Centauri Dreams

I don’t usually post comments at the top of the site, but I’m making an exception here for a couple of reasons. The recent paper I reviewed by Clément Vidal and colleagues covering technosignatures and strategies for detection is a significant work, the kind of consolidation of resources the field needs as the original radio and optical-oriented SETI expands into new realms. We now have options calibrated for intelligence via archival and observational detection of megastructures, planetary or stellar engineering, or other projects far beyond our own level of technology. Dean Zierman’s thoughts on the Vidal paper open a number of issues and highlight assumptions we’ll always need to examine. Dean is a telecommunications expert specializing in radio frequency communications, one who has been deployed to over 150 disasters and dangerous events including earthquakes, hurricanes, tsunamis and the 9/11 attacks on the United States. He has served as a subject matter expert on communications in hostile or austere environments for multiple agencies and organizations. Herewith his thoughts on the technosignature hunt and the recent review paper, which I hope will feed further energy into our conversation.

by Dean Zierman

Observations in relation to this paper. I’ll be quoting frequently from the document.

“Another limitation is the meaning of the lifetime L of a civilization. What does it mean for an interstellar civilization seeding life or colonies, or for galactic colonization models? Some colonies might go extinct, while others could transform so much that the link with parent civilization or others would be lost.”

This does not consider how long a specific technology lasts.

It also does not consider how different types of technology interact with each other or with civilization. To be fair, these ideas have not yet been covered in the literature.

“This galactic and stellar context shows that there is ample time for advanced civilizations to have explored the galaxy systematically.”

“It means that there could have been up to 4500 opportunities for visitation by one single spacefaring civilization over the lifetime of the Earth.”

The windowing issue remains unaddressed. Specifically, it is necessary to determine the duration during which an advanced civilization might explore or be detectable within the Earth’s solar system, as well as the period during which an Earth based civilization has existed in the solar system. It is also important to assess whether there is any temporal overlap between these two windows.

“Even before searching for traces of past extraterrestrial visitation, we can ask whether we are the first advanced civilization in the geological history of Earth. It turns out that traces of a civilization –even one going through an anthropocene phase– would be very challenging to detect because of erosion dynamics such as surface weathering or plate tectonics. These constraints are discussed through the Silurian hypothesis (Schmidt and Frank 2019), and the authors conclude that artefacts or fossilized examples of a population older than 4 million years (Ma) would be unlikely to be found.”

“Still, what could be searched for in this 4 Ma window? Here are a few examples. We could look for evidence of large scale agriculture that would have led to a disruption of the soil nitrogen cycle. Another line of research could look for evidence of mining, such as anomalous geological structures 7 that would be indicative of large mining operations.”

This suggests that on Earth, a civilization eventually produces enough pollution to be noticed. In the past 12,000 years, about 70 civilizations have collapsed.

These might be the same technological signs we could detect on planets outside our solar system.

“According to Schmidt and Frank (2019), all of the pollution of the anthropocene would fit within 1 cm of sediment layer, which makes sense given how short our industrial civilization has existed on geological timescales. This explains why even if there was a pre-human civilization which went through its own anthropocene, we might not have noticed it in our sediment analysis yet, while also leaving open the possibility that we could discover such a layer in the future.”

Over the last 12,000 years, only a few have reached a level where they may have produced detectable pollutants. Of those, only the most recent may have caused changes large enough to be detected in isotope ratios or radioactive isotope production.

“This makes sense if we look at the Barrow scale that proposes that civilizations progress by increasing their ability to manipulate, manufacture, and control smaller and smaller scales (see Barrow 1998, Vidal 2014, and the Barrow scale section 4.1.0).”

Another way to look at this is that as a civilization’s ability to manipulate and manufacture at smaller scales increases, it becomes less necessary to do the same thing at larger scales. This smaller scale also means that what is built at a larger scale is likely much more efficient and harder to detect at interstellar distances.

“Although there are reports of unidentified flying objects (UFO) dating back over millennia (Stothers 2007), the first modern sighting to popularize UFOs was reported by Kenneth Arnold in 1947:”

“How many reports are really unidentified or anomalous? Out of 12,000 reports analyzed in Project Blue Book, 6% of them remained unexplained. How are UAP reports categorized? 90%-95% end up as (1) explained phenomenon. The remaining 5-10% of reports could end up (2) unexplainable due to lack of credible data. Those that do have credible data imply either (3) an unknown physical mechanism, or (4) an unknown manifestation of extraterrestrial intelligence (see Fig. 4).”

This would seem to be a straightforward method for looking for UFOs, or the modern term UAPs. Unfortunately, due to the rise of our own technology, the noise level is rising exponentially faster than our ability to screen it out of any possible signal. This is similar to the phenomenon of using radio to look for SETI.  Given that we are generating UAPs with our own technology, the technology we’re using is also generating more noise.
 
It has been observed, somewhat jokingly, that the resolution of our digital cameras has greatly increased, leading one to believe that the resulting pictures of UAPs would be much clearer and thus more definitive.  What has happened is that we have increased our ability, with the increase in the number of possible detectors, to detect more UAPs, which are also at the edge of their detection resolution.  In short, we have many more fuzzy, questionable photos.

“Villarroel et al. (2021, 2022a) conducted an analysis of 1950s archival photographic plates from the Palomar Observatory Sky Surveys (POSS-I, 1948-1958 and POSS-II, 1980s-1990s).”

This is another example of our technology generating noise that is becoming impossible to filter out. If it weren’t for all the “stuff” we have sent into space, this would be a straightforward and definitive technique. But we have already FUBARed this to a near hopeless extent. The latest ground-based telescopes coming online should be able to detect these objects if they existed. Oops, I guess that’s not happening.

An idea occurs to me. The massive array of Starlink satellites has started using their star trackers to look for other satellites above them and track them for collision avoidance. It might be possible for someone to obtain copies of this data and, using modern algorithms, identify satellites in much higher orbits that are ignored for tracking, and that could be ETI satellites’ orbits.

“To sum up, many present and future observational facilities such as the Vera Rubin Observatory or the Nancy Grace Roman Space Telescope as well as planned missions in the solar systems that together with machine learning techniques have the potential to enable a much more systematic and comprehensive search for anomalies and possibly artefacts in our own solar system (see also Haqq-Misra et al. 2022c).”

Almost definitionally the search for anomalies is the 1st step in the scientific process. So just searching for anomalies should be considered an adequate scientific reason to perform many of the studies discussed.

The issue with the Kardashev scale, and even more so with the Barrow scale, is that they do not focus on what a civilization is trying to achieve with its technology. For example, in Larry Niven’s novel The Ring World Engineers, a civilization spends vast resources and time building a single massive structure. But do they really need something that big? Another problem is that you can’t use the structure until it’s finished. Since it’s just one structure, everything depends on it, so if it fails, everything is lost. It’s like putting all your eggs in one basket. If the goal is just to have more space, wouldn’t it be smarter to build several Banks orbitals? Or maybe it would make more sense to create a lot of O’Neill cylinders inside asteroids. These cylinders could even be used as slow ships to travel to other star systems.

The whole question of Big Dumb Objects begs the question of why build a BDO?  Almost anything you could think of as a reason to build them, you could do much better with other technology. BDO’s then become the ultimate SETI MacGuffin. Just because somebody can build something doesn’t mean that they would build it, especially when there are better ways to accomplish the same thing. All of this leads one to speculate that more advanced civilizations would not appear high on the K scale but would be much higher on the B scale, with smaller, very efficient, and dispersed objects with low radiation indexes that are hard to find.  This just means we should be looking for smaller, darker objects rather than the big, bright, flashing ones.

4.2. Surface Technosignatures
4.3. Atmospheric Technosignatures
4.4. Orbital Technosignatures

These sections, in my opinion, show the paper’s biggest limitation. The information is technically correct but lacks a clear framework for how technology creates civilizations that then modify technologies, which in turn change civilizations. It misses an important time and scale element.

As I mentioned above, civilizations and their interdependent technologies have risen and fallen. The vulnerability of civilizations and technology to collective and cumulative risks creates a myopic view of the detectability of these technologies in both time and scale. For 10,000 years, you would’ve been looking for just fire on earth, for example.  How would you be able to tell the difference between civilization and a natural phenomenon?

As technology advances, detection actually becomes harder. For example, we once had FM radio stations that broadcast in all directions with over a megawatt of power in the hundred megahertz range. This created a window of about 60 to 80 years when this technology could be detected. Now, we use higher frequency cellular systems that offer two way communication for many more people at much higher data rates. Instead of a few powerful narrowband transmitters that could be picked up with large antennas (perhaps on the backside of the moon), we now have many low power wideband transmitters. These blend into the background noise, making them much harder to detect.

Many technologies can be detected during their rise and fall, sometimes even within the short lifespan of a single civilization. One example not covered in the paper is the development of laser communications between satellites and ground stations. Although this technology is just emerging, it is unclear how long we will remain detectable. Among the technologies discussed, this type of laser communications may offer the best chance to find evidence of technological activity. It could be the easiest to detect and might be picked up by current or future detectors. It may also have the longest detectability period and the largest scale compared to the other technologies mentioned.

4.2. Surface Technosignatures
4.3. Atmospheric Technosignatures

Many of these might have such short detection windows that they approach the probability asymptote.

4.4. Orbital Technosignatures
4.5. Exoplanetary System Technosignatures
4.6. Multiplanetary Systems and Terraforming
5. Stellar Technosignatures

This brings up concepts the same as BDO’s becoming more SETI MacGuffins.
SETI MacGuffins with low detection windows that approach the probability asymptote.

6. Interstellar Technosignatures

I found this section to be the most interesting and comprehensive. That’s not surprising, as this is the field I’m most familiar with, other than resisting the fall of civilization: communications.

Although the authors were very comprehensive, I did notice two aspects of communication they did not discuss. The mentioned modulation schemes, but they did not mention encoding or spread-spectrum schemes. These are actually related.  For example, you could use a frequency-hopping system to spread your data over a wider bandwidth, which can give you some advantages in certain propagation conditions. You could also use direct-sequence spread spectrum, which has its own advantages, and you can combine the two.
 
The authors also mentioned at the beginning that it was assumed that all communications would be essentially noiseless. In reality, that doesn’t exist. All modern communication systems either have inherent resistance to this noise or incorporate some form of error correction into the modulated data. This could be bidirectional error correction, which is what is used mostly on the Internet, for example, but would be impractical at interstellar distances, or more likely, some form of forward error correction.

I am a little disappointed but not surprised that they did not raise the issue of toxic information, as that concept is the antithesis of most scientists’ ideologies.

7. Travel Technosignatures

Although interstellar travel is often downplayed, aside from the time window problem, it might be the most likely technical signature to be detected. Sadly, it is usually dismissed as impossible by those who want to show their supposed intelligence over others’ ignorance. This mostly shows them cherry-picking facts and lacking imagination. The time window problem is discussed further up, as in when they have visited while we could also observe them. In this particular case, it’s when they are traveling that we can observe them.

“In comparison to chemical rockets, a nuclear fission source of energy is ∼ 105 more efficient, a nuclear fusion ∼ 106 and the absolute theoretical maximum, matter-antimatter annihilation is ∼ 108 times more efficient (Mallove and Matloff 1989). This means that crossing the threshold from chemical rockets to nuclear fission propulsion leads to a gain of 5 orders of magnitude in efficiency, while going from fusion to matter-antimatter means ’only’ gaining 3 orders of magnitude.”

I hadn’t come across this fact before, and it’s pretty interesting. It suggests that interstellar travel might not require matter-antimatter after all. Maybe it could be done with technology we already have or are close to developing.

I wish the authors had extended the year scale further. That way, it would be easier to see where the Starshot project fits on the growth line.

“As Heller (2017) noted, reaching 0.1c would not happen before 150 years from now, assuming this exponential growth continues unabated. In that sense, the project might have been a few centuries ahead of its time!”

I have some doubts about Heller’s projection. It assumes that a project like Starshot would face the same energy limits as missions that are restricted by the rocket equation.

“In the case of directed energy, Lubin (2016) proposed a general search for directed energy beaming activities, and Guillochon and Loeb (2015) proposed to look for leakage from a light sail spacecraft traveling between planets of a given stellar system. This search can be done in synergy with optical laser SETI searches. However, note that if the beam matches perfectly with the size of the sail, then there is no leakage to detect, so we would be looking for leakage from a system designed to minimize it, which may be hard.”

This idea assumes that leakage only happens in the forward direction. However, a system like this would actually have a lot of leakage in the backscatter, since the light needs to go that way for the system to function. It could also be harder to detect if the launch trajectory is directly opposite the launching star system.

7.4. Ramjet
7.5. Planet engines
7.6. Stellar engines
7.7. Newtonian gravitation for propulsion

More SETI MacGuffins.

7.8. Spacetime manipulation for propulsion Spacetime – Bubble Propulsion System -Traversable Wormholes

These ideas might turn out to be possible one day, but right now they are beyond what physics can explain. Since they do not fit into our current knowledge, guessing whether we could ever detect them is just speculation. For now, they are as unlikely as Harry Potter’s magic wand. Still, it is fun to imagine and share stories about them. After all, dreaming about the impossible has sometimes helped us turn fantasy into reality.

8. Galactic and Beyond

“Given the tremendous distances involved, the magnitude of energy usage that could feasibly be observed by astronomers here on Earth would have to be immense, implying that such technosignatures would have to be produced by Type III civilizations or beyond.”

This puts the whole concept firmly in the realm of SETI MacGuffins with low detection windows that approach the probability asymptote. If a civilization like that existed, we wouldn’t need to search for them because we would already be part of it.

8.4. The Simulation Hypothesis

In the end, all of this is really about the idea of living in a dream. This is more of a philosophical view, since if reality were a simulation, we could only notice it if the simulation let us.

9. Discussion
9.1. Biosignatures and technosignatures

“Technological fossils—traces of a previous civilization on Earth (see Section 2)—or technological trash, such as inactive, broken probes in our solar system, broken Dyson spheres (Loeb 2023), and as Holmes (1991) noted more generally, rubbish, debris, defunct equipment, and defunct spacecraft are also potential technosignatures. For attempts to quantify this longevity factor of technosignatures, see Lingam and Loeb (2019) and Ćirković et al. (2019).”

This might be our best chance to find signs of previous advanced civilizations on earth. It is also among the best chances to find ETI. The chances of finding anything on earth due to geology and environmental factors become vanishingly small as you approach deep time.

“This is a blind spot in traditional natural sciences that seeks to study causal effects in a detached and objective way, and thus neglects or avoids the complexities of modelling agents (see Frank et al. 2024).”

“Thomas Kuhn (1996), who wrote in his foundational The Structure of Scientific Revolutions: “If all members of a community responded to each anomaly as a source of crisis or embraced each new theory advanced by a colleague, science would cease. If, on the other hand, no one reacted to anomalies or to brand-new theories in high-risk ways, there would be few or no revolutions.”

This issue affects all areas of science. Science operating as a business rather than a method often discourages ideas that challenge current thinking. As a result, the business side of science is a major reason our understanding of the world has not progressed much in the last 50 years and is becoming stagnant.

This section offers useful information on different ways to detect anomalies. In the end, finding ETI anomalies among all the noise will probably be the most important part of SETI.

“Arguably the ‘purest’ approach to signal analysis involves the use of Turing machines (Turing 1937) that represent the most general and universal of all computational devices.”

Science fiction

“The role of imagination is key to the scientific process. The core difference between science and science fiction is that science fiction aims to create emotional and engaging stories for human entertainment, while science tries to gain new insights, knowledge, and understanding, highly constrained by its methods and criteria. A systematic study of major science fiction novels to derive technosignature strategies would be worthwhile, although outside the scope of this paper. There is a rich interplay and synergy between science and science fiction (see Nováková et al. 2023): many new ideas start in science fiction and inspire scientists, while new scientific theories and discoveries inspire hard science fiction authors. However, science fiction is a double-edged sword for academic SETI. On the negative side, it contributes to the “giggle factor,” creating implicit associations between entertainment and serious science. On the positive side, science fiction addresses the question of extraterrestrial life and intelligence, which is so popular and fascinating that it is a huge opportunity for science education and outreach to draw people of all ages towards science.”

Scientists need to get over themselves and leave their ivory towers. The ivory towers are not reality, and they must stop hiding behind the business of science. This is where Carl Sagan excelled and did more than anyone before or since to draw people of all ages toward science. They need to build real world baloney detectors as Carl Sagan advised, not ones based on the business of science or their view from ivory towers. With a real world baloney detector, they would be equipped to understand and distinguish between something to giggle at and something to investigate.

Looking for extraterrestrial life in the form of biosignatures will involve peering into the constituents of a planetary atmosphere and identifying out of equilibrium gases that tell us something biological is going on. But here’s the problem, as demonstrated recently in work on the exoplanet K2-18b. We’ve identified dimethyl sulfide at this world, which might just be a life detection. On Earth, dimethyl sulfide comes from dimethylsulfoniopropionate (DMSP), a compound that is produced by phytoplankton and has a clear role to play in marine ecosystems. The problem is that subsequent research on K2-18b has pointed to possible instrumental errors and the relatively low level of statistical confidence in the detection.

Nothing turned up in a SETI check of this world with the Very Large Array (New Mexico) and MeerKAT (South Africa, precursor to the Square Kilometre Array), which isn’t particularly surprising. We’re going to be getting more biosignature candidates in the coming decades as our instrumentation keeps improving, but even with the Habitable Worlds Observatory, the result of any interesting detection is going to be a race to figure out ways to produce the same gases without biology. Microbial life may be all over the galaxy (I suspect that it is), but I’m less and less sure we’re going to get any biosignature detections that anyone will consider ironclad this way.

Emergence of the Technosignature

That makes technosignatures more interesting than ever, especially since there is a good case to be made that any civilization we detect will be substantially older than ourselves, and thus gifted with technological powers we may not be able to imagine. Astroengineering is but one wonderful example of what we might encounter, but would we recognize it? Asking the same about vast structures like Dyson spheres or swarms is a hot topic because we do have current tools for observing them, and also archival data that may just contain evidence of them. But we have to know what to look for.

To that end, a new paper has just arrived that is going to be a touchstone for technosignature research for some time to come. Clément Vidal (Vrije Universiteit Brussel, Brussels, Belgium) and a large team of co-authors have produced the critical review document needed to consolidate what has been done so far and help newcomers to the work orient themselves with the directions that will be needed next. This is a satisfying event, because it also comes at a time when we are about to get the first graduate-level text on interstellar flight, which should itself inspire future careers. We’ve also just had Jason Wright’s first-rate textbook on SETI, which means that we are preparing the way for interstellar issues to become embedded in college and graduate school curricula. And that’s how we get the next generation of scientists.

Image: Philosopher Clément Vidal’s background in logic and cognitive sciences has led him into new formulations for SETI, as in his 2014 book The Beginning and the End: The Meaning of Life in a Cosmological Perspective. The current paper is an in-depth examination of past and present searches for technosignatures, with suggestions on the path forward. Credit: Clément Vidal.

Both these books are splendid, and I’ll have more on each as soon as the former is publicly released (very soon now). For now, though, let’s dig into the Vidal paper, which acknowledges the new Wright text and its coverage of the theory and practice of technosignature science as well as SETI itself. What Vidal and team set out to do is to create a definitive reference for the kind of technosignatures that have emerged thus far in the field, and the methods we might use to detect them. Those of us who think in terms of distant stars as possible sites for technosignatures may be surprised at the spatial scale strategy here, which actually begins with technosignature searching on Earth, then out to the Moon, the inner Solar System, the Oort Cloud and into interstellar space.

As I’m always interested in archival searches, I want to note the work of Beatriz Villarroel and team on plates from Palomar Observatory from the 1950s. This is of course before the satellite era, so it’s intriguing to find a number of unexplained point sources that disappear from subsequent plates. To guard against artifacts of the photographic emulsion, the team found a statistically significant (22 sigma) deficit of transients in Earth’s shadow. In other words, emulsion flaws are unlikely to be the source of the detected transients. Conceivably they could have been reflective objects that disappeared when entering the shadow. The findings are still being debated in the literature, but they point to the prospect of future searches on even older astronomical plates.

Image: This is Figure 5 of the paper. Caption: Nine simultaneously occurring transients on April 12th 1950, from Villarroel et al. (2021): 10 x 10 arcmin field shown in POSS-1 and POSS-2 red bands. In the POSS-1 image we see a number of objects that cannot be subsequently found, marked with green circles. Purple circles are artifacts during the scanning process. About 9 objects are present in the POSS-I E image (left) from the 12th of April, but not in the POSS-2 image (right) from 1996. One slightly larger circle host two transients. In addition, the 9 objects are neither visible in the blue POSS-1 taken half an hour earlier, nor in a second POSS-1 red image taken six days later on April 18th. The 9 transients are not caused by a difference in depth or spectral sensitivity. The images are based on the DSS digitizations of the Palomar plates.

I won’t go through all the levels the authors mine other than to say that we move from searches for past Earth visitation and current research into Unidentified Aerial Phenomena through the question of ‘lurker’ or Bracewell probes in nearby space and even explore the Solar Gravitational Lens before moving into the kind of exoplanetary and stellar technosignatures that have thus far commanded the most attention in the field. We can’t limit this to our own galaxy because productive work has been done, especially by Wright’s team at Penn State, in examining numerous galaxies for the possible infrared signature of Dyson spheres or other large scale technologies.

Expanding the Already Daunting Search Space

Whereas the original Cocconi and Morrison paper on SETI (1959) assumed a signal deliberately sent in our direction by a species wanting to announce its presence, technosignatures demand no such intent to communicate, and rather than confining ourselves to particular slices of the electromagnetic spectrum, we should consider the possibility of going well past current laser strategies into areas only now coming online. Thus the emergence of low-freqency SETI via the multi-site LOFAR stations in Europe. Or consider the benefits of high-energy photons via X-ray lasers, which can can offer high rates of data transmission. Let me cite the paper on this:

In particular, X-ray lasers are capable of producing highly focused and intense X-ray beams with a very narrow divergence angle which allows for highly energy-efficient interstellar communication. While natural astrophysical sources of X-ray emissions are generally characterized by specific spectral lines, we could search for free electron lasers, which accelerate free electrons to nearly the speed of light, directing them through an alternating magnetic field in a way that produces highly coherent X-ray pulses (see Figure 20). Although above our present technological capabilities, fusion-powered X-ray lasers are another possibility to generate X-ray pulses.

Image: This is Figure 20 from the paper. Caption: Figure 20. A schematic illustration of a X-ray Free Electron Laser (XFEL). An electron gun fires a beam of electrons that are directed through an undulator after being accelerated through a particle accelerator. The beam of electrons then passes through an undulator, which is a periodic arrangement of magnets whose function is to produce the highly coherent X-ray pulses/beam. Diagram courtesy of Wikipedia, based on (Patterson and Abela 2010).

Another advantage: Lower background radiation as compared to radio waves, which make signals in this frequency range easier to detect against natural sources. Finding patterns of X-rays that do not jibe with natural sources would be sufficiently anomalous to catch our attention. X-rays also have advantages over longer wavelengths like radio waves because phenomena like scintillation are far less of a problem. I was interested to see that there have been archival searches through X-ray data. Michael Hippke and Duncan Forgan found in a 2017 paper that 19 candidate signals were present but could most likely be traced to astrophysical causes.

Bear in mind that at our current level of technology, a spectrum via the Chandra X-ray instrument takes five days to build. One suggested path forward is to move toward highly sensitive instruments with the necessary spectral resolution to detect the kind of narrow X-ray emissions such communications would represent. So it’s good to know that beyond Chandra and XMM-Newton we can look toward efforts like the European Space Agency’s Advanced Telescope for High ENergy Astrophysics (Athena), an X-ray telescope armed with X-ray Integral Field Unit (X-IFU) for high-resolution spectroscopy. A NASA flagship mission based on a concept called the Lynx X-ray Observatory made it into the 2020 Decadal Survey but as far as I know is not yet a confirmed mission.

So as we explore problematic options like X-rays (and the paper notes that G-class stars are good candidates here because they do not produce strong X-ray emission lines), we also push into little considered options like gamma rays, perhaps a signature of advanced propulsion. We also find interesting discussion in the paper on expanding the range by looking for communications signals happening within a target exoplanetary system, particularly as we begin to shift our own deep space communications into the laser range. Directed energy systems of the sort we have often considered here would produce a detectable signal, as would planetary radars used for self defense purposes.

Neutrinos, Gravitational Waves and Other Exotica

And here’s an interesting thought. As far back as Philip Morrison in a 1962 paper, neutrino communication has been suggested for an advanced civilization. Neutrinos react only slightly with matter, meaning that most of the Sun’s outer layers would be transparent to them, with only the dense core layers capable of absorbing them. That means the Solar Gravitational Lens effect for neutrinos starts in the range of 30 AU, roughly the orbit of Neptune. A search for a Bracewell probe is thus possible at a distance much closer than a photon-based signature from a probe at 550 AU.

Image: This is Figure 8 from the paper. Caption: The Solar Gravitational Lens (SGL) is a region where gravitational and neutrino radiation starts to focus (respectively at 22.45 AUs and 29.6 AUs) while the focus of electromagnetic (EM) rays starts from 547 AUs. Human or ETI observational or transmitting probes placed at these regions would benefit orders of magnitude of gains. Figure adapted from (Maccone 2009, xxxi). Credit: Vidal et al.

The authors note that neutrinos have been proposed for communications with submarines as well as interstellar uses. From the paper:

Their extremely low interaction cross-section makes them good candidates for interstellar communication, since they rarely interact with matter: they can travel through interstellar dust, gas clouds, planetary and stellar objects, or even strong magnetic fields surrounding pulsars and neutron stars with negligible attenuation. In other words, neutrino emissions are immune to common interstellar communication issues like dispersion, scattering, absorption, or polarization rotation (problems prevalent with electromagnetic signals). This enables neutrino signals to propagate across interstellar distances while maintaining coherence and fidelity.

Supposing an interstellar civilization wanted to create an aeon-spanning beacon of the sort imagined by some SETI advocates, a neutrino signal would have the advantage of standing out as distinctly structured in whatever modulation scheme chosen. The energy demands of a system like this are unimaginably beyond our own, but searching for technosignatures demands thinking in extravagant terms. With neutrinos the senders free themselves from issues of dispersion and scattering, producing a signal that can reach across the galaxy and remain coherent. I also want to mention Centauri Dreams regular Al Jackson’s take on such a technology in A Neutrino Beam Beacon, based on his 2019 paper. Al has also published with Greg Benford on gravitational wave transmitter concepts. It would be startling to find that the actual galactic conversation was taking place via gravitational wave methods.

My talking about X-rays, gamma rays and neutrinos is just a way of opening the window into the range that this lengthy paper covers. Who knew, for example, how much work had already gone into the theoretical detection of a starship? The various angles into the matter include analyzing motivations for starflight itself, the chief of which must surely be the continuing existence of a species. From the paper:

Survival motivations include avoiding a death threatening supernova or migrating towards a nearby star as the home star fades away (Zuckerman 1985; Hansen and Zuckerman 2021). A pioneering study by Hansen (2022) looked for close stellar encounters in the solar neighborhood. The strategy is then to look for active interstellar migration, where “generation ships” are sent during a close encounter window, hitting this window because it would cost orders of magnitude less time and energy than crossing the otherwise vast interstellar spaces. Hansen proposes this method as a way to constrain search targets because a lot of heat or communication signatures might be associated with such migration.

Interesting, to be sure, but look how many starship technosignature ideas spin out of it. Let’s assume two habitable zone planets around the same star, or perhaps in a binary system, so that civilization has expanded to set up technologies on both worlds. Here’s prime fodder for a technosignature search, and indeed TO!-2267 is a recently discovered example. We might then look for both travel signatures as well as communications, a particularly interesting idea when both planets transit.

Image: This is Figure 25 from the paper. Caption: Illustration of a gravitational machine (Dyson 1963) for accelerating spacecraft using binary star orbital energy. Diagram based on Mallove and Matloff (1989, p. 141). Credit: Vidal et al.

Researchers have considered three-body interactions that result in high-velocity ejections, or even waste signatures (‘interstellar contrails’), perhaps to be found in archival data. Robert Zubrin has studied cyclotron radiation caused by the interaction of the interstellar medium with a magnetic sail, while Ulvi Yurtsever and Stephen Wilkinson have worked on the interactions of a relativistic spacecraft with Cosmic Microwave Background (CMB) photons.

The list could go on, and I haven’t even gotten into Alcubierre-class ‘warp’ drive vessels and the perplexing technosignatures these might produce. Well, this I just have to quote, as temporal matters have their own fascination. Here the authors are discussing what they call a ‘bi-modal signal’ unique to a warp drive craft, for the bubble of spacetime as we observe it is moving faster than the speed of the emissions it is sending out:

This mechanism is a purely craft motion effect, since the craft is moving super-luminally, essentially outrunning the signals it produced earlier in its path. Thus, a distant observatory would record emissions that occurred at two different times simultaneously. One signal would move in the apparent direction of the craft’s motion, showing the emissions occurring in the correct, forward order in time. The second highly unusual signal would move in the opposite apparent direction, presenting the craft’s emissions in a reversed temporal order. This technosignature is considered a key observable (Lentz and Felton 2024) because there is no known natural phenomenon that could produce such a signal.

Image: This is Figure 26 from the paper. Caption: The York-time representation of an Alcubierre spacetime bubble, showing a localized region of warped space with contracted space ahead and expanded space behind. Credit: Vidal et al.

Getting Technosignatures into the Universities

Whether a lightsail, a ramjet, or even a planetary or stellar engine, the interstellar craft has been examined in terms of observational consequences as what we often call ‘Dysonian SETI’ evolves. The unusual waveform of a starship undergoing velocity changes is worth noting as well, again a matter of developing the future tech in the form of sufficiently sensitive gravitational wave detectors. The authors point out that natural objects are also in the mix. Could ejected rogue planets be carrying interstellar colonists, a huge generation ship that might be identified through analysis of its trajectory?

We should have plenty to work with closer to home with the upcoming availability of the Vera Rubin Observatory and the Nancy Grace Roman Telescope looking for objects on hyperbolic trajectories that may be cometary or conceivably technological debris or even active probes. Anomalous occultations in the outer Solar System are obvious targets for existing resources, and radio searches of the Solar Gravitational Lensing region between the Sun and Alpha Centauri have already been conducted. Given the proximity of the outer regions of the Oort Cloud with what may be a comparable region around the Alpha Centauri stars, technosignature searches here seem warranted as well.

I send you to this paper with enthusiasm. Its 118 pages are packed with ideas and as you can see, hardly limited to what we might detect on an exoplanetary surface, although those settings do of course come into play. Given how exciting it has been to witness the birth of direct exoplanet observation since the mid-90s, the extension and consolidation of new ideas for SETI is moving along a similarly fast track, with the obvious and overwhelming exception that it has yet to uncover the kind of observable its practitioners are hoping to find. The massive upgrade in available data that Breakthrough Listen has provided has resulted in no detections. The notion that we have only begun to search is wearing thin. As Jim Benford puts it, “It is too late to say that it is too early to tell.” Clearly, the Fermi question maintains its vitality, and its implications.

The paper is Vidal et al., “The Search for Technosignatures: a Review of Possibilities,” begun as a collective workshop at the Penn State SETI Symposium (PSETI 2023) and now available as a preprint.

While I’ve been going through early extraterrestrial ideas like those of Ronald Bracewell I’ve run back into that most unpronounceable of stellar objects, Przybylski’s Star. This one is worth a return look and I was reminded of it by author and futurist John Michael Godier on his Event Horizon podcast. I do few interviews but I’ve always admired John and Ross’s work on Event Horizon so much that I made an appearance last week. It was John who summoned up Przybylski’s Star as we moved into the broader topic of technosignatures.

Image: Antoni Przybylski in the early 1960’s. Credit: Mike Bessell (via Charles Cowley’s site).

David Kipping does a gallant job of pronouncing Przybylski in one of his Cool Worlds videos, and Wikipedia recommends pʂɨˈbɨlskʲi, which is itself a challenge. Try jebilskee, which is what University of Michigan astronomer Charles Cowley heard when he asked Przybylski himself how to say his name way back in 1964 (the reference is now offline, as Cowley unfortunately passed away in 2024). In any case, suppress the initial ‘p.’ I can’t resist reprinting an old pre-X Twitter post on the matter:

PRZYBYLSKI'S STAR (HD 101065) Blue dwarf with a peculiar spectrum showing an almost complete absence of vowels.

— FSVO (@FSVO) November 22, 2012

This star is more than a curiosity. As a matter of fact, if I were to declare the one most intriguing object in the technosignature hunt, it’s this one, although I’ll hasten to add that we’d need a lot more evidence before making that call. Przybylski’s Star is roughly 350 light years out in Centaurus, discovered in 1873 but gaining attention in 1961 when the Polish astronomer Antoni Przybylski examined its spectrum to discover that it didn’t fit our normal stellar classification scheme. I’ve seen it pegged as an F3-class star but also as an F0p, with the p standing for peculiar. If we go by effective temperature, we come up with early F-class, but its spectrum separates it from all else in that category.

It’s also referred to as an Ap star (this is Kipping’s preference), and whereas F0p is a designation based on temperature, Ap refers to stars larger and hotter than the Sun and possessed of intense magnetic fields and slow rotation rates. What to make of the star’s spectrum? It’s laced with oddball elements like europium, gadolinium, terbium and holmium. Moreover, while iron and nickel appear in low abundances, the stellar atmosphere shows the presence of short-lived ultra-heavy elements like actinium, plutonium, americium and einsteinium.

The latter were identified in 2008. Called actinides, these are elements with atomic numbers from 89 to 103 on the periodic table. They force the question of how radioactive elements with half-lives on the order of centuries or even decades could be there. How are these reactions being sustained on the surface of a star? The reference here is an important if strangely obscure one. The work of a Ukrainian team under V. F. Gopka, the paper is “Identification of absorption lines of short half-life actinides in the spectrum of Przybylski’s Star (HD 101065)” (citation below). David Kipping (on an earlier Event Horizon podcast) and Jason Wright (Penn State) have both mused on the lack of follow-up to it, even though the work seems solid and has implications in terms of technosignature searches.

We also have a 2017 paper by Vladimir Dzuba (University of New South Wales) that offers an interesting solution. The idea is that the short-lived actinides in Przybylski’s Star are the result of undiscovered superheavy elements (a theoretical ‘island of stability’ on the periodic table) which can survive for millions of years. In this model, it is the decay of these elements that produce lighter ‘daughter’ products that are found here, including such things as plutonium and uranium. A possible origin for such superheavy elements is a nearby supernova explosion whose shockwave would have fed this matter directly into the forming star.

Image: Przybylski’s star, image center. By Vizzualizer – Own work, CC BY-SA 4.0,

What we’ve seen in intervening years is discussion of whether the spectral data have simply been misinterpreted, or whether a nearby neutron star might be bombarding the atmosphere of Przybylski’s Star, but there is no observational evidence for such a companion. The island of stability idea has yet to be confirmed in the laboratory, although this work continues. I’ll also mention the star HD 25354, another ‘peculiar’ star, this one in Perseus, which is now being investigated. It contains unstable radioactive elements in its upper atmosphere.

So we have an ongoing mystery, one with tantalizing reminders of a 1980 paper from Daniel Whitmire and David Wright called “Nuclear waste spectrum as evidence of technological extraterrestrial civilizations.” Here the concept is using a star as a repository for radioactive waste. The authors homed in on A stars as being likely candidates. I suspect they were thinking about Sagan and Shklovskii in their book Intelligent Life in the Universe (Delta, 1968), where the authors speculate on the possibility of ‘salting’ a star to call attention to it, a kind of interstellar beacon. Look, a civilization is saying, there is intelligence near this star.

I mention Sagan and Shklovskii pointedly because while both are frequently fused into a single entity in later descriptions of their era, the duo had profound disagreements on a lot of things, especially the kind of SETI embodied in the Drake Equation. On the matter of ‘salting’ a star, it’s Sagan who references Shklovskii as well as a separate Drake paper on the concept, not claiming it for himself. From the book:

Drake and Shklovskii envision the dumping of a short-lived isotope – one which would not be ordinarily expected in the local stellar spectrum – into the atmosphere of the star. In any case, the material of the marker should be of a type that is difficult to explain, except as a result of intelligent activity…. Remarkably enough, the spectral lines of one short-lived isotope, technetium, have in fact been found in stellar spectra… This example illustrates one of the difficulties with such a marker announcement of the presence of a technical civilization. We must know a great deal more than we do about both normal and peculiar stellar spectra before we can reasonably conclude that the presence of an unusual atom in a stellar spectrum is a sign of extraterrestrial intelligence.

So could the spectrum of Przybylski’s Star actually be a technosignature? You can see how difficult this problem is considering the rarity of this kind of star. Even so, a look at The Catalog of Ap, HgMn, and Am Stars reveals over 8,000 ‘peculiar’ stars, ranging across the temperature scale. I dug into the catalog with AI to learn that Ap and hotter Bp stars are the largest subgrouping, both characterized by unusually strong magnetic fields. There is much work here for aspiring graduate students because we need to learn whether the actinides at Przybylski’s Star are simply a rare natural phenomena or a technical marker.

Przybylski’s original paper on the star is “HD 101065-a G0 Star with High Metal Content,,” Nature Vol. 189, Issue 4755 (1961) 739 (abstract). Jason Wright’s three-part essay on Przybylski’s Star is well worth your time. The paper identifying actinides in this star is Gopka et al., “Identification of absorption lines of short half-life actinides in the spectrum of Przybylski’s star (HD 101065),” Kinematics and Physics of Celestial Bodies Vol 24, Issue 2 (April 2008) 89-98 (abstract). The Whitmire and Wright paper is “Nuclear waste spectrum as evidence of technological extraterrestrial civilizations,” Icarus Vol. 42, Issue 1 (April 1980), 149-156 (abstract). Vladimir Dzuba’s paper is “Isotope Shift and Search for Metastable Superheavy Elements in Astrophysical Data,” Physical Review A 95 (30 June 2017), 062515 (abstract).