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Gregory Benford: Further Thoughts on ‘Lurkers’

Because I’ve been re-reading Gregory Benford’s Galactic Center sequence (now into Furious Gulf), I want to quickly mention the galactic center simulation available here, which offers a 360-degree, ultra-high-definition view based on Chandra X-ray observations as massaged by NASA supercomputers. It’s lively stuff, showing “the effects of dozens of massive stellar giants with fierce winds blowing off their surfaces in the region a few light years away from the supermassive black hole known as Sagittarius A* (Sgr A* for short).” Just remember Greg got there first. But back to the probe question we’ve been examining. Jim Benford’s take on a SETI search for ‘lurkers,’ probes that fit into the Bracewell category, examines targets known as Earth co-orbitals, as we saw on Friday. UCI physicist Greg Benford’s comments about his brother’s article examine the question of what the presence of such a probe in our system might imply. The possible scenarios take us into the realm of what Greg has called ‘deep time.’

by Gregory Benford

Consider time scales. Some tech society within a few hundred light years may have sent a Lurker which remains operating now. But our routine radio signals are only a bit more than a century old. If a Lurker reports back to its origin, it may well have not gotten an answer yet about how to proceed.

This means we should consider searches over decades-long time scales. In any case, we can carry out interrogations by radar and consider missions to orbit nearby targets and survey for Lurker sites, which may have gone dead in the distant past, or be intermittently active now.

We humans measure our eras in millennia at most. Within a century or so we may plausibly have made computer minds that could manage a Lurker that voyages for centuries and watches a biologically active world for more millennia still. If societies can persist for very long times, they will have alien artificial intelligences.

If ETI dispatches Lurkers here, these may report back any attempt at contact but do no more. Or Lurkers may be free to respond, if they were instructed so by their home society (or societies) earlier. Or further still, Lurkers from the far past may have done their duty and slowly failed, finally, into their own demise.

On scales of millennia, the ruins of Lurker installations, including mining for resources on nearby orbiting sites, may be visible, even though their animating intelligences are long gone. Surveying, exploring satellites could see these ruins and learn much from them. Thus a lack of response to our pings and messages may simply imply a needed further study through exploring probes.

This implies a staging of research. At each level, we disprove a hypothesis. Moving on in this way will disallow models of alien societies ranging through ever-larger distances, and ever longer past eras. This program can go forward for decades, as our capabilities grow. Our understanding of the near-orbital objects will grow as well, giving us a gamut of possibilities we can imagine and then test.

These minds must be made for social species; otherwise it seems unlikely that loner species like our carnivores will care much for speaking to distant others, a point made by E.O. Wilson. As Wittgenstein said, “If a lion could speak, we could not understand him.”

Translation would be unlikely if an ET mind were a single integrated intelligence, without social species drives; yet a Lurker may well be that. As well, a Lurker may be thrifty in energy use. If it came here using a nuclear drive or energy source, it would have to mine for radioactives to keep itself running—or erect large solar panels. (This is another reason to lurk near Earth: higher solar flux.) It may well be an occasional watchman, not continuous. Or it may have a small alert system to summon the larger intelligence when certain events or symptoms occur—such as a hail from Earth that repeats and contains self-explaining images that convey interest, such as a picture of the Lurker itself seen from afar. All these possibilities speak for a repeating observation and hailing pattern. This should be a long game, over years at least.


Comments on this entry are closed.

  • Harry R Ray March 26, 2019, 9:50

    If an ETAI civilization would have the motive to go to all that trouble to deposit Lurkers in our solar system, I see no reason why they they would not also send Refreshers VERY FREQUENTLY to extract information from them, monitor their health, and then transmit Lurker data back to “Home Base”(whatever that may be)at an appropriate time. These “Refreshers” would most likely NOT be traveling at velocities much faster than natural objects, because, unlike the Lurkers, once they finish transmitting Lurker data, their mission automatically terminates! Then, within a generation or so, Another Refresher enters the inner solar system, and the process repeats itself INDEFINITELY! CASE IN POINT: If `Oumuamua were such a Refresher, ALL of its radio transmissions(to the Lurker AND to “Home Base”) would have been done PRIOR to perihelion passage and its mission terminated IMMEDIATELY THEREAFTER, a signifigant time period BEFORE we even DISCOVERED it! A final set of commands may have set it tumbling. That may be the reason why no radio transmissions were never received from it. If Avi Loeb is right, there may be a quadrillion of these Refreshers within the outer Oort Cloud heading inward at speeds not much faster than Oort Cloud comet speed. They may be quadrillions MORE hanging out at LSR points throughout the galaxy just waiting for our solar system to “crash” into them!

    • ljk March 26, 2019, 11:34

      Why would an ETI have to send many Refresher probes, or even one for that matter? Just get the original probe to build itself an advanced copy from materials in the planetoid belt or a small moon.

      The ETI can transmit to them new software or engineering plans if the situation calls for it, just as NASA was able to reprogram Voyager 2 to handle Uranus and Neptune in the 1980s.

      Makes more sense economics-wise than having to keep launch a fleet of new probes so often.

      • ljk March 27, 2019, 9:11

        If you are smart enough to send a probe to dwell in another solar system and not just a fast fly through, you should also be smart enough to design it so that the probe can repair and make copies of itself at the target system:


        • Paul Gilster March 27, 2019, 9:28

          Freitas has been at this for a long time, though let’s also mention that his views have significantly changed (moving toward nanotech) since his early work on REPRO.

          • J. Jason Wentworth March 27, 2019, 10:19

            Yes, REPRO (a self-replicating version of the Daedalus starprobe) was, even when Freitas presented it, a proof-of-concept design, not seriously put forward as something to shoot for. As Arthur C. Clarke might have said, “The fact that we can envision REPRO today means that future generations, who will have technological capabilities far beyond ours, will be able to do very much better.”

            • ljk March 27, 2019, 13:01

              I used the REPRO page as a detailed example. That it has been succeeded by more advanced plans does not take away the basic concept being promoted – that a smart probe would be capable of using local resources to fix, upgrade, and make copies of itself to continue its observations.

              • Paul Gilster March 27, 2019, 13:18

                Indeed. I think the biggest issue over astronomical timeframes would be energy.

        • Alex Tolley March 27, 2019, 17:01

          For really deep time, the one mechanism that we know can self-reproduce is life. It is therefore not impossible that life is seeded, that in a few rare instances a species becomes intelligent and can understand the universe, and that species then seeds life to other worlds.

          So rather than one civilization needing to last millions of years to learn the details of the universe, different species evolve from a common source who take up the baton of science and pass it on.

          If such was the mechanism, then we might possibly expect galactic libraries to be unearthed to help the newcomers. Such libraries may be the “lurkers”. IOW, these probes are not there to do anything but simply to help bootstrap the species’ understanding of the universe and its history. If the expectation is that billions of years are needed before such a species arises from its seeding, then there may be no point in trying to leave a library as it cannot survive the lengths of time to discover it in a functioning manner and its knowledge will be billions of years out of date.

          • J. Jason Wentworth March 28, 2019, 17:44

            Alex, that is the main idea behind “Star Trek” (and it also conveniently explains, within the storyline [studio budgets are the real-world reason :-) ], why nearly all intelligent aliens in that world are humanoids). An ancient humanoid race, called the Preservers, seeded promising primordial planets with a DNA code that would direct the evolution of life on those worlds toward a form similar to their own, and:

            If we ever met intelligent extraterrestrials and they turned out to be basically similar to us (genetically as well as in physical form), the possibility that such seeding might have occurred in real life would be a viable hypothesis. (The hypothesis that the humanoid form is simply the best one for such creatures would be another.) Also:

            Even here on Earth, the basic humanoid form is found–with modifications for specific functions–among primates, ungulates (deer, moose, elk, goats, cattle, sheep, rhinoceroses, horses, etc.), canines, felines, bears, and so on. If we found multiple ETI races that were all humanoids, the “the humanoid form is simply the best one for intelligent beings” hypothesis would be strongly supported. If they also had DNA that was largely similar to ours (double- rather than multiple-helix, two sexes, mostly the same proteins and protein sequences, etc.), then the possibility of purposeful seeding in the distant past would be a strong possibility (especially if the basically similar DNA arose despite significant differences in the ETI’s planets’ primordial surface and atmosphere conditions [with or without strong global magnetic fields, for example] and compositions, significant differences in their suns’ ultraviolet and X-ray outputs, etc).

          • Alex T. April 3, 2019, 10:24

            …the one mechanism that we know can self-reproduce is life…
            Life is not the good system to collect and transfer information, life mostly collecting analog noise because it is suffering from entropy, bad choice for an super-intelligent entity that can dare to plan for billions year period. If you will add to this equation an evolution, so you even cannot predict final result.
            I know that evolution theory was not a best friend of SETI founders, but in any case it should be accounted somehow.

        • Adam March 30, 2019, 21:06

          Hi Larry
          Freitas’ REPRO paper, until “Project Icarus” came along, was one of the few online sources with the detailed mass budget for “Daedalus”. One key assumption – that the REPRO needs to cruise at 0.12c and decelerate under fusion power from that speed – means the total mass to be mined is ~10,700,000 tonnes. Almost 1,000-fold the REPRO’s double-mass “Daedalus” First & Second Stage Core. The original gas mining architecture would’ve deployed 128 balloon mines over 20 years in the atmosphere of Jupiter to mine 50,000 tonnes of fuel. A REPRO would need a ~20-fold larger industrial base to collect the needed 10,000,000 tonnes in 1,000 years. However the needed industrial effort would be much less if the cruise speed was just ~0.03-0.04 c.
          Of course Freitas has since moved to 1 kg REPRO starprobes in his nanotech writings. Probes that small could number in their trillions in our Solar System and we’d never notice them.

      • Harry R Ray March 27, 2019, 9:45

        I look at it as a question of distance. If, either, ONE: ETI “home base” is in the outer reaches of our Oort Cloud(or even a nearby star system), or, TWO: ETI has developed an Ender’s Game “ansible”-like FTL communication system, you are absolutely right. However, if ETI “home base” is thousands or tens of thousands of light-years away, and FTL communication is ABSOLUTELY physically impossible, in my opinion, Refreshers are an absolute necessity if the Lurkers are exceptionally small. I’m not ETI, but my Lurker strategy would be to send one micron-sized Von Neumann machine Lurker either to the Moon or a NEO(that is why I am REALLY interested in 2019 BE5)that can MOVE through the lunar or asteroid dust and make millions of copies and distribute them all over the lunar or asteroid surface as to present a larger “target” for the Refresher transmitter. Of course, there may be BOTH nearby and far away ETI “home bases, so both your scenario and my scenario may be in effect. It would be interesting to find out Gregory Benford’s take on this.

        • Martin March 29, 2019, 17:31

          If the civilization has priority in stealth (watch, but do not be discovered), the probes might start close (in orbits of planets), but be moving farther away (Moon surface, Kuiper belt, Oort cloud, …?) once they detect signs of technology that could detect them.

          At our current technological level, probe on/near moon is in high risk of being detected by us, probe in earth orbit would be almost impossible to hide (unless “they” have some really advanced stealth technology), while passively watching probe somewhere in kuiper belt or even oort cloud is quite safe from detection.

  • David Horacek March 26, 2019, 10:27

    There are several strategies that make sense for alien lurker builders to follow. Observational data has already ruled out the existence of what I think are the a priori most plausible lurkers. These would have set up some sort of a maintenance infrastructure for redundancy and renewal of the lurker systems. We would have noticed those already. (If I were asked to design a lurker system and the mission spec was that it had to keep watch for at least a billion years, I’d turn in a design that looks a lot like life.) But maybe aliens can come up with something more modest that does the job. When it comes to Bracewell probes and Von Neuman machines, I expect that ones most likely to be everywhere are the most primitive models that actually work: these first-gen-to-work models will have first mover advantage, and while the society who sent them may later refine the technology, the first released wave will already be spreading through the galaxy. We also need to consider that later probes could for some reason hunt and disable the earlier generations, but would they themselves be hunted by the next model? And is this galactic conflict likely to produce the sort of quiet galaxy that we’ve been observing? To me, it argues for multiple successive visitations of our system, by probes that employ multiple propagation strategies, including very unsubtle ones. My point here is that even if I can vaguely imagine a quiet Bracewell probe, I can’t imagine that *an early model* would be quiet, and early models are the most probable to have spread here. The only way this could be not so is if later, faster models chased down, destroyed, and cleaned up after the early waves. While not impossible, it quickly begins to sound like a hopelessly ad hoc conspiracy theory. (Are the Bracewell probes that we will eventually build going to follow this pattern? First we launch, then we grab our heads and scream “What have we done?” and then we launch a cleanup mission?)

    Sure, some conspiracies are real, but it makes for a weak foundation of SETI: Alien civilizations are deliberately trying to hide their existence from us, but for some reason, we think we can see through their futuristic concealment measures if you give us money.

    • Mephane April 8, 2019, 2:51

      > these first-gen-to-work models will have first mover advantage, and while the society who sent them may later refine the technology, the first released wave will already be spreading through the galaxy. We also need to consider that later probes could for some reason hunt and disable the earlier generations, but would they themselves be hunted by the next model?

      A civilization sending Bracewell/Von Neumann probes would need to “hunt down” older models, but transmits design for later, improved models to the probe network. Since the probes would all move at a speed less than c while the new designs would be transmitted at c, the oldest models would only be found closer to their home world, and the further away it gets, the more advanced the probes will be because they’d always have much more recent iterations of the design at hand when the time comes to produce copies.

  • Ron S. March 26, 2019, 11:26

    Building further on what David says, I am finding the whole concept of lurkers excessively contrived. One can say they are there but not detectable and deliberately hidden and actively hiding, but if we do look hard enough we can find them. They have fascinating abilities to monitor us, but for no discernible reason since we are, due to relative development levels, not a threat and probably not even very interesting. Or we come up with contrived reasons whey they’d monitor us, communicate with home base yet otherwise ignore us, and that they may have done so for millennia.

    One swipe with Occam’s razor and it all falls away. It’s an interesting intellectual exercise that is very likely to be nothing more than just that.

    • J. Jason Wentworth March 27, 2019, 11:17

      A big assumption–which may or may not turn out to be possible; all we know for sure is that its attainment is beyond us today–is than self-replicating machines can actually be made and work as advertised. Like mind-uploading, the realization of self-replicating machines (*real* ones that can make copies of themselves from raw, unprocessed resources in space, rather than from batches of pre-fabricated modular parts) will require many, very hard steps, any one of which might be a “show-stopper.” Until they are actually proved to be do-able, these concepts are “nifty ideas that would be great if we could implement them,” but nothing more that that until then. Turning from the nifty-if-possible to the more prosaic methods that we *know* work, because we use them today:

      Ronald Bracewell pointed out in 1974 that solid-state electronics have individual component lifetimes of millennia (systems of components, of course, are less enduring because they have more things that can fail, but lower, steady temperatures greatly extend overall electronic lifetimes [even submarine trans-Atlantic telephone cables containing many *vacuum tube* booster amplifiers spaced along them, he pointed out, have lifetimes of 20 years and more, even though no one component was ever given a 20-year test]). Given this, we could instead launch larger numbers of smaller, quantity-produced (or even mass-produced, with today’s technology) starprobes, which could–like the Voyagers–be equipped with spare circuit cards that a simple STAR (Self-Test And Repair) computer could switch to if any of them failed or became damaged. The Pioneer Jupiter/Saturn probes didn’t even ^have^ computers (they were flown from the ground, which simple starprobes need not be [even the Pioneers “flew themselves” much of the time]), but just had command registers that could store and execute–in sequence–five stored commands, if memory serves, and:

      By exercising–as the wise old unicorn says–“Patience, ye mortals!”, more starprobe options open up, regarding propulsion, impact shielding, production and launching costs, etc. Settling for a lower cruise velocity lowers the impact shielding requirements, and enables the use of natural energy sources (Saturn-Jupiter-Sun gravity assists, Sun-grazing solar sails, solar thermal rocket perihelion burn Oberth maneuvers [or even all of these]) to send the probes on their ways (with multiple probes to each star of interest being affordable if desired). As well:

      None of the above-mentioned concepts invalidate the Lurker hypothesis, which need not depend on self-replicating probes. The Lurker concept, at bottom, may depend on the psychology, lifespans, and technological (and/or even economic/industrial) capabilities of the aliens involved. There are things that make perfect, intuitive, and self-evident sense to human beings that may utterly baffle aliens, and the reverse could be equally true.

  • ljk March 26, 2019, 11:48

    SETI Is Making a New ‘Mixtape for Aliens’, and You Can Contribute

    By Vanessa Bates Ramirez

    March 20, 2019

    Over 40 years later, as the Voyagers continue to drift through space, the SETI Institute’s artist-in-residence program is kicking off a brand-new, 21st-century version of the Golden Record. Unveiled at the South By Southwest conference last week, the EARTHLING Project aims to create a global musical composition that represents the human species—and launch it into the great beyond.

    In a panel titled “SETI, EARTHLING, and the Collective Musical Voice,” composer and SETI artist-in-residence Felipe Perez Santiago, entrepreneur and Singularity University faculty fellow Ola Kowalewski, and musician and software architect Rob Baker shared details of the project, as well as their thoughts on space art, the search for life off Earth, and music as a tool for uniting humanity.

    Full article here:


    To quote:

    The final musical works will be sent into space, Golden-Record style, as an updated representation of humanity. The way we make music has changed since 1977, after all—and it will continue to change with the technological tools being developed right now.

    “We’re living through a period of technological shift,” Baker said. “I started making music in a time when there was almost perfect isolation. Now there’s universal connectivity; the tools we have at our fingertips allow us to communicate and collaborate with anyone around the world.”

    Those tools, Kowalewski pointed out, have thus far been used to create music for human audiences. In fact, when creating any form of art, artists typically have a certain audience in mind. “A distinctive aspect of the EARTHLING platform is that it’s about composing for outer space,” Kowalewski said. How might composing for a (very) unknown audience impact the process?

    Sending the final composition into space is highly symbolic, Baker said. “Humans have created art for each other, and for deities, but there’s something really unique about identifying an unknown other. It allows humanity to become a unified body speaking to something else. Hopefully it makes us want to show our best selves as a species.”

    • Alex T. April 3, 2019, 10:41

      >>It allows humanity to become a unified body speaking to something else
      Wandering, how many bodies are unified in this composition?
      can suppose quantity – will be far from represent all Humans…

  • Gary Wilson March 26, 2019, 12:27

    The Lurker idea gives us yet another reason to investigate near Earth orbits I suppose. It would be monumental to find one but the probability would seem to be very low. Do the Lurkers communicate with “home base”? If so why haven’t we intercepted signals? Are they tight beam transmissions? Would that require a lot of power to travel interstellar distances? We need to constrain this idea to understand what to look for if anything, other than a piece of extraterrestrial tech living nearby. As with Ron S. I’m highly skeptical. Are we really that interesting? I suppose a highly curious extraterrestrial species might have sent out a Lurker swarm in all directions from its home and we might be one of the eventual arrival points. The idea is definitely loaded with sci fi possibilities anyway :).

    • J. Jason Wentworth March 27, 2019, 11:36

      I wouldn’t bet on the Lurker hypothesis being true, but I wouldn’t bet against it being true, either. We simply can’t know what aliens are capable of, much less how they think. Our Solar System is a very large place (Arthur C. Clarke’s 1968 comment that “We may yet discover that ours are not the first footprints on the Moon” is just as possible today as it was then, so little have we explored at resolutions that would show such things), and our system was born a very long time ago. Also:

      Aliens could have landed, walked (or driven or flown) here eons to millennia ago, and there could easily be no surviving, recognizable evidence of their coming to be discovered on Earth today. When people explore the airless moons of other planets (and ^really^ explore our own Moon), I will not be shocked–in the very unlikely event that I’m still alive by then–if instrument packages and/or abandoned equipment left by interstellar expeditions (perhaps even in the distant past) are found.

      • J. Jason Wentworth March 27, 2019, 11:40

        Argh–I meant (and forgot) to include “and/or inert or even active starprobes” in the sentence, “When people explore the airless moons of other planets (and ^really^ explore our own Moon), I will not be shocked–in the very unlikely event that I’m still alive by then–if instrument packages and/or abandoned equipment left by interstellar expeditions (perhaps even in the distant past) are found.”

    • Harry R Ray March 28, 2019, 9:37

      “If so why haven’t we intercepted signals?” This is PRECICELY why the “added expense” of Refreshers would not be a hinderance to ETAI. The only way we could intercept transmissions between Lurkers and Refreshers would be if Earth(or one of our spacecraft)were aligned in a way where you could draw a STRAIT LINE through the three points designating their positions. If “home base” were very distant, ANY radio transmissions, no matter how “tight” the beam, could be detected with an offset of up to thirty degrees.

    • J. Jason Wentworth March 28, 2019, 18:47

      Gary, even a simple Lurker (“I’m neither fer ’em er again’ em, just pointing out what they could do if they do exist in our Solar System) could report back to its home base without us ever accidentally intercepting its radio or laser signals. Now:

      They are thought to abide in the outer Solar System, so their signals–unless they felt the need to report home *across* the inner Solar System, depending on their home stars’ locations with respect to our Sun–would always travel away from the Earth.

      Due to their very long lives, the Lurkers could wait a few months or even years to send reports home, so that their signals need never cross the inner Solar System. Also, doing that would tend to attenuate the signals due to the asteroids and planets in or near the paths of the probes’ beams (just as the Sun’s radio noise drowned out a significant amount of Pioneer 11’s Saturn and Titan data and spin-scan pictures; its trajectory and Saturn flyby schedule made its disadvantageous proximity to the Sun [in Earth’s sky] necessary). Also:

      Even if Lurkers are holed up in the inner Solar System (in the Vulcanoids zone near and inside Mercury’s orbit [or maybe even on Mercury itself], for example–there, they would have plenty of solar energy *and* be hard to detect from Earth), avoiding having their signals intercepted by those meddling Earthlings would be easy to arrange. Such probes would certainly have optical sensors (and a camera or cameras), to detect, study–and avoid hitting, or passing too close to–the target star’s planets. Once Earth was found to be a source of artificial signals, the homeward-aimed transmissions could be time to occur only when the Earth was well outside of the line-of-sight to the probes’ home stars. As well:

      The optical sensors/cameras and/or radio science instruments of (human-made) deep space probes (such as the Voyagers, New Horizons, or the Mars orbiters) might accidentally detect transmissions from Lurkers, but only by the wildest chance. The interstellar transmission beam from a Lurker–either a “pencil beam” radio signal or a laser signal–would be very narrow, making the odds of accidental Earth or spacecraft detection of such a signal very low, and:

      Even then–due to the quite possibly very brief nature of such transmissions (because of a Lurker’s high level of technology), which would probably be detected only once by a moving spacecraft–such an anomalous detection might easily be dismissed as being just a momentary glitch aboard the spacecraft (just as the never-repeated WOW! signal has largely been dismissed as probably having been a glitch). For these reasons, Lurkers–while I personally doubt that they exist (silent, dead alien probes in our Solar System seem more likely)–can’t be ruled out. Their operational strategy might make perfect sense to aliens with different psychologies, and/or with tragic historic experiences from openly contacting belligerent (to them) other aliens (if so, at least some Lurkers might be dispatched for defense reasons, to keep watch on other potential invaders [we wouldn’t have a terribly high “Peace Quotient” score in their eyes…]).

  • ljk March 26, 2019, 12:35

    Perhaps we should be looking for large magnetic anomalies under the lunar surface. No, it is not an original idea, but the following comment from the article got me thinking that it is hardly implausible that an observing ETI may be doing to so to alert its makers when the human species or some other organism reaches a certain level of development:

    “Or it may have a small alert system to summon the larger intelligence when certain events or symptoms occur.”

    Now whether that alert would be to inform the ETI of a new species to invite into the Galactic Club, or warn them that they have yet another competitor for galactic resources and worlds, or some other reason we have yet to fathom, is another matter.

  • Alex Tolley March 26, 2019, 13:08

    Mention of “deep time” reminded me that I have a copy of Greg Benford’s book: Deep Time: How Humanity Communicates Across Millennia

    Most is not relevant to this discussion, but I do wonder if any of the more esoteric ideas might be dusted off for communication between ETI and ourselves. We tend to assume that communication would be by em radiation (probably radio), or by visual and aural media (e.g. Voyager golden disks). But what if there are other ways to convey information across deep time that preserves well and is not likely to be ambiguous? One might be lifeforms in some form of stasis. This would give us a lot of knowledge about life on the home world. Maybe the lifeforms have been collected from many worlds, each set with different biologies that we can investigate. Perhaps this includes artificial life forms, or samples of molecules at various stages before abiogenesis.

    We know DNA can be preserved over 10’s of millennia under natural conditions, so it might be stable a lot longer under artificial conditions. Similarly for microbial life. Such a trove would be an immense gift, and one that ETI need not worry about exposing itself if so desired.

    I am sure that other long lasting communication methods with varying levels of information content and formal rules are possible too.

  • Al Jackson March 26, 2019, 13:45

    Worth nothing in this topic is the long term stability of Earth orbit Trojans and Horseshoe orbits.
    TK7 seems to have been ‘captured’ (inserted?) 30,000 years ago, and could be around for .25 million years.
    That orbit stability is interesting.
    2016 HO3 seems to have a lifetime of a million years tho I am not clear when it was ‘captured’ into a quasi-orbit.
    Long-term stability of horseshoe orbits
    Ćuk, Matija; Hamilton, Douglas P.; Holman, Matthew J.
    Monthly Notices of the Royal Astronomical Society, Volume 426, Issue 4, pp. 3051-3056. 2012
    Orbital stability of Earth Trojans
    Zhou, Lei; Xu, Yang-Bo; Zhou, Li-Yong; Dvorak, Rudolf; Li, Jian
    Astronomy & Astrophysics, Volume 622, id.A97, 14 pp
    Long term stability of Earth Trojans
    Marzari, F.; Scholl, H.
    Celestial Mechanics and Dynamical Astronomy, Volume 117, Issue 1, pp.91-100

  • Adam Byrne March 26, 2019, 14:14

    I will try the Galactic Centre sequence next Paul.

    Thanks for the recommendation.

    I have just finished Liu Cixin brilliant Remembrance of Earth’s Past trilogy and have now started the first book in Stephen Baxter’s Xeelee Sequence.



    • Paul Gilster March 27, 2019, 9:29

      Yes, Liu Cixin is fascinating. I share your enthusiasm. You’ll like Galactic Center.

  • Neil S March 26, 2019, 16:53

    1. If a lurker is from a civilization hiding from the surrounding dark forest, why is it here? Sent by an outlier faction? Even launching it might be thought dangerous. Would it dare tight-beam reports back? Such a lurker should be extremely hard to find.

    2. If it’s from a more open civilization, what’s it waiting for? Unless we’re not considered worth talking with, shouldn’t it be contacting us before some other lurker can? Or do we assume all the lurker-senders are cooperating? And it would perhaps be easier to find, but under what circumstances would it contact us? If we “scan” it? If we’re about to self-destruct? If we show strong signs of civility? Never; it’ll wait for us to find it?

    3. What would have incited “someone” to have launched a probe to here centuries or millennia ago? Atmospheric composition (ac)? If they detected something such as high Δac or nuclear explosions about earth more recently they probably weren’t from too many light years away and they probably arrived no more than decades ago. Or they could still be in transit.

    4. What known-to-us modes of transportation would allow it to arrive in our solar system and maneuver so as to hide in the asteroid belt with low probability of being spotted by us with current tech? And how could it report back without us detecting that unless it used tight beam? Or tech we have no clue about.

    • Keith Cooper March 27, 2019, 7:12

      “If it’s from a more open civilisation, what’s it waiting for?”

      I remember Doug Vakoch once pointing out to me, while trying to justify METI, that we study various wildlife and animals on Earth, like zebras for example, but we don’t try talking to zebras – we just observe them in their environment. But if a zebra suddenly started coming out with a sequence of prime numbers, we’d notice very quickly and communicate with it. Maybe the lurker probes, should they exist, are just waiting for us to do something interesting?

      It may also be a case of following some kind of prime directive not to interfere with other civilisations. If advanced ETI is benevolent, then the prime directive idea is one that I actually quite like. Here’s a question that other commentators might know: did the idea of the prime directive originate with Star Trek, or does it predate Star Trek?

      “What would have incited someone to have launched a probe to here centuries or millennia ago?”

      Scientists who have tried to model galactic colonisation have shown that, in theory, the entire galaxy can be pretty much explored in a few million years at sub-light speed velocities. The likeliest candidates to do such exploring – perhaps as reconnaissance for a wave of future colonisation – are probes, particularly von Neumann probes that can self-repair and replicate in order to continue their mission through deep time. Alternatively, Milan Circovic has come up with the idea of ‘city state’ civilisations, which basically stay at home and send out robotic probes to do their exploring – a bit like what we are doing at the moment. Either way, it’s plausible that many probes have passed this way, and have either moved on because there’s no available colonisation sites, or have stayed for scientific study.

      • ljk March 27, 2019, 10:16

        The 1991 episode of Star Trek: The Next Generation titled “The Nth Degree” had ETI which sent out probes to find suitable species to study and then bring those samples to them:


        Presumably these aliens were very careful about whom they selected. Then again, they appear to be advanced enough that if they did pick up some unruly creatures, they could be handled.

        • Keith Cooper March 27, 2019, 16:25

          That’s an interesting example. In that episode the ETI temporarily increased the IQ of Barclay, allowing him to take over the Enterprise and bring the ship to them. I guess in a way it was a kind of ‘uplift’, a bit like what David Brin writes about, only this time done to a human.

          • ljk March 28, 2019, 9:25

            If nothing else it had the merit of being a different perspective on the subject.

            Even though at the end Barclay lost most of his enhanced knowledge, the technology he had developed to accomplish traveling to the Cytherian homeworld near the galactic center almost instantly should still be available. Whatever became of that?

            As bad as the 2016 sequel to the film Independence Day was, it did do one thing right: Humanity had kept and utilized the alien technology left after the invasion to their advantage not only to improve our civilization but to be prepared for another alien attack. It did not just disappear after the invasion.

    • J. Jason Wentworth March 27, 2019, 11:48

      To Reply to your #4, here (see: https://www.centauri-dreams.org/2015/06/22/yarkovsky-and-yorp-effect-propulsion-for-long-life-starprobes/ ) is a novel, no-moving-parts, and possibly stealthy natural forces-utilizing propulsion system that could be used by long-life starprobes, including Lurkers, operating in exoplanetary systems.

  • Gary Wilson March 26, 2019, 18:47

    I don’t really understand the motive behind a Lurker probe. Or is it unknowable given its alien origin? If they are aware of us surely they know our level of technology at least approximately? We can’t really do much to them. We haven’t even visited our own moon (at least in person) for over 40 years. We don’t currently possess the technology to investigate even the closest star system. If they are not afraid of us are they then observing some protocol which allows us to develop (or destroy ourselves) without intervention or interference? What happens if this so-called Lurker is disguised as a rock and is currently asleep for some unspecified period? Which rock should we look at carefully first? Would it be a rock on the Earth facing side of the moon, or a rock in a stable orbit around us? And how big would this specific rock need to be? I don’t see this as a very practical exercise.

  • Ken March 26, 2019, 20:02

    What if the advanced ET society is a long-lifespan one, with lives spanning hundreds or thousands of our years? Perhaps travel of a few 10’s of light years is equivalent of me taking month long cruise. Perhaps comms are via some sort of quantum entanglement or other unknown & unguessed method.

    Parking, what to us, appears to be an inert rock might work very well for them… Finding & recognizing that rock would likely happen only by chance. Be pretty cool stuff though!

  • Tom W. Bell March 26, 2019, 20:08

    The analysis seems to assume away social carnivores far too quickly. Lions, wolves, and cetaceans form groups that predate together. Ditto those most dangerous of omnivores, humans.

    No responsible person should advocate attempting to initiate contact with aliens without more careful consideration of the downside risks. Lurkers might serve as the sensitive strands of a galaxy-spanning web, with an alien and hungry intelligence at its center….

  • Patient Observer March 26, 2019, 21:54

    If there could be lurkers then there could also be flashers:)

    A lurker/flasher probe would generate a signal when it detects something interesting like an I Love Lucy broadcast (why is that always picked as representative of Earth’s development?). The signal would be tailored to be detectable by a newbie technical civilization such as on Earth today. The earthly response to the signal could be of keen interest to the lurking civilization – are we aggressive, inquisitive, hostile, passive? The probe, if discovered, would be designed as to leave no clue where it may have originated to protect the identity of the lurker in case we take exception to their spying and unfriend them on the galactic Facebook.

    • Paul Gilster March 27, 2019, 8:53

      But Patient Observer, as an I Love Lucy fan, let me speak in favor of using it as a ‘standard candle’ for extraterrestrial reception of our signals ;-)

      • J. Jason Wentworth March 27, 2019, 13:10

        Also, “I Love Lucy” was one of the first, if not THE first, television show that was widely broadcast on the air for multiple years (including long after its first run, to this very day), making it more likely to be seen–or at least detected–by alien radio astronomers or SETI researchers.

        • ljk March 28, 2019, 9:29

          So ETI will get to learn that human females are not allowed by their male partners to have a career outside the home and will conduct wacky schemes with their best friend to circumvent these primitive cultural restrictions. All of which will fail spectacularly within 30 minutes until next week when the cycle is repeated.

    • ljk March 27, 2019, 9:34

      Why does SETI always use I Love Lucy as their go-to television series that aliens will presumably see first as it travels through the Milky Way galaxy?

      Because someone decades ago writing about SETI used the series as a popular and well-known example and it stuck, just like how the media keeps focusing on the Wow! Signal of 1977, even though there have since been far better examples and modern SETI practitioners will even tell you that with today’s technology the signal probably would have been dismissed as a false read.

      Obviously ILL was not the first television program to get past Earth into deep space and there were certainly plenty of other programs from all over the globe that escaped the ionosphere too. Do you recall this famous scene from the 1997 SF film Contact:


      According to the 1985 novel, apparently the coronation of King George VI of Great Britain was another strong TV signals that went into space, but in terms of dramatic effect such an event could not compete with seeing Hitler on the screen opening the 1936 Olympic Games in Berlin.

      Once again Westerners assume only their transmissions will be picked up before anyone else’s and that somehow the aliens will prefer them.

      • Harry R Ray March 27, 2019, 10:47

        Maybe my coffee consumption has been a bit too high recently, but that ‘stuff” that was “conveniently” ejected from Asteroid Bennu ELEVEN TIMES in time periods where Osiris Rex was in an EXCELLENT position to image them seems pretty “flashy” to me. Has any SPECTRA of this “stuff” been attained? I know, its pretty close to April 1, but what the heck!

      • Michael C. Fidler March 27, 2019, 10:53

        I would imagine that WWII early radars using magnetrons would be the first signals pick up by ET. It used a concentrated beam and was normally aimed at the horizon. The EM radio signal was pulsed, and repetitive like a pulsar from rotation of antenna. There would also be daily recurrence when the beam would sweep past the planet or spacecraft as the earth rotated. Very obviously an artificial signal coming off a planet.

      • J. Jason Wentworth March 27, 2019, 13:27

        But how long was King George’s televised coronation ceremony–or Hitler’s TV appearance at the 1936 Olympics’ opening ceremony–on the air, and how often–and widely–has either one been repeated? Not very long (just minutes) or very widely or often, I’d wager, while “I Love Lucy” has been on the air all over the world, day and night, since 1951, making it far more likely to have been detected (or even decoded and seen and heard, if the aliens have big enough antennas and good enough amplifiers [better that what we’ve got at present]). Also:

        It would be *really* interesting (if anyone wants to spin an SF yarn based on this, I have no objections) if one day we received a signal–or even visitors–who were seeking to communicate with Mister Ed, who certainly seemed to be much smarter than the human beings around him. He appeared to run the world from his barn, with his human minions doing his bidding! :-) (Like “I Love Lucy,” his show has also been widely broadcast since it began, in 1961, making it also a ‘common artificial signal’ spreading outward from the Earth.)

        • Paul Gilster March 27, 2019, 14:09

          A ‘Mr. Ed’ listening civilization might also find links to Swift’s Gulliver’s Travels. A true human theme!

          • Ron S. March 27, 2019, 14:19

            I can just imagine the little green men landing and asking the first human they meet: “Take me to your horse!”

            • ljk March 28, 2019, 9:37

              You folks do realize that most other nations also have their own television and radio programs, many of which have also been broadcast for decades now.

              Since virtually none of it was ever aimed at space in a coherent direct beam, most of it will be electromagnetic noise that a recipient will have a heck of a time trying to untangle and decipher.

              They will know there is some kind of intelligence on Earth from these signals, but understanding it will be another matter.

              The strongest such transmissions sent from our planet have been radar beams used by the military for tracking missiles and by astronomers to study NEOs. While those too are not aimed at any precise target beyond our Sol system and contain no coherent information, they will tell anyone smart enough out there that someone with at least some technological knowhow is here.

              • J. Jason Wentworth March 28, 2019, 19:25

                Of course–we just can’t normally receive the ionosphere-piercing FM and TV signals from other lands because they are (except under freak conditions, such as rare VHF and UHF ducting along the ground) strictly line-of-sight, with a little knife-edge diffraction, which doesn’t extend the reception range by much, and:

                A distant observer would receive anomalously narrow-band signals in a thin “sheet” of radio noise, corresponding to the Earth’s limbs, in which a jumble of signals would appear at one limb and disappear at the opposite limb as the planet rotates; it would regularly go up and down in intensity, as station-rich and station-sparse areas of the Earth rotate over and beyond the limbs. The alien receive-ers would notice that the radio noise occurs in a few bands (FM, TV, radar, etc.). If they had–or developed, after detecting the decidedly non-natural signals (the Earth outshines the Sun at some radio frequencies!)–big enough antennas (or antenna arrays) and sensitive enough receivers (which would be a tall technological order, but isn’t fundamentally impossible), they could hear our voices and music and radar “pips,” and see our television images.

            • Harry R Ray March 28, 2019, 9:44

              Or humpback whale?

          • J. Jason Wentworth March 28, 2019, 19:05

            …Especially if they happened to also pick up a broadcast of the 1996 television version starring Ted Danson; the Houyhnhnms’ superiority to human beings was well portrayed.

    • Harry R Ray March 27, 2019, 10:30

      Patient Observer: Please read the PDF for “A Shiney New Method for SETI: Specular Reflections from Interplanetary Artifacts.” by Brian C Lacki ASAP, and then read about A10bMLz on Pseudo-MPEC for A10bMLz-Project Pluto. The reasons I believe that A10bMLz may BE a Flasher are, ONE: Its EXTREMELY bizarre RETROGRADE trans-lunar orbit, and TWO: The VERY FIRST observation of A10bMLz showed the object to be MUCH BRIGHTER than than any SUBSEQUENT observation did, leading astronomers to believe that its perigee was ONLY ~300 miles as to the now confirmed 334,000 miles!

  • Curious March 26, 2019, 23:32

    If I were designing a probe and had technology say 50-100 years more advanced than our own, an approach might be the following:
    1) a large quantity of intelligent scouts to target a region that has promise, at say a solar system level, even if no signals have been emitted from there. For example like we are looking at Kepler-22.
    2) probe would be accelerated from my system through combination of expendable propellant/accelerator, gravity assists, and then deploy some type of sail. Lets say this sail is double the size of Oumuamua when unfurled, and the scout is a nanomachine of 0.1gram. Physics still matters even for ETI so there would be no reason to make it bigger than the absolute minimum. This has added advnategs of being virtually impossible to detect, being so light that it can be readily decelerated by using even a sparse atmosphere. Its main content is programming/AI with instructions housed within protective shell, from which some scilia could be grown out maybe using nearby materials to allow motion and external interfacing once on the target planets surface
    3) Scout is dormant until it detects RF which is a great indicator of intelligent life if it is patterned, perhaps the RF energy itself is also gathered by the sail structure for power to awaken the probe, it would only need to gather sufficient energy to activate the scout to life, with the scout having some type of long lived instant on battery (e.g. a thermal battery, though molecular) and once awakened it aims toward the RF intensity to seek out the source and then it sheds the sail and enters the world, now seeking out the strongest electrical field to land within. In this way the scout could be dormant for very long times and only awaken when there is promising RF.
    4) the nanomachine scout finds and interfaces with the electric power grid to power itself and use the grid infrastructure as an antenna to transmit signals to its homeworld its job perhaps then complete. Alternately it might have the capability to hack/implement instructions to search information systems that are connected to the power system and glean planetary and maybe civilization characteristics found there and decide what news to transmit home.

    So in this scenario it is unlikely we would ever find the scout, just the sail whizzing by… We might be able to detect a broadcast that is not our own, perhaps this would be brief but repeating bursts.

  • Robin Datta March 27, 2019, 0:16

    Lurker probes could collect data on wtat’s happening in distant reaches of time and space, just as we set out sensors and even cameras to monitor wildlife – in the wilderness. Filling in such datasets may be considered worthwhile, but it is also conceivable that some civilization(s) may have such advanced techniques that to them lurkers may be rather primitive.

    • ljk March 28, 2019, 10:26

      Robert Bradbury once said that advanced ETI – the kind that can build Dyson Shells – could also reconstruct moons into giant space telescopes that could easily distinguish surface features on worlds in other solar systems. The Dyson Shells themselves could also be utilized for similar purposes.

  • ericSECT March 27, 2019, 6:56

    Regardless of the “how’s” and “why’s”….we should scour our solar system bodies for evidence of life, panspermia or a second genesis …..AND also look for lurker probes or evidence there of, past or present.

  • ljk March 27, 2019, 9:46


    The Messenger: a galactic centre gravitational-wave beacon

    Marek Abramowicz, Michal Bejger, Eric Gourgoulhon, Odele Straub

    (Submitted on 25 Mar 2019)

    Our existence in the Universe resulted from a rare combination of circumstances. The same must be true for any advanced extraterrestrial civilisation. If there exist any in the Milky Way, they are likely scattered over large distances in space and time, however, they must be aware of the unique property of the Galactic centre: it hosts the closest massive black hole to anyone in the Galaxy.

    A sufficiently advanced civilisation may have placed material in orbit around this black hole to study it, extract energy from it, and/or for communication purposes. In either case, its orbital motion will necessarily be a source of gravitational waves.

    Here we show that a Jupiter-mass “Messenger” on the innermost stable circular orbit around the black hole can be sustained for a few billion years by the energy output of a single star and emits an unambiguously artificial (continuous) gravitational wave signal that will be observable with LISA-type detectors.

    Comments: 7 pages, 2 figures, currently under review, comments and questions welcome

    Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:1903.10698 [astro-ph.HE]
    (or arXiv:1903.10698v1 [astro-ph.HE] for this version)

    Submission history

    From: Odele Straub [view email]

    [v1] Mon, 25 Mar 2019 15:16:25 UTC (98 KB)


    • Al Jackson March 27, 2019, 17:02

      ljk, that was a bit of a surprise.
      Greg Benford and I have a paper to appear in JBIS soon, that we put up on arXiv last June.
      A Gravitational Wave Transmitter

      There will also be a popular article in Analog about it this year too.

      • ljk March 28, 2019, 9:45

        Al – Perhaps your two groups should contact each other for a form of collaboration? All those extra minds working together may come up with something even bigger and better.

  • Andrew Palfreyman March 28, 2019, 0:06

    I’m afraid I find the concept of a Lurker probe vanishingly unlikely. Developmental differences between space-savvy civilisations are likely to be tremendous especially considering the twin timescales of technological development (look at what’s occurred with us over a scant 100 years) and civilisation potential age differences (billions of years). The probability of an “overlap of interest” between these two timescales is vanishingly small. Positing a Lurker probe for this solar system would be tantamount to us deciding to install webcams on every anthill. It is a concept founded in hubris, and no more than that.

  • Alex Tolley March 28, 2019, 10:17

    …tantamount to us deciding to install webcams on every anthill.

    Perhaps more like a webcam and analysis software on a representative anthill of every ant species. That might be quite instructive, especially if simple experiments can also be carried out. Tiny, IoT cams and comms tech will make this quite possible via “citizen science” projects.

  • Gary Wilson March 28, 2019, 12:49

    Although a species which had advanced only a couple of hundred years beyond where we are now may have decided Lurker probes were a great idea Andrew. When in time that might have happened we would have no idea, but perhaps a probe or probes 10’s or 100’s of thousands or even millions of years old may be sitting somewhere on Earth or in orbit or on any number of nearby sites now, waiting to be found. That is part of the great potential and excitement of deep time.

    • hiro March 28, 2019, 23:45

      Very unlikely. The highest probability is some “random” object that happens to look like rock orbiting around Sol from a distance 550 AU. Even if it had the side of the largest football stadium, we would still fail to detect it big time.

  • Gary Wilson March 28, 2019, 14:50

    Would the Starshot probes as currently conceived be considered Flasher probes? I suppose anything we send out could initiate First Contact. The odds are certainly extremely low but we should definitely be thinking about it. Probes may have similar issues as METI signals do. What are we telling possible receivers of probes or messages about ourselves?

  • Paul451 March 28, 2019, 16:48

    Trivial pedantry, but lions are social animals.

  • Adam April 3, 2019, 4:17

    Hi Paul
    On the question of Lurker signals to their Home Bases, I do wonder if the local Nodes of a Galactic Network might be parked around nearby White Dwarf stars, to maximise their potential as gravitational lenses. In which case, perhaps Sirius is where Lurkers are beaming their findings? Elliptical orbits around a White Dwarf will precess rapidly, covering much of the sky in a predictable manner, allowing a single facility to receive programmed signals from multiple nearby systems.

    The additional question is: what would we see if a signal was being sent back from the Node? Could that explain the “Red Sirius” phenomena seen in ancient times?

  • Harry R Ray April 5, 2019, 10:09

    Paul Gilster: Not many comments with SPECIFIC reference to `Oumuamua since the Seligman, Laughlin , and Batygin paper FINALLY came out and essentially(in my opinion PRIOR to this comment)killed the necessity for a non-natural solution. NOW THIS: “Evidence against non=gravitational acceleration of I1/2017 U1 `Oumuamua.” by J. I. Katz. THE ENTIRE ABSTRACT: “Micheli et al reported that a seven parameter fit to the orbit of 1I/2017 U1 `Oumuamuaindicated a non-gravitational acceleration in the anti-Solar direction, and attributed it to recoil from from comet-like outgassing. The implied gas to dust ratio is 100 times greater than of known Solar System coments. The reported collapse of the scatter of nearly contemporaneous co-ordinate residuals upon inclusion of the non-gravitational term in the orbital fits is difficult to understand. There are grounds for skepticism.” My take on this is that either, ONE: There is NO non gravitational acceleration AT ALL(which I VERY SERIOUSLY DOUBT due to the 8+ sigma degree of confidence), or TWO: There is no non-gravitational acceleration in ANY COMET OUTGASSING SCENARIO ONLY, essentially KILLING the Seligman, Laughlin, Batygin premise entirely, leaving ONLY the thin film(lightsail), comet dust reminant, and ice fractal extremely low mass planetessimal, solutions(ALL based on solar radiation pressure driven non-gravitational acceleration)on the table! Can you please ask Avi Loeb what he thinks about this ASAP? Thanks.