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An Archaeological Approach to SETI

Changing approaches to SETI are getting public attention these days, as witness a new article in The Economist that makes reference to the probable cause of the interest, the publication of Paul Davies’ The Eerie Silence (Houghton Mifflin Harcourt, 2010). Sub-titled ‘Renewing Our Search for Alien Intelligence,’ Davies’ book is making accessible to the general public the kind of discussion we’ve often had in these pages, looking at the question of whether our SETI strategies at radio and optical wavelengths aren’t too limited for any chance of success. The Economist is just one sign of the new interest.

After all, technologies like spread spectrum encoding are already masking straightforward radio communications, while conventional broadcasting is giving way to such heavy use of fiber-optics that a planet like ours may go dark at radio wavelengths within a relatively short time as civilizations go, and no more than an infinitesimal flicker in cosmological terms. Thus the interest in alternatives like hunting up Dyson spheres, the search for which Dick Carrigan has so actively championed. The few searches for such spheres have been unsuccessful, but we’ve only begun to look for signs of the technologies of vastly more powerful cultures.

Dyson Spheres and Their Signature

In any case, Dyson spheres would be tough to locate, although it’s interesting to note that the Allen Telescope Array will investigate thirteen of what Carrigan calls the ‘least implausible Dyson sphere candidates’ once it becomes fully operational. Carrigan built up a list of such candidates by going through data from the Infrared Astronomy Satellite (IRAS), which spotted hundreds of thousands of infrared sources. A Dyson sphere should radiate in the infrared, but would have to be distinguished from natural sources like stars with thick dust shells, whose own infrared signal might closely mimic that of an artificial construct.

Carrigan goes into all this on his Web site, where he notes that the Calgary Group has classified all the available low-resolution spectra from IRAS into categories including some that would help identify a Dyson sphere candidate. Thus U stands for a type of object with an ‘unusual spectrum showing a flat continuum,’ while F stands for ‘featureless’ objects that may correspond to O or C stars with small amounts of dust. The C category flags possible late-type cool giant stars with circumstellar shells of carbon dust and emission in the infrared, while H is for HII regions, encompassing various kinds of nebulae.

All these could be considered Dyson sphere look-alikes, and Carrigan notes that more data are becoming available through the Spitzer Space Telescope and surveys like 2MASS. The key is to pick out an actual Dyson sphere signature from all the noise, one Carrigan describes this way:

…an advanced civilization inhabiting a solar system might break up the planets into very small planetoids or pebbles to form a loose shell that would collect all the light coming from the star. The shell of planetoids would vastly increase the available “habitable” area and absorb all of the visible light. The stellar energy would be reradiated at a much lower temperature. If the visible light was totally absorbed by the planetoids a pure Dyson Sphere signature would be an infrared object with luminosity equivalent to the hidden star and a blackbody distribution with a temperature corresponding to the radius of the planetoid swarm. For the case of the Sun with the planetoids at the radius of the Earth the temperature would be approximately 300 ºK.

Signs of a Kardashev Type II Culture

But back to The Economist, which notes that Dyson spheres are but one thing an advanced civilization might spend its time creating. A Kardashev Type II civilization is one capable of exploiting all the energy output of its star, leading to the possibility of interesting forms of stellar engineering. From the article:

Any civilisation that has built a Dyson sphere will have to have been around for a long time, of course—and in the very long run its star will start to change in unpleasant ways, ballooning to form a red giant. Another signature of advanced technology would be an attempt to slow this process down. Red giants are created when a star exhausts its supply of hydrogen at its core, with the result that the inner layer contracts and the outer layers expand, forming a redder and much larger star. If the star’s outer layers could be mixed into the core, that would slow the process of inflation down. And, presumably, a sufficiently advanced civilisation would try to do that if it could.

Let’s assume so, and fortunately for distant observers, such an engineering feat would show a particular signature, even if one that, like Dyson spheres, can be confused with natural phenomena:

Such a star would look odd, though. It would be bluer than it should be and would be of a type known to astronomers as a “blue straggler”. Although, again, there are perfectly natural reasons these might form. The universe, though, is an ancient place, so many civilisations could be very old indeed. Perhaps, then, it will be a sign like this—of a technological civilisation millions of years old—that is seen, rather than some upstart that has not even got its radio waves under control.

Leaving Assumptions Behind

New technologies will help us pursue other observational lines of research. Using space-based telescopes and, recently, instruments on the ground, we’re learning how to characterize the atmospheres of extrasolar planets. It’s a process that’s sure to pick up as we refine our techniques, but in the not so distant future we may be able to spot telltale signs of industrial activity on a distant world as well as probing its atmosphere for the chemical signs of life. Carrigan suggests chlorofluorocarbons (CFCs) as one possibility, a type of molecule that would have to correspond to a technology because no natural process produces it.

All of these approaches take us in a new direction, one that, as Dick Carrigan notes, makes no assumptions about the motivation of the originating civilization. Indeed, he likens the hunt to a form of archaeology, one closer to the search for exoplanets than the attempt to snag a radio signal. For more, see Carrigan, “Starry Messages: Searching for Signatures of Interstellar Archaeology,” FERMILAB-PUB-09-607-AD (abstract), which I’ll be discussing in more detail soon, along with cross-references to Davies’ The Eerie Silence. For as Davies shows, there are many more ways to conceive how the activities of an extraterrestrial civilization could modify the heavens.


Comments on this entry are closed.

  • tps April 20, 2010, 12:42

    Perhaps we should be looking for the amounts of asteroids and comets in other systems. Those are very good resources and the lack of them in a system could indicate that someone has been using them.

  • Administrator April 20, 2010, 14:12

    Interesting you should mention that, tps, as Davies explores the idea of looking for obvious gaps in circumstellar systems as a possible venue for discovery. How about magnetic monopoles? Not found because harvested by a passing star-faring culture? Some wild but energizing speculation in The Eerie Silence.

  • kurt9 April 20, 2010, 14:31

    That’s quite good for the Economist.

    Any case, I’ve always thought the archeological approach was the best one ever since I was in high school. It would also make sense to look for artifacts in our own solar system. There might not be any aliens today, but there may have been, say, a billion years ago and they may have come a-visiting back then. Its possible that a civilization develops, industrializes, and maybe expands into space for a few centuries or millennium, before they go “post physical”.

    I think post-physical civilizations are going to be really hard to find. It makes more sense to look for artifacts, both near and far.

  • Bounty April 20, 2010, 15:04

    It might be fun to discover a large transiting ‘planet’ that doens’t cause a wobble. Low mass, or large solar array?

  • NS April 20, 2010, 16:18

    For an occasionally hilarious discussion of why we might not have detected ET civilizations (even if they exist), see here:


    A more “serious” version of the same article:


  • Eric April 20, 2010, 17:32

    Well, I’m an archaeologist, so I’ll bite on this.

    I think there could be some interesting inferences on the nature of an ETI society in some plausible detection scenarios. For instance, archaeologists make a pretty big deal of “settlement pattern analysis” to look at issues of political and economic centralization. Situations with one really big site, where the next biggest sites are much smaller, tend to go with very centralized societies. Situations where you see lots of relatively equally sized sites tend to show more dispersed power and decentralization. Sometimes you can even see evidence for “no-man’s land” areas between patches of settlement, which can mean intensive warfare.

    Depending on how we detect ETIs, I think settlement pattern analysis may be the first archaeological methodology to consider using.

  • tps April 20, 2010, 19:50

    I actually got the concept from Stephen Baxter’s “Manifold: Space” and “Ark” novels.

    Maybe we should look for the signs deeper in the galaxy. The stars are closer and there’s a lot more energy available then out here in the boonies. There’s also the chance that we showed up too late for the party.

  • Ronald April 21, 2010, 5:29

    I agree with kurt9 that Interstellar Archaeology in the broadest sense (including all sorts of beacons and artefacts) is probably the most effective way to look for any ETI, simply because it makes use of a MUCH wider window of opportunity than just contemporaneous ETIs.

    At the same time I find the search for Dyson sphere like objects much too limited and speculative, because it is based on the as yet totally unfounded premise that an advanced civilization will have the complete harnessing of its sun’s power as an ultimate objective, a premature and rather weak assumption which is also true for the Kardashev scale in its original form.

    The basic idea of the Kardashev scale (expanding utilization of resources) is probably valid, but not so narrowly focused on energy consumption.
    I have previously suggested using a modified Kardashev scale (they probably exist already), one not focusing on energy consumption per se, but rather on the *level of expansion* of a civilization: single-planet, multi-planet within own planetary system, multi-planetary system, galactic, multi-galactic, …

    Availability of energy as a predominant issue might appear to be very 20th/21st century human. It is likely that for any truly advanced civilization energy is simply no longer a seriously limiting factor in its ambitions. Rather like a fundamental ingredient that lubricates the machinery of a civilization and its ambitions, a necessary means to an end, but not a goal in itself.

    With due respect for Kardashev’s original concept and brilliance, the idea of a K1 civ even attempting to harness its sun’s total output before moving on the a ‘next level’ seems almost laughable to me, even more so than an (early) earthly civilization attempting to cultivate ALL land on earth of fish ALL fish from the sea before moving on to a higher level of technological state. Some calculations on my other favorite website, NextBigFuture, show that even a totally-spoiled-and-wasteful-multi-billionaire level of energy consumption (hey, why not another space vacation) for all people on earth (at some 10 billion inhabitants level) would only use a minute fraction of a K1 level of energy consumption.

    The same might be true in a slightly different way for the attempted red-giant-postponement: moving to and engineering another planet in the same system (Mars or even Jovian moons, and Titan, in our case) would probably be vastly more cost-effective. Plus the notion that a civ that would be able to perform such a feat would likely also be able to make it to another nearby planetary system.

    Real question remains what makes an advanced ETI tick, what would be their true objectives. Dispersing through their galaxy and spreading their life and civilization might be a reasonable guess, also with a view to the tremendous added advantage of risk-spreading and survival.

  • rich April 21, 2010, 7:11

    It is very difficult to step out of our anthropomorphic frame of reference when speculating on who aliens might be, what they might be like, what kind of “civilization” they would have etc. That’s why it seems to me to be wild speculation that aliens would think of Dyson spheres – is doing so the ONLY solution to harvesting energy from a star? Or is it one that we, as humans with our own biases and frames of reference, have conceptualized as a star-harvesting response?

    There are several philosophical and sociological problems associated with our theorization of life elsewhere that start with our imagining others as social, with civilizations, that they are humanoid, or that they are amoeba-like, etc. Such beings may resemble nothing like what we imagine them to be. In our human imaginary (constructed by biology, or social and material structures, or by our human ideas, name-your-human-evolution-theory, etc.), are we constructing search tools that reflect such human biases? For example, couldn’t “pollution” be some sort of human invention, something that happened because of our particular relationship to natural process (our collective attitudes and social history toward waste, disposal, tolerance of garbage, etc.)? How do we remove our human “lens” on constructing/conceptualizing both the universe and beings that are in it? I think once we succeed at bracketing off our anthropomorphic conceits, we’ll have a better toolset by which to both search for “aliens” and to conceptualize them.

    Yet there may be verifiable evidence in space that supports the claim that beings (sentient, nonsentient?) are there. So isn’t it more plausible to simply expand the search sets of radio/electromagnetic wave spectra (and I realize that “simply” is actually quite complex), so that we are looking at all waves, rather than those that human theoretical Dyson spheres might generate? The idea of looking for pollution in atmospheres is an excellent next step, but perhaps we should be looking for ANY artificially created substance (like the idea of looking for CFCs), or artificial radioactivities…. Beside radio signals and pollution, what would an hypothetical, advanced life-form observer several light years away detect in our system that would signal “life”? And what would a non-metallic, non-plastic, non-CFC generating “civilization” look like on our sensors?

    There’s also the sad and existential possibility that there just is nothing else but us. Or perhaps rather than the idea that “we showed up too late for the party,” perhaps we’re just too early!

  • Lionel W April 21, 2010, 14:50

    My sense is that with our continually improving computational power, and better algorithms, approaches to SETI should be continually widened. So often discoveries are of things that we never even considered.

    Of course it’s sensible to spend some of our resources looking for what we know already from our own civilisation and owns concepts – radio signals, dyson spheres and so on. But in general, until we have some precedent for an ETI, we need the wide beam of the floodlight, rather than the narrow beam of the spotlight.

    I don’t know much about the tools available. While this is a basic comment,l I think that the most comprehensive analysis of a star system, combined with our latest algorithms to search for unusual combinations of abundances or material depletion, is the way to go.

    Also, Rich made the point above that maybe pollution is a human invention, but I think that the overriding precedent is that life does alter its environment in major ways. However, I absolutely agree that we should not assume that an ETI will alter the environment in the way which we have, neither in ways that we have imagined. ETI technology will at least in part be based on discoveries which we haven’t made.

  • kanomi April 21, 2010, 14:57

    Why would a civilization at that level of achievement bother fooling around with building Dyson Spheres when they could just go to the next star – of which there are hundreds of billions?

    And don’t give me that nonsense about nobody bridging the vast gulfs of space. A technology base that grows exponentially over a million years is going to put that tiresome canard to bed.

    And the idea that a civilization a million years advance of us is guided by Gee-Whiz! Engineering projects is the biggest load of anthropomorphic tosh you could toss up there.

    For all you know, they evolved into intangible, immortal fourth dimensional entities – or uploaded themselves into a quantum computer, either of which sounds more likely than the Interstellar Army Corps of Engineers.

  • Mark Meier April 22, 2010, 13:26

    Another hat tossed into the fray:

    What if solar power turns out to be too inefficient for the advanced civ to bother with? Maybe there’s another — better — option? Maybe someone out there has perfected a Casimir Generator, or Stargate’s ZPM. No need for such passe ancient tech like converting hydrogen into helium, regardless of in a power station or a star. And antimatter? HOW GAUCHE!

  • Eniac April 22, 2010, 23:58

    Or maybe the advanced civilization have moved their minds into superconducting circuitry, where they would require very little power, can live a thousand years per second, and enjoy an endless variety of virtual realities in their copious leisure time. Oh, and the entire civilization fits inside a regular sized asteroid floating in intergalactic space the better to maintain the superconductivity.

    Just kidding… (or not?)

  • Tobias Holbrook April 23, 2010, 9:15

    “There’s also the sad and existential possibility that there just is nothing else but us. Or perhaps rather than the idea that “we showed up too late for the party,” perhaps we’re just too early!”
    Sad? I prefer to consider that the best possibility ever, since it means we get the entire galaxy to ourselves. Or rather, us and out creations, plus an ET species we find and uplift. Forerunners, anyone?

    My personal opinion is that we’re alone in the galaxy, and that any search for ET’s artifacts are a waste of time. If we do find some, it will probably be Sealed Evil in a Can, anyway – some kind of grey goo that wiped out the previous civilisation that came across it.

  • Lionel W April 23, 2010, 20:21

    Well, we should add techniques for identifying planets of Grey Goo to our SETI wish list.

    For those that believe that advanced intelligence and technologies will typically result in a civilisation wiping itself out, the SETI search should be changed to SEETI – Search for Extinct Extra-Terrestrial Intelligence

  • ljk April 24, 2010, 11:31

    April 18, 2010

    Unknown Object in Nearby Galaxy Sending Mysterious Radio Waves (Weekend Feature)

    There is something strange is lurking in the galactic neighborhood. An unknown object in galaxy M82 12 million light-years away has started sending out radio waves, and the emission does not look like anything seen anywhere in the universe before except perhaps by Ford Prefect. M82 is starburst galaxy five times as bright as the Milky Way and one hundred times as bright as our galaxy’s center.

    “We don’t know what it is,” says co-discoverer Tom Muxlow of Jodrell Bank Centre for Astrophysics near Macclesfield, UK. But its apparent sideways velocity is four times the speed of light. This “superluminal” motion occurs usually in high-speed jets of material bursting out by black holes.

    “The new object, which appeared in May 2009, has left us scratching our heads – we’ve never seen anything quite like this before,” said Dr Muxlow.

    “The object turned on very rapidly within a few days and shows no sign of decaying in brightness over the first few months of its existence. The new young supernova explosions that we were expecting to see in M82 brighten at radio wavelengths over several weeks and then decay over several months, so that explanation seems unlikely.”

    Full article here:


  • Adam April 25, 2010, 4:34

    The immense time-spans likely to pass between civilizations means we’ll be picking over their technological ‘bones’ regardless of their end-state. Robert Silverberg’s “Across a Billion Years” springs to mind.

  • Lionel W April 25, 2010, 18:15

    Humans certainly don’t set any precedent for creating long lasting civilisations (not yet anyway), but who knows whether an ETI would be like us.

    I think the instability of human civilisations is partly due to the finite human life span. innovating immortality would probably lead to civilisations that lasted much longer

  • Brian May 5, 2010, 7:55

    Dyson spheres get most of its attention through fiction, rather than realistic expectations. There are probably as many dyson spheres in this universe as there are teleportation devices and time machines. Realistically, energy would not be the most critical need of an advanced alien civilization. That is more a projection of our own insecurities, much like primitive man might imagine “limitless food” as the ultimate goal of civilization.

    An advanced alien civilization – say a million years more advanced than ours – would probably be limited more by social, military, and resource (physical matter) factors than energy ones. For example, once people became nearly immortal, how to deal with new generations of children (ban them? eat them? send them off to other solar systems?)

    It needs to be said that a very advanced civilization probably would be millions of times more energy efficient. For example, travel between planets would take almost no energy with space elevators and stable orbits that pass beside planets. The most powerful computer might take less power than the weakest computer in our primitive arsenal.

    Just as monkies and primitive man fantasized about limitless food, modern man fantasizes about limitless energy. Our imagination could do better.

  • ljk May 26, 2011, 13:17

    Hubble Finds “Oddball” Stars in Milky Way Hub

    by Jason Major on May 25, 2011

    Astronomers using the Hubble Space Telescope to peer deep into the central bulge of our galaxy have found a population of rare and unusual stars.

    Dubbed “blue stragglers”, these stars seem to defy the aging process, appearing to be much younger than they should be considering where they are located. Previously known to exist within ancient globular clusters, blue stragglers have never been seen inside our galaxy’s core – until now.

    The stars were discovered following a seven-day survey in 2006 called SWEEPS – the Sagittarius Window Eclipsing Extrasolar Planet Search – that used Hubble to search a section of the central portion of our Milky Way galaxy, looking for the presence of Jupiter-sized planets transiting their host stars. During the search, which examined 180,000 stars, Hubble spotted 42 blue stragglers.

    Of the 42 it’s estimated that 18 to 37 of them are genuine.

    Full article here: