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.
