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Remote Observation: What Could ET See?

As we puzzle out the best observing strategies to pick up a bio- or technosignature, we’re also asking in what ways our own world could be observed by another civilization. If such exist, they would have a number of tools at their disposal by which to infer our existence and probe what we do. Extrapolation is dicey, but we naturally begin with what we understand today, as Brian McConnell does in this, the third of a three-part series on SETI issues. A communications systems engineer, Brian has worked with Alex Tolley to describe a low-cost, high-efficiency spacecraft in their book A Design for a Reusable Water-based Spacecraft Known as the Spacecoach (Springer, 2015). His latest book is The Alien Communication Handbook — So We Received A Signal, Now What? recently published by Springer Nature. Is our existence so obvious to the properly advanced observer? That doubtless depends on the state of their technology, about which we know nothing, but if the galaxy includes billion-year old cultures, it’s hard to see how we might be missed.

by Brian McConnell

In SETI discussions, it is often assumed that an ET civilization would be unaware of our existence until they receive a signal from us. I Love Lucy is an often cited example of early broadcasts they might stumble across. Just as we are developing the capability to directly image exoplanets, a more astronomically advanced civilization may already be aware of our existence, and may have been for a long time. Let’s consider several methods by which an ET could take observations of Earth:

  • Spectroscopic analysis of Earth’s atmosphere
  • Deconvolution of Earth’s light curve
  • Solar gravitational lens telescopes
  • Solar system scale interferometers
  • High speed flyby probes (e.g. Starshot)
  • Slow traveling probes that loiter in near Earth space (Lurkers, Bracewell probes)

Spectroscopic Analysis

We are already capable of conducting spectroscopic analysis of the light passing through exoplanet atmospheres, and as a result, are able to learn about their general characteristics. This capability will soon be extended to include Earth sized planets. An ET astronomer that had been studying Earth’s atmosphere over the past several centuries would have been able to see the rapid accumulation of carbon dioxide and other fossil fuel waste gases. This signal is plainly evident from the mid 1800s onward. Would this be a definitive sign of an emergent civilization? Probably not, but it would be among the possible explanations, and perhaps a common pattern as an industrial civilization develops. Other gases, such as fluorocarbons (CFCs and HFCs) have no known natural origin, and would more clearly indicate more recent industrial activity.

There is also no reason not to stop at optical/IR, and not conduct similar observations in the microwave band, both to look for artificial signals such as radars, but also to study the magnetic environment of exoplanets, much like we are using the VLA to study the magnetic fields of exoplanets. It’s worth noting that most of the signals we transmit are not focused at other star systems, and would appear very weak to a distant observer, though they might notice a general brightening in the microwave spectrum, much like artificial illumination might be detectable. This would be a sure sign of intelligence, but we have not been “radio bright” for very long, so this would only be visible to nearby systems.


Even if we can only obtain a single pixel image of an exoplanet, we can use a technique called deconvolution to develop a low resolution image of it by measuring how its brightness and color varies as the planet rotates. This is not unlike building an image by moving a light meter across a surface to build a map of light levels that can be translated into an image. It won’t be possible to build a high resolution image, but it will be possible to see large-scale features such as oceans, continents and ice caps. While it would not be possible to directly see human built structures, it would be clear that Earth has oceans and vegetation. Images of Pluto taken before the arrival of the New Horizons probe offer an example of what can be done with a limited amount of information.

Comparison of images of Pluto taken by the New Horizons probe (left) and the Hubble Space Telescope via light curve reconstruction (right). Image credit: NASA / Planetary Society.

Svetlana Berdyugina and Jeff Kuhn presented a presentation on this topic at the 2018 NASA Techno Signatures symposium where they simulated what the Earth would look like through this deconvolution process. In the simulated image, continents, oceans and ice caps are clearly visible, and because the Earth’s light curve can be split out by wavelength, it would be possible to see evidence of vegetation.

Solar Gravitational Lens Telescopes

A telescope placed along a star’s gravitational lens focal line will be able to take multi pixel images of exoplanets at considerable distances. Slava Turyshev et al show in this NASA NIAC paper that it will be possible to use an SGL telescope to image exoplanets at 1 kilometer per pixel resolution out to distances of 100 light years. A SGL telescope pointed at Earth might be able to see evidence of large scale agriculture, urban centers, night side illumination, reservoirs, and other signs of civilization. Moreover, pre-industrial activity and urban settlements might be visible to this type of instrument, which raises the possibility that an ET civilization with this capability would have been able to see evidence of human civilization centuries ago, perhaps Longer.

A simulated image of an exoplanet as seen from an SGL telescope. Image credit: NASA/JPL

A spacefaring civilization that happens to have access to a nearby black hole would have an even better lens to use (the Sun’s gravitational lens is slightly distorted because of the Sun’s rotation and oblate shape).

Solar System Scale Interferometers

The spatial resolution of a telescope is a function of its aperture size and the wavelength of the light being observed. Using interferometry, widely separated telescopes can combine their observations, and increase the effective aperture to the distance between the telescopes. The Black Hole Event Horizon Telescope used interferometry to create a virtual radio telescope whose aperture was the size of Earth. With it, we were able to directly image the accretion disc of galaxy M87’s central black hole, some 53 million light years away.

Synthetic microwave band image of M87’s central black hole’s shadow and nearby environment. Image credit: Event Horizon Telescope

Now imagine a fleet of optical interferometers in orbit around a star. They would have an effective aperture measuring tens to hundreds of millions of kilometers, and would be able to see small surface details on distant exoplanets. This is beyond our capabilities to build today, but the underlying physics say they will be possible to build, which is to say it is an expensive and difficult engineering problem, something a more advanced civilization may have built. Indeed, we began to venture down this path with the since canceled SIM (Space Interferometry Mission) and LISA (Laser Interferometer Space Antenna) telescopes.

A solar system scale constellation of optical interferometers would be able to resolve surface details of distant objects at a resolution of 1-10 meters per pixel, comparable to satellite imagery of the Earth, meaning that even early agriculture and settlements would be visible to them.

Fast Flyby Probes

Fast lightsail probes, similar to the Breakthrough Starshot probes that we hope to fly in a few decades, will be able to take high resolution images of exoplanets as the probes fly past target planets. Images taken of Pluto by the New Horizons probe probably give an idea of what to expect in terms of resolution. It was able to return images at a resolution of less than 100 meters per pixel, smaller than a city block.

The primary challenges in obtaining high resolution images from probes like these are: the speed at which the probe flies past its target (0.2c in the case of the proposed starshot probe),and transmitting observations back to the home system. Both of these are engineering problems. For example, the challenge of capturing images can be solved by taking as many images as possible during the flyby and then using on board post processing to create a synthesized image. Communication is likewise an engineering problem that can be solved with better onboard power sources and/or large receiving facilities at the home system. If the probe itself is autonomous and somewhat intelligent, it can also decide which parts of the collected imagery are most interesting and prioritize their transmission.

The Breakthrough Starshot program envisions launching a large number of cheap, lightweight lightsails on a regular cadence, so while an individual probe might only be able to capture a limited set of observations, in aggregate they may be able to return extensive observations and imagery over an extended period of time.

Slow Loitering Probes (Lurkers and Bracewell Probes)

An ET civilization that has worked out nuclear propulsion would be able to send slower traveling probes to loiter in near Earth space. These probes could be long lived, and could be designed for a variety of purposes. Being in close proximity to Earth, they would be able to take high resolution images over an extended period of time. Consider that the Voyager probes, among the first deep space probes we built, are still operational today. ET probes could be considerably more long lived and capable of autonomous operation. If they are operating in our vicinity, they would have been able to see early signs of human activity back to antiquity. One important limitation is that only nearby civilizations would be able to launch probes to our vicinity within a few hundred years.

The implication of this is not just that an ETI could be able to see us today, they could have been able to study the development of human civilization from afar, over a period spanning centuries or millennia. Beyond that, Earth has had life for 3.5 billion years, and life on land for several hundred million years. So if other civilizations are surveying habitable worlds on an ongoing basis, Earth may have been noticed and flagged as a site of interest long before we appeared on the scene.

One of the criticisms of SETI is that the odds of two civilizations going “online” within an overlapping time frame may be vanishingly small, which implies that searching for signals from other civilizations may be a lost cause. But what if early human engineering projects, such as the Pyramids of Giza, had been visible to them long ago? Then the sphere of detectability expands by orders of magnitude, and more importantly, these signals we have been broadcasting unintentionally have been persistent and visible for centuries or millennia.

This has ramifications for active SETI (METI) as well. Arguments against transmitting our own artificial signals, on the basis that we might be risking hostile action by neighbors, may be moot if most advanced civilizations have some of the capabilities mentioned in this article. At the very least, they would know Earth is an inhabited world and a site for closer study, and may well have been able to see early signs of human civilization long ago. So perhaps it is time to revisit the METI debate, but this time with a focus on understanding what unintentional signals or techno signatures we have been sending and who could see them.


{ 36 comments… add one }
  • Jeff Wright December 24, 2021, 14:17

    I suspect the Great Daylight Fireball of 1972 was the aerobrake of a Bracewell that was supposed to do a resonant return in two decades-though a Titan launched tug might wrangle it for shuttle orbiter return…making it a Glomar.
    Far fetched?
    Yes-but look how smooth its contrail was on that Super-8 footage!

  • Alex Tolley December 24, 2021, 15:24

    Depending on resolution, pattern recognition, and analysis, a Lurker could detect the controlled use of fire for at least a million years BP. While not a detection of civilization, it might be a signal to indicate that a possible civilization might emerge in a tiny fraction of the Earth’s history and the history of terrestrial animals living on dry land.

    We shouldn’t Lurkers need stay in space during such periods. Surface sensors, just as we place cameras on jungle trails and in hides at waterholes, might well be employed. Aerial drones would also be possible. Unlike orbital imaging, surface imaging provides a better perspective for seeing animals as well as far higher resolution. Clarke’s 1953 short story, Encounter in the Dawn about an alien explorer meeting a hominid would more likely be a robot or sensor instrument given the direction our technology is headed.

    Even if galactic civilizations remain too separated in space and time to ever have 2-way communication, there is no reason why that communication might not be 1-way, to record and convey information for future civilizations to use. We cannot time travel to the past, but what if probes in our system have recorded a details of their civilization, knowledge, and our civilization going back many millennia, providing a record for us? Such a probe may offer us parts of the record that we can safely use depending on our level of development, with more knowledge as we discover the keys to unlock more. This deeper time length of operation of a probe was a higher probability of success compared to traditional SETI argument advanced by Jim Benford to search for Lurkers in our system, perhaps even on the Moon. A Drake Equation for Alien Artifacts.

    • Jeff Wright December 26, 2021, 1:58

      Early lurkers with a more limited service life is what we may find first. If Ouamuamua was a flyby probe…it is likely a wreck.

      Miranda interests me. ET could strip mine on a whole world that looks strip-mined and hide quite well.

    • David Jernigan December 26, 2021, 18:47

      To expound on the subject of detecting the use of fire, it should be noted that, given only weeks of observing time, it would be possible to detect a culture with deliberate campfires by the spectral effects of light, heat, or even smoke. Combining even a very weak signal with diurnal, weather, and seasonal patterns, and geographic distribution, would reveal the deliberate nature of campfires at resolutions definitely inside 100m and perhaps inside of 10km. That’s for a million-plus year window of time.

      For a 50,000 year window of time, the smoke from deliberate burns to perform hunts and clear land for grazing and farming would alter the atmosphere enough to be easily detectable with single observations at even 10km resolution, and imaginable at single-pixel resolution. And note that this activity has been continuous somewhere on earth throughout that time frame.

    • Anthony Mugan December 29, 2021, 5:57

      This is a very interesting discussion, and I agree we would probably want to use surface sensors and aerial drones if we get to investigate an inhabited world in the future, so it does seem reasonable to suspect others may have done that to us. This would be for both scientific reasons but also military and intelligence reasons (ferret type operations to test systems, procedures and capabilities would be highly likely if we were doing it, for example).
      That rather raises the question of how could we achieve positive identification of such a drone and avoid dismissing it as just another UFO / UAP report / misidentification? This is tricky as we don’t know what technology we’d be looking for and therefore diagnostic signatures are hard to pin down.
      As far as I’ve got over the last few days would be something were we could definitively confirm the presence of a physical object and performance characteristics beyond current technological limits.
      Would you have any additional thoughts on what level of evidence would be needed and if there could be other diagnostic signatures we could consider?

      • GARY WILSON December 30, 2021, 20:42

        But what are the current human performance characteristics of human made aircraft/spacecraft Anthony? Due to the secret nature of military aircraft/spacecraft we don’t really know. And how do you catch a UFO/UAV to determine its physical characteristics? I think these are huge quandaries for us. I’m not saying it’s impossible but so far I’m not sure how you would go about doing it.

        • Anthony Mugan January 1, 2022, 5:52

          Hi Garry
          Yes, I entirely agree. We would probably have to wait at least 10 years to be sure an observation wasn’t of some new classified project (indeed that raises issues around even discussing such observations publically).
          There may be some use in going over archival datasets although data quality is generally abysmal from what I can see. There may be some useful data in there somewhere.
          It woukd indeed be necessary to predefined the criteria for accepting an observation as physically real. There would need to be sufficient data to positively eliminate all known sources of misidentification also. I doubt there would be many events where the data would be wide ranging enough and of sufficient quality to do it, but it might be possible.
          In concept this is similar to mining old astronomical datasets with new techniques, but the challenge would be finding data iof sufficient quality….

  • GARY WILSON December 24, 2021, 17:49

    Warmest regards and season’s greetings to everyone on here. A special thanks to Paul and Alex and all the other article contributors. The site has greatly improved my understanding of various fields and just cheered me up in general :).

    • Paul Gilster December 25, 2021, 20:48

      All best holiday wishes to you as well, Gary. Thanks for your presence and the kind words. Great to have you here.

  • H. Floyd December 24, 2021, 21:56

    “Arguments against transmitting our own artificial signals, on the basis that we might be risking hostile action by neighbors, may be moot if most advanced civilizations have some of the capabilities mentioned in this article.”

    Wouldn’t arguments in support of transmissional METI be mooted similarly?

    All things being equal, the lesser-risk path seems the more responsible. It’s one thing to be passively “flagged as a site of interest,” another thing for one generation of one species to speak for a planet and its posterity.

    • Alex Tolley December 25, 2021, 12:59

      The anti-METI (stealth) argument assumes that ET will only respond if they detect something technological from Earth. If they are anything like us, then our Age of Exploration did not depend on detecting indigenous populations. Pacific islanders could not have avoided contact by being quiet on their terms. If ET is out there, and has any interstellar travel capability, they might well visit Earth regardless. Their visitations may never coincide with humans at all, let alone our current technological civilization. We already know what our real existential threats are. The probability of an alien invasion is vanishingly small.

      • Andrei December 26, 2021, 7:23

        The potential risk of alien invasion is non-existent. The risk of having a world that will do a preventive strike with impactors or even destablilise the central star for any of any civ that do not meet their criteria cannot be ruled out. We’re not very likely to qualify for being peaceful, nice etc. Rather greedy, xenophobic and even a threat even to members of our own species. Add that we might fail on meeting any of other criteria one not even have thought about.
        ‘Greetings most revered official of peace love and eternal cosmic wisdom. This new system we found where our scopes show cities like military camps where they live like in one anthill. We listened in and all our worst fears was confirmed. Their space activities are almost all military, and just listen to this music – it’s all percussion.’
        “The non loving meat eaters? Holiest cow! You are right Mr Peaceful flower meadow. Percussion mean military rallying, we need to sterilize this one right off by tilting the energy production in their primary, launch up the degenerate matter generator!”

        • Alex Tolley December 26, 2021, 11:46

          The risk of having a world that will do a preventive strike with impactors or even destablilise the central star for any of any civ that do not meet their criteria cannot be ruled out.

          True, we cannot rule it out. However the probability that we get hit with an asteroid is far higher due to:
          1. We make a mistake doing an asteroid deflection mission
          2. Deliberate sabotage of such a deflection mission, as a subplot in Clarke’s The Hammer of God.
          3. Deliberate targeting of Earth as a plot by Marco Inaros’ belter war against Earth.

          Human agency that we have a deep history and certainty of, is far more probable than some possible, but unknown aliens. We have ample examples of human maliciousness and stupidity that could initiate an existential crisis, rather than speculate about about a risk that may not even exist, however fun it may be, and fictional versions get paying eyeballs to view.

        • Mike Serfas December 26, 2021, 12:31

          Alien invasion could make perfect sense, because “sense” is an *extremely* malleable notion. When quantum computing cracks all the Bitcoin passwords, what do you suppose the new proof-of-work for Starcoin will be? Picture debtors and protestors, oddballs and undesirables (such as members of backward species), rounded up mind-chipped and digitized/quantumized, their minds scanned, towers of torture the length of Ganymede churning away at the atomic level, billions of years a second, generating unfalsifiable tokens collecting all their sufferings as NFTs to be tossed on the baccarat tables of the wealthy and thrown as precious tips to performers of the penultimate social class. Into this mixture, a new planet foolishly toots a clarion call – and the miners are on their way. (I hope not, but you never know)

          • Alex Tolley December 26, 2021, 15:34

            Very Alistair Reynolds/Iain M Banks-like scifi concept.

    • Antonio December 26, 2021, 3:56

      “Wouldn’t arguments in support of transmissional METI be mooted similarly?”

      No. Why?

      “All things being equal, the lesser-risk path seems the more responsible.”

      Things aren’t equal at all. In one path you contact with ET, in the other not.

  • Robin Datta December 25, 2021, 2:48

    If “others” at whatever proximity (or remove) have been observing us, if they are in proximity, it has been with suprb surreptitiousness. If from afar, their technology has to get much of the credit.

    Looking out towards “them”, one question would be how much are they aware of us? Perhaps sone interstellar myrmecologist has documented the emergence of life on this planet aeons ago, and the next review is not due for several more millennia?

  • Ron S. December 25, 2021, 12:17

    Not entirely off topic, it was great to see JWST launch and pass the first of many critical procedures on its way to becoming a working observatory next year. Merry Xmas.

    • Paul Gilster December 25, 2021, 20:48

      Beautiful liftoff, and one that fortifies my admiration for the Ariane 5 and those who work on it. Too bad it was lost so quickly in the clouds on the way up, but what a liftoff!

      • Andrei December 26, 2021, 18:54

        I fully agree, and congratulations to ESA for a perfect launch. While ESA is a minor partner in the telescope itself, providing just a few of the instruments. The critical contribution this time was the launch vehicle. (Canada provide the fine guidance sensor and one spectrograph.)
        All happy that the first critical step have been accomplished, this since it could have been fatal at launch. Now we have to wait quite some months before first light.

  • Michael Fidler December 25, 2021, 22:11

    Solar System Scale Interferometers.
    That would be the easiest to develop not the hardest, individual telescopes can be pointed in any direction and combined to give the highest resolution images of tens of thousands exoplanets. All the others would be one system bets, even solar lens would take decades to move from star systems to star systems.
    ET has been watching us since well into the last ice age and could even use such a system to give us a very nice light show laser display, even some UFO/UAPs may be some advanced holographic display to mimac and educate us… even crop circles! A lot of data and a lot of power to contact without leaving their solar system would make this the best instrument for long duration exploration and study of many many exoplanets.

    • Andrei December 27, 2021, 10:00

      If crop circles would be found everywhere, it might be a phenomenon that required some attention. But they’re not found at all in my country, while most turn up in the Anglosaxian world. Meaning they’re rather is a human cultural phenomena.
      As for UFO, and this the reason I reply to this, is that I got one report only two days ago, and looked into it. The person was happy to learn that it was nothing unearthly at all, but the ISS that had been spotted.

      • Michael Fidler December 27, 2021, 19:41

        About two weeks ago I saw a FB post about the ISS over flight here in the Philippines about two minutes before it happened. I went outside and watched it rise from the NW and just after passing overhead I spotted a satellite passing overhead to the NE and then another and another. A total of 35 satellite passed over from the SW to the NE all about 15 degrees apart for some 20 minutes Yes, StarLink is making for a large number of such reports also.

  • Jean Schneider December 26, 2021, 4:40

    A solar gravitational lens telescope is a bad idea.
    It would not be flexible at all since, at 550 AU, it would take months or even years to go from one target to another.

    On the contrary, a very large hypertelescope (under study by Labeyrie et al.: http://hypertelescope.org/
    https://doi.org/10.1007/s10686-021-09747-3 )
    will indeed make high resolution images of exoplanets.

    Amother approach for transiting planets is an Earth-Moon intensity interferometer:
    It could detect and measure the height of mountains of transiting planets.

    • Brian McConnell December 26, 2021, 21:23

      I agree that a large scale interferometer would be superior because it can look at many targets whereas a SGL telescope is limited to a single target. The purpose of the article was to explore what they’d be able to see via a variety of plausible technologies.

  • Mike Serfas December 28, 2021, 18:33

    If a culture had “sufficiently advanced” computing resources, would it be possible to simply fling a trillion antennae (maybe carbon nanotubes) into random orbits in the Solar system; measure the precise currents many times an attosecond; use a computing network to make a model their orbits/rotations/internal oscillations; do interferometry; and process that absurd volume of data into a stereoscopic view of everything in every direction at every frequency from gamma rays to radio at very high resolution?

    • Alex Tolley December 28, 2021, 23:52

      While it would require a moon or planet made of computronium to process the data, and the stereoscopic image couldn’t be real-time, although predictions might be possible to integrate the data), it is an interesting idea.

      Who was it who provided a link to that mirror made of randomly oriented reflective specks that could be computationally transformed into an image? It was suggested that this approach could be used on natural objects in swarms.

      Seems like your idea is an extension of that concept.

      [I think the antennae need a small amount of sensor and computation to indicate position and direction of maximum sensitivity, but conceptually it might just work.]

      There is always some fascinating imaging work going on. MIT has developed an AI that can create real-time holograms from 2D images and video. Imagine if that can really work how movies could be reconstructed to be 3D. Could all that imaging from planetary rovers be made 3D without needing stereoscopic glasses or viewers? It is a technology that supposedly runs on smartphone CPUs. If so, I hope it eventually becomes a consumer device to play with.

    • NS December 29, 2021, 5:30

      What you’re suggesting sounds like a highly advanced version of this:


      Sadly there doesn’t seem to be any work being done on this project. I don’t know if similar projects are being developed elsewhere.

    • NS December 29, 2021, 5:40

      My mistake. It appears that the Ohio Argus array is still providing data, but that plans for expansion are on hold.

      • ljk December 29, 2021, 12:41

        I hope Argus doesn’t get demolished and turned into a golf course and condos like they did to Big Ear…




        Speaking of other SETI projects…

        Anyone know if The Planetary Society is still running their Optical SETI observatory? Their official page on the subject is not very helpful in that regard:


        I know the area it was in, Oak Ridge, was abandoned and left to rot by Harvard University with hardly a cry from the professional community. What a waste. What a crime.



      • Mike Serfas December 29, 2021, 15:18

        Wow! Thanks for some interesting links and ideas here.

        I found the mirror paper here: https://arxiv.org/pdf/1601.00033.pdf (non-accessible report here: https://ieeexplore.ieee.org/abstract/document/6999482/authors ) An example of this application used craft store glitter. I am not enthusiastic what AIs are using this for in regard to social media or conference video with sparkly backgrounds… but I wonder if you could do the same thing with Earth’s ocean waves as the mirror?

        A comment I might try to disagree with though is the ‘computronium moon’. A distributed telescope of any sort would observe a fairly finite amount of data and might output a fairly finite amount of data. We assume the calculation in the middle is unimaginably complex; but what if there is a way to ‘simplify’ it with quantum technology? I’m thinking https://qiskit.org/textbook/ch-algorithms/quantum-fourier-transform.html smells a little similar to this. If it is possible to reliably send coherent quantum states across the Solar system, maybe this can be done without exceptional expense? However it happens, a continuation of “Moore’s Law” is something spoilsport physicists haven’t ruled out for us.

        • Alex Tolley December 29, 2021, 23:33

          There have been several examples of using ML to reduce computational loads. The MIT real time holography is just the most recent. We have seen ML make fast orbital track determination, and I believe there was work to improve fluid dynamic predictions. Whether this can be applied to constantly changing arrangements of trillions of moving antennae IDK. It will still have to deal with time lags. The data flow may need to be stored for hours to match up time synchronization. Maybe quantum computation will solve the processing problem although there still seems to be issues proving quantum supremacy.

          But it is still an interesting idea. Can it be tested with perhaps 100 – 1000 antennae under lab conditions to determine viability?

  • Tom Mazanec December 30, 2021, 13:22

    I see that a space telescope array the size if the Earth’ Hill Radius could “see” as good as a SGL mission and probably be easier.

  • Stephen Smyth January 2, 2022, 8:30

    Hi all.

    Very interesting and educational article and comments, I’m so glad I came across the blog!

    Admittedly I’m only a very interested amateur but surely evidence of an intelligent, technological species on our planet can only be detected by ET life within a 200-500 light year bubble of Earth? Even intelligence a billion years ahead of ours, living closer to the galactic centre on “our side” of the Milky Way, can only see our last ice age if they are looking at us today?

    • Brian McConnell January 5, 2022, 19:59

      If they can image the Earth at resolutions of ~1 meter / pixel, they would be able to see the effects of urbanization and agricultural development going back millennia. That would put the radius of detectability out to a few thousand light years, not trans galactic distance, but a much greater volume of space than 50-100 light years, the assumed distance early radio signals might have been picked up.

  • Wojciech Jacyk January 11, 2022, 10:19

    A very well written review of the methods of detection. It is becoming rather clear that ideas about Dark Forest theory and warnings about METI are completely flawed. This is also puts radio SETI in doubt, as an advanced civilization would know about Earth-like planets, biospheres and potential civilization quite early using its own observations rather than rely on passive listening.

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