A Dyson Sphere makes an extraordinary setting for science fiction. In fact, my first knowledge of the concept came from reading Larry Niven’s 1970 novel Ringworld, a book that left such an impression that I still recall reading half of it at a sitting in the drafty little parlor of a house I was renting in Grinnell, Iowa. Ringworld had just come out as a Ballantine paperback with the lovely cover you see below. I was hooked after about three pages and read deep into a night filled with wind and snow.
It could be argued, of course, that a ring made out of planetary material, a habitat so vast that it completely encircles its star, is actually one of the smaller Dyson concepts. It was in 1960 that Freeman Dyson suggested how a civilization advanced to the point of such astro-engineering might use everything it found in its solar system to create a cloud of objects, a swarm that would make the most efficient use of its primary’s light. And as you keep adding objects, you point to the ultimate outcome, a Dyson Sphere that completely envelopes the star from which it draws its energy.
A Dyson Sphere Search with IRAS
Last April I looked at Dyson spheres in the context of an article by Bruce Dorminey that considered new SETI strategies. Now I see that Richard Carrigan, a retired physicist from Fermilab, has added a new paper to the arXiv site, one that discusses the work reviewed in that earlier story. Carrigan has been examining sources identified by the Infrared Astronomical Satellite (IRAS), the idea being to look for objects that seem to be radiating waste heat in such a way that they might be Dyson Spheres of one kind or another. A fully enveloped star won’t be visible to the eye, but Carrigan’s infrared search covers the blackbody temperature region from 100 to 600 degrees Kelvin for full or partial Spheres.
The data come from an IRAS database that covers 96 percent of the sky and includes some 250,000 sources. Exciting stuff on the face of it, because unlike a conventional SETI search, a hunt for Dyson Spheres involves no necessary intent to communicate on the part of the civilization in question. And when you’re dealing with SETI, the fewer preconceptions you bring to the dance, the better. Here’s the thinking behind Carrigan’s attempt:
For a Dyson Sphere the stellar energy from the star would be reradiated at a lower temperature. If the visible light was totally absorbed by a thin “shell” a pure Dyson Sphere signature would be an infrared object with luminosity equivalent to the invisible star and a Planck or blackbody distribution with a temperature corresponding to the radius of the spherical shell formed by the cloud of objects. For a sun-like star with the shell at the radius of the Earth the temperature would be approximately 300º K.
Sorting the Evidence
A distinct signature? You would hope so, and if that is the case, we can dig through our data practicing what Carrigan delightfully calls ‘cosmic archaeology,’ using data that cover the 8 to 100 micron infrared range needed to study a Dyson Sphere’s emissions under these assumptions. Yet an identification runs into immediate problems, not the least of which is the need to differentiate any candidate from natural sources that show much the same signature. A cocoon of gas and dust around a young star, for example, might mimic an artificial source.
Carrigan goes through the possibilities — protostars, planetary nebulae, dying stars — and weighs their telltale infrared identity against a true Dyson Sphere, with notes on how to tell the natural from the potentially artificial. Here he considers the methods (italics mine):
A Dyson Sphere candidate with a blackbody distribution can have several characteristics such as a blackbody temperature, the distance from our Sun, magnitude in the infrared, and variability. It may also have a stellar signature in the visible or infrared. Slysh (1985) notes, “The confusion between red giants with thick circumstellar envelopes and possible Dyson Spheres in the IRAS survey is a serious problem, and to differentiate the two we need additional data.” …[S]ome of the source types discussed above populate the same region of an infrared color-color plot as a Dyson sphere candidate would. Non-Dyson Sphere objects can be eliminated using discriminants like spectral lines in the infrared or radio regime, implausible blackbody temperatures, established classifications, and statistical departures from a blackbody distribution.
A Dwindling List
So we still have a chance to find a true Dyson Sphere, assuming one or more are out there. If I had more money to burn, I would ring up Tibor Pacher with an offer to make another bet, this one saying that no Dyson Sphere will be found in this century. Tibor is bound to take that one, but I’ve lost several other bets recently and had better put down my cards (our other bet, on the date of the first true interstellar mission, is viewable at the Long Bets site; feel free to comment on either side of that one).
Image: A Dyson Sphere as envisioned by the producers of Star Trek: The Next Generation, from the episode “Relics.” Credit: Paramount Pictures.
Bet or no, the process of working through the database is fascinating, but the list of candidates quickly dwindles in Carrigan’s discussion. We wind up with a scant seventeen possibilities, none of them particularly promising, though worthy of further study. Carrigan comments:
This search has shown that at best there are only a few quasi-plausible Dyson Sphere signatures out of the IRAS LRS sample in the 100 < T < 600 ºK temperature region. This limit includes both pure and partial Dyson Spheres. With several possible exceptions all the "good" sources identified in this search have some more conventional explanation other than as a Dyson Sphere candidate. In spite of the fact that there are many mimics such as stars in a late dusty phase of their evolution good Dyson Sphere candidates are quite rare!
Next Steps
Where do we go from here? Compiling more on the list of seventeen Dyson candidates would be the logical next step (Carrigan discusses how). And we can search further using the more powerful Spitzer Space Telescope, an instrument with greater angular resolution than IRAS and three orders of magnitude better sensitivity in the infrared ranges needed for this work. This would extend the survey out past the center of the galaxy, but we’ll lose some of the IRAS sources, which are too bright for Spitzer’s camera to avoid saturation. And only one of the seventeen candidates Carrigan finds would be covered by such a Spitzer study.
This intriguing work reminds us how early we are in the study of Dyson Spheres, and the broader attempt to identify astro-engineering on this vast scale. The Low Resolution Spectrometer aboard the IRAS satellite was only sensitive enough to track solar-sized Dyson Spheres out to a range of some 300 parsecs, which includes a million solar-type stars. Extending that reach, and finding ways to either rule out or strengthen the case for some of Carrigan’s seventeen candidates, is work that extends our existing radio and optical SETI methods. Beefing up our infrared tools will help us determine whether a concept once considered outrageous might conceivably flag an extraterrestrial presence.
The paper is Carrigan, “IRAS-based Whole-Sky Upper Limit on Dyson Spheres,” available online.
The propulsion technology the human characters conceive in the Netflix version of Liu Cixin’s novel The Three Body Problem clearly has roots in the ideas we’ve been kicking around lately. I should clarify that I’m talking about the American version of the novel, which Netflix titles ‘3 Body Problem,’ and not the Chinese 30-part series, which is also becoming available. In the last two posts, I’ve gone through various runway concepts, in which a spacecraft is driven forward by nuclear explosions along its route of flight. We’ve also looked at pellet options, where macroscopic pellets are fired to a departing starship to impart momentum and/or to serve as fusion fuel.
All this gets us around the problem of carrying propellant, and thus offers real benefits in terms of payload capabilities. Even so, it was startling to hear the name Stanislaw Ulam come up on a streaming TV series. Somebody was doing their homework, as Freeman Dyson liked to say. Ulam’s name will always be associated with nuclear pulse propulsion (along with the Monte Carlo method of computation and many other key developments in nuclear physics). It was in 1955 that he and Cornelius Everett performed the first full mathematical treatment of what would become Orion, but the concept goes back as far as Ulam’s initial Los Alamos calculations in 1947.
Image: Physicist and mathematician Stanislaw Ulam. Credit: Los Alamos National Laboratory.
Set off a nuclear charge behind a pusher plate and the craft attached to that plate moves forward. Set off enough devices and you begin to move at speeds unmatched by any other propulsion method, so the deep space concepts that Freeman Dyson, Ted Taylor and team discussed began to seem practicable, including human missions to distant targets like Enceladus. Dyson pushed the concept into the interstellar realm and envisioned an Orion variant reaching Alpha Centauri in just over a century. So detonating devices is a natural if you’re a writer looking for ways to take current technology to deep space in a hurry, as the characters in ‘3 Body Problem’ are.
Johndale Solem’s name didn’t pop up on ‘3 Body Problem,’ but his work is part of the lineage of the interstellar solution proposed there. Solem was familiar with Ted Cotter’s work at Los Alamos, which in the 1970s had explored ways of doing nuclear pulse propulsion without the pusher plate and huge shock absorbers that would be needed for the Orion design. Freeman Dyson explored the concept as well – both he and Cotter were thinking in terms of steel cables unreeled from a spacecraft as it spun on its axis. Dyson would liken the operation to “the arms of a giant squid,” as cables with flattened plates at each end would serve to absorb the momentum of the explosions set off behind the vehicle. Familiar with this work, Johndale Solem took the next step.
Image: Physicist Johndale Solem in 2014. Credit: Wikimedia Commons.
Solem worked at Los Alamos from 1969 to 2000, along the way authoring numerous scientific and technical papers. In the early 1990s, he discussed the design he called Medusa, noting in an internal report that his spacecraft would look something like a jellyfish as it moved through space. He had no interest in Orion’s pusher plate because examining the idea, he saw only problems. For one thing, you couldn’t build a pusher plate big enough to absorb anything more than a fraction of the momentum from the bombs being detonated behind the spacecraft. To protect the crew, the plate and shock absorbers had to be so massive as to degrade performance even more.
The solution: Replace the pusher plate with a sail deployed ahead of the vehicle. The nuclear detonations are now performed between the sail and the spacecraft, driving the vehicle forward. The sail would receive a much greater degree of momentum, and it would be equipped with tethers made so long and elastic that the acceleration would be smoothed out. I quoted Solem some years back on using a servo winch in the vehicle which would operate in combination with the tethers. Let’s look at that again, from the Los Alamos report:
When the explosive is detonated, a motorgenerator powered winch will pay out line to the spinnaker at a rate programmed to provide a constant acceleration of the space capsule. The motorgenerator will provide electrical power during this phase of the cycle, which will be conveniently stored. After the space capsule has reached the same speed as the spinnaker, the motorgenerator will draw in the line, again at a rate programmed to provide a constant acceleration of the space capsule. The acceleration during the draw-in phase will be less than during the pay-out phase, which will give a net electrical energy gain. The gain will provide power for ancillary equipment in the space capsule…
This is hard to visualize, so let’s look at it in two different ways. First, here is a diagram of the basic concept:
Image: Medusa in operation. Here we see the design 1) At the moment of bomb explosion; 2) As the explosion pulse reaches the parachute canopy; 3) Effect on the canopy, accelerating it away from the explosion, with the spacecraft playing out the main tether with its winch, braking as it extends, and accelerating the vehicle; 4) The tether being winched back in. Imagine all this in action and the jellyfish reference becomes clear. Credit: George William Herbert/Wikimedia.
Second, a video that Al Jackson pointed out to me, made by artist and CGI expert Nick Stevens, shows what Medusa would look like in flight. I recall Solem’s words when I watch this:
One can visualize the motion of this spacecraft by comparing it to a jellyfish. The repeated explosions will cause the canopy to pulsate, ripple, and throb. The tethers will be stretching and relaxing. The concept needed a name: its dynamics suggested MEDUSA.
Bear in mind as you watch, though, that Solem’s Los Alamos report speaks of a 500-meter canopy that would be spin-deployed along with 10,000 tethers. The biggest stress that suggested itself to readers when we’ve discussed Medusa in the past is in the tethers themselves, which is why Solem made them as long as he did. Even so, I became rather enthralled with Medusa early when I first encountered the idea, an interest reinforced by Greg Matloff’s statement (in Deep Space Probes): “Although much analysis remains to be carried out, the Medusa concept might allow great reduction in the mass of a nuclear-pulse starprobe.” With Dyson having given up on Orion, Medusa seemed a way to reinvigorate nuclear pulse propulsion, although to be sure, Dyson’s chief objection to Orion when I talked with him about it was the sheer impracticality of the concept, an issue which surely would apply to Medusa as well.
Like so much in the Netflix ‘3 Body Problem,’ the visuals of the bomb runway sequence are well crafted. In fact, I find Liu Cixin’s trilogy so stuffed with interesting ideas that my recent re-reading of The Three Body Problem and subsequent immersion in the following two novels have me wanting to explore his other work. I haven’t yet attempted the Chinese series, which is longer and presents the daunting prospect of dealing with a now familiar set of plot elements with wholly different actors. I’ll need to dip into it as Netflix ponders a second season for the American series.
Anyway, notice the interesting fact that what you have as a propulsion method on ‘3 Body Problem’ is essentially Medusa adapted to a nuclear bomb runway, with the sail-driven craft intercepting a series of nuclear weapons. As each explodes, the spacecraft is pushed to higher and higher velocities. I’m curious to know how the Chinese series handles this aspect of the story, and also curious about who introduced this propulsion concept, which I still haven’t located in the novels. I’m not aware of a fusion runway combined with a sail anywhere in the interstellar literature. Nice touch!
The Los Alamos report I refer to above is Solem’s “Some New Ideas for Nuclear Explosive Spacecraft Propulsion,” LA-12189-MS, October 1991 (available online). Solem also wrote up the Medusa concept in “Medusa: Nuclear Explosive Propulsion for Interplanetary Travel,” JBIS Vol. 46, No. 1 (1993), pp. 21-26. Two other JBIS papers also come into play for specific mission applications: “The Moon and the Medusa: Use of Lunar Assets in Nuclear-Pulse Propelled Space Travel,” JBIS Vol. 53 (2000), pp. 362-370 and “Deflection and Disruption of Asteroids on Collision Course with Earth,” JBIS Vol. 53 (2000), pp. 180-196. To my knowledge, Freeman Dyson’s ‘The Bolo and the Squid,’ a 1958 memo at Los Alamos treating these concepts, remains classified.
How long before we can send humans to another star system? Ask people active in the interstellar community and you’ll get answers ranging from ‘at least a century’ to ‘never.’ I’m inclined toward a view nudging into the ‘never’ camp but not quite getting there. In other words, I think the advantages of highly intelligent instrumented payloads will always be apparent for missions of this duration, but I know human nature well enough to believe that somehow, sometime, a few hardy adventurers will find a way to make the journey. I do doubt that it will ever become commonplace.
You may well disagree, and I hope you’re right, as the scenarios open to humans with a galaxy stuffed with planets to experience are stunning. Having come into the field steeped in the papers and books of Robert Forward, I’ve always been partial to sail technologies and love the brazen, crazy extrapolation of Forward’s “Flight of the Dragonfly,” which appeared in Analog in 1982 and which would later be turned into the novel Rocheworld (Baen, 1990). This is the novel where Forward not only finds a bizarre way to keep a human crew sane through a multi-decade journey but also posits a segmented lightsail to get the crew home.
Image: The extraordinary Robert Forward, whose first edition of Flight of the Dragonfly was expanded a bit from the magazine serial and offered in book form in 1984. The book would later be revised and expanded further into the 1990 Baen title Rocheworld. The publishing history of this volume is almost as complex as the methods Forward used to get his crew back from Barnard’s Star!
Forward was a treasure. Like Freeman Dyson, his imagination was boundless. Whether we would ever choose to build the vast Fresnel lens he posited in the outer Solar System as a way of collimating a laser beam from near-Sol orbit, and whether we could ever use that beam to reflect off detached segments of the sail upon arrival to slow it down are matters that challenge all boundaries of engineering. I can hear Forward chuckling. Here’s the basic idea, as drawn from his original paper on the concept.
Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The ‘paralens’ in the image is a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus. Study the diagram and you’ll see that the sail has three ring segments, each of them separating to provide a separate source of braking or acceleration for the arrival, respectively, and departure of the crew. Imagine the laser targeting this would require. Credit: Robert Forward.
I tend to think that Les Johnson is right about sails as they fit into the interstellar picture. In a recent interview with a publication called The National, Johnson (NASA MSFC) made the case that we might well reach another star with a sail driven by a laser. Breakthrough Starshot, indeed, continues to study exactly that concept, using a robotic payload miniaturized for the journey and sent in swarms of relatively small sails driven by an Earth-based laser. But when it comes to human missions to even nearby stars, Johnson is more circumspect. Let me quote him on this from the article:
“As for humans, that’s a lot more complicated because it takes a lot of mass to keep a group of humans alive for a decade-to centuries-long space journey and that means a massive ship. For a human crewed ship, we will need fusion propulsion at a minimum and antimatter as the ideal. While we know these are physically possible, the technology level needed for interstellar travel seems very far away – perhaps 100 to 200 years in the future.”
Johnson’s background in sail technologies for both near and deep space at Marshall Space Flight Center is extensive. Indeed, there was a time when his business card described him as ‘Manager of Interstellar Propulsion Technology Research’ (he once told me it was “the coolest business card ever”). He has also authored (with Gregory Matloff and Giovanni Vulpetti), books like Solar Sails: A Novel Approach to Interstellar Travel (Springer, 2014) and A Traveler’s Guide to the Stars (Princeton University Press, 2022), as well as editing the recent Interstellar Travel: Purpose and Motivations (Elsevier, 2023). In addition to that, his science fiction novels have explored numerous deep space scenarios.
Image: NASA’s Les Johnson, a prolific author and specialist in sail technologies. Credit: NASA.
So there’s a much more optimistic take on the human interstellar guideline than the one I gave in my first paragraph, and of course I hope it’s on target. We’re probably not going to be going to what is sometimes called an “Earth 2.0,” in Johnson’s view, because he doubts there are any such reasonably close to us. That’s something we’ll be learning a great deal more about as future space instrumentation comes online, but we can bear in mind that the explorers who tackled the Pacific in the great era of sail didn’t set out thinking they were going to find another Europe, either. The point is to explore and to learn what you can, with all the unexpected benefits that brings.
Johnson’s early interest in sails, by the way, was fired not so much by Forward’s Rocheworld as by Larry Niven and Jerry Pournelle’s novel The Mote in God’s Eye (Simon & Schuster, 1974), where an incoming laser sail from another civilization is detected. The realization that unusual astronomical observations point to a technology, and a laser-beaming one at that, is an exciting part of the book. Here the authors’ human starship crew describes the detection of a strange light emanating from a smaller star (the Mote) in front of a much larger supergiant (the Eye):
“…I checked with Commander Sinclair. He says his grandfather told him the Mote was once brighter than Murcheson’s Eye, and bright green. And the way Gavin’s describing that holo – well, sir, stars don’t radiate all one color. So -”
“All the more reason to think the holo was retouched. But it is funny, with that intruder coming straight out of the Mote…”
“Light,” Potter said firmly.
“Light sail!” Rod shouted in sudden realization…”
For more on all this, see my Our View of a Decelerating Magsail in these pages. It’s not surprising that Niven and Pournelle ran their lightsail concept past Robert Forward at a time when the idea was just gaining traction. We all have career-changing literary experiences. I can remember how a childhood reading of Poul Anderson’s The Enemy Stars (J.B. Lippincott, 1959) utterly fired my imagination toward the idea of leaving the Solar System entirely. It was a finalist for the Hugo Award that year following serialization in Astounding, though I didn’t encounter it until later.
Johnson’s work at Marshall Space Flight Center takes in the deployment of a large solar sail quadrant for the Solar Cruiser mission that was first unfurled in 2022 to demonstrate TRL 5 capability, and has just been deployed at contractor Redwire Corp.’s facility in Longmont, Colorado to demonstrate TRL 6. In NASA’s terms, that means going from “Component or breadboard validation in relevant environment” (TRL 5) to “System or subsystem model or prototype demonstration in a relevant environment (ground or space).” In other words, this is progress. At TRL 6, a system is considered “a fully functional prototype or representational model.” Says Johnson in a recent email:
“25 years ago, when I first met Dr. Forward, he inspired me to plan a development program for solar sails that would eventually lead us to the stars. With Bob’s help, I laid out a milestone driven roadmap that began with the space flight of a 10 m² solar sail, which we did in 2010 with NanoSail-D.
“Next on the plan was the development of something an order of magnitude larger. This was achieved with the development and launch of the 86 m² Near Earth Asteroid Scout solar sail in 2022 and the soon to be launched ACS-3 sail. The Solar Cruiser sail is an order of magnitude larger still at 1653 Square meters. The next step is 10,000!”
Image: NASA and industry partners used two 100-foot lightweight composite booms to unfurl the 4,300-square-foot sail quadrant for the first time Oct. 13, 2022, at Marshall Space Flight Center, making it the largest solar sail quadrant ever deployed at the time. On Jan. 30, 2024, NASA cleared a key technology milestone, demonstrating TRL6 capability at Redwire’s new facility in Longmont, Colorado, with the successful deployment of one of four identical solar sail quadrants. Credit: NASA (although I’ve edited the caption slightly to reflect the TRL level reached).
Solar sails are becoming viable choices for space missions, and the Breakthrough Starshot investigations remind us that sails driven not just by sunlight but by lasers are within the bounds of physics. A key question that will be informed by our experience with solar sails is how laser-driven techniques scale. Theoretically, they seem to scale quite well. Are the huge structures Forward once wrote about remotely feasible (perhaps via nanotech construction methods), or is Johnson right that fusion and one day antimatter may be necessary for craft large enough (and fast enough) to carry human crews?
Speculating about the diffusion of intelligent species through the galaxy, as we’ve been doing these past few posts, is always jarring. I go back to the concept of ‘deep time,’ which is forced on us when we confront years in their billions. I can’t speak for anyone else, but for me thinking on this level is closer to mathematics than philosophy. I can accept a number like 13.4 × 10⁹ years (the estimate for the age of globular cluster NGC 6397 and a pointer to the Milky Way’s age) without truly comprehending how vast it is. As biological beings, a century pushes us to the limit. What exactly is an aeon?
NGC 6397 and other globular clusters are relevant because these ancient stellar metropolises are the oldest large-scale populations in the Milky Way. But I’m reminded that even talking about the Milky Way can peg me as insufferably parochial. David Kipping takes me entirely out of this comparatively ‘short-term’ mindset by pushing the limits of chronological speculation into a future so remote that elementary particles themselves have begun to break down. Not only that – the Columbia University astrophysicist finds a way for human intelligence to witness this.
You absolutely have to see how he does this in Outlasting the Universe, a presentation on his Cool Worlds YouTube channel. Now Cool Worlds is a regular stop here because Kipping is a natural at rendering high-level science into thoughtful explanations that even the mathematically challenged like me can understand. Outlasting the Universe begins with Kipping the narrator saying “We are in what you would call the future…the deep future” and takes human evolution through the end of its biological era and into a computer-borne existence in which a consciousness can long outlive a galaxy.
Image: Astrophysicist, author and indeed philosopher David Kipping. Credit: Columbia University.
Along the way we remember (and visit in simulation) Freeman Dyson, who once speculated that to become (almost) immortal, a culture could slow down the perceived rate of time. “Like Zeno’s arrow,” says Kipping, “we keep dialing down the speed.” The visuals here are cannily chosen, the script crisp and elegant, imbued with the ‘sense of wonder’ that brought so many of us to science fiction. Outlasting the Universe is indeed science fiction of the ‘hard SF’ variety as Kipping draws out the consequences of deep time and human consciousness in ways that make raw physics ravishing. I envy this man’s students.
With scenarios like this to play with, where do we stand with the ‘zoo hypothesis?’ It must, after all, reckon with years by the billions and the spread of intelligence. Science fiction writer James Cambias responded to my Life Elsewhere? Relaxing the Copernican Principle post with a tight analysis of the notion that we may be under observation from a civilization whose principles forbid contact with species they study. This is of course Star Trek’s Prime Directive exemplified (although the lineage of the hypothesis dates back decades), and it brings up Jim’s work because he has been so persistent a critic of the idea of shielding a population from ETI contact.
Jim’s doubts about the zoo hypothesis go back to his first novel. A Darkling Sea posits an Europa-like exoplanet being studied by a star-faring species called the Sholen, who are employing a hands-off policy toward local intelligence even as they demand that human scientists on the world’s sea bottom do the same. Not long after publication of the novel (Tor, 2014), he told John Scalzi that he saw Prime Directives and such as “ …a mix of outrageous arrogance and equally overblown self-loathing, a toxic brew masked by pure and noble rhetoric.” The arrogance comes from ignoring the desires of the species under study and denying them a choice in the matter.
…we deduce that you can’t hide a star system which contains a civilization capable of large-scale interstellar operations, which the Zookeepers are by definition. They’re going to be emitting heat, EM radiation, laser light, all the spoor of a Kardashev Type I or higher civilization. And the farther away they are, the more they’re going to be emitting because they need to be bigger and more energy-rich in order to have greater reach.
This gives us one important lesson: if the goal of a Zoo is to keep the civilizations inside from even knowing of the existence of other civilizations, the whole thing is impossible. You can’t have a Zoo without Zookeepers, and the inhabitants of the Zoo will detect them.
Jim’s points are well-taken, and he extends the visibility issue by noting that we need to address time, which must be deep indeed. For a civilization maintaining all the apparatus of a protected area around a given star has to do so on time frames that are practically geological in length. Here we can argue a bit, for a ‘zoo’ set up for reasons we don’t understand in the first place might well come into existence only when the species being studied has reached the capability of detecting its observers.
I referenced Amri Wandel (Hebrew University of Jerusalem) on this the other day. Wandel argues that our own industrial lifespan is currently on the order of a few centuries, and who knows what level of technological sophistication a ‘zoo-keeping’ observer culture might want us to reach before it decides it can initiate contact? That would drop the geological timeframe down to a more manageable span, although the detectability problem still remains. So does the issue of interaction with other star-faring species who might conceivably need to be warned off entering the zoo. Cambias again:
If Captain Kirk or whoever shows up on your planet and says “I’m from another planet. Let’s talk and maybe exchange genetic material — or not, if you want me to leave just say so,” that’s an infinitely more reasonable and moral act than for Captain Kirk to sneak around watching you without revealing his own existence. The first is an interaction between equals, the second is the attitude of a scientist watching bacteria. Is that really a moral thing to do? Why does having cooler toys than someone else give you the right to treat them like bacteria?
This is lively stuff, and speculation of this order is why many people begin reading and writing science fiction in the first place. A hard SF writer, a ‘world builder,’ will make sure that he or she has thought through implications for every action he attributes not only to his characters but the non-human intelligences they may interact with. One thing that had never occurred to me was the issue of visibility when translated to the broader galaxy. Because a zoo needs to be clearly marked. Here’s Jim’s view:
If you’re going to exclude other civilizations from a particular region of the Galaxy, you have to let them know. Shooting relativistic projectiles or giant laser beams at incoming starships is a very ham-fisted way of communicating “keep out!” — and it runs the risk of convincing the grabby civilization that you’re shooting at to start shooting back. And if they’re grabby and control a lot of star systems, that’s going to be a lot of shooting.
Jim’s points are telling, and the comments on my recent Centauri Dreams posts also reflect readers’ issues with the zoo hypothesis. My partiality to it takes these issues into account. If the zoo hypothesis is the best of the solutions to the Fermi question, then the likelihood that other intelligent species are in our neighborhood is vanishingly small. Which lets me circle back to the paper by Ian Crawford and Dirk Schulze-Makuch that set off this entire discussion. It asked, you’ll recall, whether the zoo hypothesis wasn’t the last standing alternative to the idea that technological civilizations are, at the least, rare. It’s not a good alternative, but there it is.
In other words, I’d like the zoo hypothesis to have some traction, because it’s the only way I can find to imagine a galaxy in which intelligent civilizations are common.
Consider the thinking of Crawford and Schulze-Makuch on other hypotheses. Interstellar flight might be impossible for reasons of distance and energy, but this seems a non-starter given that we know of ways within known physics to send a payload to another star even in this century. A slow exploration front moving at Voyager speeds could do the trick in a fraction of the time available given the age of the Milky Way. The lack of SETI detections likewise points to technologies that are physically feasible (various kinds of technosignatures) but are not yet observed.
Is the answer that civilizations don’t live very long, and the chances of any two existing at the same brief time in the galaxy are remote? The nagging issue here is that we would have to assume that all civilizations are temporally limited. It takes only one to find a way through whatever ‘great filter’ is out there and survive into a star-faring maturity to get the galaxy effectively visited and perhaps colonized by now. Crawford and Schulze-Makuch reject models that result in volumes of the galactic disk being unvisited during the four billion years of Earth’s existence, considering them valid mathematically but implausible as solutions to the larger Fermi puzzle.
Many of the hypotheses to explain the Great Silence go even further into the unknowable. What, for example, do we make of attempts to parse out an alien psychology, which inevitably is seen, wittingly or not, as reflecting our own human instincts and passions? Monkish cultures that choose not to expand for philosophical reasons will remain unknowable to us, for example, as will societies that self-destruct before they achieve interstellar flight. We can still draw a few conclusions, though, as Crawford and Schulze-Makuch do, all pointing at least to intelligence being rare.
Although we know nothing of alien sociology, it seems inevitable that the propensity for self-destruction, interstellar colonization and so on must be governed by probability distributions of some kind. The greater the number of ETIs that have existed over the history of the Galaxy, the more populated will be the non-self-destructed and/or pro-colonization wings of these distributions, and it is these ETIs that we do not observe. On the other hand, if the numbers of ETIs have always been small, these distributions will have been sparsely populated and the non-observation of ETIs in their expansionist wings follows naturally.
Image: Are ancient ruins the only thing we may expect to find if we reach other star systems? Are civilizations always going to destroy themselves? The imposing remains of Angkor Wat. Credit: @viajerosaladeriva.
Likewise, we still face the problem that, as Stapledon long ago noted, different cultures will choose different priorities. Why assume that in a galaxy perhaps stuffed with aliens adopting Trappist-like vows of silence there will not be a few societies that do want to broadcast to the universe, a METI-prone minority perhaps, but observable in theory. We have no paradox in the Fermi question if we assume that aliens are rare, but if they are as common as early science fiction implied, the paradox is only reinforced.
So Crawford and Schulze-Makuch have boiled this down to the zoo hypothesis or nothing, with the strong implication that technological life must indeed be rare. I rather like my “one to ten” answer to the question of how many technological species are in the galaxy, because I think it squares with their conclusions. And while we can currently only speculate on reasons for this, it’s clear that we’re on a path to draw conclusions about the prevalence of abiogenesis probably in this century. How often technologies emerge after unicellular life covers a planet is a question that may have to wait for the detection of a technosignature. And as is all too clear, it’s possible this will never come.
The paper is Crawford & Schulze-Makuch, “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Published online in Nature Astronomy 28 December 2023 (abstract). James Cambias’ fine A Darkling Sea (Tor, 2014) is only the first of his novels, the most recent of which is The Scarab Mission (Baen, 2023), part of his ‘billion worlds’ series. Modesty almost, but not quite, forbids me from mentioning my essay “Ancient Ruins” which ran in Aeon a few years back.
John Barrow has been on my mind these past few days, for reasons that will become apparent in a moment. In my eulogy for Barrow (1952-2020), I quoted from his book The Left Hand of Creation (Oxford, 1983). I want to revisit that passage for its clarity, something that always inspired me about this brilliant physicist. For it seemed he could render the complex not only accessible but encouragingly pliable, as if scientific exploration always unlocked doors of possibility we could use to our advantage. His was a bright vision. The notion that animated him was that there was something in the sheer process of research that held its own value. Thus:
Could there be any shortcuts to the answers to the cosmological questions? There are some who foolishly desire contact with advanced extraterrestrials in order that we might painlessly discover the secrets of the universe secondhand and prematurely extend our understanding. Such a civilization would surely resemble a child who receives as a gift a collection of completed crossword puzzles. The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination.
You can see that for Barrow, the question of values was not separated from scientific results, and in a sense transcended the data we actually gathered. He goes on:
About 50 years ago a group of eminent cosmologists were asked what single question they would ask of an infallible oracle who could answer them with only yes or no. When his opportunity came, Georges Lemaître made the wisest choice. He said, “I would ask the Oracle not to answer in order that a subsequent generation would not be deprived of the pleasure of searching for and finding the solution.”
Image: Cosmologist, mathematician and physicist John D. Barrow, whose books have been a personal inspiration for many years. Credit: Tom Powell.
Leave it to Lemaître (and Barrow to quote him) as we reach beyond the immediately practical to unlock what it is about human experience that compels us to push into new terrain, whether it be physical exploration or flights of the imagination as we pursue a new hypothesis about nature. Barrow comes to mind because we’ve just been talking about the scales by which a civilization can be measured. Some of these are well established, as for example the Kardashev scale, with its familiar Types I, II and III keyed to the scale of a civilization’s energy use. In Clarke’s The Fountains of Paradise we find an alien scale based on the use of tools. It’s possible to imagine other scales, and Barrow’s own contribution takes us into the nano-realm.
As best I can determine, Barrow first floated the scale in his 1998 book Impossibility: The limits of science and the science of limits (Oxford University Press). Inverting Kardashev, Barrow was interested in a civilization’s ability to control smaller and smaller things, relying on the observed fact that as we have explored such micro-realms, our technologies have proliferated. Nanotechnology and biotechnology are drawn out of our ability to manipulate matter at small scales, and in fact the development of nanotech is one marker for a Barrow scale IV culture.
Barrow I: The ability to manipulate objects at the same scale as the person or being involved. In other words, simple activities involving basic tools.
Barrow II: The control of genetic information.
Barrow III: The ability to control molecules.
Barrow IV: The ability to control individual atoms.
Barrow V: The manipulation of atomic nuclei..
Barrow VI: Control of elementary particles.
Barrow Omega (Ω): The ability to control fundamental elements of spacetime.
Table: Energetic and inward civilization development. Kardashev’s (1964) types refer to energy consumption; Barrow’s (1998, 133) types refer to a civilization’s ability to manipulate smaller and smaller entities. Credit: Clément Vidal.
I’ve drawn the above table from a paper by French philosopher and SETI scientist Clément Vidal, who is one of the few who have explored this realm (citation below). Here we get both Kardashev and Barrow at once, a convenience, and central to Vidal’s argument that black holes are going to draw advanced civilizations to extract their energies and explore what he calls “the computational density of matter.” On this score, it’s interesting to note that Freeman Dyson proposed in 1979 that a civilization exploiting time dilation effects near black holes could survive effectively forever (a later revision had to take into account the accelerating expansion of the universe).
What all this means for SETI is intriguing – almost punchy – and I’ll send you to Vidal’s superb The Beginning and the End: The Meaning of Life in a Cosmological Perspective (Springer, 2014) for a deep dive into the concepts involved. But consider this for a starter: Dysonian SETI assumes civilizations far more advanced than our own, the reasoning being that their works should be apparent even at astronomical scales. Thus searching our astronomical data as far back as we can could conceivably flag an anomaly that merits investigation as a possible civilizational marker.
What Clément Vidal has been investigating is where such markers would turn up, and for this he deploys the scales of both Kardashev and Barrow. I think the easiest assumption is that we would find an alien civilization at its home world, but of course this needn’t be the case. Vidal speaks of ‘attractors’ as those sources of energy that an advanced civilization would increasingly exploit. Take a culture a billion years older than our own and ponder energy needs that might require it to exploit things like the energies of close binary neutron stars or black holes themselves. Such a civilization would be far flung, with operations well beyond its local group of stars.
Now ponder Barrow Type Ω. This ‘omega’ culture is free of the constraints of spacetime, having achieved the ability to manipulate both. It’s anyone’s guess whether such a civilization would be noted by achievements on a truly celestial scale, or whether its works would actually be embedded in the nature of space and time themselves, so that to us they appear the simple functioning of nature. In this mode of thinking, the more advanced a civilization becomes as it moves up the Barrow scale, the more it begins to effectively disappear. Barrow thus channels Richard Feynman and anticipates Lee Smolin’s notions about cosmological evolution, a kind of self-selection for universes.
I’m going to swipe the chart below from Vidal’s 2010 paper on black hole attractors, showing the entertaining fact that as he puts it, “from the relative human point of view, there is more to explore in small scales than in large scales.”
Table: That humans are not in the center of the universe is also true in terms of scales. This implies that there is more to explore in small scales than in large scales. Richard Feynman (1960) popularized this insight when he said “there is plenty of room at the bottom”. Figure adapted from (Auffray and Nottale 2008, 86). Credit: Clément Vidal.
Futurist John Smart has dug into what he calls STEM Compression, with STEM in this case meaning Space/Time/Energy/Matter, and the compression being the idea that in terms of density and efficiency, we can as Vidal puts it “do more with less.” For going deeper into the Barrow scale, we see that as things get smaller, we are not hampered by the speed of light problem. In fact, our endgame barrier is at the Planck scale. A Kardashev II civilization extracting energy from a rotating black hole using technologies far up the Barrow scale may well be indistinguishable from an X-ray binary of the sort that has been cataloged in the astronomical literature.
Such speculations are on the far edge of SETI (and again, I refer you to Vidal’s book), but it’s also true that whether or not extraterrestrial civilizations exist, our own ability to chart futures for an expanding civilization may well come in handy if we can somehow punch through whatever ‘great filter’ may be out there and become a species that survives on the scale of deep time. There is no knowing whether this is even possible, and it may be that the galaxy is filled with the ruins of those who have gone before us.
It is also true, of course, that no one may have gone before us. Maybe N really does equal 1. But I return to Barrow: “The human search for the structure of the universe is more important than finding it because it motivates the creative power of the human imagination.” And the human imagination is currency of the realm in matters like these.
The Vidal paper is “Black Holes: Attractors for Intelligence?” presented at the Kavli Royal Society International Centre, “Towards a scientific and societal agenda on extra-terrestrial life”, 4-5 Oct 2010 (abstract). The Dyson paper is “Time Without End: Physics and Biology in an Open Universe,” Review of Modern Physics 51: 447-460 (abstract). My eulogy for Barrow is On John Barrow. John Smart contributed a fascinating essay on cosmic evolution in these pages in The Goodness of the Universe.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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