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Technosignatures and the Age of Civilizations

Given that we are just emerging as a spacefaring species, it seems reasonable to think that any civilizations we are able to detect will be considerably more advanced — in terms of technology, at least — than ourselves. But just how advanced can a civilization become before it does irreparable damage to itself and disappears? This question of longevity appears as a factor in the famous Drake Equation and continues to bedevil SETI speculation today.

In a paper in process at The Astronomical Journal, Amedeo Balbi (Università degli Studi di Roma “Tor Vergata”) and Milan Ćirković (Astronomical Observatory of Belgrade) explore the longevity question and create a technosignature classification scheme that takes it into account. Here we’re considering the kinds of civilization that might be detected and the most likely strategies for success in the technosignature hunt. The ambiguity in Drake’s factor L is embedded in its definition as the average length of a civilization’s communication phase.

Immediately we’re in shifting terrain, for in the early days of SETI, radio communication was the mode of choice, but even in the brief decades since Project Ozma, we’ve seen our own civilization drastically changing the radio signature it produces through new forms of connection. And as Balbi and Ćirković point out, the original L in Drake’s equation leaves open a rather significant matter: How do we treat the possibility of civilizations that have gone extinct?

These two authors have written before about what they call ‘temporal Copernicanism,’ which leads us to ask how the longevity of a civilization is affected by its location in our past or in our future. We are, after all, dealing with a galaxy undergoing relentless processes of astrophysical evolution. As we speculate, we have to question a value for L based on a civilization (our own) whose duration we cannot know. How can we know how far our own L extends into the future?

Image: Messier 107, a globular cluster around the disk of the Milky Way in the constellation Ophiuchus, is a reminder of the variety of stellar types and ages we find in our galaxy. What kind of technosignature might we be able to detect at a distance of about 20,000 light-years, and would ancient clusters like these in fact make reasonable targets for a search? Many factors go into our expectations as we formulate search strategies. This image was taken with the Wide Field Camera of Hubble’s Advanced Camera for Surveys. Credit: ESA/NASA.

Thinking about these matters always gets me thinking of Arthur C. Clarke’s 1956 novel The City and the Stars, set in the city of Diaspar a billion years from now. How do we wrap our heads around a civilization measured not just in millennia but in gigayears? Speculative as they are, I find a kind of magic in playing around with terms like Γ, cited here as the average rate of appearance of communicating civilizations (with L, as before, their average longevity), so that if we take Γ as constant in time, its value can be estimated as the total number of technosignatures over the history of the galaxy (Ntot) divided by the age of the galaxy (TG). Thus Balbi and Ćirković cite the equation:

From the paper:

It is apparent that the number of technosignatures that we can detect is a fraction of the total number that ever existed: the fraction is precisely L/TG. Because TG ∼ 1010 years, L/TG is presumed to be generally small; any specific precondition imposed on the origination of technosignatures, like the necessity of terrestrial planets for biological evolution, will act to reduce the fraction. This is the quantitative argument that justifies one of the most widely cited assertions of classical SETI, i.e. that the chances of finding ETIs depend on the average longevity of technological civilizations. (In fact, it is well-known that Frank Drake himself used to equate N to L.)

The equation clarifies the idea that SETI depends upon the average longevity of technological cultures, but the authors point out that another way to look at the matter is this: L needs to be large, for we’re requiring a high number of technosignatures indeed to have any chance for detecting a single one. Spread out over time, many such signatures need to have existed for us to make a single detection, or at best a few, with our present level of technology.

And here is where Balbi and Ćirković take us away from the more conventional approach derived above. Is the number of detectable technosignatures, N, static over time? From the paper:

…both Γ and L are average quantities, and there is an implicit assumption that N is stationary over the history of the Galaxy. There are good reasons to believe that this is not the case. Of course, it is unrealistic to assume that Γ is constant with cosmic time. Even if we limit ourselves to the last ≃ 10 Gyr of existence of thin disk Pop I stars which are likely to harbour the predominant fraction of all possible habitats for intelligent species, their rate of emergence is likely to be very nonuniform. One obvious source of nonuniformity is the changing rate of emergence of planetary habitats, as first established by Lineweaver (2001) and subsequently elaborated by Behroozi & Peeples (2015), as well as by Zackrisson et al. (2016). This nonuniformity can be precisely quantified today and some contemporary astrobiological numerical simulations have taken it into account (Ðošović et al. 2019).

We should assume, the authors argue, that the appearance of technosignatures varies with time. They are interested less in coming up with a figure for N — and again, this is defined in their terms (not Drake’s) as ‘the number of detectable technosignatures’ — than in spotlighting the most likely type of technosignature we can detect. Their classification scheme for technosignatures as filtered through the lens of longevity goes like this:

Type A: technosignatures that last for a duration comparable to the typical timescale of technological and cultural evolution on Earth, τ ∼ 103 years

Type B: technosignatures that last for for a duration comparable to the typical timescale of biological evolution of species on Earth, τ ∼ 106 years

Type C: technosignatures that last for for a duration comparable to the typical timescale of stellar and planetary evolution, τ ∼ 109 years

The scheme carries an interesting subtext: The longevity of technosignatures does not have to coincide with the longevity of the species that created the detectable technology. Here we’re at major variance from the L in Frank Drake’s equation, which had to do with the lifetime of a civilization that was capable of communicating. Balbi and Ćirković are tightly focused on the persistence not of civilizations but of artifacts. Notice that a technosignature search is likewise not limited to planetary systems — an interstellar probe could throw its own technosignature.

We might assume that technosignatures of long duration could only be produced by highly advanced civilizations capable of planetary engineering, say, but let’s not be too sure of ourselves on that score, for some technosignatures might be left behind by species well down on the Kardashev scale of civilizations. Consider Breakthrough Starshot, for example. Let’s push its ambitions back a bit and just say that perhaps within a century, we may be able to launch flocks of small sailcraft to nearby stars using some variation of its methods.

These would constitute a technosignature if detected by another civilization, as would remnant probes like Voyager and Pioneer, as would some forms of atmospheric pollution or simple space debris. A single civilization could readily produce different kinds of technosignatures over the course of its lifetime. As the authors note:

Our species has not yet produced Type A technosignatures, if we only consider the leakage of radio transmissions or the alteration of atmospheric composition by industrial activity; but its artifacts, such as the Voyager 1 and 2, Pioneer 10 and 11, and New Horizons probes, could in principle become type B or even C in the far future, even if our civilization should not survive that long. Similarly, a Type C technosignature can equally be produced by a very long-lived civilization, or by one that has gone extinct on a shorter time scale but has left behind persistent remnants, such as a beacon in a stable orbit or a Dyson-like megastructure.

Persistent remnants. I think of the battered, but more or less intact, Voyager 2 as it passes the red dwarf Ross 248 at about 111,000 AU some 40,000 years from now (Ross 248 will, in that era, be the closest star to the Sun). That’s a technosignature waiting to be found, one produced by a civilization low on the Kardashev scale, but it bears the same message, of a culture that explores space. I wonder what kind of a technosignature Clarke’s billion year old civilization in Diaspar would have thrown?

Whatever it might be, it would surely be more likely to be detected than our Voyager 2, a stray bit of flotsam among the stars. That said, I keep in mind what we learned from the TechnoClimes workshop — and Jim Benford’s continuing work on ‘artifact’ SETI — making the point that we can’t rule out a local artifact in our own system. And, of course, if Avi Loeb is correct, we may already have found one, though suitably ambiguous in its interpretation. Clearly, if we did detect technosignatures close to home, the implication would be that they are found widely in the galaxy, and that would dramatically change the nature of the hunt.

So the scope for technosignatures is wide, but drawing the lessons of this paper together, the authors find that the technosignature we are most likely to detect with present technological tools is a long-lived one, meaning in Balbi and Ćirković’s terms, one with a duration of at least 106 years. Technosignatures younger than this may be detectable but only if it turns out they are common, as thus relatively nearby and easier for us to find. Of course we can search for them, but the authors believe these searches are unlikely to pay off. Their thought:

This suggests that an anthropocentric approach to SETI is flawed: it is rational to expect that the kind of technosignatures we are most likely to get in contact with is wildly different, in terms of duration, from what has been produced over the course of human history. This conclusion strengthens the case for the hitherto downplayed hypothesis (which is not easily labeled as “optimistic” or “pessimistic”) that a significant fraction of detectable technosignatures in the Galaxy are products of extraterrestrial civilizations which are now extinct.

How to proceed? The authors’ focus on longevity leads them to conclude that our most likely targets may well be rare and they may flag extinct civilizations, but the value N that Balbi and Ćirković are talking about is different than classical SETI’s N, which needs a large value to ensure detection. It only takes one technosignature, and a few of the Type C signatures would be much more likely to be detected than a spectacularly high number of Type A signatures:

Dysonesque megastructures, interstellar probes, persistent beacons—as well as activities related to civilizations above Type 2 of the Kardashev scale, or to artificial intelligence—should be the preferred target for future searches. These technosignatures would not only be ‘weird’ when measured against our own bias, but could arguably be less common than short-lived ones. Such [a] conclusion deflates the emphasis on large N (and human-like technosignatures) that informed much of classical SETI’s literature.

If this sounds discouraging, it need not be. It simply tells us the kind of strategy that has the greatest chance for success:

…the supposed rarity of long-lived technosignatures should not be regarded, in itself, as a hindrance for the SETI enterprise: in fact, a few Type C technosignatures, over the course of the entire history of the Galaxy, would have much higher chance of being detected than a large number of Type A. Also, possible astrophysical mechanisms which could lead to a posteriori synchronization of shorter lived technosignatures should be investigated, to constrain the parameter space of this possibility, if nothing else.

Civilizations that appeared long ago and survived have conceivably found a way to persist, and therefore may still be active, but for detection purposes their existence now is less significant than what they may have left behind. Just how they grew to the point where they could begin the construction of detectable technosignatures is explored in the paper’s discussion of ‘phase-transition’ scenarios via a mathematical framework used to model longevity. “Achieving such form[s] of institutions and social structures might count as an advanced engineering feat in its own right,” as the authors note.

Technosignature work is young and constitutes a significant extension of the older SETI paradigm. Thus modeling how to proceed, as we saw both here and in the previous post on NASA’s TechnoClimes workshop, is the only path toward developing a search strategy that is both sound in its own right and also may have something to teach us about how our own civilization views its survival. The kinds of insight technosignature modeling could produce would take us well beyond the foolish notion of some early SETI critics that its only didactic function is as a form of religion, looking for salvation in the form of the gift of interstellar knowledge. To the contrary, the search may tell us much more about ourselves.

The paper is Balbi and Ćirković, “Longevity is the key factor in the search for technosignatures,” in process at The Astronomical Journal (preprint).


Comments on this entry are closed.

  • Alex Tolley March 17, 2021, 14:32

    I would really like to read this paper when it becomes available. I really like the classification of types, but I would like to read the math on detection probabilities of few C vs many A.

    Long-lived technosignatures, when translated into artifacts, are often the plot point of Jack McDevitt’s Alex Benedict novels. A universe that appears devoid of ETI, but occasionally an artifact comes to Benedict’s attention and the hunt is on. Most artifacts and mysteries are of human origin, but just occasionally…

    I think that a civilization able to construct long-lived artifacts in space, e.g. a Dyson Swarm or Sphere, could also well be starfaring. If so, that civilization will rapidly increase those artifacts/signatures as they travel from star to star. While the suggestion that Starshot Breakthrough could become a technosignature, it seems to me that a motivation to explore other star systems might lead to more substantive artifacts. Von Neumann machines are the classic example, but we are not far from being able to build mining robots and 3D printers able to build a range of devices, many of which might produce detectable technosignatures in another star system.

    The more interesting problem is what technosignatures are detectable. Our expansionist human civilization still has scale and size as a deep-seated meme – from building huge structures (pyramids, cathedrals, walls, skyscrapers), to amassing territory (imperial cultures have abounded – a genetic drive.) We extrapolate what we could do – build geosync satellite swarms (or graveyards), or Dyson swarms. We are rather like neo-Victorians in this regard. Will we follow this scaling-up path, or will we try another approach?

    Yet we know that economic growth requiring concomitant energy growth is not sustainable, at any level. The growth rate is inversely proportional to longevity, and must surely be logistic in the long run. Yet if rapid growth is required to produce such technological artifacts, then indeed it is the long-lasting artifacts that will be detectable after we are long gone.

    Heath Rezabek had written some CD posts on his “Vessels” to house all knowledge and needed artifacts to reboot civilization after a collapse, putting some in space. Others have suggested similar ideas. It may be that such containers may be more likely to be discovered by an alien intelligence than a post-collapse civilization. If so, then we should construct such vessels with the equivalent of the Pioneer and Voyager plaques but far more extensive to help such intelligence understand the contents. Rosetta stones in spades.

    This post started with Clarke’s “The City and the Stars”, but I wonder if a more appropriate story would be his short, “The Star”.

    • Alex Tolley March 17, 2021, 21:16

      Scanning the paper I think the authors assume all technosignatures are equally detectable irrespective of distance. Their primary constraints are time and teh light cones. If I understand this correctly, finding an alien artifact on the beach is no more or less detectable than a globular cluster where all the stars are surrounded by Dyson swarms, or that a distant star has a planet with artificial lighting. The authors just assume detectability, but not the ease of detectability. The range of detectability is presumably very large, but I am not confident of their analysis and the range of artifact or communication method possibilities.

      Pre-moderns could certainly find an alien artifact on Earth, even one as recent as a Monolith no more than a few million years old, but be unable to detect a Dyson swarm around Proxima. We, OTOH, could probably detect that Dyson swarm whilst failing to discover a buried monolith. (Recall that we didn’t detect the KT asteroid until the late 20th century and that only recently have asteroid particles been retrieved at the Chicxulub crater. We are still discovering buried ancient cities, sometimes on land that has been occupied for many centuries.) IOW, ease of detection with the requisite detection equipment (from eyeballs to telescopes to microscopes) is relevant to the probabilities.

  • ljk March 17, 2021, 14:48

    The paper is online here:


    Longevity is the key factor in the search for technosignatures

    Amedeo Balbi, Milan M. Ćirković

    It is well-known that the chances of success of SETI depend on the longevity of technological civilizations or, more broadly, on the duration of the signs of their existence, or technosignatures.

    Here, we re-examine this general tenet in more detail, and we show that its broader implications were not given the proper significance. In particular, an often overlooked aspect is that the duration of a technosignature is in principle almost entirely separable from the age of the civilization that produces it.

    We propose a classification scheme of technosignatures based on their duration and, using Monte Carlo simulations, we show that, given an initial generic distribution of Galactic technosignatures, only the ones with the longest duration are likely to be detected.

    This tells us, among other things, that looking for a large number of short-lived technosignatures is a weaker observational strategy than focusing the search on a few long-lived ones.

    It also suggests to abandon any anthropocentric bias in approaching the question of extraterrestrial intelligence. We finally give some ideas of possible pathways that can lead to the establishment of long-lived technosignatures.

    Comments: Accepted for publication in The Astronomical Journal

    Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP); Popular Physics (physics.pop-ph)

    Cite as: arXiv:2103.02923 [astro-ph.IM]

    (or arXiv:2103.02923v1 [astro-ph.IM] for this version)

    Submission history

    From: Amedeo Balbi [view email]

    [v1] Thu, 4 Mar 2021 10:08:25 UTC (405 KB)


  • Robin Datta March 17, 2021, 15:17

    If indeed interstellar debris reaches terra firma in the form of meteorites, consider the case of a meteorite with an embedded equivalent of an alien USB-bluetooth “thumb drive” found by a hiker or homesteader in a remote location and investigated by that person with the pliers, hammer, screwdriver, etc. prior to discarding it.

    Or investigated by an advanced lab with methods equivalent to stone tools in comparison to the device. Or data successfully extracted but the whole affair kept secret because the data puts TPTB eons ahead of their contemporaries…

    Would any of these be plausible scenarios? And what can be speculated about the sample returns from Mars and the asteroid? And how do we (and should we) shoehorn such stuff into the Drake equation?

    • Alex Tolley March 17, 2021, 20:44

      Sounds suspiciously like you are describing the plot of the current tv series “Debris“.

      • Robin Datta March 18, 2021, 12:40

        I don’t have a TV and I quit watching TV shortly after I returned from Desert Storm.

        • Alex Tolley March 19, 2021, 11:23

          I don’t watch “Keeping Up With the Kardashians” but the news media have made me aware of Kim Kardashian and Kanye West are divorcing. ;)

          While “Debris” is pitched as an X-Files styled show, it has clearly jumped on the ‘Oumuamua sighting of 2017 and the speculation it is an alien ship with the added twist that it was disintegrating and parts are falling to Earth. Those parts have strange effects and are a clear, if unacknowledged, reference to the novel/movie Roadside Picnic/Stalker.

          Your comment could easily have been the elevator pitch for the show.

          • Robin Datta March 19, 2021, 23:55

            I get my news from individuals’ and small groups’ blogs, except local news, from Google “Your Local News”, 99+% of the time on an iPad Mini.

  • ljk March 17, 2021, 15:24
    • Alex Tolley March 18, 2021, 11:26

      We continue to try to communicate as best we can. With luck, we may even understand them. If nothing else, we should learn to empathize with all animals and treat them better.

  • Geoffrey Hillend March 17, 2021, 16:32

    Quote from Technosignatures and the Age of Civilizations: “This is the quantitative argument that justifies one of the most widely cited assertions of classical SETI, i.e. that the chances of finding ETIs depend on the average longevity of technological civilizations.” Quantitative arguments are subordinate to the bias and limitations of the idea or qualitative theory one is trying to prove. I don’t agree with the argument above. It does not include the psychological, intuitive and intellectual advancement of the viewer, so that one’s world view can change if any of the three above become more advanced, i.e., the world view of the observer which can be limited to a definite technological period.

    For example: If we assume that no technological ET civilization has ever died then we can’t lump them all into a world view limited to the technology of today. Once a civilization reaches a million years of technological advancement, it may have FTL technology, etc. so they can see us and know where we are, but today we can’t see them. We have not done this yet today but hope to do it in the near future. This brings up the question if there are ET’s with FTL in our solar system observing us right now, then why don’t they contact us? More than one reason can be found to support that hypothesis. This is important because the advancement of the technological civilization can determine ones belief about how common life is in our galaxy. If the nearest civilization near our level of advancement is ten thousand light years away from us we will have a long wait before we can hear from them. Even if it is only one thousand or five hundred light years away there still we be a long wait. Consequently, we can conclude the universe is empty and we are the only ones in the galaxy since the FTL ET’s are invisible to us and don’t communicate to us. This of course is only a hypothesis, but it at least we can’t get carried away with a false sense of superiority with this theory that we think we are so special and superior to all other life. Are false superiority has been used in more than one science fiction film by the ET’s with the superior technology.

  • Cosmic Inquirer March 17, 2021, 16:55

    Also worth noting this article which highlights a similar point https://arxiv.org/abs/2010.12358

    • ljk March 18, 2021, 11:28

      There is also the possibility of encountering a younger society that is similar to us in the sense of they are simultaneously eager and naïve about the wider Cosmos and want to make new friends in their galactic neighborhood.

      The Young and the Restless ETI as I call them might be a more legit possibility compared to a much older and presumably more mature civilization. The latter would know what their cosmic neighborhood is like and as a result not only be more cautious but less inclined to go leaping into some Star Trek style adventure because they are already long familiar with who and what are out there.

      These mature species could settle for monitoring stations, either placed about the galaxy or using powerful instruments situated in their solar systems to keep tabs on things. They might only respond if there is some urgent event requiring their direct attention.

      To use a real analogy from human history: In the European Renaissance period, eager and hungry nations emerging from the Middle Ages sent sailing ships over most of Earth in search of new riches, land, and international clout. Meanwhile, China, which had done some exploring on its own, decided that no other human society matched it and decided to sit back. In a few centuries, Europe became a global force to reckon with, while China would not take a similar route until the last century or so.

  • James Benford March 17, 2021, 18:50

    “If we did detect technosignatures close to home, the implication would be that they are found widely in the galaxy…” But ‘live’ planets may be very rare, so may not be ‘found widely in the galaxy’. If we find interstellar probes near Earth (on the Moon, Earth Trojans, Earth co-orbitals,), it will likely be because the life of Earth has been detected because of our out-of-equilibrium atmosphere. The high oxygen level here has been evident for billions of years. Hence we could have been attracting probes long before human civilization arrived. But it’s another question how widespread life is.

  • Law March 17, 2021, 22:22

    We want to learn something new from detecting alien civilizations.
    Finding microbes or a human-like civilization with fossil fuels and emissions is not very useful information.

    The most obvious civilizations (the biggest and most understandably technological ones) yield the most valuable data, and may well be easiest to detect at intergalactic distances. Hell yeah searches should be focused on them.

    We need to know whether we share the cosmos with obvious Kardashev III and II civilizations, or at least, rule out the obvious ones. This information will immediately inform our prospects for long-term survival and the prospects for any human policy of galactic conquest/expansion, and thusly feed information into how much governments should spend on space and technology development.

    It may be hard to admit, but the overwhelming majority of people are risk-averse. Nobody was willing to invest in reusable launch until SpaceX demonstrated it, at which point everyone and their dog in the US and China poured billions into reusable launch.

    “It hasn’t been done before, so it cannot be done” is an extremely convincing argument… but so is “the Americans did it, so we can do it too! It’s not like Americans are smarter than us or anything.”. Likewise, if aliens built a K3 civilization… well, so can we. The detection of a Dyson sphere, a K3ed galaxy, any large-scale engineering really, will utterly revolutionize how governments invest in technology and space. It is of paramount importance that we rule out obvious K2s and 3s.

    But since we haven’t detected anything yet, I suspect that we may well be missing something about how civilizations develop, the limited efficacy of space settlement, or some unusual technological development pathways…

    • Alex Tolley March 18, 2021, 11:23

      We want to learn something new from detecting alien civilizations. Finding microbes or a human-like civilization with fossil fuels and emissions is not very useful information.

      Apart from dismissing biologists and just about every academic field of the humanities, you might just want to consider whether civilizations are no more advanced than we have something useful to say. For sheer longevity, cultures with little technology have far exceeded us so far. Despite our technological prowess, we seem to have learned little about making our societies work for everyone. While low-tech societies might have lived lives “brutish and short”, we (i.e. in the West) just seem to live lives “brutish and long”. A stable, long-lived culture with technology not that different from ours might teach us a lot about how to best operate. We may not like their solutions, but it is very useful nonetheless.

      Suppose we find K2 or K3 civilizations and it turns out they are all no longer biological, or they are the Borg. Then what?

      Nobody was willing to invest in reusable launch until SpaceX demonstrated it

      Huh? Were you asleep when we had the Space Shuttle? Don’t get distracted with shiny things. What SpaceX have done is show that low cost access to space is possible. Their rockets have limited reusability. We don’t scrap airliners after a dozen flights.

      But since we haven’t detected anything yet, I suspect that we may well be missing something about how civilizations develop, the limited efficacy of space settlement, or some unusual technological development pathways…

      Don’t exclude the possibility that we are first, or even alone.

      • Law March 19, 2021, 6:15

        While the scientific and anthropological return from detection of microbes on Proxima would be impressive, such a detection would not immediately spur a multitrillion dollar technological race to discover new technological principles by observing the K3 civilization by all means available. It would not ring alarm bells in Zhongnanhai and the Pentagon. It would not cause the NRO to spend fifty billion dollars on space telescopes immediately to outcompete the Chinese.

        The amount of money governments and military-industrial complexes would shift around should indicate how “significant” a certain discovery might be.

        Sure, news a K3 civilization might be received with a shrug, but news of a primitive hunter-gatherer civilization 100ly distant would receive an even more shruggish shrug.

        • Alex Tolley March 19, 2021, 11:10

          The amount of money governments and military-industrial complexes would shift around should indicate how “significant” a certain discovery might be.

          An interesting observation, but the effect of such a shift might be quite adverse. Civilian scientists might get extra funds for conservation and study, but it is more likely that this would be an excuse for the military to demand more funding, with a concomitant reduction in spending on now downgraded priorities. Politically there might be demands that the Earth “goes silent”, of that democracy must be shrugged off in favor of authoritarian forms of government. A K3 civilization might also inspire new cults and religions, and contemporary religious institutions might not like that.

          Now consider the difference in response to a nation that manipulates another through disinformation and propaganda, but denies culpability, and a nation that openly threatens another with weapons. The former might get some saber-rattling, the latter a potential war that is economically exhausting to the threatened nation (and likely destruction to itself). Suppose advanced ETI is already manipulating us to destroy ourselves, can you imagine how much quicker that destruction might be if they suddenly revealed themselves?

          Be careful what you might wish for.

          • Law March 19, 2021, 20:57

            Indeed. Nobody ever said first contact would be painless. Nonetheless, given the potential technological insights (and “investment advice” discovery of a K2 or K3 civilization would provide, there would seem little reason not to rule out the most obvious ones. If they exist in easily detectable form, they will be found sooner or later; not really much of a point postponing their discovery by actively restricting passive SETI.

  • Andrew Palfreyman March 18, 2021, 1:49

    When we begin to acknowledge this possibility that the longevity of an artifact could vastly exceed that of its parent civilisation, it seems to me that our extant METI protocols would require revision. For now we have to fold in the probability that the successful reception of an intelligent message from ourselves may trigger a possibly long-dormant artifact to spring to life and to reply. The resultant effect on our METI protocols would then tend to encourage a far more active program.

    • Alex Tolley March 18, 2021, 11:32

      The reply from any METI will belong in the future. The best possible situation for a reply is a dormant artifact in our solar system that wakes up and replies to us. If however it wakes up and sends a message to its makers, then perhaps not so good.

  • Michael Fidler March 18, 2021, 2:07

    I see some major problems that have not been assessed because we look at alien civilization’s in our own context. The idea of mortality of the individual is something we may overcome in the not too distant future. The alien civilization’s may all be immortal and could possibly be millions to a billion years old as individuals. This seems logical since increasing lifespan has been a goal of humans before leaving the caves. The ability of surviving major catastrophes and common sickness plus better diet has lengthened life from in the 20’s to late 70’s now. The ability to protect individuals from supernovas and impactors would and genetics should give rise to immortals. AI may protect them also by seeing what the odds of their actions may put them in danger. This is what we should be looking at when researching what alien civilization’s will be like because a billion year old individual would be very wise and may look like a human in their prime of 25 years if humanoid. We have not looked at it from their viewpoint and may be why we are still alone. The Fermi paradox is on their side, for they have all the time in the universe!

  • Simon March 18, 2021, 10:19

    People might be interested in Robin Hanson’s recent writings on “Grabby Aliens”:

  • Mike Serfas March 18, 2021, 13:12

    Ancient atmospheric signatures could be hard to find, but evidence in the rock may never fade. I still remember that first image from Dawn, where the facula in Occator Crater made its debut. If another species had ever been in the Solar system, they must have mined; if they mined, they would mine the asteroid belt; if they mined the asteroid belt, they would mine Ceres, leaving behind ancient workings, factories, spacecraft … and *there it was*. After half a day it looked more like a volcano of Epsom salt or baking soda … a pity, I think.

    But every astronomical body has its own chance of surprises. The outermost dwarf planets, objects like Sedna, captured moons, rogue planets if we can spot one – anything that might have come from outside the Solar system, considering how much the stars of our galaxy have been stirred, could have come from far across the galaxy. Any one of these might contain treasures beyond the dreams of Croesus and Mansa Musa, or the nightmares of Lovecraft. All we need to do is knock on the right door.

  • Gary Wilson March 18, 2021, 15:11

    If intelligent organic life is rare then it could be argued that AI will be even rarer. This may in part account for the apparent lack of detected technosignatures to date and the apparent (over a very short time scale) lack of ET’s visiting Earth (in part because we don’t have a very long recorded history and we’re rightfully distrustful of anecdotal evidence). If the requirement for extended time periods for biological sentience to arise due to the slow rate of evolution from microbes to primitive plants to animals finally including modern humans is accepted as a general rule then billions of years may be required for other sentient species to arise. Then there are the negative feedback loops such as environmental degradation, overpopulation, large scale warfare and so on. Taken together we probably have a sparsely populated galaxy with intelligent species arising, competing and disappearing over tens to hundreds of thousands of years. Long lived technosignatures will be very rare. I think McDevitt may have it reasonably correct. It may take us hundreds to thousands of years or more to find the first such artifacts. I’m not discouraged by that :).

  • Adam March 18, 2021, 16:26

    Civilizations could send their cultural archives to other star systems in probes, potentially lasting for gigayears if engraved in a durable medium like zircons. No latency and no diffraction losses.

  • ljk March 18, 2021, 17:28

    Thanks for all the fish

    The search for dolphin intelligence and the quest for alien life have moved in historical lockstep. What does the future hold?

    Despite putting questions to nature, as the philosopher Francis Bacon once suggested, science has revealed to us a strangely silent world. Where once divinity’s lessons were read out from the structure of the cosmos itself, now outer space answers our outward-bound signals with only stony stillness. There is no response to our call. Where once the bestiary and fable allowed each and every animal to sing to us its instructive lesson, we now recognise that we have never yet held a philosophical dialogue with another species. The animals don’t speak to us. Where once we felt convinced that we were in harmonious conversation with nature writ large, where there might be miscommunications but the back-and-forth would go on interminably, now there is one particular silence we fear the most: that of our own potential extinction.

    These three modern silences – of outer space, of other beings, and of our own end – can weigh upon us heavily. But there are important and hopeful lessons here, to be found in the story of our attempts, failed or not, to communicate with this Universe we find ourselves within.

    Full article here:


  • Steve Muise March 18, 2021, 19:06

    “If we did detect technosignatures close to home, the implication would be that they are found widely in the galaxy…”

    Not necessarily. It may just mean that we’ve found a technical civilization fairly close to us. There’s no guarantee that other technical civilizations are evenly distributed across the Mily Way.

  • Ryan Carthy March 19, 2021, 8:10

    I wonder how the entire longevity question changes in a formula if you incorporate the theory that gamma ray bursts which were more numerous in the past prevented life from coming out from oceans across the galaxy and becoming terrestrial:

  • ljk March 19, 2021, 12:47

    The Enigmatic Objects of Kepler-51

    Anomalous Planets, or Alien Megastructures?

    Brandon Weigel

    Jan 29, 2021


    • Michael Fidler March 23, 2021, 10:15

      Interesting, Wikipedia wrote that the star is 500 million years old but in the observational data says 2.14 billion years with no reference. The article; The Featureless Transmission Spectra of Two Super-Puff Planets, https://arxiv.org/abs/1910.12988 has been used in another article; Kepler-51 is Home to Three Super-Puff Exoplanets, http://www.sci-news.com/astronomy/kepler-51-super-puff-exoplanets-07936.html to say they are losing mass fast.
      That is not the case:
      “We modeled the evolution of the masses and radii of Kepler 51b and 51d from 10 Myr to 5 Gyr using a photoevaporation and core contraction model (Lopez & Fortney 2014). We found that these planets, while currently possessing unusual densities, will both contract and lose some atmospheric mass over time. In the end, Kepler 51b will evolve to a fairly typical sub-Neptune while Kepler 51d will remain with a slightly lower than normal density.”

      So we have three very strange planets that have no spectrum features that may be artificial and a sun like star that could also be artificially processed if that benefits the culture there. The paper uses nothing but other examples and models that try to fit these ultra low density planets at low temperatures into the normal planetary ranges. This is one system that needs to be observed as soon as JWST becomes operational for we may be in for a very big surprise…

      See image below:

      Planetary density data from NASA’s exoplanet archive showing the anomaly of the Kepler-51 system (green), compared with Hot Jupiters (red) and planets in systems less than 100 million years old (blue). Only HAT-P-67 b has a comparable density to the planets of Kepler-51, which yields an equilibrium temperature of over 1500 K.

  • Mike Jude March 20, 2021, 15:53

    Interesting analysis on the persistence of civilizations. However, I think that the authors have opened a much larger barn door than they realize. If we are talking about detectable extraterrestrial civilizations, I think the N value can only increase over time, if for no other reason that the speed of light ensures that we would be in a position to detect civilizations at various stages of their evolution. Even a civilization that goes extinct, if it is far enough away, would still appear as a detectable signal. Also, N, it seems to me, is single planet or system biased. What if a civilization is star faring and plants daughter civilizations over a number of nearby stars. The mother civilization could go extinct, but it could foster civilizations that would live on and foster other civilizations over time. If the rate of creation is even on the order of one every thousand years or so, the number of detectable civilizations will increase over time. But this analysis makes you think!

  • ljk April 2, 2021, 16:08
  • ljk May 10, 2021, 10:39

    We Could Detect Extraterrestrial Satellite Megaconstellations Within a few Hundred Light-Years


    Starlink is one of the most ambitious space missions we’ve ever undertaken. The current plan is to put 12,000 communication satellites in low-Earth orbit, with the possibility of another 30,000 later. Just getting them into orbit is a huge engineering challenge, and with so many chunks of metal in orbit, some folks worry it could lead to a cascade of collisions that makes it impossible for satellites to survive.

    But suppose we solve these problems and Starlink is successful. What’s the next step? What if we take it further, creating a mega-constellation of satellites and space stations? What if an alien civilization has already created such a mega-constellation around their world? Could we see it from Earth?

    This is the idea behind a recent article posted on the arXiv. It’s based on an idea about how civilizations might grow over time, known as the Kardashev scale. It’s based on the level of energy a civilization can tap into, where Type I uses energy on a global scale, type II a star’s worth of energy, and so on. By some estimates, Earth is roughly at level 0.7. If a Type I built a Starlink-like constellation of satellites, it would push the limits of orbital technology.

    Full article here:


    To quote:

    There have been searches for Kardashev scale civilizations before with no success. But most of these studies have focused on highly advanced Type II or Type III civilizations which would leave evidence on a galactic scale. What makes this idea interesting is that it focuses on Type I, which is much closer to where we are now. With luck, we could become a Type I civilization in a millennium. We would have a much better chance of recognizing the evidence of a civilization like ours rather than a hyper-advanced civilization that spans a galaxy.

    Reference: Osmanov, Zaza. “From the SpaceX Starlink megaconstellation to the search for Type-I civilizations.” arXiv preprint arXiv:2103.07227 (2021).