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Detection Possibilities for Optical SETI

The Laser SETI campaign we looked at on Friday is one aspect of a search for intelligent life in the universe that is being addressed in many ways. In addition to optical methods, we look of course at radio wavelengths, and as we begin to characterize the atmospheres of rocky exoplanets, we’ll also look for signs of atmospheric modification that could indicate industrial activity. But we have to be careful. Because SETI looks for evidence of alien technology, it is a search for civilizations about whose possible activities we know absolutely nothing.

So we can’t make assumptions that might blind us to a detection. Getting the blinders off also means extending our reach. If successful, the Laser SETI project will do two things we haven’t been able to do before — it will scan the entire sky and, because it is always on, it will catch optical transients we are missing today, and tell us whether any of these are repeating.

In radio terms, think of the famous WOW! signal of 1977, detected at Ohio State University’s Big Ear radio telescope. Seeming to come out of the constellation Sagittarius, it fit our ideas of what an extraterrestrial signal could look like, but we can’t draw any conclusions because we’ve never seen it again. If the signal intrigues you, Robert Gray’s book The Elusive WOW (Palmer Square, 2011) goes into it in great depth, including Gray’s 1987 and 1989 attempts to find it. Gray would search again in the mid 90’s using the Very Large Array, and again in 1999 with the University of Tasmania’s Mount Pleasant Radio Observatory, with null results.

The Elusive WOW is a splendid page-turner that captures the drama of the hunt. It also reminds us how frustrating a transient can be — here today, gone in moments, never seen again. Did the WOW signal reappear at some time that we weren’t pointing our instruments at it? Is it repeating on some schedule we haven’t figured out?

All-sky surveys like Laser SETI weren’t on the mind of Giuseppe Cocconi and Philip Morrison when they wrote their ground-breaking paper “Searching for Interstellar Communications” in Nature (1959), one that is mostly commonly cited as launching SETI. But for optical SETI’s origins, we can look back with equal admiration at R. N. Schwartz and Charles Townes’ “Interstellar and Interplanetary Communication by Optical Masers,” which ran two years later in the same journal. The author’s vision encapsulates the idea:

We propose to examine the possibility of broadcasting an optical beam from a planet associated with a star some few or some tens of light-years away at sufficient power-levels to establish communications with the Earth. There is some chance that such broadcasts from another society approximately as advanced as we are could be adequately detected by present telescopes and spectrographs, and appropriate techniques now available for detection will be discussed. Communication between planets within our own stellar system by beams from optical masers appears a fortiori quite practical.

Image: Charles Hard Townes, at the National Institute of Biomedical Imaging and Bioengineering’s 5th Anniversary Symposium, held in June 2007. Credit: NIBIB.

Optical SETI Scenarios

We saw Friday that a petawatt laser of the kind that has been built at Lawrence Livermore National Laboratory could be transformed into an optical SETI beacon, working in conjunction with a huge mirror like that found on our largest telescopes. Indeed, the Sun can be outshone by a factor of 10,000, a bright and, one would assume, obviously artificial beacon. But the complexities involved in targeting another star — and aiming the beam to lead the moving target, one that will be many light years away, make targeted laser beacons difficult.

Surely the challenges of laser beacons — not to mention their cost — could be overcome by advanced civilizations, although the idea of a less targeted beacon seems to make more sense; i.e., a beacon that sweeps a region of the sky on a recurrent basis, assuming the intent here is simply to announce the presence of the extraterrestrial civilization as widely as possible. But perhaps it’s much more likely that, if we do detect a laser signal from another civilization, it will be in the form of a chance interception of a technology at work.

Image: The power of laser technology even today. Credit: Eliot Gillum/SETI Institute.

Detecting communications within an exoplanetary system presents serious problems of geometry, given that these optical beams would be broadcast to specific targets and are unlikely to be pointing by chance at the Earth. But there is a scenario that could work: We’ve learned all about exoplanet detection through planetary transits from the Kepler mission. A planetary system that was co-planar with our own could produce a communications beam between its own planets that swept past us with each orbital revolution. Even then, the target planet would likely absorb enough of the signal that detection would be unlikely.

But there are other kinds of detections. James Guillochon and Abraham Loeb have looked at the possibility that beaming to interstellar sailcraft would produce leakage that might be observable to our detectors (see SETI via Leakage from Light Sails in Exoplanetary Systems). Both interplanetary as well as interstellar transportation systems leave possible signatures.

And consider Boyajian’s Star (KIC 8462852), whose odd light curves drew it to the attention of citizen scientists at the Planet Hunters project and subsequent worldwide scrutiny. Numerous natural phenomena have been put forward to explain what we are seeing here, but light curves like this could also be the sign of an extraterrestrial civilization working on some kind of massive project (a Dyson sphere inevitably comes to mind, but who knows?)

It made sense, then, to make Boyajian’s Star a SETI target, which is why the SETI Institute used the Allen Telescope Array to search for radio emissions, a two-week survey that produced no evidence of artificial radio signals coming from the system. For more on this investigation, see Jim and Dominic Benford’s Quantifying KIC 8462852 Power Beaming, which analyzed the ATA results at radio wavelengths. But note the following, which summarizes what the Benfords believe would be detectable given the instruments used in the attempt. As you can see, not all detectable signals would come from power beamed, for example, to an interstellar mission. Some of them definitely include applications within the target system:

  • Orbit raising missions, which require lower power, are not detectable at the thresholds of the Allen Array.
  • Launch from a planetary surface into orbits would be bright enough to be seen by the 100 kHz observations. However, the narrow bandwidth 1 Hz survey would not see them.
  • Interplanetary transfers by beam-driven sails should be detectable in their observations, but are not seen. This is for both the narrow 1 Hz and for the “wideband” 100 kHz observations.
  • Starships launched by power beams with beamwidths that we happen to fall within would be detectable, but are not seen.

Image: Power beaming to drive an interstellar lightsail. Credit: Adrian Mann.

But let’s move back into the optical. Nate Tellis (UC-Berkeley) recently worked with astronomer Geoff Marcy to analyze Keck data archives on 5,600 stars observed between 2004 and 2016, using a computer algorithm fine-tuned to detect laser light (see A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars,” preprint here). The search was an excellent way to put thousands of hours of accumulated astronomical data to work — who knows what discoveries may lurk within such datasets? As a part of the effort, the astronomers studied Boyajian’s Star, again finding no detectable signals. Potential candidates that did emerge in the survey all turned out to be the result of natural processes.

But power beaming is a possible observable as any local civilization goes about moving things around in its own system. Leakage from a beamed power infrastructure is something we’ve focused on here frequently (see, for example, Power Beaming Parameters & SETI re KIC 8462852). Power beaming could be what enables a space-based infrastructure, one that would be capable of large-scale engineering and also of producing the kind of power beams that could drive spacecraft at high velocity to other stars.

But we needn’t exclude communications entirely. Jim Benford has pointed out that any civilization using large-scale power beaming would be aware that its activities could be visible to others. If it had the desire to communicate on such a random basis, the ETI civilization could embed a message within the beam. A kind of interstellar message in a bottle, thrown into the cosmic sea with each sweeping power beam that does local work.

All of this should reinforce the key issue that the Laser SETI project addresses — such beams, working within their own planetary system, would appear in our sky as transients. We return to the core issue, the need for an all-sky survey that observes continuously. Making no assumptions about any desire to communicate, such a survey nonetheless is capable of spotting the signs of a working civilization going about its business. It should, I would wager, also pick out new astrophysical phenomena that will add to our knowledge of the galaxy.


{ 34 comments… add one }
  • Tom Mazanec July 17, 2017, 14:23

    Since transients exist in radio (Fast radio bursts) and gamma (gamma ray bursts), it seems likely their would be transients in the optical between them.

  • Brenda Kalt July 17, 2017, 15:35

    Phys.org contained a report that the WOW! signal had been identified. Here is the first paragraph (search “WOW signal”):

    A team of researchers with the Center of Planetary Science (CPS) has finally solved the mystery of the “Wow!” signal from 1977. It was a comet, they report, one that that was unknown at the time of the signal discovery. Lead researcher Antonio Paris describes their theory and how the team proved it in a paper published in the Journal of the Washington Academy of Sciences.

    Read more at: https://phys.org/news/2017-06-wow-mystery-space.html#

    A beautiful mystery just got solved!

    • Paul Gilster July 17, 2017, 15:38

      I’m afraid the comet theory is highly controversial and not widely accepted.

    • ljk July 18, 2017, 9:28

      Astronomers are picking apart the recent claim of comets causing the Wow! Signal of 1977, and with good reasons:


      To quote:

      “Regarding the criticisms of his theory, the unflappable Paris is not concerned. “I suspect that SETI, who have used the Wow! signal as a source of revenue, are nervous,” he says. However, this is disputed by the fact that Shostak and his fellow scientists in the SETI community have frequently talked down the importance of the Wow! signal, pointing out that their experiments find signals like the Wow! signal, in that they are transient and never seen to repeat, all the time.”

      As usual, everyone has more than one agenda when it comes to aliens, especially if they want to take them out of the equation. Not only are Paris’ credentials rather suspect, he is or was apparently part of some UFO group. They tend to prefer their aliens coming right to our doorstep, rather than sending us long-distance messages.

      Humans either want their aliens to save us or they make convenient boogeymen. This is likely just more projections of our own ignorance and fears upon the Cosmos, which definitely could not care less about what humans want or think. And as with creationists, the cloak of science is ironically being used to make the idea look legit. Sadly the science-ignorant mainstream media is eating it up.

      The one good thing out of this is that most of the general public will forget this matter in due course, if they even bother to read about it at all with all the other nonsense going on across this planet right now. The real scientists will hopefully continue to do what they do best, which this Paris person seems to be having more than a little trouble with, further shedding light on the strong possibility he has an agenda that has squat to do with real science. Whenever someone starts claiming persecution and such by the science community, you can be pretty sure what we have going on here is pseudoscience cult behavior.

      An excellent scientific tearing apart of this comet issue here:


  • Harry R Ray July 17, 2017, 16:28

    Strange radio signals detected in the vicinity of Ross 128(if you remember,this is one of 3 stars being monitored by the Red Dots campaign)by the Aricebo Radio Telescope on May 22! Re- observed yesterday. Results to be presented by the end of the week, but the rumor mill is buzzing that the signals HAVE been detected again, proving that they are of astronomical origin, instead of Earthly interference(high altitude orbiting spacecraft)which was the prevailing theory. Still probably of natural origin, but ET NOT ruled out YET>

    • Zachary July 18, 2017, 0:11

      Harry, where are you seeing the rumor mill buzz? I want to check it out.

      This is so interesting.

      Great posts as always, Paul, thank you.

  • Dale Jacobs July 17, 2017, 19:14

    Excellent review of possible contact methods. I continue to think about how gravitational waves or even multi dimensional methods might be used? Distance and time might not a travel problem with extra dimensional communications? Where am I going with this? Think about a billion year old civilization? Where might such a civilization have taken the ‘laws of physics’?

  • Haxo Angmark July 17, 2017, 20:42

    blasting out high-power laser signals just to “announce ourselves” would be the height of folly. You do not want to attract the attention of a Conquistadore-type sector civilization. It would crush us like ants, perhaps sparing a few specimens for the Galactic Zoo. And then strip-mine Urth.

    • Wojciech J July 18, 2017, 6:29

      “blasting out high-power laser signals just to “announce ourselves” would be the height of folly. You do not want to attract the attention of a Conquistadore-type sector civilization.”

      Our biosphere does this for us. Any advanced civilization would be able to detect life on Earth since eons. The same applies to city lights being detected by possible hypertelescopes. Also probably changes to atmosphere.

      The Earth as a campfire in a dark forest theory is flawed. There is no forest, and any planet with biosphere like ours is visible to its neighbourhood like full moon in the sky at the night.

      • Michael July 18, 2017, 14:12

        There may be many planets out there with life signatures, but if life on one of those planets fires a zillion watt laser it would surely peek their interest ! Let them think we are still swinging through the trees.

    • Shaun July 18, 2017, 8:05

      Just because you are paranoid, doesn’t mean they aren’t out to get you!

      • David Cummings July 18, 2017, 14:17

        Or in the immortal words of Frank Burns:

        “I’m not paranoid! You’re just all out to get me!”

    • ljk July 18, 2017, 9:23

      There are 400 billion stars in the Milky Way galaxy. Thanks to Kepler it has been determined that nearly all suns have planetary systems, which include planetoids and comets. What is not so certain is that every one of them also have life, intelligent or otherwise.

      Does it makes sense to travel all the way across interstellar space just to get something you can probably get right at home? And even if you do need to travel, would you pick a system that is already occupied, even if the natives are no match for you? An extra expensive of time and resources plus danger to occupy an inhabited system.

      Not saying there are not potentially dangerous ETI out there. If anything, humans might be the most dangerous conquering species of them all. But there are some things people worry about when it comes to aliens that with a little logic fall apart outside the venue of a B-grade science fiction story.

      • DJ Kaplan July 18, 2017, 13:28

        It’s presumptuous to assume that we know what would drive an alien civilization to announce itself or not. We are limited to detecting the types of signals that we can, and if we detect them, then we detect them.

        • ljk July 25, 2017, 11:20

          When one method of detection hasn’t delivered the good after six decades, it’s time to check other methods. And we are capable of doing more than just Radio SETI, see here:


  • Andrew Palfreyman July 18, 2017, 5:42

    Just as per the movie Contact, the simplest message indicating artificiality is a prime series encoded in unary, repeating.

    • ljk July 18, 2017, 13:49

      For our way of thinking, yes, but alien minds may assume something else is universal and patently obvious to intelligent minds. Even mathematics are not done the same by every human culture.

      When the Jesuits went to the New World to convert the Native Americans around the Canadian side of the Great Lakes in the 17th Century, they assumed based on logic from Saint Thomas Aquinas and Saint Augustine of Hippo that all rational human minds would eventually come to the Western view of God, they just might need a little guidance.

      Turned out the natives had their own way of thinking about the supernatural et al, and guess what, it did not match up with the Jesuits preordained logic at all. See more here:


    • ljk July 18, 2017, 13:55

      And here is a fascinating look at how some human languages actually affect the way some societies think about the flow of time as circular and not linear:


      If this sounds familiar, think of the film Arrival.


  • Wojciech J July 18, 2017, 6:37

    “Jim Benford has pointed out that any civilization using large-scale power beaming would be aware that its activities could be visible to others. If it had the desire to communicate on such a random basis, the ETI civilization could embed a message within the beam. A kind of interstellar message in a bottle, thrown into the cosmic sea with each sweeping power beam that does local work. ”

    There is a fundamental error in this thinking. Such a civilization would already have capacity to detect probable life-bearing worlds in its cosmic neighbourhood and as such wouldn’t have to resort to beaming messages at random.
    Another option is that that left-over beacons would exist pointing out to sites with stored cultural information and data by civilizations fearing they will eventually demise. Sort of a time capsules on galactic scale. But would civilizations capable of building such works really be afraid of vanishing ?

    I think the final resolution of this problem will be that life is rare in the universe, while intellect and technological civilization exceedingly unique and differences in both time and distance make communication on equal basis impossible. Thus it is avoided. I wouldn’t though go against the possibility that we do occasionally glimpse some left over activity of Kardashev scale civilizations that have passed away at least into inactivity.

    A fascinating time, as we finally are getting tools to test our theories even if it they are still at the very initial stage of development.

    • Haxo Angmark July 18, 2017, 16:01

      “…time capsules on a galactic scale”. Cf. Larry Niven’s Tales of Known Space novels, re artifact-containing “Slaver stasis boxes” left by an extinct sector civilization. Given the billions of years our galaxy has been in existence, and the overwhelming distances between a scattering of high-tech civilizations at any given point in time, we might be more likely to find something like this – or some other long-lasting artifact – rather than detect a signal.

      • NS July 19, 2017, 19:50

        I don’t know if you’re a Star Trek fan, but there’s an episode of the animated series that sounds like it was based on the novels you mention (which unfortunately I haven’t read). Members of the Enterprise crew find an ancient “stasis box” containing a “slaver weapon”, an artifact of a war that once destroyed all intelligent life in the galaxy. The crew is then captured by an enemy that tries to use the weapon, with devastating results.

        • ljk July 20, 2017, 13:54

          Yes, written by Larry Niven and adapted from his story “The Soft Weapon”.

          While the animated Star Trek series was not perfect and did not get its initial due credit, in large part because Gene Roddenberry wrongly called them not canon, some episodes were very well done and as canon as anything else done in the franchise since. And they used most of the voices of the original ST as well as script writers, so again I don’t quite know how or why Roddenberry did not give them proper credit – unless he wasn’t making enough money off them.

          You can thank the animated series for revealing what the T stands for in Kirk’s middle name.

          The details here:


          And yes, if we want to preserve our culture for distant eras, putting multiple time capsules across deep space are our best option. This is the fortieth anniversary of the Voyager Interstellar Records, after all.


          • NS July 20, 2017, 19:57

            Thanks for the Memory Alpha link. A very interesting account!

            • ljk July 21, 2017, 9:48

              You are welcome. Memory Alpha has ridiculous amounts of information on just about everything in the Star Trek franchise.

        • Haxo Angmark July 21, 2017, 0:31

          no, not a trekkie. I found the Rainbow propaganda off-putting. Also, Roddenberry stole the basic concept from the 1950’s J. Vance novel To Live Forever: which includes, as a secondary character, an intersteller planet “Locater” whose ship is named the…Star Enterprise. Credit where credit is due, please.

        • ljk July 21, 2017, 13:55

          Here is a new reason why we need to preserve our knowledge and culture and get it off this planet for genuine safe keeping:


  • ljk July 18, 2017, 13:08

    Let us not have Ross 128 go under the radar – or radio, or laser beam, as it were:

    ‘Peculiar’ radio signals emerge from nearby star


    The Strange Radio Signals Coming From a Nearby Star

    Astronomers have detected a mystery transmission at a frequency they haven’t observed before.


  • Michael July 20, 2017, 8:42

    Just thinking about how we could ‘stop’ or slow down another alien civilisation development.

    If we had very powerful free electron x-ray/gamma lasers at the solar focal point we could in theory render their home planets atmosphere smoggy creating a global winter. It could also be enough to blind their satellites in orbit or ships heading our way without their knowledge.

    • ljk July 20, 2017, 13:58

      And why would we want to do that? And why should we assume they would not respond in kind or worse if we did such a thing to them?

      Want to slow down or grind a culture to a halt? Just give them incompetent, greedy leaders and keep the public undereducated, that seems to work quite well on us.

      • Michael July 21, 2017, 6:32

        These stellar corridors are going to become very important for our expansion into space not only with communication and transport but also our defence. If we accelerate sail discs from the solar focal line they could create very dangerous projectiles to impact hostile craft along these stellar highways.

        • ljk July 21, 2017, 10:08

          While it is naturally bothersome to think that our first interstellar probes might also serve as weapons against enemies we do not even know exist yet, we should also assume again that what we can conceive of, an even more advanced mind may have already done so as well.

          That being said, even I have considered the possibility that should the nuclear-pulse propulsion vessel Orion ever become a reality and run into trouble while exploring the galaxy, they could consider using some of their fuel supply – the nuclear fission bombs – as defensive weapons. Again, smart ETI who would already be traversing the stars may have already considered such things too.

          I would rather not make our first forays into the Milky Way be offense ones in several senses of the word, unless we know for certain in advance we do not live in a nice neighborhood. All the more reason to really ramp up SETI.

  • ljk July 20, 2017, 8:47

    Charles Townes original paper on Optical SETI from 1961:


    Had the radio SETI folks not dominated the conversation for decades, we could have been looking for optical signals for as long as we have radio ones, sporadic as it would have been.

    Who here knows that the Soviet Mars 7 probe was doing Optical SETI in 1974?! Sure its lander missed the Red Planet completely, but it was still able to search for aliens.

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