Centauri Dreams

‘Big dumb objects’ (BDOs) appear to great effect in science fiction. They come in all manner of sizes and shapes and they fulfill a wide range of functions. An early favorite of mine was Cordwainer Smith’s “Golden the Ships Were Oh! Oh! Oh!,” which I snagged on a long ago trip to a Chicago newsstand, where it appeared in an issue of Amazing Stories. It’s probably found most easily these days in The Rediscovery of Man: The Complete Short Science Fiction of Cordwainer Smith (NESFA Press, 1993), a collection that should be on every science fiction fan’s shelf.

Smith (a pseudonym for Paul Myron Anthony Linebarger, whose life was as remarkable as his fiction) goes to work on structures that are millions of miles long. I won’t say more for fear of spoiling the story for newcomers. More recent BDOs are better known, Dyson spheres and Dyson swarms are no strangers to these pages, and have been the subject of intense scrutiny by Jason Wright and his colleagues at Pennsylvania State University. The G-HAT (Glimpsing Heat from Alien Technologies) project scanned data from the Wide-field Infrared Survey Explorer satellite looking at tens of thousands of galaxies for the waste heat signature of possible Dyson spheres. The idea that megastructures might interest a hugely advanced civilization is reasonable, but we have yet to find evidence that Dyson spheres exist.

Larry Niven’s Ringworld posits a structure that circles an entire star but does not encompass it. A transit signature might give this one away if ever found; imagine the lightcurve. Niven and Gregory Benford later come up with the ‘shipstar’ concept that Greg described some years back on Centauri Dreams. This was an unusual re-thinking of the original ‘Shkadov Thruster,’ a device that could be used to move an entire star. See the Bowl of Heaven trilogy for more.

The work of Russian physicist Leonid Shkadov in 1987, the thruster design used asymmetric light pressure from a huge mirror to move an entire planetary system to a new destination. The physics works, but we’re moving at slow speeds, on the order of 20 meters per second after a million years. On the other hand, a truly long-lived species might find waiting a billion years to reach 20 kilometers per second, with a whopping 34,000 light years shift in position, to be plausible. Shipstar would be able to move considerably faster.

Image: An artist’s conception of the Benford/Niven ‘shipstar’ concept. Think of the ‘bowl’ as half of a Dyson sphere curved around a star whose energies flow into a propulsive plasma jet that moves the entire structure on its journey. Here the notion of living space may remind you of Niven’s Ringworld, that vast structure completely encircling a star, though not enclosing it. The difference is that in the ShipStar scenario, most of the ‘bowl’ is made up of mirrors, with living space just on the rim. Credit: Don Davis.

In conversations with Benford about his shipstar concept a few years ago, I learned that a solid Dyson sphere is unstable, and would need constant adjustment to maintain its position. Concerns over stability plague BDOs. Colin McInnes (University of Glasgow) looks at the problem in a recent paper, noting this about the Shkadov design:

In its simplest form a stellar engine can be considered as a single ideal ultra-large rigid reflective disc in static equilibrium above a central star… As the disc accelerates due to radiation pressure from the star, the centre-of-mass of the gravitationally coupled star-reflector system accelerates, leading to a displacement of the star.

Image: This is Figure 1 from a paper by Duncan Forgan (citation below). Caption: Diagram of a Class A Stellar Engine, or Shkadov thruster. The star is viewed from the pole – the thruster is a spherical arc mirror (solid line), spanning a sector of total angular extent 2ψ. This produces an imbalance in the radiation pressure force produced by the star, resulting in a net thrust in the direction of the arrow. Credit: Duncan Forgan.

That seems straightforward, assuming a civilization so advanced that it could build mirror structures of the needed size. Here too, though, we have stability problems. The McInnes paper is highly interesting, examining megastructure concepts and the possible ways of stabilizing them. While a uniform, rigid reflective disk proves unstable as a star-moving engine, a disk with its mass concentrated at the edges can be stable. Instead of a flat disk, we are looking at something much closer to the shape of a ring. Here passive stability is what we want – i.e., the object does not need continual adjustment by other technologies to maintain its position and function.

In the case of the Schkadov engine, we have this consideration:

…for an ideal reflector subject to gravitational and radiation pressure forces the gradient of these forces across the reflector will induce stresses. While the direction of the radiation pressure force is always normal to the reflector, the direction of the gravitational force will vary across the reflector moving from the centre to the edge. Therefore, while the component of the gravitational force normal to the reflector can in principle be balanced by the radiation pressure force, there will be an in-plane component of the gravitational force which will generate a compressive stress. A thin reflector will clearly be unable to support such compression. However, in principle a zero-stress reflector can be configured for a non-homogeneous, partially reflecting rotating reflector…

The math for a stellar reflector and a stellar ring are laid out in the paper’s appendices.

McInnes thinks that stability is useful as we investigate possible technosignatures in our SETI work, whether they be star-moving thrusters or energy-gathering Dyson objects. The assumption is that passive stability will be sought after because it is efficient and economical, not requiring control systems that must continually adjust position. Remember, too, that in searching for technosignatures, we have the possibility of finding megastructures like these that have survived the demise of their creators. Passive stability is essential for these objects to remain intact and detectable.

What McInnes calls a ‘Dyson bubble’ can likewise be stabilized. Here we’re talking not about a solid Dyson sphere but a constellation of discs, a ‘power swarm’ that allows a civilization to exploit most of the output of its star. The terminology can be confusing but bear with me. The author distinguishes between a cloud of small reflectors in orbit around the central star – huge in number, these form a so-called ‘Dyson swarm’ – and a ‘Dyson bubble,’ by which he means a smaller number of large reflectors in ‘statite’ configuration, so that instead of orbiting, radiation pressure exactly balances gravity. In other words, the ‘bubble’’ components stay stationary relative to the star.

Self-stabilizing techniques are challenged not only by gravitational and radiation pressure but also collisions between the myriad orbiting disks as well as outside perturbing forces. Over large timeframes, passing stars can disrupt the gravitational dance, while interstellar comets, whose numbers are likely to be huge, present a similar risk of disruption. Even so, there are ways around this:

…the Dyson bubble can remain stable when its self-gravity and a simple model of a diffuse background of scattered radiation are included in the dynamics defined in Section 6.4. However, there are now regions of the parameter space where instability can occur, primarily at the edge of the Dyson bubble driven by the diffuse background radiation. In addition, it has been shown that the self-gravity of the Dyson bubble is in itself sufficient to ensure passive stability in the absence of the diffuse background radiation, and indeed it enhances the stability of the Dyson bubble when the diffuse background of scattered radiation is included.

A Dyson swarm if properly implemented can also ensure passive stability. Reflectors must always be configured ‘normal’ (perpendicular) to the central star “…using slighting conical reflectors with the centre-of-pressure displaced behind the centre-of-mass.”

So there are ways of doing these things as long as we abandon the Shkadov concept of a uniform reflector disc in favor of a ring supporting the reflector, or in the case of the two Dyson options McInnes looks at, a dense cloud of reflectors stabilized through orbital mechanics, or a smaller assembly of reflectors in static equilibrium with radiation pressure from the star exactly balancing gravity. But here I’m more interested in the consequences in terms of hunting for technosignatures:

A Dyson swarm can be expected to generate a different technosignature to a passively stable Dyson bubble discussed above. For example, the motion of the discs in a swarm would imply a flickering of the observed luminosity of the central star, with a larger variation expected from a small number of ultra-large discs relative to a large number of small discs. Finally, while an orbiting swarm of reflectors will be susceptible to collisions (B. C. Laki 2025), collisions within a Dyson swarm could in principle be minimised using families of displaced non-Keplerian orbits, where the orbit planes of the reflectors can be stacked in parallel rather than being inclined relative to each other (C. R. McInnes & J. F. L. Simmons 1992).

And what of Shipstar? A recent conversation with Jim Benford reminded me that his brother Greg had worked out a way to stabilize the induced flare on the central star through intense magnetic fields, but as far as I know, this concept has never been rigorously investigated. From the technosignature standpoint, McInnes’ paper reminds us that stability problems can be overcome should an advanced civilization choose to build Dyson-class structures, or undertake star-moving of the Shkadov variety. How to engineer the stability of BDOs should continue to provide insight into possible technosignatures, even if the lack of any trace of Dyson structures despite intensive work at G-HAT remains puzzling. Next week I want to look at an even more recent stellar engine concept as presented by Illinois State University’s Michael Caplan.

The paper is McInnes, “Stellar engines and Dyson bubbles can be stable,” Monthly Notices of the Royal Astronomical Society 546 (2026), 1-18 (full text). The Shkadov paper is “Possibility of Controlling Solar System Motion in the Galaxy,” presented at the 38th Congress of the International Astronautical Federation (IAF) in Brighton, UK. An English translation of the original paper was published in the Journal of Solar System Research Volume 22, Issue 4, pp 210–214 under the title “Possibility of Control of Galactic Motion of the Solar System.” The Forgan paper mentioned above is “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves,” Journal of the British Interplanetary Society, Vol. 66, no. 5/6, 2013 pp. 144–154. Preprint.

Lately we’ve been discussing interstellar probes, the kind that an extraterrestrial civilization might use to explore the galaxy. Ronald Bracewell’s analysis of such probes dates back to 1960 and was all but coterminous with the emergence of SETI. The problem with Bracewell probes is that we would expect to have one in our Solar System if they exist. Rather than using that notion to add stress to the Fermi question, I’m going to point out that there is a lot of real estate waiting to be searched.

Case in point: What might our ongoing study of the lunar surface through images from the Lunar Reconnaissance Orbiter pick up as we use AI models that have already identified human-made space debris from various missions? A closer look at this project reminds us that while the Moon is an obvious place to look for a ‘lurker’ probe, we can’t discount other locations even though earlier work on the various Lagrange points, a good place for long-term observation of our planet, came up empty (see below). Our capabilities are so much more advanced not only in terms of instrumentation but analytical tools that a continued hunt for artifacts is reasonable.

I’m getting picky here given the wide variety of possible probes, tapping the definition that Bracewell used in his original article. That’s a probe we probably would have noticed by now if it were active. In 1960, Bracewell was offering an alternative to the SETI goal of detecting an interstellar radio signal aimed at Earth. His physical probe would arrive in a planetary system to look for signs of life and technology, duplicating any radio signals it heard so as to re-transmit them to the originators, thus establishing contact. Sagan uses the notion in his novel Contact (1979), where Adolf Hitler’s opening speech from the 1936 Berlin Olympics is found embedded within the message, along with much else.

How would we respond to hearing a signal sent back to us from space? Bracewell thinks we would experiment with it to see what would happen next:

To notify the probe that we had heard it, we would repeat back to it once a:gain. It would then know that it was in touch with us. After some routine tests to guard against accident, and to test our sensitivity and band-width, it would begin its message, with further occasional interrogation to ensure that it had not set below our horizon. Should we be surprised if the beginning of its message were a television image of a constellation?

Bracewell’s notions of dispatching a physical object as opposed to sending a radio signal take advantage of the ‘information density’ available to a physical probe. This is the familiar notion that a box of DVDs in a truck moves information at a far higher rate than fiber-optic cable. But of course you have to get the truck to its destination, and in the case of interstellar flight the latency is huge – perhaps thousands of years or more. A long-lived civilization, thought Bracewell, may nonetheless see purpose in seeding nearby stars if the travel time is a small fraction of its likely civilizational life.

Swarming and Reproducing

Bracewell’s ideas jibe nicely with the Breakthrough Starshot concept of swarms of sails investigating nearby stars. We might imagine the descendants of such tiny flyby probes scattered to all interesting stellar systems within, say, 100 light years. With concepts like Bracewell’s entering the literature, it was left to Robert Freitas to run the first scientific search I am aware of for such probes (citation below). Freitas made a series of visual observations of the various LaGrange points in the early 1980s. But in the early days of SETI (and Bracewell was writing even before the Green Bank meeting in 1961 that produced the Drake Equation), other ideas about how interstellar probes might operate had begun to surface. Ancient probes sent by civilizations far more advanced than ours might still be live, waiting and reporting on our activities (Clarke’s sentinel ‘slabs’ from 2001: A Space Odyssey come to mind . Or they might be long-dead relics.

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When Michael Hart went to work on this in 1975, he amplified the probe concept and changed the game. He produced, in fact, what Jason Wright (Pennsylvania State) has dubbed “The most influential formulation of the Fermi Paradox…,” one that compresses the conundrum by homing in on the fact that we observe no intelligent beings on our planet, something Hart called Fact A. The fact that they are not observed tells us that despite the amount of time available for long-lived cultures to have colonized the galaxy, none evidently have. This is no small problem, for as Wright calculates in his new textbook on SETI, even a ‘wavefront’ of probes moving outwards from star to star at Voyager-like speeds would have been able to reach every star within 2 billion years.

Move the dial up in terms of speed to, say, 0.5 c and the numbers get considerably shortened. Imagine relativistic ships that close on lightspeed and we find exponential growth saturating the galaxy in 150,000 years, all contrasting with an Earth that is 4.5 billion years old. Hart saw nothing in the laws of physics that prohibited starflight, and he found the idea that ETI was uninterested in Earth to be unconvincing. What David Brin coined the ‘Principle of non-Exclusiveness’ boils down to the idea that alien species will not all behave the same way. All that is needed is for one civilization to decide to send out probes, and by now such probes should have reached every star.

Image: How quickly would a single civilization using self-replicating probes spread through a galaxy like this one (M 74)? Moreover, what sort of factors might govern this ‘percolation’ of intelligence through the spiral? The answers affect our view of the Fermi question, and thus our own place in the cosmos. Image credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.

Advances in computing led Frank Tipler to push Hart’s views even more strenuously, bringing John von Neumann’s work on self-replicating machines to bear. His insight was to ask what would happen if an extraterrestrial culture began seeding stars with self-reproducing probes, each capable of not only studying a new world but building another probe that could reach yet another star, and so on. Here the numbers become even more telling. Such probes could use local resources in each system to build their next generation, thus nullifying the resource problem. Here’s Tipler on the matter:

…if the motivation for communication is to exchange information with another intelligent species, then as Bracewell has pointed out, contact via space probe has several advantages over radio waves. One does not have to guess the frequency used by the other species, for instance. In fact, if the probe has a von Neumann machine payload, then the machine could construct an artifact in the solar system of the species to be contacted, an artifact so noticeable that it could not possibly be overlooked. If nothing else, the machine could construct a “Drink Coca-Cola” sign a thousand miles across and put it in orbit around the planet of the other species. Once the existence of the probe has been noted by the species to be contacted, information exchange can begin in a variety of ways.

As to the cost of such a vast exploration program, Tipler has this to say:

Using a von Neumann machine as a payload obviates the main objection to interstellar probes as a method of contact, namely the expense of putting a probe around each of an enormous number of stars. One need only construct a few probes, enough to make sure that at least one will succeed in making copies of itself in another solar system. Probes will then be sent to the other stars of the galaxy automatically, with no further expense to the original species.

A ‘Catastrophic’ Answer to Fermi?

Tipler suggested a timeframe of 300 million years to fill the galaxy with these devices, in an argument that drew fire from Carl Sagan and William Newman, who argued in 1983 that his approach was ‘solipsistic’ because the idea that we were alone in producing a technological civilization was anti-Copernican. And here we need to pause on a concept that has surfaced repeatedly in SETI studies not just in the western nations but also the Soviet Union. The idea of ‘mediocrity’ troubled attendees at the Soviet SETI meeting at the Byurakan Astrophysical Observatory in 1964, to be discussed again in a second meeting (with American scientists as participants) in 1971.

Do we just take the Copernican principle as a given? Sagan clearly thought so. His ‘co-author’ on Intelligent Life in the Universe, Iosif S. Shklovskii was far less sanguine on the matter:

Since we do not adequately understand the factors leading to the evolution of intelligence and technical civilizations, we cannot reliably estimate the probability that intelligence and technical civilizations will emerge.

Here I’m drawing on Mark Sheridan in his 2023 book SETI’s Scope (How The Search For Extraterrestrial Intelligence Became Disconnected From New Ideas About Extraterrestrials). Sheridan homes in on the philosophical disagreement between emerging Soviet SETI and the ideas in the Drake Equation. At Byurakan, Soviet mathematician A. V. Gladkii challenged the idea, accepted by Sagan, that mathematics could be a recognizable common ground between all intelligences across the stars. And Sheridan quotes Theodosius Dobzhansky, a Ukrainian-born geneticist later working in the U.S., who in a 1972 paper cast doubt on Sagan’s insistence that because intelligence had arisen on our planet, it must arise everywhere life exists. In his view, the principle of mediocrity was being taken several steps too far. Quoting Dobzhansky:

“Natural scientists have been loathe, for at least a century, to assume that there is anything radically unique or special about the planet Earth or about the human species. This is an understandable reaction against the traditional view that Earth, and indeed the whole universe, was created specifically for man. The reaction may have gone too far. It is possible that there is, after all, something unique about man and the planet he inhabits.”

In a fascinating 2009 paper, Milan Ćirković examines the Fermi question in the context of our basic premises about science. As amplified in his later book The Great Silence: Science and Philosophy of Fermi’s Paradox (Oxford University Press, 2018), the Serbian astronomer points to the focus the ‘where are they’ question places upon both Copernicanism and gradualism. In the former, as clearly stated by Sagan as by many other of the early SETI practitioners, the assumption is that we occupy no privileged place in the cosmos, and thus should expect other civilizations to exist, some of which would be far more advanced than ourselves. Yet we do not observe them.

Many answers can be offered to Fermi’s question, of course, but as we continue probing the cosmos, the silence takes on escalating significance. Must we envision a future in which we abandon Copernicanism and assume that we do not, in fact, occupy a relatively common niche in the cosmos, but rather a rather special one?

Or should we give up on gradualism, the idea that geophysical processes proceed in the future more or less as they did in the past? The concept is foundational to 18th Century geology and remains a commonplace in current thinking. But ‘catastrophism’ is an obvious factor in the development of life, as extreme ruptures like the K–T extinction event that ended the era of the dinosaurs make clear. Are there common factors that could affect planets throughout what is thought of as the Milky Way’s habitable zone?

The question is the focus of recent work on gamma ray bursts and implies, as Ćirković notes, a ‘reset’ of the clock. That could explain our lack of detections, as it would imply that living worlds, no matter their geological age, have had only about the same amount of time we have had to develop intelligence. The Fermi question highlights both of these key assumptions, while our lack of a solution keeps the tension tight.

The Bracewell paper is “Communications from Superior Galactic Communities,” Nature Volume 186, Issue 4726 (1960), pp. 670-671. Abstract. On the LaGrange search, see Freitas, “A search for natural or artificial objects located at the Earth-Moon libration points,” Icarus, Volume 42, Issue 3 (June, 1980) p. 442-447 (abstract). Michael Hart’s paper on galactic expansion is “Explanation for the Absence of Extraterrestrials on Earth,” Quarterly Journal of the Royal Astronomical Society, Vol. 16, p.128 (full text). Frank Tipler’s paper on self-reproducing probes is “Explanation for the Absence of Extraterrestrials on Earth,” Royal Astronomical Society, Quarterly Journal, vol. 21 (Sept. 1980), p. 267-281 (full text). Milan Ćirković’s paper on Fermi and Copernicanism is “Fermi’s Paradox – The Last Challenge for Copernicanism?” Serbian Astronomical Journal 178 (2009), 1–20. Preprint.