The European Planetary Science Congress ends today in Rome even as scientists and engineers on the astronautical side of things head for Prague, where the International Astronautical Congress convenes on Monday. I’ll be keeping an eye on events in Prague and wishing I could join the gathering of Tau Zero practitioners that will be taking place there — Marc Millis will be presenting four papers, and many of the Project Icarus team members are also making the journey, so we should be getting regular updates on matters interstellar.
Nor do I want to neglect the Royal Society meeting on extraterrestrial life, coming up early in October in Buckinghamshire in the UK. Emails from James Benford (Microwave Sciences) and Richard Carrigan (Fermilab) tell me both will be speaking at the session, which reminds me that it was way back in April that I promised more on Carrigan’s notions of interstellar ‘archaeology,’ a form of SETI that makes no assumptions about the originating civilization. It’s time to honor that pledge by looking at the kind of artifacts an advanced civilization might create that could be detectable. Today I want to focus on an extreme possibility, a civilization that spans its galaxy.
Kardashev Type III and Its Traces
What would happen if a true galaxy-spanning civilization went to work on the kind of astroengineering it would be capable of? We call this a Kardashev Type III civilization, one that could exploit the power resources of an entire galaxy, and my assumption has always been that such a culture would be blindingly obvious, its projects so vast that our astronomers would be able to detect them by noting anomalies that could hardly be natural occurences. Imagine, for example, a galactic culture that encloses each individual star in a Dyson sphere.
A Dyson sphere or ‘shell’ would absorb all of the visible light from a star, re-radiating stellar energy at infrared wavelengths. A Dyson ‘ring’ would use planetary materials that would mask only part of the star’s light. Dick Carrigan used a list of interesting infrared sources from the Infrared Astronomy Satellite (IRAS) in his own search, coming up with no strong Dyson sphere candidates. But Dyson spheres remain interesting, vastly increasing the habitable area around a star. What would a Type III civilization do with technologies that could create Dyson spheres not only in one place but across the galaxy?
Image: M81, a spiral galaxy in Ursa Major. A ‘wavefront’ Dyson sphere culture might spread across such a galaxy, causing stars to drop out of visible light entirely, one by one, to be detected in the infrared. Credit and copyright: Giovanni Benintende.
Whatever the answer, you would think it would be clearly noticeable. Freeman Dyson himself has said that “…a type III (Kardashev civilization) in our own galaxy would change the appearance of the sky so drastically that it could hardly have escaped our attention.” In a recent paper, Carrigan also quotes James Annis, who has studied anomalous galaxies in quest of signs of a Type III civilization. Annis’ view: “It is quite clear that the Galaxy itself has not transformed into a type III civilization based on starlight, nor have M31 or M33, our two large neighbors.” But Carrigan wonders whether we should take these statements as definitive:
…what would happen for a civilization on its way to becoming a type III civilization, a type II.5 civilization so to say? If it was busily turning stars into Dyson spheres the civilization could create a “Fermi bubble” or void in the visible light from a patch of the galaxy with a corresponding upturn in the emission of infrared light. This bubble would grow following the lines of a suggestion attributed to Fermi… that patient space travelers moving at 1/1000 to 1/100 of the speed of light could span a galaxy in one to ten million years. Here “Fermi bubble” is used rather than “Fermi void”, in part because the latter is also a term in solid state physics and also because such a region would only be a visible light void, not a matter void.
A Wavefront of Dyson Spheres
This is long-term thinking in the richest sense. We currently have spacecraft moving out of our Solar System at a speed of roughly 0.0001 of the speed of light. If we stay well within physics as currently understood, we can expend 100 times the energy to raise that velocity from 42 kilometers per second to 400 kilometers per second, in the neighborhood of 10-3 c. Now we’re talking galactic travel times on the order of a hundred million years and less, half the time the Sun takes to circle the center of the galaxy. A patient, long-lived civilization could, as Carrigan notes, envelop a galaxy on a time-scale comparable to or shorter than the rotation period of the galaxy.
Turning stars into Dyson spheres as it went, such a culture should leave vast Fermi ‘bubbles’ whose infrared signature would flag their existence. But detection might still elude us. Carrigan points out that we see M51, the Whirlpool galaxy, face-on at a distance of 30 million light years. We can say with some confidence that we see no unexplained voids larger than about five percent of M51’s area, but any void features below this level would be hard to identify because of spiral galaxy structure. Elliptical galaxies might be better places, says James Annis, to look for Fermi bubbles, because they display little structure, and potential voids should be far more pronounced.
Image: The elliptical galaxy Centaurus A. Are ellipticals the best place to search for Type III civilizations? Credit: NASA/CFHT.
Even so, separating artificial structure from natural phenomena is not easy. I was fascinated to learn from Carrigan’s paper that we have identified several relatively dark dwarf galaxies, including Virgo HI21, the Ursa Major dwarf spheroidal UMa dSph and others. Anyone making the case that these objects show evidence of Dyson sphere activity runs into more plausible natural explanations. Virgo HI21, for example, may be showing the effects of tidal shredding. These so-called ‘dark galaxies’ demand a thorough understanding of star populations and types and exquisitely precise radio astronomy measurements to help fix the rotation rate. What might have seemed an easy search for a Dyson infrared signature becomes exceedingly complicated.
When Annis ran his search for anomalous galaxies, he took advantage of the fact that galactic optical brightness can be measured against the maximum rotation velocity and radius of the galaxy. The relationship between these factors is relatively consistent, allowing the scientist to look for sources below the normal trend line in a sample of 137 galaxies. Annis found no candidate galaxies in this admittedly small sample, but the work allowed him to set a limit on dark galaxies below one percent of the population. Larger-scale searches will doubtless be attempted one day.
The Uses of Interstellar Archaeology
Even though we have only begun to tune up our methods for detecting an extraterrestrial civilization, there is much we can learn. Our existing SETI methods and near-term observations of exoplanets may be the best chance at finding a genuine life signature elsewhere, but we should keep the possibility of a truly advanced civilization in mind. Says Carrigan:
Breaking up a planet to build a Dyson sphere completely transcends the will, ability and long term dedication of contemporary civilization. There is not even a mildly plausible technique for stellar engineering. Surprisingly the step from a Dyson sphere to a galaxy filled with Dyson spheres only involves a strengthened capability for space travel. On the time scale of Dyson Sphere construction this type of relatively slow space travel can be discussed even now. To recapitulate, SETI and the search for planetary signatures of life and intelligence currently appear to be the most accessible signatures. For Dyson spheres and stellar engineering work should mostly be directed toward investigating engineering techniques and limitations.
I come back around to the premise behind interstellar archaeology, that unlike conventional SETI it does not require a civilization to have any intention of contacting us. There are numerous ways to proceed, involving the kind of Dyson sphere search Carrigan has himself conducted within our own galaxy, or looking at planetary atmospheres in hopes of finding not only biosignatures but the markers of an advanced industrial or post-industrial culture. As we continue the SETI hunt, keeping in mind how planetary change or deliberate decisions to expand into the galaxy could leave visible traces allows us to hunt for things advanced intelligence might do.
How many civilizations in our galaxy, for example, have already faced the end of their main sequence star’s lifetime? If the number is high, it may be that we can find evidence of their response in the form of planetary or stellar engineering, making stars of this description interesting targets for future searches. In any case, our model of SETI is changing as not only our technologies but our assumptions become more sophisticated, leaving us to ponder a universe in which the need for expansion or simple survival may have left its own detectable history.
The paper is Carrigan, “Starry Messages: Searching for Signatures of Interstellar Archaeology” (preprint). The Annis paper is “Placing a limit on star-fed Kardashev type III civilisations,” JBIS 52, pp.33-36 (1999).
Comments on this entry are closed.
In an interesting coincidence, here’s an article on a hard-to-detect alternative to a Dyson Sphere:
“The Solar Wind Power Satellite as an alternative to a traditional Dyson Sphere and its implications for remote detection”
Thanks, Tulse. What an interesting concept! I want to read this one carefully and will probably write it up here soon.
I think the Kardashev scale needs to be replaced by a better measure of a civilization’s technological capability. The current I, II, III scale seems way too optimistic in its assumptions about the kind of technology obtainable by us or hypothetical alien civilizations. For example, although it’s plausible on paper, I don’t think its at all realistic to assume any civilization can ever reach a Kardashev Type III (a civilization able to harness all of the power available from a single galaxy).
I think a more modest/realistic scale should be adopted… perhaps something based on a civilization’s energy requirements to travel great distances (as opposed to their ability to harness all energy available), i.e.:
***Type I*** has power available to travel throughout own solar system (able to accelerate ~ 1000 kg to about 0.0001c — this is us). ***Type II*** power available to travel to local star systems (able to accelerate ~ 1000 kg to about 0.01c). ***Type III*** power available for travel throughout a single galaxy (able to accelerate ~ 1000 kg. to about 0.1c). ***Type IV*** power available for intergalactic travel (can achieve speeds arbitrarily close to c — where relativistic effects dominate). To me, a scale such as this makes much more sense.
It strikes me that we’re using the strategy of looking for what’s easy to see, akin, as has been stated elsewhere to looking for one’s lost keys under the streetlamp.
It seems to me that logically, Kardashev II civilizations should be more common than K III. I for one would be interested in a future article or articles that also explore our potential ability to detect and strategies for detection of K II civilizations in our own galaxy. I presume this would be by detection of telltale signatures in stellar spectra, but I’d love to hear other ideas.
M81 shown above may not be such a random pick. According to Robert Burnham’s Celestial Handbook, “M81 is among the densest galaxies known, containing about 0.1 solar mass per cubic parsec; it has possibly about twice the density of a large typical Sb spiral like M31.”
And: “…the integrated spectral type is about G3.”
The images of this beautiful spiral do suggest tighter spacing: the stars are more neighborly. In that wise, they would be more beckoning to native questing intelligence.
Once our own civilization began to penetrate space, its methods tended to advance, and the newer technologies were applied. If an extragalactic civilization has the will and the means, its operations in interstellar space could engender expansion, much like the early European fleets looking to circumnavigate the planet, discover new lands, establish new and profitable trade routes… and colonies.
Perhaps truly advanced cultures that do not annihilate themselves have a different view of expansion than we do? Consider the level of restraint necessary to avoid nuclear confrontation. Even a humanist or a materialist can talk about this as a sort of “spiritual” advancement to not immediately destroy one’s enemies with such a powerful weapon. Perhaps the greed for energy will be obsolete by then? Is it reasonable to expect an advanced civilization to have an industrial mindset of immediately assimilating resources, when we already know that, in the end, such a pursuit will not stop the heat death of the universe, for instance? Will not man (or other intelligent life) advance “spiritually” as well as technologically? Is it possible advanced civilizations may prefer to preserve galaxies in their natural state of galactic evolution, like we preserve National Parks?
Just a thought. I don’t consider myself an environmentalist, but it sure makes it sound like I am!
There are other reasons I can see why Type III civilizations could already exist and not be obvious:
-Alternatives to Dyson spheres/rings could be created that are so closely aligned with physics that are not apparent as an anomaly to us Type <1 folks.
-Technological singularity could cause a civilization to become increasingly energy-efficient and less biologically-based which would widen the range of habitability and lower the energy requirements for expansion. This would drastically reduce any infra-red signature needed for detection.
Also, can we say for sure that a given brown dwarf star or dark body is not already a Dyson sphere?
Now this is just educated guessing but for a Class III civilization to exist wouldn’t a form of FTL communication via signaling or FTL travel be needed? Just simple light speed communication would take 100,000 years to get from one side of the galaxy to the other, the change and separation between cultures would be vast between transmission and reception. Not only must they be long lived but change very little culturally over thousands of years. So either we have FTL or a very stagnant, immortal civilization.
More interestingly: how much of what we think is “natural” galactic structure could be the work of intelligent beings? Maybe Kardashev III civilizations like to arrange the stars of their galaxies into spiral shapes, and only elliptical galaxies are “undeveloped.” Or vice versa.
I’m glad I read Istvan’s comment: “It strikes me that we’re using the strategy of looking for what’s easy to see, akin, as has been stated elsewhere to looking for one’s lost keys under the streetlamp.”
This led me to recall a use of Bayesian Statistics that states the best place to look is not only where you suspect to find something, but also one where you can most easily see what you are looking for. Here’s a link: http://en.wikipedia.org/wiki/Bayesian_inference#Search_theory
The article is in reference to establishing a search-grid to look for a lost US nuclear submarine (the USS Scorpion). Here’s an excerpt that articulates the main point: “The probability attached to each square was then the probability that the wreck was in that square. A second grid was constructed with probabilities that represented the probability of successfully finding the wreck if that square were to be searched and the wreck were to be actually there. This was a known function of water depth. The result of combining this grid with the previous grid is a grid which gives the probability of finding the wreck in each grid square of the sea if it were to be searched.”
The Bayesian approach eventually worked in finding the ship; but the most notable difference in using this approach to find a sunken ship versus finding an ET is that they knew the ship was down there somewhere.
Gerald Nordley’s alternative to a Dyson Habitat is the Dyson Power-Sphere – thin and immense solar collectors close in to a star to provide power for an active interstellar civilization. IMO that provides another means of “extraterrestrial archaeology” – search for large artificial structures causing transits.
Which raises a point – how would we distinguish different shaped transitting objects? A sphere versus a circular disk for example. Of course there are some very clever spectral analyses of transitting bodies which pretty strongly indicate they’re planets, and independent mass determinations make the case even stronger for natural objects – but if a transit is detected, yet no corresponding mass is detectable, would it be written off as a “starspot” rather than a synchronous orbit solar collector? Of course it would. Thus we could be seeing ETIs hard at work collecting energy for their starfleets – and we’d be none the wiser!
I must agree with Robotbeat in that I don’t understand the fascination among certain SETI-philes with Dyson Spheres/Shells/Rings, energy dominance, Kardashev Scales, and generally all things materialistic. Assigning 21st century human motivations to a billion year old civilization…makes me extremely skeptical. I don’t think I would use the word “spiritual” because of the new age baggage, but I think I understand what Robotbeat means. The most powerful thing in the universe is not spacial conquest or harnessing ever-larger quantities of energy, but intelligence itself. Maybe these far advanced minds just want to understand the reality they find themselves in and leave some for those who follow.
Along the lines of what several posters have said, here’s an interesting speculation (one of many out there I’m sure) that we may no more be able to detect a truly advanced intelligence than an ant is able to detect us:
“Any sufficiently advanced intelligence is indistinguishable from nature.”
To Adam, it might be possible (not for me though!) to mine Kepler Mission data for the type of scenario you describe.
About that solar wind power scheme, as far as I can understand the authors suggestion in
would not work at all (i.e. the ‘pre-wire’ can’t be magically ‘folded’ so that it wouldn’t cancel the ‘main-wire’ field)..
Technical details aside, as far as I can understand, the purpose of the device would be to capture solar wind’s kinetic energy with some kind of MHD-generator type of scheme. Interesting idea.
btw, quick back-of-the-envelope shows that at 1 AU solar wind carries around ~ 1mW/sqm kinetic energy. Just saying.. as an aspiring Kardashev scale I civilization I might just rather want to go after that full 1300W/sqm solar output before long.
I just want to thank you for expanding my Seti horizons more in the last 5 days than in the last 10 years with your “Interstellar Archaeology” and “Ecliptic” articles. The comments to this article are very interesting, but my biggest concern remains the Fermi Paradox. From my observations in my house (two sons in high school) we will devolve into a robotically supported culture of virtual realists having orgasms.
Ok, that last sentence was extreme, but the FP and our future and any advanced civilizations prospects remain a huge concern to me. So, therefore, Seti prospects as well.
I think of the very long term civilizations on our planet that lasted for 10K+ years and did not leave any discernable trace from orbit. I too am skeptical about the whole Dyson sphere thing. I don’t see that as being the way to go for a civilization that has 1) interstellar flight capability and 2) a very very long term view of things. If they needed to expand they’d just travel to another planet, moon, or star system. Why go to the trouble to build a super- complicated engineered shell when there are billions of planets to colonize? Spanning timescales far longer than the lives of the suns they’re enclosing, it’d be a waste to build that Dyson shell for only a “temporary” containment of the energy. As a famous spacefaring TV robot once said: “DOES NOT COMPUTE.”
I am with Greg on the futility of interstellar civilization without FTL communication.
Going further, it can be argued that advanced intelligences will have found ways to speed up their thought processes dramatically, making light speed an even more formidable limit. Such “accelerated” civilizations would tend to miniaturize and crowd together onto small worlds such as asteroids, in order to reduce the latency of their discourse. If they have an alternative energy source, they may even want to be far away from any stars, to avoid thermal and radiation damage and to better take advantage of superconducting circuits.
Thomas: I whole-heartedly agree.
Scott G: All searches of any kind are done with the purpose of finding something of a particular class. If you don’t know what you’re looking for, how do you know you’ve found it? Still, to be surprised by the unexpected you must still search. The thing is, will you recognize the unexpected when you see it in the data? That isn’t a trivial question, but crucial. I think it was Asimov who said about scientific discovery that the really interesting discoveries came from those who collected data while searching for one thing and then found themselves saying, ” hmm, that’s funny…”.
Regarding Dyson spheres and their like: I just don’t buy it. Any civilization that could build such a construct doesn’t need it. That kind of knowledge and power would more likely be applied elsewhere. For example, if they can build such a thing they don’t need a star to fuel their “public works” projects. And if they did, I doubt there’d be any waste heat; they would just take the photons and stellar wind particles to make stuff or just store it away as bulk matter, just as they used the same technology to initially build the construct.
As a directed search strategy this seems pointless to me. But then, as I said above, an infrared search may turn up surprises of other kinds while other, productive science is being accomplished.
I agree that environmental concerns might make a Dyson sphere seem highly dubious to an ethical species. One thought struck me: What if simple lifeforms existed in the outer system – like, say, an acetylene- or hydrogen-consuming organism on a moon like Titan? Would enclosing a star around the ‘habitable zone’ shut off energy vital for their development and, if so, should they be accorded an inalienable right to a share of solar energy?
bhb2048, thanks for the supportive words! You’re probably already aware of the Benfords’ work on interstellar beacons, but if you haven’t run into it in your SETI search, be sure to check out their ideas. A quick search under ‘benford’ in the search engine here will pull them up, or try these for starters:
Ignoring the phlebotinium (e.g. slipspace), Bungie – yes, Bungie, creators of the Halo series – have created in my opinion a much better system for classifiying the level of a civilisation, http://halo.wikia.com/wiki/Technological_Achievement_Tiers
Personally, I refer a system based on several technological “streams”, where they would be classified on things such as power source (i.e. human, animal, coal+steam, nuclear), power storage (i.e. clockwork, steampower, I.C.E), and computing power (i.e. clockwork, transistors, quantum). Assuming we can’t get free energy, the most advanced civilisation would be found around balck holes, converting matter directly into energy (both from the tidal effects, which is around 50% efficient at most, and the Hawking radiation), and be using this power for quantum computing – if that’s the computational limit. The presupposition of Dyson Spheres suffers from the flaw that any civilisation capable of enclosing the galaxy in them can probably take the stars apart and either fuse the matter in a more efficient way that doesn’t result in 90% of the star remaining unburnt, or take it apart and convert it directly into energy.
Alastair Reynolds has a rather nightmarish version of the kind of Galactic scale transformation that this sort of SETA effort is looking for. In his “Galactic North” anthology self-replicating terraforming robots are unleashed accidentally and begin converting a large part of the Galaxy into “Solaria” style Dyson Swarms – if anyone has read Marshall Savage’s “The Millennial Project” you’ll know what I mean, but simply it means all the mass in a system is converted into spherical habitats filled with plant life. Thus in the fictional disaster the machines turn large parts of the Galaxy green…
I’d be more interested in we could find ways to detect species with relatively low technology, because I would argue that such species are likely to be in a stage that lasts longer than more technological civilizations. Are there ways to detect such species effects on a local or planetary scale? This might be relevant if we can start to do detailed planetary imaging and analysis within the next century several thousand nearby stars.
I don’t believe in the existence of Dyson spheres, for good reason : nuclear fusion technology would come long before that kind of technology. Indeed, a civilization capable of producing Dyson spheres would in all likelihood have miniature stars inside their cellphones first, which would be quite scalable.
“This might be relevant if we can start to do detailed planetary imaging and analysis within the next century several thousand nearby stars.”
Once they start altering their atmosphere significantly, and provided our telescopes are advanced enough, we should be able to detect them. It might require them to be going through an Industrial revolution, though.
A “low-tech” culture that was (say) burning wood for fuel might not produce an atmospheric signature that was distinguishable from natural forest fires. The same might be true of a “high-tech” culture that was consciously minimizing its footprint. Maybe it’s only “in-between” cultures that produce detectable signatures.
Pete, though I entirely agree with your point regarding energy sources, there are additional considerations for Dyson spheres or other structures – lebensraum and resource extraction. Energy isn’t the only constraint, and a few more billions of us will come close to exhausting the resources accessible on our local rock. Our searches may benefit from improved ability to examine spectra of dusty halos, because it’s reasonable to assume that some fraction of a civilization moving toward Kardashev II will be based among their planetary system’s asteroid and dust belts.
I agree with the implied point that fusion and possibly more advanced energy sources are in fact necessary for this kind of engineering. While a Dyson sphere may not end up as the engineering artifice of choice, when looking, we might be able to spot a Solaria-type swarm or other engineered structure almost as easily. An interplanetary civilization will need habitats, and the home planet is not an efficient one due to the delta-v needed getting in and out.
A random side speculation – in the area of detectability and things that may be easy to spot (or not): we’re now beginning to use electric-ion propulsion for our probes. Assuming a civilization making very substantial use of such propulsion, what would it take, in terms of advancing our detection ability or in terms of the extent of the civilization’s use of ion propulsion, for us to be able to detect a signature of ionized matter in the interplanetary halo of a star, sufficient to believe that signature is artificial? Alternatively, posit a putative fusion-derived propulsion in use, which would also expel radiating exhaust?
‘How many civilizations in our galaxy, for example, have already faced the end of their main sequence star’s lifetime?’
According to Kardashev, Type III civilization can evolved from a Type 0 civilization in thousands of years. Star’s lifetimes are counted in billions of years. And a Type III civilization is no longer linked to a single star. So, I cannot see the point of that question.
If there has been any civilization on our galaxy that has endured billions of years, then it would have attained the Type V or Type VI status.
I agree with Scott G (and others) and argued before that the Kardashev scale is probably outdated and needs revision because it is based on the very 20th century premise that maximum energy harnessing is the ultimate goal of any advanced civilization., whereas to such a civilization energy utilization may just be a means to another end.
More realistic indeed to use the technological level of such a civilization (particularly types of energy used) or its level of expansion: from own planet via own planetary system, interstellar, and galactic, to intergalactic.
Hmm. Compare many photographs of galaxies and do statistics on the number of galaxies bearing life forms that eventually become galactic scale dyson sphere empires.
Black Holes: Attractors for Intelligence?
Authors: Clement Vidal
(Submitted on 21 Apr 2011)
Abstract: The Search for Extra-Terrestrial Intelligence (SETI) has so far been unsuccessful and needs additional methods. We introduce a two-dimensional metric for civilization development, using the Kardashev scale of energy increase and the Barrow scale of inward manipulation.
To support Barrow’s scale limit, we contend with energetic, societal, scientific, computational, and philosophical arguments that black holes are attractors for intelligence. An application of the two-dimensional metric leads to a simple, consistent and observable hypothesis to test the existence of very advanced civilizations.
We suggest that some already observed X-Ray binaries may be unnoticed advanced civilizations, of type KII-Bomega. The appendix provides an argumentative map of the paper’s main thesis.
KEYWORDS: SETI, black holes, Kardashev scale, Barrow scale, star lifting, XRB
Comments: Presented at the Kavli Royal Society International Centre, “Towards a scientific and societal agenda on extra-terrestrial life”, 4-5 Oct 2010. 16 pages, 1 figure and 2 argumentative maps
Subjects: General Physics (physics.gen-ph)
Cite as: arXiv:1104.4362v1 [physics.gen-ph]
From: Clément Vidal [view email]
[v1] Thu, 21 Apr 2011 22:29:17 GMT (579kb)
Sure this is fiction, but it is no worse a guess than I have seen for the real possible reasons for no evidence of ETI yet:
First we should check that magnetic anomaly in the crater Tycho:
Who will be looking at the LRO images for artifacts, and how can we keep this from turning into the fringe fest that happens with this kind of SETI?
Has anyone ever proposed to look for extant civilizations using spectroscopy or other means?
It seems to me a lot of thinking and research has gone into to looking for living civilizations at our level of technology (radio SETI), just beyond our level (optical SETI), well beyond our level, (infrared searches for Dyson Spheres, Dyson Shells, Galactic or Extra-Galactic level engineering,) etc.
But with the Fermi Paradox seemingly unanswered (an assumption I disagree with but that’s another post entirely) why do we not look for failed civilizations??
Beyond looking for “funeral pyre” Brentford Beacons I can think of a number of other things which could befall a civilization which might be detectable at interstellar distances with some future advance in astronomical data collection.
Looking for astro-archeology of extant civilizations could be anything from looking for signs of past (or current) nuclear conflagration, looking for the signal from a formerly habitable planet with an artificially induced runaway greenhouse effect, the signature of a self-replicating nano-machine meltdown (ie: the “grey goo” scenario), potential victims ( of an interstellar disassembler (ie: a terrestrial planet being dismantled by a species alien to that star system), the gamma ray signature of large quantities of antimatter coming into contact with matter on a planetary surface, the ionization over time of a terrestrial planet’s atmosphere due to some form of technology, the list could go on but you see what I am getting at.
Sagan when he talked about SETI with skeptics in Congress in the 80s during the Cold War often referred to the factor L in the Drake Equation and the very real possibility that SETI may not have heard anyone because L is low due to species wiping themselves out in atomic conflict.
So a good study of what it would take to detect such a civilization (presumably less intelligent than we have been -so far-) would be in the spirit of Carl Sagan. What would the signal be from a global nuclear conflict both as it happened and 100s or thousands of years later? How long would a “nuclear winter” exist and be detectable? What other potential long term (ie: billion year or more) effects would be detectable from a nuclear conflict?
All good questions in my mind.
Discovering such extant civilizations could very well serve to increase the L factor of our own civilization by providing a cautionary tail of ones who didn’t make it.
The investigation into how to detect such civilizations might also bear fruit in providing technologies or yielding techniques for discovering existing, living super civilizations who have been previously undetectable.