Thissame evidence weights, somewhat less conclusively, but stilloverwhelmingly, against the existence of a past Type 3 civilizationas well. Thelatterhypothesis compounds theproblembypostulatingextinction ofagalactic-sizecivilization. 8Completelyneglecting averygermaneproblemofwhatcouldbeaconceivable reasonforextinction ofagalaxy-widespecies (while retaining local habitability, as testified by our existence). Such literallycosmological agency seems to be beyond our currentimagination;see,however,Egan(1997), Reynolds (2008). 6 KARDASHEV’SCLASSIFICATIONAT50+ Recently, we have proposed a concept of as- baryonicdarkmatterdistributedthroughthehaloof trobiological landscape as a useful way of thinking theMilkyWay–andotherlargeluminousgalaxies– aboutbiologicalevolutioninthemostgeneralcosmo- as either energy source or other industrial purposes, logicalcontext (Círković2012, Církovićand Vukotić thiswouldnotonlyallowmuchlargerandmorecom- pletecontrolofphysicalenvironment,butalsowould 2013; for a similar concept see also Ashworth 2014). lead to civilizational trajectories not bound to stars This can be imagined as a (hyper)surface consist- andtheirdistribution. Inotherwords,therecouldbe ing of all evolutionary trajectories starting with the 2.x-levelcivilizationsnotlocated(primarily)nearlu- “dead space” taking up most of the universe’s vol- minousstars. Onlyadvancesinfundamentalphysics ume (e.g. the intergalactic space) and the epochs and successful detection of CDM particles (for a re- before any abiogenesis took place anywhere. Subse- view,seeCheungetal. 2012)willthrowsomefurther quent emergence and evolution of life, as well as the light on this part of the overalldesign space. appearanceofintelligence(noogenesis)representeda Why put an emphasis on 2.x types and not, particularmanifestationsofthe increaseincomplex- for example, on 0.x or 1.x which are equally legiti- ity as functions of the spatio-temporal coordinates mate as fractional Kardashev’s Types? The answer andprobablyother,yetpoorlyunderstood,physical, is twofold. One aspect is purely practical: we might chemical,etc.,parameterswhichdrivethechangesin wish to concentrate on what is most likely to be de- complexity. Only a part of this vast set of possible tected and yet be viable. Suppose that we are plan- evolutionary trajectories will be realized within our ning a practical SETI project and that, counterfac- cosmological horizon, in analogy with the fact that tually,we startin the tabula rasa state, withoutany onlyasmallsubsetofallevolutionarypossibleforms empirical information about the real universe and, will be realized in the actual course of the biologi- especially, the real Milky Way. What would be the cal evolution on Earth. But in each case, we do not bestpossibletargetcivilization,theoneeasiesttode- havereasontosuspecttheexistenceofthevastover- tect? Clearly, that would be the Type 3 Milky Way archingsetofconsistentpossibilities(astrobiological civilization. But, clearly, such thing does not exist landscapeorthemorphologicalspaceofevolutionary at present on empirical grounds. In the other ex- biology). treme, Type 0.x civilizations are clearly empirically It is exactly within the landscape framework allowedatourpresentlevelofknowledgeandinlarge that Kardashev’s scale can be usefully interpreted quantities;indeed,itisstillnotpossibletodecisively as a set of attractors of evolutionary trajectories in reject the possibility of technologically very primi- thehigh-complexity(“intelligent”)partoftheoverall tive civilizations of this sort even around the closest astrobiological landscape. Therefore, a task for the stars. So, there is a see-saw situation between de- true SETI theory,which willundoubtedly emerge at tectability and empirical constrains, well-known to somefuturepoint,wouldbetoexplainthedynamics themoreoriginalSETIthinkers: whatwouldbeeas- of evolving different Kardashev Type civilizations, iesttodetectistheleastempiricallyviable,andmore presumably as a function of the age of a galaxy and empirically viable targets are proportionally harder the domicile planetary systems and its other astro- to detect. Such a situation implies the existence of physicalproperties,inadditiontoanumberofchem- anoptimumpositionfor meaningfultargets,the one ical/biological/cognitivevariables. While this might which is easiest to detect when all empirical con- seematallorderatthisjuncture,itisnotthatmuch strains are taken into account. It is my conjecture different from the programme of explaining the dis- here that this optimum lies in the Type 2.x domain, tribution of algae in terms of marine chemical pat- presumably toward the lower part of the scale. terns and water temperature histories (e.g. Van den One admittedly extreme, but still instructive Hoek 2008), to give just one “mundane” example. way of thinking about the constraints of detectability is imposed by the Dyson-sphere model for Kar- dashev’sType2civilizations. Followingpioneersug- 3. REFINING: TYPES 2.x gestion by Dyson (1960),we could expect that suffi- ciently advanced civilizations will tend to use more and more of the “naturally available” nuclear fu- While Kardashev’s study was pioneering in sion energy released by its parent star. This would, linking the abstract and vague concept of the “level to an outside observer, look like bigger and bigger of development” of a civilization with something part of the stellar short-wavelength UV and optical as specific and quantifiable as the energy consump- flux being convertedinto high-entropyinfraredradi- tion and management, it does have some limitations ation corresponding to low working temperature of whichcouldbeovercomebysimplerefinements. One thesupposedalientechnology. Therefore,anomalous part of the limitations comes from the very nature infrared sources are excellent targets for the Dyso- of the distribution of (baryonic) matter in the uni- nian SETI attempts and some searches have been verse. While itseemsclearthatatleastafractionof made in the Solar neighborhood (e.g. Jugaku et al. baryonicmatter is usedinbuilding anadvancedciv- 1995, Jugaku and Nishimura 2003, Timofeev et al. ilization, it is not obvious that non-baryonic matter 2000, Carrigan 2009); continuation of this tendency isentirelyunusableinthisrespect. Incontrast,there consistsinalreadymentionedextragalacticsearches, are some indications that annihilation of CDM particles and antiparticlescouldbecome a viable source along the lines set by Annis (1999). of energy in physical-eschatological future (Adams The other side of the problem deals with the measure of detectability at each particular step (or and Laughlin 1997). If advanced technological civi- lizationsarecapableofutilizingvastamountsofnon- tier) of the scale. Whatever the best estimator of 7 M.M.CÍRKOVIC´ Fig. 2. A schematic representation of Kardashev’s types in an extended framework. While the energy management is largely determined by the fraction of cosmic resources at the disposal of a civilization, the scale of increasing complexity is far less certain to establish since smaller structures could, in principle, be morecomplex than thelarge ones. Type4 pertains totheentireuniversewithin our horizon –anything above it is necessary non-empirical and at best dependent on extremely speculative theories. (Courtesy of Slobodan Popović Bagi.) detectability is – and we do not have a clear view Note that this conclusiondoes not imply that of this in the general case, although of course each the complexity measure is superadditive as well. In particular SETI activity by definition has some esti- contrastto mostofour everydayexperience,smaller mator built in it – we may reasonably argue that it structures could, from the physicalpoint of view, be must be superadditive. Namely, if d is our adopted morecomplex thanthe largeones. Infact, the ubiq- m measure of detectability of a civilization managing uitous process of spontaneous symmetry breaking the set of resources m (including the spatial volume maycausethetotalcomplexityofthewholeuniverse controlled), we may argue that it holds that, on the to be quite small (Tegmark 1996)! While this is ad- average: mittedly an extreme view, it still remains uncertain whether realisticcivilizations,humanor extraterrestrial, possess a well-defined complexity measure and dm+n ≥dm+dn. (3) whether such measure behaves superadditively. Itisclearwhatthis meansforpracticalSETI: specificsearchesshouldmakeacompromisebetween So, for example, a civilization managing 5 planetary the volume and duration surveyed on one hand, systemsismoredetectablethaneither5civilizations and detectability measures on the other. Obsession managing a single planetary system, or two civiliza- with large number of civilizations, including prim- tions,onemanaging3andtheother2planetarysys- itive ones similar to humanity, evolving in parallel tems. Obviously, communications and transport be- (large values of N in the Drake equation) should be tween the managed systems are “extra” potentially toned down, and traces and manifestations of inter- detectable processes. In the worst case, those could stellarcolonizationandappropriateenergyconsump- be undetectable, so that we would have equality in tionshouldbegivenhigherpriority. Thespectrumof Eq.(3)above;butinrealisticcaseweexpectinequal- possible targets should be increased by novel, origi- ity, even very strong one. Quantity does not make nalandcreativetheoreticalwork,conservativehand- up for quality – or a lack of it. waiving notwithstanding. 8 KARDASHEV’SCLASSIFICATIONAT50+ 4. EXTENSIONS AND MODIFICATION shev’s types, but presentsstrong philosophicalargu- ments why we should not hope to detect it at all, While the galaxy-size civilization managing even if it exists; some of those were prefigured in a resourcesofseveraltimes1011ormorestellarobjects superb story-essay of Stanislaw Lem entitled “New and possibly even the CDM content may seem as a Cosmogony” (Lem 1993; the original was written in crown of achievement of any intelligent community, 1971). NotethatLem’sconclusionismoreoptimistic thisisnotnecessarilyso. AlreadyOlafStapledon,in in this respect, since he speculates that it is exactly Star Maker envisioned an even greater control over the influence of universe-wide civilizations on effec- physical and cosmological environment as a possi- tive local laws of physics which could give evidence ble next stage in the universalevolutionofcomplex- of their existence, while the “classical” scenario of ity.9 Stapledon’s narrator has a vision in which he detection or communication is, of course, excluded. travelsthroughspacevisitingaliencivilizationsfrom In a subsequent work, Galántai (2007) rejects Kar- the past and the future, before finally encountering dashev’sclassificationentirelyandarguesthatatax- theeponymousStarMaker,an“eternalandabsolute onomy more appropriate for both SETI and future spirit” who has created all these worlds in a succes- studies would be one based on robustness toward sion of experiments. Each experiment is a universe, various catastrophic risks. So, from level I where a andeachservesdesigningthenextonealittlebetter. puny local disaster can destroy civilization, to level While creationof “basementuniverses” is a possible V where potentially immortal civilizations, immune activity of advanced civilizations which has already to all kinds of threats, could be found, we have a been discussed a bit by humans (e.g. Sato et al. wide spectrum of possibilities. Galántai’s scheme 1982, Farhi and Guth 1987, Holt 2004), one should is open to criticism that robustness of this kind is keep in mind that this of itself does not guarantee rather unlikely to be detected from afar; in addi- further ascent on the Kardashev’s scale. Depending tion, the author is forced to admit that higher levels on the energy requirements for such feat – if pos- of robustness are connected to wider spatial range, sible within the framework of the correct theory of i.e. to the same process of interstellar colonization quantum gravity – it is perhaps not necessary for a and resource utilization on which Kardashev’s scale is based. In a sense, this is an unavoidable compro- civilization to be particularly high on Kardashev’s miseGalántai–togetherwithotherKecskes,Zubrin, ladder to create a “basement universe”. Again, as and other authors – is forced to make with Karda- in the case of possible utilizing of CDM for indus- shev, if the proposed taxonomy is to retain a degree trialpurposes,onlyfurther advancesinfundamental of relevance for practical SETI projects. physics will be able to judge to what extent these We should also consider those aspects of cul- possibilities are realistic; up to that moment, we in- turalevolutionleadingtotheinwardbounds. British clude them here in an extended classification (Table astrophysicist John D. Barrow suggests adding a 1 below) as logically possible. number of civilization types based on the level of In the overall context of rather limited re- control of the microworld:10 search interest for SETI issues, it is not surprising that the alternatives sometime proposed have not Type I-minus is capable of manipulat- really become comparable in popularity in spite of ing objects over the scale of themselves: being potentially more realistic and useful. For in- buildingstructures,mining,joining and stance, Kecskes (1998, 2009) proposes a complex breaking solids; level-basedhierarchyofcivilizationsorderedinterms Type II-minus is capable of manipulat- of transport, communications, material, and energy ing genes and altering the development resources. It contains four basic types (“planet oflivingthings, transplantingorreplac- dwellers, asteroid dwellers, interstellar travellers, in- ingparts of themselves, reading and en- terstellar space dwellers”), to which six additional gineering their genetic code; types have been subsequently added. The motiva- Type III-minus is capable of manipulat- tion is to avoid some the pitfalls and weaknesses of ingmolecules and molecular bonds, cre- Kardashev’s scale, notably, what Kecskes and many ating new materials; others have perceived as “bigger is better” error. Type IV-minus is capable of manipu- While the motivation is sound (and can be illus- lating individual atoms, creating nan- tratedbyexamplesofdefeatingscenarios;seeSection otechnologies on the atomic scale and 5 below), the resulting scheme is too complicated creating complex forms of artificial life; to gain wide acceptance any time soon. The dis- cussionof whatKecskescallsmeasure of“advanced- Type V-minus is capable of manipulat- ness” clearly shows, however, that much multidisci- ing the atomic nucleus and engineering plinaryresearch,includingthatinsocialsciencesand the nucleons that compose it; economy,isnecessarytoachieveanythingmorereal- Type VI-minus is capable of manip- istic and fine-grained than what Kardashev’s crude ulating the most elementary particles scheme offers. of matter (quarks and leptons) to cre- Galántai(2004)suggestsadoptingdesignation ate organized complexity among popu- Type 4 for the natural extension of previous Karda- lations of elementary particles; 9Stapledon (1937), esp. chapters 9–11. 10Barrow(1999), p. 133. 9 M.M.CÍRKOVIC´ culminating in: and energy-efficient pieces of hardware containing Type Ω-minus is capable of manipu- a finite number of individuals sharing, at the ba- lating the basic structure of space and sic level, the same virtual reality (“polises”). Such time. piecesofhardwareareverysmall–incomparisonto astronomical and even geological length scales –and On this scale, the present-day humanity is might be undetectable directly over interstellar dis- about Type II-minus, with some aspirations toward tances even if no stealth is desired or implemented. Type III-minus in fields like graphene or nanotube In terms closer to the spirit of Kardashev’s scale, research. Obviously, levels of Barrow’s scale are not their efficiency might be so high that no significant entirelydecoupledfromKardashev’sTypessince,for amount of waste heat and other products could be example, it is only reasonable to assume that Bar- detectable above the natural background. Since a row’s Type V-minus civilizations could achieve nu- particular ideal of “perfection” has been achieved, clear fusion in more efficient way than just to utilize nosignificantinteractionwiththephysicalsurround- natural stellar fusion via Dyson-shell-type construc- ing is necessary – and it might even be regarded as tions. Possible or likely interrelations between the culturally or morally undesirable. twoscalespresentafascinatingtopicforexploratory Ofcourse,theIntrodusscenarioisinspiredby engineering (Inoue and Yokoo 2011, Vidal 2014), Greg Egan’s polises in his imaginative and intrigu- which lies beyond the scope of the present review. ing1997novelDiaspora. Itisalsorelatedtoconcepts suchasJohnSmart’s“transcension”(Smart2012)or Kurzweil’s “singularity” (Kurzweil 2005, Chalmers 5. SOME DEFEATER SCENARIOS: 2010), although these concepts possess other impor- INTRODUS AND THE tant elements which are irrelevant from the point KRELL MACHINE of view of SETI. The Introdus is characterized by extremely low detectability of a civilization – or an assembly of civilizations – which nevertheless pos- Inordertofacilitatefurtherdiscussionoflimi- sesses advanced scientific knowledge and capacity to tationsoftheKardashevscale,Ifinditusefultocon- performlargefeats ofastro-engineeringifsodesired. sider possible scenarios of systematically erroneous Thatcapacity remains,as far as Egan’sfictional ex- perception of an advanced technologicalcivilization, ample is concerned, undeployed since there is no escaping the clauses of the taxonomy. Those de- impulse, or imperative to do so; even in the face feater or outlier scenarios might be of immense in- of external catastrophe which prompts the epony- terest, since their study could, at least in principle, mous exodus or “diaspora”, posthuman polises re- provide deeper, “low-level”explanations for the reg- main stealthy and undetectable. Their cross-section ularitiesunderlyingKardashev’sscale. Asisusualin science,wearrivetosatisfactoryinsightintolaw-like for any actual or even conceivable detection and/or regularitiesbyinvestigatingcounterfactualcasesand communication technology are extremely low – in exceptions from the rule. The fact that inspiration dramatic contrast to their actual level of civilizational advancement. This could be generalized to a for these is found in pop-cultural references should wider class of evolutionary trajectories in which op- not be cause for any greater hesitation in consider- timization of research, technology and daily life be- ingthemcomparedtothecaseofconceivingthemas pure thought experiments. On the contrary, exam- comes the stable end-value (Círković and Bradbury ples from literary or cinematographic fiction are ad- 2006). vantageousoverthepurethoughtexperiments,since The Krell Machine: A complementary sce- often develop significant details (thus guarding us nario is the one in which large physical power is fromthe coherence gapproblem,see Havel1999). It actually deployed but for some reason is hidden or is also likelier that they will provoke a serious de- remains undetected. In the classical science-fiction bate on the structure of the problem and possible movie Forbidden Planet (Wilcox 1956), humans be- resolutions. comeawareofthe extinctraceofadvancedbeings of Ihavelabeledthesescenariosdefeatingscenar- theplanetAltairIV,onlyknownasthe“Krell”. The ios (ordefeaters),sincetheyrepresentcasesofdisso- Krellhadreacheda stageoftechnologicalandscien- nancebetweentherealstatesofaffairsandourprac- tific development so advanced that they were able tical prospects of detection in SETI projects, while to construct a vast underground machine – about led by Kardashev’s scale. In the sense emphasized 33,000 km3 of operational volume – with virtually in the introductory section Kardashev’s scale is re- unlimitedpower;amachinethatcouldturnthoughts gardedasapracticaltoolforthinkingandsearching; intorealityandprojectthat realityanywhereonthe if the reality is dramatically differentfrom any cases planet. The energy resources of the Krell machine included in an anyway flexible and wide framework, make them safely in the Type 1.x if not Type 2 do- then the framework is essentially defeated from the main. (In the same time, the Krell perfected cogni- practical point of view (while it might retain some tive enhancementto the levelthat their devices, one theoretical relevance). While necessarily subjective, ofwhichisshowninthemovie,wereabletoincrease these two scenarios – or two bunches of scenarios intelligenceandimpartknowledgetoanysentientbe- –encapsulate major points which need further elab- ingina–relatively–non-invasivemanner.) Butthey oration in more detailed, quantitative models. have vanished, a rather modest interval of 200,000 Introdus: An advanced technological civi- years ago, leaving only the artefacts and some terri- lization might consist of individuals uploaded into ble secrets, upon which reckless human protagonists virtual reality supported by extremely miniaturized unwittingly stumble. As a likely a nod to Plato’s 10