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.
