Contents lists available at ScienceDirect

Acta Astronautica

Acta Astronautica 67 (2010) 1410–1418

0094-57

doi:10.1

E-m1 In

follows

theoret

have ar

ments—

which g

SETI wo

fanciful

derogat

less scie

some of

adventu

percept

capable

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

Targets and SETI: Shared motivations, life signaturesand asymmetric SETI

William H. Edmondson

School of Computer Science, University of Birmingham, Birmingham B15 2TT, UK

a r t i c l e i n f o

Article history:

Received 5 February 2009

Received in revised form

27 September 2009

Accepted 18 January 2010Available online 5 March 2010

Keywords:

Targets

SETI

Life signatures

Beacons

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

016/j.actaastro.2010.01.017

ail address: w.h.edmondson@cs.bham.ac.uk

response to one reviewer’s concerns this c

. SETI itself is not theoretically well groun

ical predictions, no hypotheses, no experim

e scientific domains—with theories, hypot

which can be considered to be entailed by

ive the enterprise its scientific credibility. C

rkers appear willing to consider entailmen

and more like Science Fiction. I do not u

ory sense; it merely describes speculation/c

ntific than seems appropriate in SETI (and a

it). Sometimes, however, even conventional s

rous and fanciful and SETI workers need t

ion in such cases, although they may indeed

of provoking useful predictions and observa

a b s t r a c t

The argument proposed in this paper is that conventional assumptions which underpin

SETI can be revised in ways which permit a more nuanced approach to the enterprise. It

is suggested that sensible assumptions based on adventurous science include the notion

that we can conjecture helpfully about what we can know about SETI, and that probably

the ETIs for which we are looking are sending signals to us because they know they are

not alone, and are interested to help us learn that we are not alone. Additionally,

existing work using Pulsars as Beacons for SETI is reviewed in the context of what we

can now call asymmetric SETI, the term coined to reflect that we are merely seeking to

determine what ETI already knows.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Searching for Extraterrestrial Intelligence is nowwidely accepted as essentially a scientific enterprise. Itis unusual in its poverty of theory and concomitantweakness in experimental paradigm—consequently it isrich in speculation and assumption1. In the second half ofthe 20th century various attempts were made to supplythe semblance of scientific respectability to SETI, notably[1], Drake’s equation in 1961 (see [2]), and serious

ll rights reserved.

an be unpacked as

ded—there are no

ents. What we do

heses and experi-

work on SETI and

ontrastingly, some

ts which are more

se this term in a

onjecture which is

lso I enjoy reading

cience can look too

o attend to public

be reasonable and

tions.

analytic discussions of probabilities, transmission fre-quencies, travel, technical advancement, and so forth(for a discussion of the history of SETI and ideas fromBracewell, Dyson, Hoerner, Shklovskii and others, in the1960s, see [2]). However, forceful arguments were putforward in the 1970s and 1980s which claimed theenterprise was futile simply because if ETIs exist theymust be here by now, but they are not so they do not(Fermi’s conjecture from 1950 was re-energised tobecome the primary argument against SETI; on ‘‘Fermi’sparadox’’ see also [3]). Nonetheless, actual observations,and the development of various observational pro-grammes, continued and some of the enduring themesin SETI work were widely accepted at this time. Forexample, the assumption that the 21 cm wavelength willbe universally recognized—and thus is sensible forSETI—was made in the 1960s. And of course therecontinue to be both RF and Optical searches—so thereare sufficient resources available both in terms ofequipment and people to keep a scientific debate alive,if not to conduct SETI with the level of intensity somepeople would like.

More details about all these issues can be found in theliterature ([2] is a good place to look, as is [3]; for a more

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–1418 1411

recent survey see [4]), and my purpose here is not to writea history of SETI. What does concern me, however, is theneed to reconsider assumptions and the need to injectmore scientific caution, as well as adventure, into theenterprise. On the issue of transmission frequencies, andOptical versus RF searching, one can maintain a reason-able level of detachment—the searching has to startsomewhere and 1420 MHz is as good a place as any. Onecould argue in favour of the two OH lines around1666 MHz—either with or in place of the H line. In favouris the scientific belief that such frequencies will beobvious to any civilization capable of radio astronomyand of signalling to anyone. The disadvantage is that suchfrequencies are important for radio astronomy—just aswe Earthlings prohibit pollution of such frequencies onemight assume an ETI would do the same [Eric Gerard,personal communication]. Optical signalling is technicallyfeasible, and OSETI is by now unexceptional, with muchto recommend it. These issues matter as practicalexamples of a general question: How can we makescientific caution work alongside speculation to yieldplausible assumptions?

However, what matters at least as much are questionssuch as: What can we know about an ETI’s motives, waysof thinking, and the like? How can we make assumptionsabout ETIs serve SETI effectively in terms of under-standing options for signalling? In more general terms:can we engage more adventurous science in the refine-ment of speculation to yield more unusual assumptions toguide novel SETI? The remainder of this paper is in fourparts: first we look at some common assumptions(Section 2); second we reconsider those assumptions inthe light of recent scientific developments, and arrivethereby at some thinking about how to do SETI (Sections 3and 4); third we look at the details of a specific searchtechnique which reflects the reanalysis of assumptions(Section 5); finally we consider some implications of thesearch technique (Sections 6 and 7).

2. Assumptions

SETI inevitably entails assumptions, as noted above inrelation to frequencies, and before considering some novelassumptions it is necessary to look at the more conven-tional assumptions. Typically these cluster around severalthemes and we will deal with them in the following order:biology, technological advance, communication, travel,search strategy. One over-arching theme will be drawnout as the opportunity arises—assumptions are notalways explicitly stated, and are sometimes inconsistentwith one another.

2.1. Biology

A widely shared assumption—only sometimesexplicitly stated—is that what we Earthlings know ofbiology (that it is water/carbon based, and evolutionoperates) is sensibly thought of as being universal.‘‘Follow the water’’ as a search paradigm in astrobiologyis considered sensible because it is well-grounded (cf. [5]).

Instrumentation follows theoretical pre-supposition, ofcourse, so detection of non-water/carbon life on Earthwould require skill and imagination. The assumptionthat on Earth we only have water/carbon life is veryplausible—we do have the skills we need to find otherlife-forms. A more realistic concern is that these might notbe ubiquitous so lack of discovery is simply a samplingissue. Nonetheless—astrobiology presumes water/carbonlife. Additionally, astrobiology assumes the importance ofevolution in the development of life-forms (this mightseem to be a consequence of presuming water/carbon asthe basis for life, but is not). The appeal is that Earth-based science can help the search for life off-planet:‘‘astrobiology begins at home’’ might be a motto for astro-biology. More remote consequences (not often discussedexplicitly) are that life-forms will be embodied andnecessarily will include forms which have sensory organs,articulators, brains, and etc., (otherwise evolution wouldnot work). Such biological assumptions provoke usefulterrestrial science, which in turn can feed back into theastrobiology debates.

2.2. Technology

Another domain for assumptions in SETI researchconcerns the conjectured ETI’s ‘‘advanced technology’’. Itis sometimes claimed that when found ETI will demon-strate ‘‘advanced technology’’, ‘‘superior science’’, and soon and so forth, and this is usually coupled with views onthe ‘‘age’’ of the civilization envisaged (‘‘L’’ in the Drakeequation). It is not so usually noted that these terms arevery vague even whilst also being limited—for example,there is usually no mention of ‘‘advanced socio–politicalorganization or planet management’’. When the generalexpression ‘‘more advanced than us’’ is used it mostlyrefers to technology, science and medicine.

A common conjecture in Science Fiction1 in relation to‘‘advanced’’ technology and science, is that ETI will havesolved the problem of super-luminal travel (and seebelow Section 2.4). This undermines the work of moreserious scientists. Recent work on SL travel [6]—looking atthe Alcubierre warp—seems scientifically encouragingbut fails to consider the navigational issues attendant onSL travel (information is limited to light speed so SLtravellers are, essentially, lost). In addition the energyreleased when the warp collapses will surely engulf thetravellers! Other conjectures which can look like assump-tions concern prodigious energy resources. Kardashev andDyson are well known for such proposals (cf. [2, chapter7]). Sometimes the claim is made more or less explicitlythat the life-form itself will have evolved to be ‘‘superior’’to humans in various ways—‘‘more intelligent’’, forexample, leading inevitably to more advanced scienceand technology. Bracewell and Tipler probes, for example,illustrate this mode of thought. The difficulty for SETI isthat when these conjectures become assumptions used inchains of reasoning we end up with the ‘‘Fermi Paradox’’and the ‘‘Hart attack’’ (see Section 2.4). [2, chapter 7]discusses this and optimistically concludes that ulti-mately observational work was galvanized and took

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–14181412

precedence over theory. It is not clear that discussions ofprobes or of Dyson spheres merit the term ‘‘theory’’.

Set alongside the Science Fiction are assumptionswhich seem more obviously plausible—ETI will knowabout stuff we call physics, engineering, etc., and will bedoing astronomy and so forth. These assumptions areunremarkable. Those doing SETI recognize that transmis-sions are limited by light speed and assume ETI knowsthis too. A novel assumption about ‘‘advanced technol-ogy’’ will be offered below.

2.3. Communication

In addition to assuming that ETI will have a recogniz-able life-form it is generally assumed that ETI will want tocommunicate. If it is assumed that ETI has an incompre-hensible technology (magic—cf. A.C. Clarke) this couldcause difficulty for specifying observational programmes,so it is necessary to assume plausible, if not exactlyfamiliar, technology. Currently the search for ETI is,predominantly, a search for a signal. There are severalaspects to this. As noted earlier, the transmissions canbe sought at specific RF wavelengths and/or optically(more assumptions). The hardware and software devel-oped for signal detection is now very sophisticated. Ofmore concern is the issue of ‘‘content’’ and ‘‘language’’if the additional assumption is made that ETI is sendinga ‘‘message’’. This is readily examined by looking atproposals for transmissions from Earth. Typically thesefocus on intentional, or otherwise, cryptographic chal-lenges and the development of artificial languages.Enough is known about semiotics and symbol systemsto show such proposals to be worthless.

2.4. Travel

It has famously been suggested that because ETI is notwith us, they do not exist. This is the so-called ‘‘FermiParadox’’ and is best discussed in Stephen Webb’s book‘‘Where is Everybody?’’ [3]. The assumptions of ETI’sinterest in and capability for galactic travel and/orcolonization are well located in the Science Fictiondomain. Difficulties with super-luminal travel technologywere noted earlier. Ref. [3] has an informative discussionof this aspect of SETI and [3] successfully presents a widerange of ‘‘stories’’ told around the theme—but his conclu-sion (human uniqueness) is, within the domain if notmore widely, disappointing and unconvincing.

2.5. Search strategy

One widely held view in SETI is that because we knownothing about ETI (despite the assumptions consideredabove) so it must be that the only way to search isthrough surveys and without any pre-conceived notionsof targets/sources. Others prefer targeted searching ofidentified star systems with potential life-supportingcharacteristics. Further, assumptions will necessarily bemade about frequencies/techniques to be used, about thenature of signals to be expected, and etc., and even about

the motivations which might drive ETI to send signalsat all.

The ‘‘look everywhere for anything’’ strategy suppo-sedly does scientific justice to the thought that as wedo not know anything about ETI we have no reason forbeing selective or specific. The problem with this view isthat as soon as a portion of the RF/Optical spectrumis selected for the SETI activity, and a signal typeconjectured, we imply we do know something about ETIafter all—so we should really work harder to be morespecific about what we might know (this inconsistencyis addressed below). If we do not attempt to providereanalysis of the assumptions we might justly be thoughtto be looking under the street-lamp because that is wherethe light is. We will review and revise assumptionsrelevant to SETI, including those concerned with strategy,in the next section.

3. Revised assumptions

In this section of the paper we will look at revising/extending the assumptions set out in Section 2 above, toguide novel SETI. We will be addressing points broughtup in each of the above sections. Before getting into thedetails it is helpful to be clear about what is assumed tobe the overall goal of SETI. The goal is to find an interesting(negative) answer to the question ‘‘Are we alone?’’ Themotivation is well captured in A.C. Clarke’s words:

‘‘Sometimes I think we’re alone in the universe, andsometimes I think we’re not. In either case the idea isquite staggering.’’

We should note, in passing, that of course by the timewe discover a civilization elsewhere in our galaxy it mayhave ceased to exist, just as humanity may have ceased toexist by the time ETIs get to know about us. Issues ofsynchronicity do not actually undermine the questalthough they add subtlety to Clarke’s words. In addition,we need to think about ETI’s motives for transmission ofsignals, and about other ways of doing SETI. This will helpus review our own strategy for SETI (see Section 6 below).We start by looking at the domains where assumptionshave been made.

3.1. Biology reconsidered

The primary concern in this domain is just how far wecan go making assumptions about ETI. If we assume somesort of embodied life-form with brain, sensory apparatus,and etc., as the outcome of evolution, can we go further tosay something about ETI’s behaviour and motivations? Ithas been proposed that ETIs—as much as TIs—aregoverned by some General Cognitive Principles [7,8].The suggestion here is that ‘‘the brain’’ as an organ has afunctional specification which is universal. The brain’srole in an organism is to provide for the SequentialImperative, which is the inevitability of the need toexploit sequentiality in behaviour and perception. Plans,intentions, beliefs, etc., are cognitive entities which maybe ‘‘about temporal structures and events’’ but without

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–1418 1413

inherent sequence. To be expressed they must be cast intosequential form. Likewise, but in reverse, sensory stimuliare constantly in flux, but cognitive processing strips thetemporal dimension out to yield the ‘‘percept’’ which maybe about time, but not temporally structured. Thisobservation, along with some proposals for a set ofgeneral principles, imply that our understanding of ‘‘thebrain’’ and its functionality can be cast in universalterms—we can actually say something about brains(and behaviour) wherever we might suppose those brainsto be located.

The general principles include coverage of a topicknown as ‘‘Distributed Cognition’’ (which concernsteam-work, not mind-reading) and thus they licensethe assumption that ETIs will know about DistributedCognition [9]. With this assumption it can be argued thatETIs will know that we know that they know we bothshare a problem and need a shared solution. This in turnlicenses discussion about ETI’s motivations for sendingsignals (see Section 4), but here it is enough to say that ETImight do so as part of a shared solution to a sharedproblem.

An additional point to keep in mind under the generalheading of biological assumptions is that evolution willhave evolved to a stall in technologically favourablesettings, as it has for the human race (cf. the views ofSteve Jones [10]; note also [11], written independently ofJones’s work). Technology develops to neutralize evolu-tionary pressures, it can be argued, by prolonging life,reducing pressures to produce offspring, etc. Indeed, itcould even be argued that socio-technology has a primarymotivation to neutralise evolutionary pressures and thusmust stall the process; much of what we currently enjoyin technology (convenience equipment in the home,entertainment technology) just occupies the spandrels2

in the more structural socio-technological edifice. Ananalysis/history of socio-technology would need to seethis developed in detail, but the idea is useful: we cannotassume that ETI will be an organism with a vastly superiorintelligence or greater muscular strength or whatever.The science here is more adventurous than conven-tional approaches and the implications for SETI lead usin new directions. We will see this again in the discus-sions below.

3.2. Advanced technology reviewed

Within the socio-technological context set out above itcan be argued that the notion of ‘‘advanced technology’’does not have to deal with stories of super-luminalspaceships, fusion power, or whatever; we do not needto think of ‘‘magic’’. It could be enough that a society with‘‘advanced technology’’ is one which knows it is not alone

2 The allusion here is to the famous paper on evolution by Gould and

Lewontin on ‘‘The Spandrels of San Marcoy’’ [22]. Spandrels are found

in structures with arches: they are the ‘‘triangular’’ spaces typically in

the plane of an arch and defined by the vertical and horizontal elements

in the structure, and the arch itself, where the arch provides an opening

in a wall, or at the end of a corridor. The spandrels have no structural

function and exist by virtue of the structural elements.

in the universe3. This can come about in a variety of ways,including the use of extremely large telescopes for directobservation of intelligent life-form behaviour, i.e. artefacts,on other planets (a telescope/interferometer 1000 kmacross could resolve a 1000 km feature at 150 light years).Discovery does not have to be via transmission andreception of signals. This definition encompasses the ideathat more advanced societies/technologies will be olderthan ours (on the basis that we have only just reached thepoint of making plausible SETI efforts). A milestone forsuch advancement for an ETI is simply that they now knowthey are not alone. This is my preferred limit for socio-technical conjecture—at once more focussed and lesstechnically specific, and leading SETI in new directions.

If we try to specify other aspects of what it is to betechnologically advanced we are quickly in the realm offiction. It is not clear what we gain by making conjecturesin this domain, and indeed they do not form part ofmainstream SETI work. One example, not infrequentlymentioned, is that of Tipler probes: self-replicatingtechnological probes with advanced AI technology onboard. These, it is supposed, will flood the galaxy andreport back on what they find (see also [3, pp. 79–84] onBracewell probes, or Bracewell–von Neumann probes ashe refers to them). That we have not encountered themmeans they do not exist, and therefore ETIs do not exist.This is unhelpful conjecture, and indeed any suchconjecture about possible technologies reduces, in thelimit, to Science Fiction. For example, an equally plausibleScience Fiction scenario is that ETI experimented withsuch probes and found them uncontrollable and only justsucceeded in wiping them all out before they ran amok!

3.3. Communication and signalling

It was noted earlier that as part of the overall contextwithin which communication with SETI is discussed onefinds the urge to send messages ‘‘about us’’ and likewise toenvisage ways in which ETIs would be communicating aboutthemselves. Message structure and content have beenconsidered in great detail, e.g. [12] introduces a proposalfor a self-explanatory language dubbed ‘‘Lincos’’. Theassumptions underpinning such work seem weak—we knowenough about semiology to know that we could not shareexperiences with ETI (and thus not share linguistic under-standing) other than in domains where other evidence ismore readily established. For example—that ETI can doastronomy presumes an enormous background in technol-ogy and science. This does not need a linguistic account ordescription or explanation to make it known to us. Whywould an ETI seek to tell us what they must reasonablyexpect us to know already? If aspects of their world/existence are local to their circumstances then we will notbe able to understand them on the basis of a linguisticallycoded message. Other aspects will, to a significant extent, beobvious and require no extra explanation.

3 The notion here is that ‘‘advanced technology’’ can be defined as

the corollary of ‘‘advanced knowledge’’ when the latter includes

knowledge that intelligent life exists elsewhere in the universe.

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–14181414

But in point of fact, the primary question—‘‘Are wealone?’’—does not require us to know if ETI has threepurple eyes or whatever. It is addressed by signallingpresence, unambiguously, or by observation. Indeed, it isentirely plausible that for the most part other intelli-gences have answered the question observationally andthat they seek to signal their existence in order to assistany ETI to answer the question sooner rather than later.We will take up this theme later when we considermotivations for active and passive SETI—suffice to say herethat assumptions about the purpose of sending signalsneed to be reconsidered and that doing so changes ourefforts at SETI.

Technically, therefore, the issue for SETI should remainthe search for signals not messages, and it should do thison the basis of reasoned analysis about why and to whatsuch signals might be sent (see Section 3.5).

3.4. Travel revisited

It has already been noted that travel—whether usingsuper-luminal techniques, or using indirect representa-tion in the form of probes—is an unlikely option for anyETI. The primary issue is motivation: ‘‘going boldly’’anywhere is never going to be a good way to addressthe big question, whatever the makers of TV programmesmight like to think. However, assuming for the sake ofdiscussion that such travel is contemplated other issuesarise. For example, ETI could have visited Earth a millionyears ago because it looked promising a couple of millionyears ago, but found nothing exciting, and moved on. Howwould we know? Why would they leave anything for us tofind if they do not know there is going to be anyonearound to do the finding/interpreting?

Galactic travellers would have socio-technical sustain-ability issues significantly more complex than thoserequired of planetary based civilizations—and of coursethe motivation for the trip, if nothing other than answeringthe basic question, has to be sustained for as long as ittakes to get somewhere interesting and then to get back(this theme is well known to Science Fiction fans). TheDrake equation assumptions might need to be elaborated iftravel speeds are not very high—which is to say, the Drakeequation itself might need to be rethought in the context ofextended voyages involving substantial communities. Inshort, aside from motivational implausibilities the techni-cal problems seem unlikely to be worth struggling toovercome—a point made by Drake himself (see [2, p. 220]).

One might want to promote the travel option on thebasis of re-defining ‘‘advanced technology’’ to be inter-stellar travel technology. But, if we assume that energyand life-support and speed and communications techno-logyy have all been addressed as issues and the problemssolved, then we are asked to suspend belief in ourunderstanding of the world. This is not to say that ETI’sphysics will be based on an ETI equivalent of Newton orEinstein, merely that the phenomena they account forwill be familiar to us. So far our review of assumptionshas asked us to suspend disbelief—a somewhat morecautious step.

3.5. Search strategy renewed

The discussions above concerning revised assumptionsin the light of more adventurous science appear to leadSETI in new directions. For example, it seems plausible tosuppose we know more about ETI’s cognition than wemight have once thought, and that evolution will haveserved its purpose for them as much as for us. When welook for signals they should be signals directed at us, forreasons we can recover (so we, in our turn, know where topoint our instruments). To this end it seems plausible tosuppose that ETI will have answered the big question andthus will have a motive to help us answer it (ETI will knowwe exist, through observation), and that help is morelikely to take the form of signalling than travel, and morelikely to be directed signalling than omni-directionalsignalling. What remains is for us to reconsider thestrategies we use for SETI.

Long-term the pursuit of advanced observationaltechnology looks sensible, and to a degree inevitable(the timescales will reflect the motivational energy). Interms of looking for signals of one sort or another—theprimary concern is going to be the need to understandwhat it means to be the target of transmissions. We needto work out where we should be looking, and for what,with as much specificational detail as we can muster.A proposal was made [13] to use Pulsars as beacons—

relying on alignments between Habstars, Earth andPulsars to provide specifications for directions and signalproperties in a novel SETI paradigm (see the next section).There may well be other similar proposals which achievethe same goals—directing searching towards specifictargets which, it can be supposed, could harbour an ETIwhich would have identified us as a target for a signal(not a message) which we can specify and for which wecan refine our instruments.

The most radical revision of assumptions which arisesfrom the foregoing discussions is that the distinctionbetween active and passive SETI—between sending signalsin active mode, and listening/looking for such signals inpassive mode—becomes secondary. Whilst it can beargued that passive SETI is best done by thinking abouthow one would do active SETI, it turns out that a moreimportant characteristic to understand is that thesituation is asymmetric.

I believe that asymmetric SETI is the circumstance weshould assume defines the enterprise and shapes ourefforts within it. This comes from considering as plausiblethe assumption that ETI elsewhere is likely to know weexist—it will have answered the big question for itself andwill have identified target worlds to which it makes senseto send signals. Their motivation is simply to help othersanswer the question. For them it makes economic andpsychological sense to target transmissions–omnidirec-tional interstellar shouting is unlikely to appeal to them(and be too expensive of energy), and their own ‘‘babblebubble’’ of radar, TV, and radio broadcasting will havedissipated to become worthless (and in the big scheme ofthings is merely a brief RF ‘‘flash’’). Indeed—the argumentthat sending signals is a way to discover the presence ofothers seems flawed—the energies required, and the

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–1418 1415

uncertainties involved in targeting, make such an enter-prise unattractive—the debate between passive SETI andactive SETI is, on this account, sterile.

The suggestion was made earlier that shared under-standing of distributed cognition, and thus of the problemknown to be shared—we know that ETI knows that weknow that we both share the problem of finding eachother and thus need a solution known also to beshared—leads to a mutually interchangeable reciprocityof understanding, or situational symmetry. This remainsvalid if we consider specific strategies for SETI, such assignalling. However, it is not the case that thinking aboutdistributed cognition only works in cases of situationalsymmetry. We have been conjecturing that ETI’s‘‘advanced technology’’ has led it discover it is not alone,perhaps on the basis of extremely large optical telescopes.We do not have such telescopes so the situation is nottechnologically symmetric—but for ETI to deploy signal-ling technology on the basis of its understanding ofdistributed cognition renders the situation symmetric in asense. However, this is not the thinking behind the use ofthe term asymmetric SETI. This term reflects the fact thatETI knows it is not alone, and thus needs a differentmotive for its signalling effort. We Earthlings do not yetknow; ETI knows—that is the deep asymmetry whichmatters, and which leads us to suppose that ETI issignalling to help us answer the question. Of course, ETIcould simply wait for us to build bigger and betterinstruments.

4. Life signatures

There is one implication of the revised strategic viewwhich deserves comment—before we move on to con-sider details of how searching can be done using Pulsars.From the point of view of doing SETI from/on Earth, ETI ispresumed to know about our planet—much as it mighthave an inventory of what we call exoplanets. It is furtherassumed on this basis that ETI considers it worthwhile tosend signals to likely/plausible exoplanets. But the list ofsuch targets could be large, and the technical problem(and cost) of transmission unappealing without moretarget refinement. So, assume further that ETI knowsabout us, and has assessed that we are deservingrecipients of signals. ETI knows about intelligent life hereon Earth—how and when did it learn this?

Recent studies of global warming data, for example,indicate that around the middle of the 19th centuryatmospheric carbon dioxide levels started to rise abovenormal. This would be detectable at a distance withsuitable instrumentation. Does this constitute the firstreliable remotely observable evidence of intelligentbehaviour on our planet? What could be the lifesignatures we might suppose we are (or have been)projecting over the centuries? Would a sufficientlyadvanced observational technology enable observationof human activity—e.g. canal building in Europe in the18th century, de-forestation in Roman times, Egyptianpyramids thousands of years ago? This matters because ofthe issue of timing—how long ago did the Earth first

provide observable evidence that it harboured intelligent/human life? Half that length of time, in light years, is thedistance of the furthest Habstar (see below) from whichwe might suppose we could receive a signal. ETI will onlyconsider transmissions when it has good reason tosuppose that at the target there might be an intelligencecapable of receiving them. That is the profound implica-tion of considering SETI to be asymmetric.

We end up reasoning our way to two detailedprocesses for passive SETI. The first assumes ETI is actively

transmitting in order to help us with our passive SETI. Weneed to refine our ‘‘listening’’ strategy to account for this,but ideas exist and data can be collected (see thediscussion of [13] above, and see below in Section 5).We develop an interesting view on the concept of ‘‘lifesignatures’’, namely that it is the signatures of theexistence of terrestrial intelligent life-forms that weshould be thinking about. These signatures will determinehow it is that ETI might learn about us. And theconception of such life signatures leads directly to thesecond process for passive SETI—the life signatures willshape our observational studies of Habstars and exopla-nets as we look for life elsewhere in observational mode,becoming ever more technically advanced until we reachthe stage of ‘‘advanced technology’’ defined earlier whenwe discover we are not alone.

5. Pulsars as Beacons

It was noted above that a proposal had been made(Edmondson & Stevens [13]) for using Pulsars as Beacons.This introduced a targeting scheme ex caeruleo anddiscussed the production of target lists. This scheme isbased on the idea that Pulsars can be used as Beacons ifthey line up with Habstars and Earth:

(i)

Pulsars are rapidly rotating, and thus apparentlypulsing, radio sources (see [14]). Each Pulsar has itsown periodicity and periods found range from alittle more than a millisecond to several seconds.Those which are not too distant are reasonablyisotropically distributed in relation to our Sun.

(ii)

Catalogues of ‘‘habitable’’ stars have been compiled(see for example [15]) where it is understood thatsuch stars are not themselves habitable, but thatthey could support life on an appropriately orbitingplanet. These too are isotropically spread aroundus—but nothing like as far as the Pulsars. Thesestars are called ‘‘Habstars’’.

(iii)

The proposed scheme is to look for straight-linealignments featuring Earth, a Habstar, and a Pulsar.The alignment can be to arbitrary precision—E&Schose alignment within a cone of 11 radius andproduced inventories of alignments where theHabstar is between the Earth and the Pulsar,and also where the Earth is between the Habstarand the Pulsar.

(iv)

The scheme supposes that the ETI will have similarlists of Pulsars and Habstars, and will know of ourSun as a Habstar. Further, the scheme supposes that

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–14181416

the ETI will think of these straight-line alignmentsand produce lists of them, in accordance with thepresuppositions about distributed cognition in rela-tion to finding a shared solution to the sharedproblem.

(v)

The alignments provide the basis for specification ofa predictable and entirely artefactual signal fortransmission by the ETI (towards the Habstar). Allthe ETI needs to do is pick one of the ‘‘obvious’’frequencies (see above, typically one might think ofthe 1.42 GHz H line, or the OH lines, or both) andproduce a pulsed transmission at the periodicitydetermined by the relevant Pulsar in the alignment(optical transmissions of pulses are also feasible ofcourse).

(vi)

The pulsed signal, if not corrected for the ETI’s relativemotion around its star, provides information aboutthat motion—and also details like its planetaryrotation period, the presence of a moon (and itsperiod), and so forth. Should the ETI choose totransmit from some technology orbiting its planetor star then some of this information is lost, butreliability might be gained (and indeed with sufficientpower resources and automated technology thetransmitter might be left to do its work for a longwhile).

(vii)

We Earthlings work out where to point ourtelescopes, and what sort of signal to look for. Ifwe assume that the ETI transmits continuously for afew hundred years (because it knows we are likelyto exist) then we just have to ‘‘tune in’’ to each star,so to speak, looking for a known periodicity. If theETI timeshares its transmitter efforts betweendifferent Habstars then we have to observe Habstarsrepeatedly and for extended sessions to be confidentof the possibility of an overlapping moment oftransmission and reception.

(viii)

‘‘Pulsars as Beacons’’ provide a good mutuallyunderstandable scheme but it is technologicallyasymmetric. The transmitting ETI invests muchmore and learns nothing unless the recipientreturns a signal. It can be assumed that thetransmitting ETI already has a positive answer tothe basic question, and it is transmitting to help‘‘spread the news’’. This deepens the notion ofasymmetry. Indeed, it can be assumed that ETIknows that it is transmitting to an intelligentcivilization, but one which is not yet aware that itis not alone. It must make sense for an ETI to reserveits resources to start transmissions when it believesthe recipient is able to receive them—but even inthis scenario it is still required that the ETI and itstarget have some reason to identify themselves as‘‘set up’’ for such an exchange. The Pulsar as Beaconscheme serves this purpose.

Fig. 1. This illustrates the multiple alignment case involving six Pulsars

in M62 aligned with six Habstars, of which Earth is one. Distances are

given for the locations of the Habstars relative to the Earth.

The E&S Pulsar as Beacon scheme described above hasbeen couched in terms of RF transmissions and indeed hasbeen used as the basis for using the Arecibo radiotelescope. In reality this is not a necessary component ofthe scheme. Optical transmissions at the Pulsar rate

suffice as well as RF transmissions, and might in fact beeasier to set up and to detect.

6. Deployment of Pulsars as Beacons

In the original conception of the scheme the benefitwas thought to be just that a mutually intelligibletargeting strategy could be made to work for theidentification of targets in our searching—targets whichcould be targeting us. The major consequence is thatbecause the number of Habstars (potential targets) israther large it provides a principled way of cutting downthe number to be considered in detail as targets. However,the number is still large (�1800) so it becomes necessaryfor practical reasons to consider principled ways ofcutting it further. We should note also that the numberof known Pulsars is increasing, and this increases the sizeof the target list. There are some refinements that can bemade to the basic scheme which serve our purposes(e.g. only consider fast Pulsars). And of course it is alwayspossible that someone else will come up with a differentscheme considered also to be ‘‘obvious’’, but whichdelivers more directly the list of only those targets really

worth looking at.

6.1. Mutiple alignments

The scheme as described implies that finding align-ments is difficult and that each case will consist of aPulsar and two Habstars (of which our Sun is one). Threeobjects in a straight line serve to identify a target Habstar(and distance can be used to work through the set ofalignments to cut numbers further if required, in line withconceptions of life signatures, as discussed in the previoussection). In addition, to keep numbers down we restrictattention to rapidly spinning Pulsars with short periods(in E&S the list of such alignments is rather short—113).Therefore, it is remarkable that Pulsars and Habstars occurin multiple alignments. This happens because in somecases Pulsars exist in remote clusters (e.g. 47 Tuc) whichline up within the limit set for defining ‘‘straight linealignment’’, and because in other cases a chance co-alignment of Habstars provides for multiple alignments.In one particularly interesting example there are six fastPulsars in M62 which line up with six Habstars (of whichour Sun is one). See Fig. 1. This arrangement is unique inthe dataset produced by E&S and thus it constitutes aprimary case for further observational study (see below).Suffice to say here that multiple alignments serve tocut down the observational target list to manageableproportions.

The use of these alignments played a part in targetselection for a SETI programme conducted at Arecibo in2005 (see [16]). Data are still being analysed.

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–1418 1417

6.2. Exoplanets

The Pulsars as Beacons scheme can be refined in twoways by considering exoplanets. The most obvioustechnique is to look for alignments with Habstars thathave known exoplanets orbiting them. This will cut downthe number of targets independently of the considerationof multiple alignments—the shorter list is thus a differentshort-list. If there are Habstars which are present on bothshort-lists they become favoured targets for our attention.A more intriguing opportunity for refining the list ofalignments may actually increase the number of targets.The technique here is to use the set of Pulsar as Beaconalignments to produce targets for exoplanet searches—e.g. the Habstars shown in Fig. 1. This technique hasmuch to recommend it despite the prospect of moretargets, namely that the eventual list of targets will bebetter defined as plausibly life supporting.

It should be noted that the opportunity to contribute tothe exoplanet search effort, by identifying Habstars to bethe focus of specific efforts to find exoplanets, is avaluable and novel extra benefit derived from the Pulsarsas Beacons scheme. This was not part of the originalconception.

6.3. Alternative schemes

The Pulsars as Beacons scheme is the only one anyonehas thought of so far which provides a precise specifica-tion of the signal to be sought, along with identificationof the potential source (usually just a single Habstar). Itmight be thought that other schemes using, say, referenceevents of one sort or another such as gamma ray burstsor super-novae might have addressed the issues but theydo not (cf. [17] and on Synchronized SETI see [18,19];for an early proposal to use Pulsars see [20], and [21]pp. 168–170). For any new scheme the same sorts ofargument would have to be worked through as givenabove concerning source and signal specification. It seemslikely that without some sort of external reference beaconthe guarantee of artefactuality in the transmitted (sought)signal will be difficult to sustain, let alone specification ofany other characteristics of the life signal4 [4].

4 One reviewer reasonably pointed out that this is a very strong (and

for more conventional SETI somewhat destabilising) assertion. Maybe it

overstates the case, but the issue is actually rather simple. Note first that

any proposal for a search is not, technically speaking, an experiment—

there are no theoretical predictions/hypotheses, and negative results tell

us virtually nothing other than that a signal has not been detected (and,

in the case of targeted searches, that the potential source is not

signalling in the manner we considered most likely). In all cases the lack

of detected signal does not mean that ETI does not reside in the target

domain/direction. So, if one searches, as many are doing, for a CW signal

at 1.42 GHz for example, and one sets up one’s receivers and filter banks

with exquisite resolution to determine that any such signal found is

from outside the solar system, then if a signal is found one is able to say

no more than that it has certain properties and comes from outside the

solar system, and from a certain direction/source. One certainly cannot

guarantee that it is an artefact or that it is generated by an ETI. If a

pseudo-pulsar signal is found (in the manner of E&S above) then

artefactuality is guaranteed and—moreover—there is a bonus. One

learns from study of the pseudo-pulsar pulse train more about the

source (orbital characteristics of ETI’s planet, its moons, etc.). And,

7. Life signatures—pulses

The scheme described above assumes that any ETIusing it will transmit pulses at the pulse rate(s) which itknows for the Pulsar(s) in the alignment. It will have itsown calculated barycentric rate for the Pulsar, which willnot be corrected for the relative motion of its Sun withrespect to the Pulsar—just as our notion of the pulse ratewill not be corrected.

In the case where ETI’s Habstar is aligned with a Pulsarbut in the opposite direction, from our point of view, theirrelative motion could yield a pulse rate closely related tothe Pulsar rate but measurably different. This complicatesthe pulse signal detection but not unduly. In the case whereETI’s Habstar lies between the Earth and the Pulsar it willbe the case that ETI’s pulse transmissions will automati-cally be compensated for any relative motion because itsnotion of the Pulsar pulse rate will likewise be affected.

Significant data gathering needs to be attempted ifweak pulse streams from ETI are to be detected in noise.Knowing something about the desired frequency of thepulse train can help for signal processing techniques likefolding (see [14]), but ultimately conventional signalprocessing techniques using FFT are the most flexible.Knowing the desired/expected pulse rate/frequency isobviously helpful in rejecting false detections due to‘‘birdies’’ or whatever. In addition, because we knowsomething about how far away candidate sourcesare located we can make appropriate adjustments tocorrections for dispersion.

8. Summary and conclusions

Throughout this paper we have contrasted the notionof SETI research which is driven by conventional scientificassumptions with the use of more adventurous science torealign SETI with different thinking, but thinking whichdoes not distract from science by tending towards fiction.The point being made here is that the sort of scientificextrapolation we see in some SETI work (and thatadvocated here) is believable—so the reader is only askedto suspend disbelief. More fanciful Science Fiction wouldhave the reader abandon much that is needed for conce-ptualising the world as we know it—that is, to suspendbelief (in physics, for example). That can be entertaining,but does not lead to scientific progress. If, as proposed atthe outset, the reader accepts the notion that SETI is ascientific enterprise then it matters how we extrapolatescience. The consequences may be unexpected, but that isonly to be expected!

Motivations for Earthlings to do SETI need to bederived from informed/scientific conjecture about an ETI’smotivations for doing SETI, as well as conjecture about theETI’s techniques. If we are to do passive SETI we have twochoices—either we ‘‘listen/look’’ for transmitted signals

(footnote continued)

furthermore, these observations can then be checked by visual

inspection when the telescopes are built and trained on the star/planet

concerned.

W.H. Edmondson / Acta Astronautica 67 (2010) 1410–14181418

using radio/optical telescopes, or we build sufficientlypowerful instruments that we can answer the ‘‘Are wealone?’’ question observationally. If we are to attempt todetect a signal we must assume that an ETI is transmit-ting, and further that it is targeting transmissions to usdirectly. A targeting strategy is discussed in this paper andpresented in detail elsewhere [13].

In this paper we have looked at some of the relevantfactors for an ETI’s approach to SETI to work for us whenwe do passive SETI. The asymmetry in SETI reflected in theterms active and passive is simply the difference betweenwho transmits. The notion of asymmetric SETI is deeperand covers the situation where an ETI knows that it is notalone and thus is motivated to signal its existence to helpus answer the question ‘‘Are we alone?’’

References

[1] G. Cocconi, P. Morrison, Searching for interstellar communications,Nature 184 (1959) 844–846.

[2] S.J. Dick, Life on Other Worlds, Cambridge University Press,Cambridge, 1998.

[3] S. Webb, Where is Everybody? Copernicus Books, 2002.[4] J.C. Tarter, The evolution of life in the Universe: are we alone?

in: Karel A. van der Hucht (Ed.), Highlights of Astronomy, vol. 14,14–25 August 2006, IAU XXVI General Assembly, 2007.

[5] The Limits of Organic Life in Planetary Systems. National ResearchCouncil of the National Academies: Committee on the Limits ofOrganic Life in Planetary Systems (Chair: J.A. Baross); Committee onthe Origins and Evolution of Life; Space Studies Board; Board of LifeSciences. National Academies Press, Washington, 2007 (ISBN-10:0-309-10484-X; ISBN-13: 978-0-309-10484-5).

[6] R.K. Obousy, G. Cleaver, Putting the ‘‘Warp’’ into Warp Drive.Spaceflight, 50 (4) (2008), arXiv:0807.1957v2 (physics.pop-ph).

[7] W.H. Edmondson, General cognitive principles and the structure ofbehaviour, Unpublished paper for the meeting of the International

Society for Theoretical Psychology, Toronto, 2007, see: /http://www.cs.bham.ac.uk/�whe/ISTP2007.pdfS.

[8] W.H. Edmondson, General cognitive principles: the structure ofbehaviour and the sequential imperative, International Journal ofMind, Brain and Cognition, 2009, in press, see: /http://www.cs.bham.ac.uk/�whe/GCPSOBSI.pdfS.

[9] E. Hutchins, Cognition in the Wild, MIT Press, Cambridge, 1995.[10] S. Jones, much published, see wiki article here: http://en.wikipedia.

org/wiki/Steve_Jones_(biologist).[11] W.H. Edmondson, Evolution of intelligence in the context of SETI,

Bioastronomy 2004: Habitable Worlds, Reykjavik, July 12–16, 2004,Abstracts published in Astrobiology 4 (2) (2004), Abstract 60F,p. 251.

[12] H. Freudenthal, Lincos—Design for a Language for Cosmic Inter-course, Part I, North Holland, 1960.

[13] W.H. Edmondson, I.R. Stevens, The utilization of pulsars asSETI beacons, International Journal of Astrobiology 2 (4) (2003)231–271, see: /http://www.cs.bham.ac.uk/�whe/SETIPaper.pdfS.

[14] D. Lorimer, M. Kramer, Handbook of Pulsar Astronomy, CambridgeUniversity Press, Cambridge, 2005.

[15] M. Turnbull, J. Tarter, Target selection for SETI: 1. A catalog ofnearby habitable stellar systems, Astrophysical Journal SupplementSeries 145 (2003) 181–198.

[16] See /http://www.cs.bham.ac.uk/�whe/AOproposal.pdfS.[17] R.H.D. Corbet, The use of gamma-ray bursts as direction and time

markers in SETI strategies, Publications of the Astronomical Societyof the Pacific, vol. 111, Astronomy Society of the Pacific, SanFrancisco, CA, 1999, pp. 881–885.

[18] R.H.D. Corbet, /http://lheawww.gsfc.nasa.gov/�corbet/pub/astrobio.htmlS.

[19] G.A. Lemarchand, Passive and active SETI strategies using the synchro-nization of SN1987A, Astrophysics Space Science 214 (1994) 1–2.

[20] J. Heidmann, C.R. Acad. Sci., Paris 306, (1988) 1441–1445 (inFrench), The English abstract is at: /http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1988CRASM.306.1441&db_key=AST&high=3c0dab1fe501622S.

[21] J. Heidmann, Extraterrestrial Intelligence, Cambridge UniversityPress, Cambridge, 1992.

[22] S.J. Gould, R. Lewontin, The spandrels of San Marco and thePanglossian paradigm: a critique of the adaptationist programme,Proceedings of the Royal Society of London. Series B: BiologicalSciences 205 (1979) 581–598.