White Paper: Why We Should Seriously Evaluate Proposed Space Drives

Moving propulsion technology forward is tough, as witness our difficulties in upgrading the chemical rocket model for deep space flight. But as we’ve often discussed on Centauri Dreams, work continues in areas like beamed propulsion and fusion, even antimatter. Will space drives ever become a possibility? Greg Matloff, who has been surveying propulsion methods for decades, knows that breakthroughs are both disruptive and rare. But can we find ways to increase the odds of discovery? A laboratory created solely to study the physics issues space drives would invoke could make a difference. There is precedent for this, as the author of The Starflight Handbook (Wiley, 1989) and Deep Space Probes (Springer, 2nd. Ed., 2005) makes clear below.

by Greg Matloff

We live in very strange times. The possibility of imminent human contraction (even extinction) is very real. So is the possibility of imminent human expansion.

On one hand, contemporary global civilization faces existential threats from global climate change, potential economic problems caused by widespread application of artificial intelligence, the still-existing possibility of nuclear war, political instability, at many levels, an apparently endless pandemic, etc.

Image: Gregory Matloff (left) being inducted into the International Academy of Astronautics by JPL’s Ed Stone.

One the other hand, humanity seems poised for the long-predicted but oft-delayed breakout into the solar system. United States and Chinese national space programs will compete for lunar resources. Elon Musk’s SpaceX has its sights fixed on establishing human settlements on Mars. Jeff Bezos’ Blue Origin is concentrating on construction of in-space settlements of increasing population and size.

Because of the discovery of a potentially habitable planet circling Proxima Centauri and the possibility of other worlds circling in the habitable zones of Alpha Centauri A and B, one wonders how many decades it will take for an in-space settlement with a mostly closed ecology to begin planning for a change of venue from Sol to Centauri.

Consulting the literature reveals that controlled (or partially controlled) nuclear fusion and the photon sail are today’s leading contenders to propel such a venture. But the literature also reveals that travel times of 500-1,000 years are expected for human-occupied vessels propelled by fusion or radiation pressure.

Before interstellar mission planners finalize their propulsion choice, an ethical question must be addressed. In the science-fiction story “Far Centaurus”, originally published by A, E. van Vogt in 1944, the crew of a 500-year sleeper ship to the Centauri system awakens to learn that they must go through customs at the destination. During their long interstellar transit, a breakthrough had occurred leading to the development of a hyper-fast warp drive.

We simply must evaluate all breakthrough possibilities, no matter how far-fetched they seem, before planning for generation ships. The initial crews of these ships and their descendants must be confident that they will be the first humans to arrive at their destination. Otherwise it is simply not fair to dispatch them into the void.

Recently, I was a guest on Richard Hoagland’s radio show “Beyond Midnight”. Although the discussion included such topics as the James Webb Space Telescope, panpsychism, stellar engineering and ‘Oumuamua, I was particularly intrigued when the topic of space drives came up.

Richard is especially interested in possible ramifications of Bruce E. DePalma’s spinning ball experiment, which has not been investigated in depth. He later sent along a 2014 e-mail released to the public from physics Nobel Prize winner Brian D. Josephson discussing another proposed space-drive candidate, the Nassikas thruster. Professor Josephson is of the opinion that this device is well worth further study, writing this:

The Nassikas thruster apparently produced a thrust, both when immersed in its liquid nitrogen bath, and for a short period while suspended in air, until it warmed to above the superconductor critical temperature, this thrust presenting itself as an oscillating motion of the pendulum biased in a particular direction. If this displacement is due to a new kind of force, this would be an important observation; however, until better controlled experiments are performed it is not possible to exclude conventional mechanisms as the source of the thrust.

It is in this area of controlled experiments that we need to move forward. A little research on the Web revealed that there are a fair number of candidate space drives awaiting consideration. Most of these devices are untested. DARPA, NASA and a few other organizations have applied a small amount of funds in recent years to test a few of them—notably the EMdrive and the Mach Effect Thruster.

Experimental analysis of proposed space drives has not always been done on such a haphazard basis. Chapter 13 of my first co-authored book (E. Mallove and G. Matloff, The Starflight Handbook, Wiley, NY, 1989) discusses a dedicated effort to evaluate these devices. It was coordinated by engineer G. Harry Stine, retired USAF Colonel William O. Davis (who had formerly directed the USAF Office of Scientific Research) and NYU physics professor Serge Korff.

Between 1996 and 2002, NASA operated a Breakthrough Physics Program. Marc G. Millis, who coordinated that effort, has contributed here on Centauri Dreams a discussion of the many hoops a proposed space drive must jump through before it is acknowledged as a true Breakthrough [see Marc Millis: Testing Possible Spacedrives]. These ideas were further examined in the book Marc edited with Eric Davis, Frontiers of Propulsion Science, where many such concepts were subjected to serious scientific scrutiny. When I discussed all this in emails with Marc, he responded:

“The dominant problem is the “lottery ticket” mentality (a DARPA norm), where folks are more interested in supporting a low-cost long-shot, rather than systematic investigations into the relevant unknowns of physics. In the ‘lottery ticket’ approach, interest is cyclical depending if there is someone making a wild claim (usually someone the sponsor knows personally – rather than by inviting concepts from the community). With that hype, funding is secured for ‘cheap and quick’ tests that drag out ambiguously for years (no longer quick, and accumulated costs are no longer cheap). The hype and null tests damage the credibility of the topic and interest wanes until the next hot topic emerges. That is a lousy approach.

“That taint of both the null results and ‘lottery ticket’ mentality is why the physics community ignores such ambitions. I tried to attract the larger physics community by putting the emphasis on the unfinished physics, and made some headway there. When the emphasis is on credibility (and funding available), physicists will indeed pursue such topics and do so rigorously. And they will more quickly drop it again if/when the lottery ticket advocates step up again.”

Marc advocates a strategic approach, which he tried to establish as the preferred norm at NASA BPP, thus identifying the most ‘relevant’ open question in physics, and then getting reliable research done on those topics, thereafter letting these guide future inquiries. He believes that the most relevant open questions in physics deal with the source (unknown) and deeper properties of inertial frames (conjectured). Following those unknowns are the additional unfinished physics of the coupling of the fundamental forces (including neutrino properties).

In light of this pivotal period in space history and the ever-increasing contributions of private individuals and organizations, it seems reasonable to conclude that now is an excellent time to establish a well funded facility to continue the work of the Stine et al. team and the NASA Breakthrough Propulsion Physics program.

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The Persistent Case for Exomoon Candidate Kepler-1708 b-i

We started finding a lot of ‘hot Jupiters’ in the early days of planet hunting simply because, although their existence was not widely predicted, they were the most likely planetary types to trigger our radial velocity detection methods. These star-hugging worlds produced a Doppler signal that readily showed the effects of planet on star, while smaller worlds, and planets farther out in their orbits, remained undetected.

David Kipping (Columbia University) uses hot Jupiters as an analogy when describing his own indefatigable work hunting exomoons. We already have one of these – Kepler-1625 b-i – but it remains problematic and unconfirmed. If this turned out to be the first in a string of exomoons, we might well expect all the early finds to be large moons simply because using transit methods, these would be the easiest to detect.

Kepler-1625 b-i is problematic because the data could be showing the effects of other planets in its system. If real, it would be a moon far larger than any in our system, a Neptune-sized object orbiting a gas giant. In any case, its data came not from Kepler but from Hubble’s Wide Field Camera 3, and with but a single transit detection. In a new paper in Nature Astronomy , Kipping describes it as an ‘intriguing hint’ and is quick to point out how far we still are from confirming it.

This ‘hint’ is intriguing, but it’s also useful in making the case that what exomoon hunters need is an extension of the search space. While the evidence for Kepler-1625 b-i turned up in transit timing variations that indicated some perturbing effect on the planet, how much more useful would such variations be if detected in a survey of gas giants focusing on long-period, cool worlds, of which there is a small but growing catalog? These planets are harder to find transiting because, being so much farther from their star, they move across its surface as seen from Earth only in cycles measured in years. That means, of course, that as time goes by, we’ll find more and more of them.

But we now have a sample of 73 cool worlds that Kipping and colleagues analyze, bringing their exomoon detection toolkit to bear. The method of their selection homes in on worlds with a radius no less than half that of Jupiter, and with either a period of more than 400 days or an equilibrium temperature less than 300 K. A final qualifier is the amount of stellar radiation received from the star. Of the initial 73 worlds, three had to be rejected because the data on them proved inadequate for assessment.

So we have 70 gas giants and a deep dive into their properties, looking for any traces of an exomoon. The team sought planets in near-circular orbits, knowing that eccentric orbits would lower the stability needed to produce a moon, also looking for at least two transits (or preferably more), where transit timing variations could be detected. The model of planet plus moon needed to stand out, with the authors insisting that it be favored over a planet-only model by a factor of no less than 10; Kipping describes this as “the canonical standard of strong evidence in model selection studies.”

Out of the initial screening criteria, 11 planets emerged and were subjected to additional tests, refitting their light curves with other models to examine the robustness of the detection. Only three planets survived these additional checks, and only one emerged with a likely exomoon candidate: KIC-8681125.01. KIC stands for Kepler Input Catalog, a designation that changes when a planet is confirmed, as this one subsequently has been. Thus our new exomoon candidate: Kepler 1708-b-i.

Image: The discovery of a second exomoon candidate hints at the possibility that exomoons may be as common as exoplanets. Just as our Solar System is packed with moons, we can expect others to be, and it seems reasonable that we would detect extremely large exomoons before any others. Image credit: Helena Valenzuela Widerström.

We know all too little about this candidate other than the persistence of the evidence for it. Indeed, in a Cool Worlds video describing the effort, Kipping gets across just how firmly his team tried to quash the exomoon hypothesis, motivated not only by the necessary rigors of investigation but also by frustration born of years of unsuccessful searching. Yet the evidence would not go away. Let me quote from the paper on this:

The Bayes factor of the planet-moon model against the planet only is 11.9, formally passing our threshold of 10 (strong evidence on the Kass and Raftery scale). Inspection of the maximum-likelihood moon fit, shown in Fig. 2, reveals that the signal is driven by an unexpected decrease in brightness on the shoulder of preceding the first planetary transit, as well as a corresponding increase in brightness preceding the egress of that same event. The time interval between these two anomalies is approximately equal to the duration of the planetary transit, which is consistent with that expected for an exomoon . The second transit shows more marginal evidence for a similar effect. The planet-moon model is able to well explain these features, indicative of an exomoon on a fairly compact orbit, to explain the close proximity of the anomalies to the main transit…

That ‘pre-transit shoulder’ shouldn’t happen, and it turns up in all the models used. It looks remarkably like the signature of an exomoon. Here’s the figure mentioned above:

Image: This is Figure 2 from the paper. Caption: Transit light curves of Kepler-1708 b. The left/right column shows the first/second transit epoch, with the maximum-likelihood planet-moon model overlaid in solid red. The grey line above shows the contribution of the moon in isolation. Lower panels show the residuals between the planet-moon model and the data, as well as the planet-only model. BJD, barycentric Julian date; UTC, coordinated universal time. Credit: Kipping et al.

I won’t go through the complete battery of tests the team used to hammer away at the exomoon hypothesis – all the details are available in the paper – but as you would imagine, starspots were considered and ruled out, the moon model fit the data better than all alternatives, the transit signal to noise ratio was strong, and the pre-transit ‘shoulder’ is compelling. Numerous different algorithms for light curve analysis were applied to the modeling of this dataset. Indeed, the paper’s discussion of methods is itself an education in lightcurve analysis.

Describing Kepler 1708-b-i as “a candidate we cannot kill,” Kipping and team present a moon candidate that is 2.6 times larger than Earth and 12 planetary radii from its host planet, which happens to be about the distance of Europa from Jupiter. The exomoon’s mass is unknown, but constrained to be less than 37 Earth masses. The F-class host star, around which the planet orbits every 737 days, is some 5500 light years from Earth.

How such a moon might form raises a host of questions:

There are several broad scenarios for moon formation: planet-planet collisions, formation of moons within gaseous circumplanetary disks (for example the Galilean moons) or direct capture—either by tidal dissipation or pulldown during the growth of the planet. For a gaseous planet, the first scenario is unlikely to produce a debris disk massive enough to form a moon this large. The moon is also at the extreme end of the mass range produced by primordial disks in the traditional core-collapse picture of giant-planet formation, but is easier in the case where planets form by disk instability. Such models also naturally produce moons on low-inclination orbits. Direct capture by tidal dissipation is also possible, although the parameter range for capture without merger is limited. Pulldown capture can produce large moons within ~10 Jupiter radii, with a wide range of inclinations depending on the timescale for planetary growth.

In our own system, of course, we see no moons at Venus or Mercury, and it’s worth asking whether moons are rare for planets close to their host stars. Be that as it may, the supposition here is that if Neptune-sized moons do exist, they’ll constitute the bulk of our early catalog of exomoons, just as hot Jupiters dominated our early exoplanet finds. Indeed, it’s hard to see how anything smaller could be found in Kepler data.

This exomoon candidate is smaller than the previous candidate – Kepler-1625 b-i – and on a tighter orbit. While both these discoveries retain their candidate status, they hint at the possibility that large moons like these may begin turning up in JWST or PLATO data. The authors call for follow-up transit photometry for both Kepler-1708 b-i and Kepler-1625 b-i, adding “we can find no grounds to reject Kepler-1708 b-i as an exomoon candidate at this time, but urge both caution and further observations.”

The paper is Kipping et al., “An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i,” Nature Astronomy 13 January 2022 (full text).

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A Continuum of Solar Sail Development

2020 GE is an interesting, and soon to be useful, near-Earth asteroid. Discovered in March of 2020 through the University of Arizona’s Catalina Sky Survey, 2020 GE is small, no more than 18 meters or so across, placing it in that class of asteroids below 100 meters in size that have not yet been examined up close by our spacecraft. Moreover, this NEA will, in September of 2023, obligingly make a close approach to the Earth, allowing scientists to get that detailed look through a mission called NEA Scout.

This is a mission we’ve looked at before, and I want to stay with it because of its use of a solar sail. Scheduled to be launched with the Artemis 1 test flight using the Space Launch System (SLS) rocket no earlier than March of this year, NEA Scout is constructed as a six-unit CubeSat, one that will be deployed by a dispenser attached to an adapter ring connecting the rocket with the Orion spacecraft. After separation, the craft will unfurl a sail of 86 square meters, deployed via stainless steel alloy booms. It is one of ten secondary payloads to be launched aboard the SLS on this mission.

Les Johnson is principal technology investigator for NEA Scout at Marshall Space Flight Center in Huntsville:

“The genesis of this project was a question: Can we really use a tiny spacecraft to do deep space missions and produce useful science at a low cost? This is a huge challenge. For asteroid characterization missions, there’s simply not enough room on a CubeSat for large propulsion systems and the fuel they require.”

Image: NEA Scout is composed of a small, shoebox-sized CubeSat (top left) and a thin, aluminum-coated solar sail about the size of a racquetball court (bottom left). After the spacecraft launches aboard Artemis I, the sail will use sunlight to propel the CubeSat to a small asteroid (as depicted in the illustration, right). Credit: NASA.

A solar sail ought to be made to order if the objective is eventual high performance within the constraints of low mass and low volume. Reflecting solar photons and also using small cold-gas thrusters for maneuvers and orientation, NEA Scout will be investigating a near-Earth asteroid in a size range that is far more common than the larger NEAs we’ve thus far studied. It’s worth remembering that the Chelyabinsk impactor was about 20 meters in diameter, and in the same class as 2020 GE.

The 2023 close approach to Earth will occur at a time when NEA Scout will have used a gravitational assist from the Moon to alter its trajectory to approach the asteroid. Julie Castillo-Rogez, the mission’s principal science investigator at JPL, says that the spacecraft will achieve a slow flyby with relative speed of less than 30 meters per second. Using a camera with a resolution of 10 centimeters per pixel, scientists should be able to learn much about the object’s composition – a clump of boulders and dust (think Bennu, as investigated by OSIRIS-REx), or a solid object more like a boulder?

Bear in mind that NEA Scout fits into a continuum of solar sail development, with NASA’s Advanced Composite Solar Sail System (ACS3) the next to launch, demonstrating new, lightweight boom deployment techniques from a CubeSat. The unfurled square sail will be approximately 9 meters per side. As we haven’t looked at this one before, let me add this, from a NASA fact sheet:

The ACS3’s sails are supported and connected to the spacecraft by booms, which function much like a sailboat’s boom that connects to its mast and keeps the sail taut. The composite booms are made from a polymer material that is flexible and reinforced with carbon fiber. This composite material can be rolled for compact stowage, but remains strong and lightweight when unrolled. It is also very stiff and resistant to bending and warping due to changes in temperature. Solar sails can operate indefinitely, limited only by the space environment durability of the solar sail materials and spacecraft electronic systems. The ACS3 technology demonstration will also test an innovative tape-spool boom extraction system designed to minimize jamming of the coiled booms during deployment.

Image: An illustration of a completely unfurled solar sail measuring approximately 9 meters per side. Since solar radiation pressure is small, the solar sail must be large to efficiently generate thrust. Credit: NASA.

ACS3 is scheduled for liftoff later in 2022 as part of a rideshare mission on Rocket Lab’s Electron launch vehicle from its Launch Complex 1 in New Zealand. Beyond ACS3 looms Solar Cruiser, which takes the sail size up to 1,700 square meters in 2025, a mission we’ve looked at before and will continue to track as NASA attempts to launch the largest sail ever tested in space.

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The Dyson Sphere Search

The Dyson sphere has become such a staple of SETI as well as science fiction that it’s hard to conceive how lightly Freeman Dyson himself took the idea. In a 2008 interview with Slate, he described the Dyson sphere as no more than ‘a little joke,’ and noted “it’s amusing that of course you get to be famous only for the things you don’t think are serious.” Indeed, Dyson’s 1960 paper “Search for Artificial Stellar Sources of Infrared Radiation,” was but a one-page document in Science that grew out of his notion that an intelligent civilization might not have any interest in communicating. How, then, would astronomers on Earth go about finding it?

Waste heat was his answer, a nod to the laws of thermodynamics and the detectability of such heat in the infrared. Coming hard on the heels of Frank Drake’s Project Ozma (a likewise playful name, coined out of affection for L. Frank Baum’s imaginary land of Oz), Dyson saw a search for what would come to be called Dyson spheres as a complement in the infrared to what Drake had begun to do with radio telescopes. And in fact, Dyson didn’t refer to spheres at all, but biospheres. Let me quote him on this from the 2008 interview:

I suggested that people actually start looking in the sky with infrared telescopes as well as regular telescopes. So that was the proposal. But unfortunately I added to the end of that the remark that what we’re what we’re looking for is an artificial biosphere, meaning by biosphere just a habitat, something that could be in orbit around the neighboring star where the aliens might be living. So the word biosphere didn’t imply any particular shape. However, the science-fiction writers got hold of this and imagined a biosphere means a sphere, and it has to be some big round ball, so out of that there came these weird notions which ended up on Star Trek.

Changing Notions of a Dyson Sphere

Dyson told Slate he hadn’t been thinking remotely of a shell around a star, but a ‘a swarm of objects surrounding a star,’ one that from the outside would look more or less like a dust cloud. He was also quick to give credit for the concept to writer Olaf Stapledon, who as we saw in the previous post introduced it in his novel Star Maker all the way back in 1937. And indeed, we find Stapledon writing about a way to tap the energies of entire stars through “a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed…”

That Star Trek reference is to an episode of Star Trek: The Next Generation that ran on October 12, 1992 (Season 6 Episode 4). It depicted the Enterprise crew encountering a shell-like Dyson sphere. Dyson didn’t mention Larry Niven’s wonderful Ringworld (1970), which draws on the shell concept to envision a vast ring around its star, a ‘cut-through’ of a full Dyson shell. I suspect he read it somewhere along the line, as he was frequently in conversation with science fiction writers who increasingly found his work a source for good ideas.

Image: Olaf Stapledon, author of Star Maker and Last and First Men.

As far as his light-hearted approach to the things he is best remembered for, I take that as a personal quirk. When I interviewed Dyson back in 2003, I found him quick to shift credit for ideas to other people and charmingly dismissive of his own contributions. I think his was an intellect so formidable that it surprised him with ideas that seemed to well up unbidden, so that in a real sense he didn’t want to lay claim to them.

And on Stapledon’s Star Maker, a further thought. It’s fun to note that Dyson sphere hunter Jason Wright at Penn State maintains a blog he calls Astrowright. It’s a fine play on words, I assume intentionally containing a nod to Stapledon. For if a shipwright is a maker of ships, an astrowright is surely a ‘star maker.’

The paper by Wright and Macy Huston that I looked at yesterday notes the distinction between Dyson shells and Dyson spheres; i.e., between a solid spherical shell and a vast collection of objects in orbit around the star, adding that ‘for simplicity of language, we refer to any configuration of a starlight-manipulating megastructure as a Dyson sphere.’ I think that’s common usage throughout the literature, saving the intriguing work on Shkadov thrusters, which are inherently asymmetrical and don’t fit Dyson sphere modeling. But moving stars is a topic for another day.

How Dyson Sphere Searches Proceed

Playful or not, it didn’t take long for Dyson’s SETI notion to take hold. In 1966, Carl Sagan and Russell Walker delved into “The Infrared Detectability of Dyson Civilizations” in a paper for the Astrophysical Journal. This too is no more than a note, but it makes the case for looking for astronomical sources that would appear as blackbodies with a temperature of several hundred K. Such detections were possible, the authors argued, but “discrimination of Dyson civilizations from naturally occurring low temperature objects is very difficult, unless Dyson civilizations have some further distinguishing feature, such as monochromatic radio-freqency emission.”

Note that last comment, because we’ll come back to it. It’s insightful in describing the nature of Dyson sphere searches and the possible results from a detection.

Which brings me to the Russian radio astronomer Vyacheslav Ivanovich Slysh, who in the 1980s examined sources identified by the Infrared Astronomical Satellite (IRAS) in a search for just the kind of waste heat Dyson had discussed. In 2000, Slysh’s work was followed up by M. Y. Timofeev, collaborating with Nikolai Kardashev (most famous, of course, for the ‘Kardashev scale’ ranking technological civilizations).

Richard Carrigan, a scientist emeritus in the Accelerator Division at the Fermi National Accel­era­tor Laboratory, went to work on IRAS data as well and in a 2009 search, used the data on 250,000 infrared sources (covering 96 percent of the sky), looking for both full and partial Dyson spheres in a blackbody temperature region from 100 K to 600 K.

The result: Some 16 candidates with temperatures below 600 K in a field of objects out to 300 parsecs. And as Carrigan noted, most of these have non-technological explanations, and all are in need of further study before any conclusions are drawn. I should also mention the searches for Dyson spheres by Jun Jugaku and Shiro Nishimura when talking about IRAS. Their work in the 1990s found no Dyson spheres around the roughly 550 stars they surveyed within 25 parsecs.

This is a good time to mention some useful background materials. The first is a video presentation Jason Wright made to a seminar at Penn State in 2020, helpfully made available online. It’s an excellent encapsulation of the Dyson sphere concept and the investigations into it, including the subsequent searches using WISE [Wide Field Infrared Survey], with higher resolution than IRAS could provide. One problem with all of these is what Wright dubs ‘infrared cirrus,’ which basically refers to diffuse dust that greatly compromises the consequent data. Carrigan would doubtless have retrieved a much higher number of candidates if he could have worked without this background.

The second reference is Wright’s overview “Dyson Spheres,” which ran in the Serbian Astronomical Journal, Issue 200 (2020), with preprint available here. For those wanting to come up to speed on the origins and development of the idea of Dyson spheres, their purpose, their engineering, and their detectability, this is an excellent resource.

Until reading the Huston & Wright paper, I had been unaware of Massimo Teodorani, whose 2014 paper in Acta Astronautica presented what he called a ‘pragmatic strategy’ for searching for Dyson spheres involving infrared excess and anomalous light curves using Spitzer data to locate such signatures at G-class stars. A common theme in much of this work is the recognition that the goal is to identify interesting targets for further study. A detection of an interesting source would not in itself be proof of an extraterrestrial civilization, but rather identification of an object that could be followed up with more conventional methods such as laser or radio search. There is no single ‘aha!’ moment, but steady and careful analysis.

The search space for Dyson spheres has been expanding dramatically. In the late 1990s, James Annis analyzed the rotational dynamics of 137 different galaxies in the Ursa Major and Virgo galaxy clusters, looking for Kardashev Type III civilizations. He found no evidence for them, but going to this scale inevitably reminds us of the Fermi paradox. As Annis told Lee Billings in 2015:

“Life, once it becomes spacefaring, looks like it could cross a galaxy in as little as 50 million years. And 50 million years is a very short time compared to the billion-year timescales of planets and galaxies. You would expect life to crisscross a galaxy many times in the nearly 14 billion years the universe has been around. Maybe spacefaring civilizations are rare and isolated, but it only takes just one to want and be able to modify its galaxy for you to be able to see it. If you look at enough galaxies, you should eventually see something obviously artificial. That’s why it’s so uncomfortable that the more we look, the more natural everything appears.”

The mid-infrared WISE survey [Wide-field Infrared Survey Explorer] gave us far more data within which to conduct such a search. Wright’s work using WISE data has been extensively covered in these pages, including an article he wrote for Centauri Dreams called Glimpsing Heat from Alien Technologies, the name of the program he started at Penn State. The G-HAT program led to a search through WISE data that culled out some 100,000 galaxies looking for unusually strong signatures in the mid-infrared. Fifty of these galaxies showed interesting infrared properties, though as with Carrigan’s results, without any definitive signs of a technology.

I’ve quoted Wright on this result before, but that was years ago, so let me pull this out again:

“Our results mean that, out of the 100,000 galaxies that WISE could see in sufficient detail, none of them is widely populated by an alien civilization using most of the starlight in its galaxy for its own purposes. That’s interesting because these galaxies are billions of years old, which should have been plenty of time for them to have been filled with alien civilizations, if they exist. Either they don’t exist, or they don’t yet use enough energy for us to recognize them.”

Note the phrasing: This is explicitly a search for Kardashev Type III, one manifestation of which would be civilizations that fill their galaxy with Dyson spheres. The G-HAT results do not close the book on Dyson sphere searches, but they do tell us that such Type III civilizations are not detected within the energy levels we might expect.

Image: A false-color image of the mid-infrared emission from the Great Galaxy in Andromeda, as seen by Nasa’s WISE space telescope. The orange color represents emission from the heat of stars forming in the galaxy’s spiral arms. The G-HAT team used images such as these to search 100,000 nearby galaxies for unusually large amounts of this mid-infrared emission that might arise from alien civilizations. Credit: NASA/JPL-Caltech/WISE Team.

G-HAT is all about putting upper limits on energies emitted as waste heat in nearby galaxies, and while Dysonian SETI methods seem to diverge from earlier radio and laser SETI, the two approaches actually work quite well together. As the search continues, anomalous objects form a catalog which can be consulted by the entire SETI community, using its resources at various wavelengths to probe the result more deeply.

Search References

Although I’m out of time today, I want to make the point that Dyson spheres shouldn’t be thought of purely as means of energy collection, because the manipulation of a star at this level could involve changing the character of the star itself. In a future article I’ll look at why a civilization might want to do this, and who has been investigating the matter. Until then, here are references to the searches we’ve talked about today.

The Sagan and Walker paper is “The Infrared Delectability of Dyson Civilizations,” Astrophysical Journal 144 (3), (1966), p. 1216 (abstract).

The Slysh paper is “A Search in the Infrared to Microwave for Astroengineering Activity,” in The Search for Extraterrestrial Life: Recent Developments, M. D. Papagiannis (Editor), Reidel Pub. Co., Boston, Massachusetts, 1985, p. 315.

Timofeev and Kardashev wrote “A Search of the IRAS Database for Evidence of Dyson Spheres,” Acta Astronautica 46 (2000), p. 655.

Richard Carrigan’s 2009 study is “The IRAS-based Whole-Sky Upper Limit on Dyson Spheres,” Astrophysical Journal 698 (2009), pp. 2075-2086. Abstract / preprint.

The Teodorani paper is “A strategic “viewfinder” for SETI research,” Acta Astronautica Vol. 105, Issue 2 (December 2014). Abstract.

On G-HAT, see Griffith et al., “The ? Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. III. The Reddest Extended Sources in WISE,” Astrophysical Journal Supplement Series Vol. 217, No. 2, published 15 April 2015 (abstract / preprint).

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Dyson Sphere ‘Feedback’: A Clue to New Observables?

Although so-called Dysonian SETI has been much in the air in recent times, its origins date back to the birth of SETI itself. It was in 1960 – the same year that Frank Drake used the National Radio Astronomy Observatory in Green Bank, West Virginia to study Epsilon Eridani and Tau Ceti – that Freeman Dyson proposed the Dyson sphere. In fiction, Olaf Stapledon had considered such structures in his novel Star Maker in 1937. As Macy Huston and Jason Wright (both at Penn State) remind us in a recent paper, Dyson’s idea of energy-gathering structures around an entire star evolved toward numerous satellites around the star rather than a (likely unstable) single spherical shell.

We can’t put the brakes on what a highly advanced technological civilization might do, so both solid sphere and ‘swarm’ models can be searched for, and indeed have been, for in SETI terms we’re looking for infrared waste heat. And if we stick with Dyson (often a good idea!), we would be looking for structures orbiting in a zone where temperatures would range in the 200-300 K range, which translates into searching at about 10 microns, the wavelength of choice. But Huston and Wright introduce a new factor, the irradiation from the interior of the sphere onto the surface of the star.

This is intriguing because it extends our notions of Dyson spheres well beyond the habitable zone as we consider just what an advanced civilization might do with them. It also offers up the possibility of new observables. So just how does such a Dyson sphere return light back to a star, affecting its structure and evolution? If we can determine that, we will have a better way to predict these potential observables. As we adjust the variables in the model, we can also ponder the purposes of such engineering.

Think of irradiation as Dyson shell ‘feedback.’ We immediately run into the interesting fact that adding energy to a star causes it to expand and cool. The authors explain this by noting that total stellar energy is a sum of thermal and gravitational energies. Let’s go straight to the paper on this. In the clip below, E* refers to the star’s total energy, with Etherm being thermal energy:

When energy is added to a star (E? increases), gravitational energy increases and thermal energy decreases, so we see the star expand and cool both overall (because Etherm is lower) and on its surface (because, being larger at the same or a lower luminosity its effective temperature must drop). A larger star should also result in less pressure on a cooler core, so we also expect its luminosity to decrease.

Image: Artist’s impression of a Dyson sphere under construction. Credit: Steve Bowers.

Digging into this effect, Huston and Wright calculate the difference in radius and temperature between the normal and irradiated stellar models. Work on irradiated stars goes back to the late 1980s, and includes the interesting result that a star of half a solar mass, if subjected to a bath of irradiation at a tempearature of 104 K, has its main sequence lifetime shortened by approximately half. The star expands and cools overall, but the distribution of the thermal energy causes its central temperature to increase.

A bit more on this background work: The 1989 paper in question, by C. A. Tout and colleagues, has nothing to do with Dyson spheres, but provides data on stellar irradiation of the sort that would be produced by proximity to a quasar or active galactic nucleus. Tout et al. worked on isotropic radiation baths at constant temperatures up to 104 K, finding that while the effects on stars whose energy is radiative are minor, convective stars increase in size. Keep in mind that cooler stars of low-mass are fully convective; hotter and more massive stars transport their energies from the interior through a radiative zone that forms and expands from the core.

Applying this to Dyson spheres, a star surrounded by technology would have light reflected back onto the star, while at the same time the sphere would become warm and emit thermal energy. I was intrigued to see that the paper gives a nod to Shkadov thrusters, which we’ve discussed in these pages before – these are stellar ‘engines’ using a portion of a star’s light to produce a propulsive effect. See Cosmic Engineering and the Movement of Stars, as well as Greg Benford’s look at the physics of the phenomenon, as developed by himself and Larry Niven, in Building the Bowl of Heaven.

Huston and Wright model how a Dyson sphere would affect the structure and evolution of a star, incorporating Dyson sphere luminosity as returned to the surface of the star. Each star is modeled from the start of its enclosure within the Dyson Sphere to the end of its main sequence lifetime. Beyond luminosity, the authors use Wright’s previous work on Dyson sphere parameters and his formulation for radiative feedback, while deploying a tool called Modules for Experiments in Stellar Astrophysics to assist calculations.

The authors consider stars in a mass range from 0.2 to 2 solar masses while varying luminosity fractions from 0.01 to 0.50. The effects of energy feedback on stars and the calculations on Dyson sphere properties produce absolute magnitudes for combined systems incorporating a central star and the Dyson sphere around it. From the paper:

Irradiated stars expand and cool. A Dyson sphere may send a fraction of a star’s light back toward it, either by direct reflection or thermal re-emission. This returning energy can be effectively transported through convective zones but not radiative zones. So, it can have strong impacts on low mass main sequence stars with deep convective zones which extend to the surface. It causes them to expand and cool, slowing fusion and increasing main sequence lifetimes. For higher mass stars with little to no convective exterior, the returned energy cannot penetrate far into the star and therefore has little effect on the star’s structure and evolution, besides some surface heating.

The effects are observationally significant only for spheres with high reflectivity or high temperatures – remember that Dyson assumed a sphere in the ~300 K area to correspond to a planet in the habitable zone. The authors combine the spectrum of the host star and the Dyson sphere into a ‘system spectrum,’ which allows them to calculate absolute magnitudes. The calculations involve the star itself, the interior of the sphere (both would be hidden) and the exterior of the sphere, which would be unobscured.

Wright has previously developed a set of five defining characteristics of a Dyson sphere, involving the intercepted starlight, the power of the sphere’s thermal waste heat, its characteristic temperature and other factors in a formalism called AGENT. The authors run their calculations on hot Dyson spheres and their opposite – cold, mirrored Dyson spheres that return starlight to the star without significant heating. Thus we go through a range of Dyson spheres intercepting starlight, including the familiar notion:

As the classical idea of a Dyson sphere, we can examine a solar mass star with low transmission of starlight through the sphere and a Dyson sphere radius of roughly 1 AU. We see that the feedback levels are very low and that the systems will appear, relative to a bare solar mass star, to be dimmed in the optical range and reddened in both optical and infrared colors.

Notice the range of temperatures we are talking about, for this is where we can expand our thinking on what a Dyson sphere might involve:

For our 0.2 and 0.4 M stars, feedback levels above roughly 1% cause at least a 1% change in nuclear luminosity; their effective temperatures do not significantly change. For our 1 and 2 M stars, feedback levels above roughly 6% cause at least a 1% change in the star’s effective temperature; their nuclear luminosities do not significantly change. Physically, these limits may correspond with a cold, mirrored surface covering the specified fraction of the star’s solid angle. For light-absorbing, non-reflective Dyson spheres, these feedback levels correspond to very hot spheres, with temperatures of thousands of Kelvin.

Dyson spheres in the latter temperature ranges are utterly unlike the more conventional concept of a civilization maintaining habitable conditions within the shell to gain not just energy but vastly amplified living space. But a hot Dyson sphere could make sense from the standpoint of stellar engineering, for feedback mechanisms can be adjusted to extend a star’s lifetime or reduce its luminosity. Indeed, looking at Dyson spheres in the context of a wide range of feedback variables is useful in helping jog our thinking about what might be found as the signature of an advanced technological civilization.

The paper is Huston & Wright, “Evolutionary and Observational Consequences of Dyson Sphere Feedback,” accepted at the Astrophysical Journal (abstract / preprint). The paper by Tout et al. is “The evolution of irradiated stars ,” Monthly Notices of the Royal Astronomical Society Volume 238, Issue 2 (May 1989), pp. 427–438 (abstract). For an overview of Dyson spheres and their background in the literature, see Wright’s “Dyson Spheres,” Serbian Astronomical Journal Issue 200, Pages: 1-18 (2020). Abstract / preprint. Thanks to my friend Antonio Tavani for the early pointer to this work.

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