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A Neutrino Beam Beacon

If you want to look for possible artifacts of advanced civilizations, as do those practicing what is now being called Dysonian SETI, then it pays to listen to the father of the field. My friend Al Jackson has done so and offers a Dyson quote to lead off his new paper: “So the first rule of my game is: think of the biggest possible artificial activities with limits set only by the laws of physics and look for those.” Dyson wrote that in a 1966 paper that repays study today (citation below). Its title: The Search for Extraterrestrial Technology.” Dysonian SETI is a big, brawny zone where speculation is coin of the realm and the imagination is encouraged to be pushed to the limit.

Jackson is intrigued, as are so many of us, with the idea of using the Sun’s gravitational lens to make observations of other stars and their planets. Our recent email conversation brought up the name of Von Eshleman, the Stanford electrical engineer and pioneer in planetary and radio sciences who died two years ago in Palo Alto at 93. Eshleman was writing about gravitational lensing possibilities at a time when we had no technologies that could take us to 550 AU and beyond, the area where lensing effects begin to be felt, but he saw that an instrument there could make observations of objects directly behind the Sun as their light was focused by it.

Claudio Maccone has been working this terrain for a long time, and the complete concept is laid out in his seminal Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer Praxis, 2009). There is much to be said about lensing and space missions, and it’s heartening to see interest in scientists within the Breakthrough Starshot project — a sail moving at 20 percent of lightspeed gets us to 550 AU and beyond relatively quickly. By my back of the envelope figuring, travel time is just a little short of 16 days.

There would be no need for Starshot to approach 550 AU at 20 percent of c, of course. The focal line runs to infinity, but as Jackson explains when running through gravitational lensing’s calculations, we can assume beam intensity gradually diminished by absorption in the interstellar medium, though all of this with little beam divergence. Just how to use the Sun’s gravity lens (a relay for returning data from a star mission, I assume) and how to configure mission parameters to get to the lensing region and use it are under debate.

Transmitting Through a Gravitational Lens

But back to Al Jackson’s paper, which offers us a take on gravitational lensing that I have never before encountered [see my addendum at the end of this post for a correction]. What he is proposing is that an advanced civilization of the kind Dyson is interested in would have the capability of using a gravitational lens to transmit data. He’s turned the process around, from observation to beacon or other form of communication. And he’s working with neutrinos, where attenuation from the interstellar medium is negligible.

A gravitational lens does not, of course, need to be a star, but could be a higher mass object like a neutron star. In Jackson’s thinking, a KII civilization could place a neutrino beam transmitting station around a neutron star. We make neutrino beams today via the decay of pi mesons, as the author reminds us, when large accelerators boost protons to relativistic energies that strike a target, producing pions and kaons that decay into neutrinos, electrons and muons. What counts for Jackson’s purposes is that pions and kaons can be focused to produce a beam of neutrinos.

For a stellar mass gravitational lens and 1 Gev neutrinos, the wavelength is about 10-14 cm, the gain is approximately 1020! The characteristic radius of main region of concertation is about one micron; however there is an effective flux out to about one centimeter.

And as mentioned above:

This beam intensity extends to infinity only diminished by absorption in the interstellar medium, encounters with a massive object like a planet or star and a very small beam divergence.

Image: This is Figure 6 from the paper. Caption: A schematic illustration of a possible neutrino accelerator-transmitter, the accelerator and lens (nothing to scale). Credit: A. A. Jackson.

A one-centimeter beam creates the problem of focusing on a specific target, one whose phenomenal pointing accuracy could only be left to our putative advanced civilization. Even so, increase the number of transmitters and detection becomes easier. Thus Jackson:

Suppose that a K2 type civilization capable of interstellar flight can reach a neutron star it should have the technological capability to build a beacon consisting of an array of transmitters in a constellation of orbits about the neutron star. Let this constellation consist of 1018 ‘neutrino’ transmitters 1 meter in characteristic size ‘covering’ the area of a sphere 1000 km in radius with 1018 particle accelerators in orbit… At the present time there is the development of plasma Wakefield particle accelerators that are meters in size [21, 22]. It is probable that a K2 civilization may construct Wakefield electron accelerators of very small size.

Jackson has heeded Dyson, that’s for sure. Remember the latter’s injunction: “…think of the biggest possible artificial activities with limits set only by the laws of physics and look for those.”

What emerges is a ‘constellation of neutrino beam transmitters,’ 1018 in orbit at 1000 neutron star radii, increasing the probability of detection, so that the detection rate at 10,000 light years becomes approximately 5 per minute. The transmitters must be configured to appear as point sources to the gravitational lens, again another leap demanding KII levels of performance. But if a civilization is trying to be noticed, a neutrino beam that takes neutrino detection well out of the range produced through stellar events in the galaxy should stand out.

Thus we have a method for producing directed neutrino signal transmission. Why come up with such? Harking back to Dyson, we can consider the need to examine the range of possible ETI technologies, hoping to create a catalog that could explain future anomalous observations. Jackson’s beam could be used as what he calls a ‘honey pot’ to attract attention to an electromagnetic transmitter broadcasting more sophisticated information. But we would not necessarily be able to understand what uses an advanced civilization would make of such capabilities. We might have to content ourselves merely with the possibility of observing them.

As Jackson points out, a KII civilization “…would likely have the resources to finesse the technology in a smarter way,” so what we have here is a demonstration that a thing may be possible, while we are left to wonder in what other ways a neutrino source can be used to produce detections at cosmic distances. Going deep to speculate on technologies far beyond our reach today should, as Dyson says, remind us to stay within the realm of known physics while simultaneously asking about phenomena that advanced engineering could produce. We hope through the labors of Dysonian SETI to recognize such signatures if we see them.

Appreciating Von Eshleman

And a digression: When Al brought up Von Eshleman in our conversations, I began thinking back to early gravitational lensing work and Eshleman’s paper, written back in 1979 but prescient, surely, of what was to come in the form of serious examination of lensing capabilities for space missions. Let me quote Eshleman’s conclusion from the paper, cited below. I have a lot more to say about Eshleman’s work, but let’s get into that at another time. For now:

It has been pointed out that radio, television, radar, microwave link, and other terrestrial transmissions are expanding into space at 1 light-year per year (2). Another technological society near a neighboring star could receive the strongest of these directly with substantial effort and could learn a great deal about the earth and the technology of its inhabitants. The concepts presented here suggest that on an imaginary screen sufficiently far behind that star, the short-wavelength end of this terrestrial activity is now being played out at substantial amplifications. Properly placed receivers with antennas of modest size could in principle scan the earth and discriminate between different sources, mapping such activity over the earth and learning not only about the technology of its inhabitants, but also about their thoughts. It is possible that several or many such focused stories about other worlds are now running their course on such a gigantic screen surrounding our sun, but no one in this theater is observing them…

I learned about Eshleman’s work originally through Claudio Maccone and often reflect on the tenacity of ideas as we go from a concept originally suggested by Einstein to a SETI opportunity realized by Eshleman to a mission concept detailed by Maccone, and now a potential actual mission seriously discussing using gravitational lensing as a relay opportunity for data return from Proxima Centauri (Breakthrough Starshot). As it always has, the interstellar field demands long-term thinking that crosses generations in support of a breathtaking goal.

[Addendum]: Although I hadn’t heard of discussions on using gravitational lensing for transmission, an email just now from Clément Vidal points out that both Claudio Maccone and Vidal himself have looked into this. In Clément’s case, the reference is Vidal, C. 2011, “Black Holes: Attractors for Intelligence?” at Towards a scientific and societal agenda on extra-terrestrial life, 4-5 Oct, Buckinghamshire, Kavli Royal Society International Centre. Abstract here. This quote is to the point:

“For a few decades, researchers have proposed to use the Sun as a gravitational lens. At 22.45AU and 29.59AU we have a focus for gravitational waves and neutrinos. Starting from 550AU, electromagnetic waves converge. Those focus regions offer one of the greatest opportunity for astronomy and astrophysics, offering gains from 2 to 9 orders of magnitude compared to Earth-based telescopes…It is also worth noting that such gravitational lensing could also be used for communication…Indeed, it is easy to extrapolate the maximal capacity of gravitational lensing using, instead of the Sun, a much more massive object, i.e. a neutron star or a black hole. This would probably constitute the most powerful possible telescope. This possibility was envisioned -yet not developed- by Von Eshleman in (1991). Since objects observed by gravitational lensing must be aligned, we can imagine an additional dilating and contracting focal sphere or artificial swarm around a black hole, thereby observing the universe in all directions and depths.”

The author of The Beginning and the End: The Meaning of Life in a Cosmological Perspective (2014), Vidal’s thinking is examined in The Zen of SETI and elsewhere in the archives.

I’m glad Clément wrote, especially as it jogged my memory about Claudio Maccone’s paper on using lenses as a communications tool, where the possibilities are striking. See The Gravitational Lens and Communications for more. I wrote about this back in 2009 and thus have no good excuse for letting it slip my mind!

The paper is Jackson, “A Neutrino Beacon” (preprint). The Dyson paper is “The Search for Extraterrestrial Technology,” in Marshak, ed. Perspectives in Modern Physics: Essays in Honor of Hans Bethe, New York: John Wiley & Sons 1966. The Eshleman paper is “Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances,” Science Vol. 205 (14 September 1979), pp. 1133-1135 (abstract).


{ 40 comments… add one }
  • Robin Datta May 17, 2019, 15:23

    Setting up gravitational lensing receivers about the sun’s focal point to use these natural phenomena would leverage modest reception capabilities by so many orders of magnitude that it would rationally be a high-priority goal. It offers not only a possibility of resetting Fermi’s paradox, but also a close examination of regions of the cosmos that would otherwise be well beyond the grasp of the technology.

  • Michael May 17, 2019, 15:43

    Gamma ray communications would be better.

  • Geoffrey Hillend May 17, 2019, 16:20

    Neutrino beams are very stealthy and hard to detect. It might be better to use light or radiowaves to be sure it could easily be detected by another civilization in another star system. It might take a lot of energy to make neutrino communication device so that one might as well use conventional radio waves or lasers.

    • Al Jackson May 17, 2019, 20:54

      I covered beamed electromagnetism in another paper:
      Black Hole Beacon: Gravitational Lensing
      JBIS, 68, pp.342-346, 2015

      • Jeff Wright June 7, 2019, 13:36

        I wonder if the WOW signal came from Earth passing through a focal line…

  • Alex Tolley May 17, 2019, 17:06

    Neutrino beams seem rather like a solution in search of a problem. Detection currently is very hard, so it isn’t clear to me how useful this would be as a “honey pot”. With the post on high velocity astronomy indicating the value of Doppler shifting IR signals into the visible light, one might think that the best strategy is to make signals visible. Monochromatic laser light on an intensity that makes a spectral analysis of a star show a high amplitude signal in a specific wavelength seems far more appropriate. Pulsing the signals with either AM or FM is simple ans a communication medium. By all means use gravitational focusing to increase the signal strength at either or both transmitter and receiver, but using hard to detect particles seems a stretch.

    The other issue I personally quibble with is Dyson’s suggestion of maximizing size. Why should we expect civilizations to expend economic resources just to create large structures or devices? I understand that this implies that our detection of ETI by those that want to make themselves noticed is made easier, and possibly explain phenomena that seem to have no natural explanation. However it seems like a fetish to assume brute force when more effective, lower cost methods could be equally effective. I am struck by how limited our terrestrial economy can grow from here. Expansion into space by some as yet to be determined drive could get us to KII status eventually, but I wonder why we must assume that is the natural direction for ETI to take. It assumes that they can and will expand their civilization in that way, even though that expansion in turn is highly limited too. We don’t build fleets of steam ships to carry physical communications, but rather use far more efficient electronic means. Once we thought we needed crewed space stations to handle orbital communications relays, although it turned out smaller satellites were a better solution, and currently swarms of tiny satellites are being deployed do the same thing. Similarly, rather than building huge star ships like Daedalus, we are now thinking is far more affordable tiny craft to may the early exploratory visits. Even a craft as large as Cassini would be far cheaper to send that a Daedalus sized vehicle. For beacons, rather than the huge energy requirements for omni-directional transmission, proposals for directed beams cycling through targets for short periods makes far more sense for any civilization at a cost of somewhat less obvious nature. IMO, gravitational lensing meets the economic directive, whereas using neutrino beams seems to squander that advantage.

    The recent imaging of a black hole using many terrestrial telescopes is definitely in the smarter, more affordable category, than trying to build a vast telescope with similar collecting power and resolution. I hope we see far more of these innovative approaches, simply because I think we may well be reaching the spending limits due to the limits of our global economy that are already in sight. I hope we are not limited to KI status, but I think we will get a lot more done assuming that as a constraint, rather than waiting for a massive expansion of a national or global economy to support huge projects to manage objectives.

    • Al Jackson May 17, 2019, 20:50

      I agree, Dyson ‘Sphere’ is a Malthusian artifact* so to speak. It does really seem ‘brutish’ … when one thinks of an advanced civilization that can command the sophisticated instrumentality to build an astronomical sized artifact I think of a civilization that could do a Clarke-ian finesse of its energy needs. To me , now days, I think of Kardashev scaling as a measure of cleverness beyond anything we can imagine. K2 could still mean mastering a solar energy output but done with an ingenuity we can’t imagine. After all they had to survive their adolescence with a cleverness that we don’t seem to have!
      *One notes , Dyson put this forward as a thought experiment.

      • ljk May 20, 2019, 10:10

        Perhaps the main problem is that Dyson Spheres (should really be called Shells or Swarms) are still being viewed as dwelling places for presumably organic beings like us.

        Then at some point, Freeman Dyson’s original idea of a swarm of small habitats circling Sol was changed at some point into a vast solid sphere surrounding the star. Was this done on purpose, or did someone misinterpret Dyson’s original intentions from 1960, or a bit of both?

        Others such as Robert Bradbury envisioned Dyson Shells/Swarms not as habitats for organics but as entities in their own right, with computing power abilities that make all our combined efforts to date look like a joke.

        What are they calculating/thinking about? I suppose if I knew I would not be here typing this. :^) What I like about the idea is that it goes beyond the original concept into something that is probably more suitable for beings who can manipulate space on a large and long-term scale.

        Other purposes for Dyson Shells include as a concentrated beam device that could push lightsails all over the galaxy – or wipe out galactic targets many light years away. They would also make for amazingly strong and detailed interstellar communications systems.

        I am just trying to think practically here as a starting point. There could certainly be other reasons we would not have a clue about. Think of those big Medieval European cathedrals. A visiting ETI with no preconceived notions about humans might wonder why such cultures would build such massive structures that clearly took a lot of time, money, and resources with no obvious material benefits for its makers – yet built they were, many times over.

        Some reference points, starting here:


        Check the Related Articles section at the end of the above piece.

    • john walker May 18, 2019, 5:33

      “Why should we expect civilizations to expend economic resources just to create large structures or devices?” I suspect KII civilisations will not be bound by our economic strictures. At least locally with advanced AI, our own concept “cost of operations” will no longer be relevant. Assuming the resources for such a neutrino array are available within the system, it seems completely conceivable to me that an otherwise arbitrary neutron star system can be refunctioned autonomously. No conventional expenses save the seed mechanics to get the project started. The primary, virtually sole “cost” becomes time.

      • Alex Tolley May 18, 2019, 7:06

        That sounds suspiciously like “magic pixie dust”. Suppose in a post-scarcity world we can deploy extractive and manufacturing resources that are costless. The extraction and manufacturing still uses planetary resources, depletes ores, uses up other sources like water, pollutes, etc, etc. There is no reason to believe that any KII civ will not be subject to similar constraints. Even a machine civ that does not care about biospheres will eventually deplete its star systems’ resources even with very high, but imperfect recycling. At some point, doing anything new will require giving up something that already exists. On a cosmic time scale, that will happen quite quickly, well within a million years.[ Note that at even just 1% annual growth in energy demand, we harness all our sun’s output within just thousands of years. Useful materals would be depleted more slowly with recycling, but even tiny inefficiencies add up and energy is needed for recycling. KII civs will have steady state economies and will need to make resource use choices within the constraint of their star’s output.]

  • Al Jackson May 17, 2019, 18:24

    I should emphasize , even though I cite them, that Eshleman and Claudio Maccone have noted that a gravitational lens can act as a ‘transmitter’ as well as a telescope.

    Eshleman, Von R., Gravitational lens of the sun: its potential for observations and communications over interstellar distances, Science, Vol. 205, no. 4411 pp. 1133-1135. (1979)

    Claudio Maccone, Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer Praxis Books) Hardcover – 2009

    • Paul Gilster May 17, 2019, 21:20

      Al, I was the one who said I was unaware of earlier work on transmitting via gravitational lens. Even though I had written up Claudio’s work on this very point 10 years back — somehow it slipped my mind. You’re right, you correctly cited the predecessors I should have mentioned in the article.

      • Michael May 18, 2019, 15:16

        10 years back it slipped my mind…you are doing well Paul breakfast to me is a distant memory.

  • Ivan Vuletich May 17, 2019, 21:07

    How would gravitational lensing of gravity waves work? At 22.45 AU that lens point is just beyond Uranus. How much gain would you get for gravity wave lensing from 1 solar mass? Could sending a gravity wave detector to 22 AU be easier than sending a telescope to 550 AU?

    • Andrew Palfreyman May 18, 2019, 1:54

      It might work as an exoplanet detector (I have not done the maths). But as an exoplanet observer it is of no use.

    • Antonio May 18, 2019, 3:59

      To your last question: probably yes. Sending a trio of spacecrafts like LISA to 22 AU should be easier than sending something like Hubble to 550 AU.

      OT: A clever way to search for Planet Nine: https://arxiv.org/pdf/1905.06354.pdf

    • Michael May 18, 2019, 10:16

      We don’t have to go 550 AU to get a solargravilens EM image, we just need to follow the light cone with a few spacecraft. The magnification would be lower and steadily get stronger the further we go out, but the resolution gain would be very good indeed.

    • Michael May 18, 2019, 11:49

      Although the core of the sun aids the bending distance it would also cause distortion as even GW’s would be slowed down moving through the very dense region.

  • Michael Spencer May 18, 2019, 10:53

    What sort of resolution is predicted when using gravitational lensing (GL)? And at what wavelengths? My experience with antennas, gain, and such, is limited strictly to amateur radio and and of course reading about optical devices, chiefly.

    With the sorts of gain often quoted, I am (wrongly?) picturing KII folks walking about on their planet! Or perhaps, more seriously, this technology would extend our view horizon across the universe? Perhaps more to the point, if the focal point is essential limitless along a given line, we ought to find countless examples in the heavens yielding exhibiting research-quality data.

    The issue of gain of course is only one side of a difficult coin; the other side being resolution. Is there an analogy between a ‘telescope’ using GL and the ‘big buckets’ that we use for visible light?

  • Denver May 18, 2019, 13:56

    “After all they had to survive their adolescence with a cleverness that we don’t seem to have!”

    Just to note, we’re still here. We’re still here after surviving 50,000+ fusion warheads all basically aimed at something. That is, happily, not the case at present.

    • Al Jackson May 20, 2019, 8:19

      I love that quote from Contact by Dr. Ellie Arroway, probably written by Carl Sagan in the novel, picked up by the movie.

      Panel member : If you were to meet these Vegans, and were permitted only one question to ask of them, what would it be?
      Ellie Arroway : Well, I suppose it would be, how did you do it? How did you evolve, how did you survive this technological adolescence without destroying yourself?

      • ljk May 20, 2019, 12:32

        Of course this assumes that alien beings evolved on an alien world would still go through our form of technological development. What are the odds of that happening?

        Contact came from the era and group of American SETI pioneers who were focused on how to detect alien technological signals based on what we could do at the time – and largely in radio at that, even though optical was another viable option that was largely ignored until 1998. Those technosignatures that the SETI community is embracing now were given even fewer token efforts than most SETI projects were back then.

        Twentieth Century SETI was looking for versions of ourselves because that was what was easiest, even though others tried to point out the evolutionary unlikely odds of this happening. Star Trek and most other science fiction reinforced this viewpoint.

        See here for the historical details:


        How else might ETI develop elsewhere? Well of course we do not have any actual examples yet, but here is something to consider right in our own terrestrial oceans…


        Humans really need to get past their biases that high intelligence only comes in our form, or only acts in certain ways.

        • Antonio May 20, 2019, 15:59

          We have a similar problem today, with almost all people defining habitability by the presence of liquid water.

          • ljk May 21, 2019, 8:48

            I agree. although of course this does not negate the fact that water is an excellent medium for supporting life. What other mediums might support life in the absence of water?

          • Alex Tolley May 21, 2019, 11:10

            It is even more restrictive than that. It requires the possible presence of [bodies of] liquid water on the surface under the right conditions. A planet where this is not possible, for example, present day Mars (or a hypothetical planet further out, is considered outside the HZ, even if internal heat could maintain water below the surface. Similarly with icy worlds with water beneath an ice crust. All this is due to using Earth as the “proof of existence” model based on our conservative POV.

    • ljk May 20, 2019, 9:41

      While we have fewer nuclear weapons than we did at their peak in 1989, where the USA and USSR had about 55,000 such devices of mass destruction between them, the lower numbers are still more than enough to wreck our civilization and render much of Earth itself uninhabitable for centuries.

      See these latest stats here:


      It is luck that we have destroyed ourselves in this way so far, but now we have more nations than ever opting for dictators to lead them, so we will see how long our luck holds out. And keep in mind that both America and Russia have restarted developing nuclear weapons, with many other nations doing the same. Remember just two rather primitive atomic weapons wiped out two entire cities in Japan in 1945.

      • Antonio May 20, 2019, 16:00

        “and render much of Earth itself uninhabitable for centuries”

        Not really.

        • ljk May 22, 2019, 10:45

          Care to elaborate? Have you read about what the areas in and around Chernobyl are like after that nuclear reactor meltdown in 1986?

  • Michael T May 20, 2019, 6:32

    Fast Neutrino Bursts, anyone?

  • Al Jackson May 20, 2019, 8:28

    A note: This idea was part of a presentation at the Technosignatures workshop last September. I wrote up what I had presented.
    (I didn’t made the deadline for getting this into the report.)
    Paul may have posted this link earlier.

  • ljk May 20, 2019, 9:48
  • ljk May 22, 2019, 10:46

    Curtin scientists use giant telescope on sea floor to study rays from space

    Media release

    Tuesday 21 May 2019

    Curtin University researchers are part of an international project that will use a huge underwater neutrino telescope at the bottom of the Mediterranean Sea to help explain some of the most powerful and mysterious events in the universe.


  • ljk June 3, 2019, 14:52


    “First in, last out” solution to the Fermi Paradox

    Alexander Berezin

    (Submitted on 20 Mar 2018 (v1), last revised 27 Mar 2018 (this version, v2))

    No present observations suggest a technologically advanced extraterrestrial intelligence (ETI) has spread through the galaxy. However, under commonplace assumptions about galactic civilization formation and expansion, this absence of observation is highly unlikely. This improbability constitutes the Fermi Paradox.

    In this paper, I argue that the Paradox has a trivial solution, requiring no controversial assumptions, which is rarely suggested or discussed. However, that solution would be hard to accept, as it predicts a future for our own civilization that is even worse than extinction.

    Subjects: Popular Physics (physics.pop-ph)

    Cite as: arXiv:1803.08425 [physics.pop-ph]
    (or arXiv:1803.08425v2 [physics.pop-ph] for this version)

    Submission history

    From: Alexander Berezin [view email]

    [v1] Tue, 20 Mar 2018 20:13:40 UTC (175 KB)
    [v2] Tue, 27 Mar 2018 16:58:02 UTC (347 KB)


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