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Acta Astronautica

Acta Astronautica 67 (2010) 1025–1029

0094-57

doi:10.1

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journal homepage: www.elsevier.com/locate/actaastro

What ET will look like and why should we care$

Seth Shostak n

SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA

a r t i c l e i n f o

Article history:

Received 21 March 2010

Accepted 19 June 2010Available online 7 July 2010

Keywords:

SETI

Strategies

Extraterrestrial life

Artificial intelligence

65/$ - see front matter & 2010 Elsevier Ltd. A

016/j.actaastro.2010.06.028

is paper was presented during the 60th IAC

esponding author. Tel.: +1 650 960 4530; fax

ail address: sshostak@seti.org

a b s t r a c t

Our experiments to find extraterrestrial life are predicated on the assumption that it is

most likely to be found on so-called ‘‘habitable worlds.’’ These are planets and moons

where surface liquid water exists, and atmospheres of light gases are found. Our

searches presume that life on other worlds has a biochemistry at least somewhat

similar to our own.

While these postulates might be our best guide for finding biology, they could be

misleading us in the search for extraterrestrial intelligence (SETI). Timescale arguments

suggest that shortly after a sentient species invents the technology for communication,

it develops synthetic intelligence. Consequently, SETI’s targeted searches of star systems

that might have habitable planets in the conventional sense may be chasing a very

short-lived prey.

In this paper, we discuss what the implications of post-biological intelligence might

have in directing our SETI experiments.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

SETI experiments of the last 50 years have assumedthat sentient extraterrestrials will be fashioned in ourown image. Our current efforts to eavesdrop on artificiallyconstructed signals reflect this. We do not insist that thebroadcasters be biochemically identical to terrestrial life,or even that they be carbon-based. But SETI observershave almost always chosen target stellar systems that aredeemed likely to include rocky worlds, with liquid seasand thick atmospheres [1]. The only routine exception tothis are the sky surveys that give all patches of celestialreal estate equal a priori weight.

The Drake equation [2], the logical scaffolding fordiscussing the chances of finding sentience elsewhere,assumes that SETI’s prey will arise as biology on a planet,and will have progressed to intelligence and long-livedtechnical capability. But this same formulation also makesclear that the chances of finding a signal depend

ll rights reserved.

in Daejeon’’.

: +1 650 961 7099.

sensitively on the value of L—the technological lifetimeof a sentient society. Typical estimates of L range from 103

to 106 years [3].Therefore targeted SETI searches make two strong,

implicit assumptions about the type of beings they coulddetect: (a) they are the outgrowth of biological processesand (b) they are – even thousands of years after achievingtechnical ability – still inhabiting their natal world (or areemigres to another world of similar type).

This anthropocentric reasoning lies behind the enthu-siasm of SETI practitioners for NASA’s Kepler mission [4],an experiment that, by 2012, will establish the fraction ofstellar systems that have Earth-like worlds—planetsroughly the same size as our own, and orbiting their sunin the so-called ‘‘habitable zone’’, where water remainsliquid. The expectation is that such terrestrial clones willnot be rare.

For SETI, the detailed construction of the aliens isseemingly of no consequence—if these putative beingsare capable of building powerful transmitters or lasers,then that is all we require. Their appearance, physiology,or motivations, are obviously speculative and – judging bythe scarce attention given to the matter – of small interestto SETI practitioners.

S. Shostak / Acta Astronautica 67 (2010) 1025–10291026

However, this indifference to the true nature of theextraterrestrials may be adversely affecting the search.

2. Why we might care

The necessity for considering the nature of extrater-restrial sentience can be gleaned from a historicalexample, namely our changing conception of Martians.

Only a century ago, mostly thanks to the inventivemind and supple writing of Lowell [5], we imagined thatthe inhabitants of Mars were both intelligent and capableof planet-wide engineering. There was, we were told, avast, hydraulic society on Mars. By the 1930s, ourexpectations had diminished, as astronomers found thatthe Red Planet possessed only a very thin, carbon dioxideatmosphere. But even as late as the mid-1970s, we stillexpected some plant life, or at least pond scum (if notponds) on the martian surface, thus prompting theelaborate experiments carried out by the Viking Landers.At least one embryologist had speculated that finding lifeon Mars required no more than landing a mousetrap and acamera on the surface [6]. However, the Viking landers –which did include cameras – failed to find any signs of amartian biota. The biology team concluded that thelandscape was barren [7]. (Note, however, that at leastone member of the Viking team maintains that thisconclusion was marred by misinterpretation of thedata [8].)

For those who expected to find life on Mars, Viking wasa disappointment. It was not the last. In 1996, we weretold that Martians – dead ones – had been found in ameteorite, ALH 84001 [9]. This claim remains contentious,but has led most researchers to the current, modest viewof Martians—if they exist at all, they are likely to bebacteria-like creatures in underground aquifers.

In a century, we went from Martians engaged inplanet-wide civil engineering, to cryptic microbes a fewhundred feet below the planet’s dry, sterile surface.Clearly, there has been an evolution in our image of theseputative neighbors, and similarly an evolution in ourtechniques for discovering for them. We once soughtMartians using modest-size refractors in Flagstaff, Arizo-na. Today we contemplate a search using robotic drillingrigs, sent to the surface of the Red Planet. Perhaps this is anecessary, and salutary, warning for SETI. As our percep-tion of the Martians changed, so too have our techniquesto hunt them down. But our search for ET has not changedin anything like the same degree.

3. Usual assumptions

Our anthropomorphic bias about extraterrestrials isoccasionally justified on the basis of convergent evolution.Morris [10] has argued that the body plan of Homo sapiens

is near-optimal for intelligent beings, and consequentlynon-terrestrial sentients might be – on a dark night andfrom a distance – indistinguishable from ourselves. Buteven those who dispute the idea that the extraterrestrialswill be a physically close approximation to ourselvesmake the usual assumptions about their habitat: (a) on a

planet with liquid water, (b) having a non-negligibleatmosphere, and (c) of an age of at least a billion years orso (to allow time for the evolution of intelligence).

So our targeted SETI experiments, beginning withProject Ozma in 1960 [11], scrutinize star systems thatare assumed to be of the type likely to host Earth-likeplanets. Until recently, this meant stellar cousins of theSun: F-, G-, or K-type stars. Recent work [12] has enlargedthis small range of types, by noting that even closelyorbiting planets – tidally locked worlds – could havehabitable zones. Consequently, modern SETI target listsalso include the very numerous M-dwarfs.

An additional evolution in our ideas of habitability hasbeen theoretical work showing that double-star systemsmight also be suitable homes for long-lived planets [13].So multiple star systems, which had been personae non-

gratae for targeted SETI searches, are now in the candidaterace.

Today, roughly 90% of all star systems seem plausiblehomes for the type of protoplasmic aliens that we havealways pictured. This is an evolution in our strategies, butdoes not include any evolution in our picture of theextraterrestrials. However, with the overwhelming ma-jority of stars now tallied as possible environs fortechnological societies, what more could we do indeciding where to point our telescopes?

Well, here is something both obvious and tangible: westill have little idea of the planetary complement of thesestars, at least as far as habitable worlds are concerned.That is, we still have yet to find a demonstrably Earth-mass – let alone, Earth-like – exoplanet. Presumably, thiswill happen by 2012, as NASA’s Kepler mission completesits planetary inventory. At that point, we can sharpen ourtarget lists by narrowing in on the types of solar systemswhere we, ourselves, might feel at home.

All of this is either history or is in the easily predictednear future. When Frank Drake pivoted his 85 ft antennaat Tau Ceti and Epsilon Eridani, he was checking outcousins of the Sun, where he assumed he might have thebest chance to find cousins of us. And truth be told, westill engage in the same experiment, albeit fattened on thefeed of some good astronomy and excellent improvementin search hardware.

4. Different sort of extraterrestrial

Our searches continue to hunt for the same sort ofextraterrestrial we have imagined since the days ofPercival Lowell. However, a well-established truism ofSETI might serve to enlarge the range of our vision.

That truism is this—should a SETI experiment detect asignal, it is most probably coming from a species far inadvance of our own. This is easily understood by recallingthe Drake Equation, and its last, important term—L, theaverage lifetime of a technological society. If this term isnot large – is not at least 103 or 104 years or more – thenthe chances of stumbling across a signal are small. Ergo, ifwe find a signal, then ex post-facto we have demonstratedthat L is large. Consequently, the chance that we have

S. Shostak / Acta Astronautica 67 (2010) 1025–1029 1027

heard a society within, say, a century or two of our ownlevel is clearly very small.

Detected extraterrestrials will be millennia or morebeyond our own level of development. This fact motivatesthe usual approach to answering the question ‘‘what willthe aliens be like?’’ We simply extrapolate our ownexistence a few thousand years into the future. Perhaps, asis so often depicted by fiction, our descendants of theforty-first century will have taken our own physicaldevelopment farther down the road we have trod so far.We will have less body hair, fewer teeth, and onlyminimal olfactory sense. Our continuing social drifttowards a meritocracy may endow our descendants withlarger brains and less muscled bodies, since the latter areno longer essential. This is the way that many aliens of thephosphor and silver screens are depicted.

Another scenario for our future development is that wewill become bionic. The first steps in putting engineeredappliances into our bodies have already been taken. Insome cases this is just to remedy physiological defects—-

such as cochlear implants. In others, such as RFID devices,implants offer a convenience (and a possible loss ofprivacy). But if you could encode even one bit ofinformation into every atom of a memory device (anumber that is surely too conservative), then all theinformation on the internet could be kept on somethingfar smaller than a grain of sand. If some chip could makethis accessible to your brain, then a serious enhancementof both knowledge, and possibly even your ability tothink, would be achieved. Such off-loading of informationand, presumably, cognitive power is frequently estimatedto be no more than a few decades away.

However, while these speculative scenarios about ourown near future – and by extrapolation, the extraterres-trials’ past – are interesting, they would not cause a shiftin our SETI experiments. We are still looking for carbon-based chemistry on rocky, watery planets.

5. Replacement and improvement

However, another extrapolation of what lies in storefor humanity leads down a different road. That is the pathto artificial intelligence. Perhaps we are only here toinvent our successors.

This has become a popular theme of late. Whileartificial intelligence, or AI, is an endeavor many decadesold, the continued exponential growth in computerpower, typified by Moore’s law [14], implies that evenconsumer grade computers will have the processingpower of a human brain (and much more memory) bythe year 2040 [15]. This headlong and accelerating rushtowards technology that can cognitively outpace itscreators is often called ‘‘the singularity’’ [16,17]. Today,if you go to conventions on AI, the discussion seems tohave moved beyond the question of ‘‘could we ever makea thinking machine’’ to such questions as ‘‘can we ensurethat it has moral behavior’’—which is surely just aneuphemism for ‘‘could we pull the plug, if necessary?’’.

While not everyone agrees that it is possible toconstruct a functional replacement for what Morrison

[18] called ‘‘a slow-speed computer operating in saltwater’’ – namely our brain – the AI community remainssanguine. They also point out that their lack of success todate should not be confounded with a lack of progress.

If this occurs, if we succeed in producing an artificialsentience able to think as deeply as we can, then we willbe quickly outpaced thereafter. We will have lost the race,and will have no hope for a rematch. Moore’s law forcomputational machinery describes a doubling time forperformance (at a given cost) of 18 months. The exactnumbers are not important. What is important is that thisrate of improvement is enormously speedier than any wecould expect from Darwinian evolution—even making theless-than-certain assumption that such evolution wouldmove us in the direction of greater cognitive ability. Thethought that we will keep up because we are going bionicis wishful thinking, and akin to believing that improvedrunning shoes will allow you to beat out a race car. If webuild a machine with the intellectual capability of onehuman, then within 50 years, its successor is better thanall humanity combined.

Assuming that our own technical timescales are notgrossly atypical, this implies something important forSETI: once any society invents the technology that couldput them in touch – once they reach a level that’scomparable to our own, and become detectable with ourlistening experiments – they are at the most only a fewhundred years away from changing their own paradigm ofsentience to artificial intelligence.

It is possible, of course, that the development of AIwould not substantially affect the continued existenceand activities of its biological ancestors, in which case theliving extraterrestrials could still be there to find. But itseems reasonable to suppose that the artificial sentiencewill inevitably outlast and outperform its predecessors.Making this assumption, we can conclude simply on thebasis of ‘‘technological lifetimes’’ that the aliens – at least,any we hear – will be machines.

6. Behavior of advanced sentients

To understand how a shift in our conception ofsentient extraterrestrials from biological to technologicalmight affect our SETI searches, we would be well served tounderstand their motivation and behavior. What is it thatthey would find interesting to do?

While there has been speculation – mostly in the sci-filiterature – about what would be important to a thinkingmachine, there are only three things that seem sufficientlyplausible that we might call them likely:

(a)

High speed computation requires a compact config-uration. Consequently, the machines are unlikely tospread out over vast distances—either by usingdistributed computing via a widespread array ofcentral processors or a swarm of nanobots. An optimalcomputer would be functionally malleable, andcompactly packaged—made up of (still hypothetical)‘‘computronium’’ [19]. In other words, the principalartificial sentients will be localized.

S. Shostak / Acta Astronautica 67 (2010) 1025–10291028

(b)

Because of the very short timescale for improvement,a ‘‘winner take all’’ situation would probably obtainfor thinking machines. That is, the first one to arisewill dominate, at least within a volume of spacethrough which communication (to its competitors) isstill small enough that a ‘‘second genesis’’ would notget ahead.This point can be simply understood. Suppose atechnological society arises in the galaxy at point A,and eventually produces machine intelligence. Thatdevice locates another machine – one that may havebeen around a long time – at point B, which is 100light-years away. Machine B downloads its capabil-ities and knowledge to A, and both machines continueto improve, although machine A is clearly 100 yearsbehind its predecessor B (the download is old, delayedby the time-of-flight). If, however, machines have asmuch as a 10% variation in speed of improvement,then within 1000 years, machine A could overtake B,and become the ‘‘alpha male’’ of that part of thegalaxy. In other words, the lead machine could shiftaround.But if there is any limit on the variation in improve-ment speed, then there is also a limit on how far fromthe first successful machine a competitor could ariseable to surpass its elder. The also-rans might never beable to catch up. So it may be that the oldest machinecultures (and a culture may consist of one machine)could successfully remain in advance of all the othersindefinitely, at least within a volume of space thatmight be thousands of light-years across.

(c)

Space is dangerous, even for machines. So some sort ofDarwinian selection will inevitably take place. If amachine exists now, it is because its mode ofexistence has kept this device from natural disas-ter—or possibly even from deliberate disaster, if sucha phenomenon exists for machines. Perhaps it makeslots of copies. Or at least a few copies, updating asnecessary. It does something to withstand inevitablecatastrophe.

7. Consequences for SETI strategy

What consequences might the scenario describedabove have for our SETI searches? We first consider theadvantages AI would have over biological intelligence:

�

far more intellectually capable, � immortal, or at least amenable to indefinite repair, � capable of Lamarckian evolution—the individual could

improve,

� not dependent on biological environments, � any new instance of an artificial intelligence could be

preloaded with knowledge, and ‘‘born smart’’.

One of the many sequelae of this laundry list of sellingpoints is that these ‘‘intelligently designed’’ aliens canundertake interstellar travel. After all, when you areimmortal, all trips are of the same length. But it is also

true that they need not live on planets. What they requirefor existence is rather simple:

�

energy on which to operate, and � some material from which to fashion new parts (either

replacements or improvements).

In other words, the restriction to watery worlds is not onethat such intelligence is required to heed. Indeed, it isunclear that they need be situated on planets at all. Theinterstellar flux in our part of the galaxy is roughly a wattper square kilometer of collector. Consequently, it is notinconceivable that efficient machines could be situatedanywhere, not just in the immediate neighborhoods ofstars.

While low-power machinery might eke out an ex-istence between the stars, the intellectual giants of theuniverse – machines able to expand their computationalactivities far beyond what we can conceive – might beprodigious consumers of energy. That suggests that thesealpha male sentients might repair to regions of highenergy density, for instance the center of the galaxy. Forthe machines, for whom the trip to the center is only aninconsequential inconvenience, and for whom the badenvironment would be tolerable, this might be a desirableplace to set up shop.

Stellar black holes are another locale offering abundantenergy. Neutron stars, or simply high-luminosity O and Bstars might also be seductive locales for high-metabolismmachinery. Cirkovic et al. [20] have suggested that AIwould prefer the outer regions of the galaxy, where thegelid cold could permit greater thermodynamic efficiency.On the other hand, the obvious shortcoming of such rurallocations is the diminished availability of matter, as thestellar density is low.

We suggest that locales that have the thermodynamicadvantages of the galactic nether regions but still lie inregions of high matter density should also be consideredas stellar targets. Bok globules, incompletely understooddark clouds of interstellar gas and dust, are examples of atarget that seems to accord with the requirements ofhigh-powered computing machinery. While the kinetictemperature in the interstellar medium is typically 100 K,Bok globules often have temperatures o10 K. In additionto offering high thermodynamic efficiency, they are alsorife with interstellar matter. As a practical matter, they arealso conducive to observation—being typically a few to afew hundred light-years in diameter, and at distances ofhundreds to tens of thousands of light-years, they arenicely congruent with the beam sizes of many radiotelescopes.

8. Conclusions

Timescale arguments suggest that the bipedal biologi-cal creatures of yesteryear may be a provincial concept forextraterrestrial intelligence. Since, on the basis of our ownanticipated future, the development of artificial intelli-gence follows the invention of radio by only a fewcenturies, it seems likely that the majority of aliens that

S. Shostak / Acta Astronautica 67 (2010) 1025–1029 1029

are beyond our level are not merely another species; theyare of an entirely different construction altogether.

The nature of our SETI experiments ensures that anysentience we detect will at least as technically advanced aswe are, and the requirement that such sentience be long-lived (the L in the Drake equation) suggests that anythingwe find will likely be centuries or millennia (or more)beyond our level of development, and therefore will sportartificial intelligence. The three factors noted above, namelya compact configuration, a Darwinian imperative, and anextreme hierarchy, seem likely for machine intelligence, andthus for alien intelligence we might find.

On the basis of what seem like obvious long-termrequirements for any deliberate intelligence – matter andenergy – we suggest that regions of high energy densityand/or low temperature should be included in future SETItargeted searches. These include the neighborhoods of hotstars, black holes, neutron stars, and the cold molecularclouds known as Bok globules.

The question for SETI is, would such intelligence producesignals we might detect? That is unclear. Perhaps suchmachines have curiosity and/or altruism. The former mightcause them to seek out worlds where intelligence may havearisen, including worlds like our own. The latter mightencourage them to bring other intelligence into their fold.

Such possibilities provide scant clues for focusing oursearch. But continuing to hunt for our analogs –technically competent biological sentients – may be anenterprise with less than promising prospect, as it focuseson a highly transient prey. Machines have no obviouslimits to the length of their existence, and consequentlycould easily dominate the intelligence of the cosmos.Perhaps we should expend some effort in trying toestablish their presence. To paraphrase Cocconi andMorrison [21], we can succeed only if we try.

References

[1] J.C. Tarter, Results from Project Phoenix: looking up from downunder, in: C.B. Cosmovici, S. Bowyer, D. Werthimer (Eds.),Astronomical and Biochemical Origins and the Search for Life inthe Universe, Editrice Compositori, Bologna, 1997, p. 633.

[2] Frank Drake, Dava Sobel, Is Anyone Out There? The Scientific Search forExtraterrestrial Intelligence, Delacorte Press, New York, 1992.

[3] Steven J. Dick, The Biological Universe: The Twentieth-CenturyExtraterrestrial Life Debate and the Limits of Science, CambridgeUniversity Press, Cambridge, 1996.

[4] W.J. Borucki, D.G. Koch, J. Lissauer, G. Basri, T. Brown, D.A. Caldwell,J.M. Jenkins, J.J. Caldwell, J. Christensen-Dalsgaard, W.D. Cochran,E.W. Dunham, T.N. Gautier, J.C. Geary, D. Latham, D. Sasselov, R.L.Gilliland, S. Howell, D.G. Monet, N. Batalha, Kepler mission status,in: C. Afonso, D. Weldrake, Th. Henning (Eds.), ASP ConferenceSeries, vol. 366, p. 309, Astronomical Society of the Pacific, SanFrancisco, 2007.

[5] Percival Lowell, Mars, Houghton-Mifflin, New York, 1895.[6] Engineering and Science 24 (6) (1961) 11 (Caltech), 1961.[7] Harold P. Klein, Norman H. Horowitz, Gilvert V. Levin, Vance I.

Oyama, Joshua Lederberg, Alexander Rich, Jerry S. Hubbard, GeorgeL. Hobby, Patricia A. Straat, Bonnie J. Berdahl, Glenn C. Carle,Frederick S. Brown, Richard D. Johnson, The Viking biologicalinvestigation: preliminary results, Science 194 (4260) (1976) 99.

[8] G.V. Levin, Modern myths concerning life on Mars, Electroneur-obiologia 11 (5) (2006) 3.

[9] McKay, David S. Gibson, K. Everett Jr., Thomas-Keprta, Kathie L. Vali,Hojatollah Romanek, Christopher S. Clemett, Simon J. Chillier,Xavier D.F. Maechling, R. Claude, Zare, N. Richard, Search for pastlife on Mars: possible relic biogenic activity in martian meteoriteALH84001, Science 273 (5277) (1996) 924.

[10] Morris Simon Conway, Life’s Solution: Inevitable Humans in aLonely Universe, Cambridge University Press, Cambridge, 2003.

[11] F. Drake, How can we detect radio transmissions from distantplanetary systems, Sky and Telescope 39 (1960) 140.

[12] J.C. Tarter, P.R. Backus, R.L. Mancinelli, J.M. Aurnou, D.E. Backman,G.S. Basri, A.P. Boss, A. Clarke, D. Deming, L.R. Doyle, E.D. Feigelson,F. Freund, D.H. Grinspoon, R.M. Haberle, S.A. Hauck, II,M.J. Heath,T.J. Henry, J.L. Hollingsworth, M.M. Joshi, S. Kilston, M.C. Liu, E.Meikle, I.N. Reid, L.J. Rothschild, J.M. Scalo, A. Segura, C.M. Tang, J.M.Tiedje, M.C. Turnbull, L.M. Walkowicz, A.L.Weber, R.E. Young 2006,/arxiv.org/pdf/astro-ph/0609799S.

[13] Elisa V. Quintana, Jack J. Lissauer, Terrestrial planet formation inbinary star systems, arxiv:0706.3444, May 23, 2007.

[14] G. Moore, Cramming more components onto integrated circuits,Electronics Magazine 19 (1965).

[15] Hans Moravec, Robot: Mere Machine to Transcendant Mind, OxfordUniversity Press, Oxford, 2000.

[16] Vernor Vinge, The coming technological singularity, vision-21:interdisciplinary science & engineering in the era of cyberspace, in:Proceedings of a Symposium held at NASA Lewis Research Center,NASA Conference Publication CP-10129, 1993.

[17] Ray Kurzweil, The Singularity is Near, Viking Penguin, New York,2005.

[18] Philip Morrison, Nothing is Too Wonderful to Be True, AIP Press,Woodbury, NY, 1995.

[19] Tommaso Toffoli, Norman Margolus, Programmable matter: con-cepts and realization, Physica D 47 (1991) 263.

[20] Milan M. Cirkovic, Robert J. Bradbury, Galactic gradients, post-biological evolution and the apparent failure of SETI, NewAstronomy 11 (2006) 628.

[21] G. Cocconi, P. Morrison, Searching for interstellar communications,Nature 183 (1959) 844.