A recent workshop at Ohio State raises a number of interesting questions regarding what is being referred to as ‘high energy SETI.’ The notion is that places where vast energies are concentrated might well attract an advanced civilization to power up projects on a Kardashev Type II or III scale. We wouldn’t necessarily know what kind of projects such a culture would build, but we might find evidence that these beings were at work, perhaps through current observations or, interestingly enough, through scans of existing datasets.
Running June 23-24, the event was titled “Bridging Multi-Messenger Astronomy and SETI: The Deep Ends of the Haystack Workshop.” ‘Multi-messenger astronomy’ refers to observations that take in a wide range of inputs, from electromagnetic wavelengths to gravitational waves, from X-rays through gamma ray emissions. Extend this to SETI and you’re looking in all these areas, the broad message being that a SETI signature might show up in regions we have only recently begun to look at and may have prematurely dismissed.
Notice that such ‘signals’ don’t have to imply intended communication. We might well turn up evidence of advanced engineering through astronomical plates taken a century ago and only now recognized as anomalous. This kind of search is deliberately open-ended, acknowledging as it does that civilizations perhaps millions of years ahead of us in their history might be far more occupied in their own projects than in trying to talk to species in their infancy.
As I mentioned in SETI at the Extremes, Brian Lacki (Oxford University) and Stephen KiKerby (Michigan State) have produced a white paper on the workshop, an overview that puts the major issues in play. The high-energy bands that we have been talking about recently have seldom been explored with SETI in mind, given the natural predisposition to think that life would be something rather like ourselves, and certainly not capable of existing on, say, a neutron star. High-energy SETI pushes the idea of astrobiology into these realms anyway, but equally significant, makes the point that whatever their makeup, advanced aliens might exploit high-energy sources whether or not they had evolved on them. Thus these energy resources become SETI targets, in the hope that activity affecting them will throw a signature.
Image: The area around Sgr A* contains several X-ray filaments. Some of these likely represent huge magnetic structures interacting with streams of very energetic electrons produced by rapidly spinning neutron stars or perhaps by a gigantic analog of a solar flare. Scattered throughout the region are thousands of point-like X-ray sources. These are produced by normal stars feeding material onto the compact, dense remains of stars that have reached the end of their evolutionary trail – white dwarfs, neutron stars and black holes. Because X-rays penetrate the gas and dust that blocks optical light coming from the center of the galaxy, Chandra is a powerful tool for studying the Galactic Center. This image combines low energy X-rays (colored red), intermediate energy X-rays (green) and high energy X-rays (blue). Credit: NASA/CXC/UMass/D. Wang et al.
Let’s acknowledge our ignorance by recognizing that the motivations of any off-Earth civilization are unknown to us, and for all our logic, we have no notion of what such a culture wants to do. It’s a helpful fact that technosignature searches don’t require futuristic off-planet observatories. Reams of observations have been recorded that have seldom if ever been actively mined. Thus high-energy SETI, exotic as it is, can proceed with existing materials, even as ongoing astrophysical research continues to produce new data that add to the mix.
As the authors note, high-energy radiation has many sources, from nuclear processes, from gamma ray emissions and neutrinos to relativistic particles, which include not only cosmic rays but particles thrown out by jets and the interaction of electrons and positrons. We can study compact sources like neutron stars and black holes (ideal for energy extraction) and relativistic flows from energetic transients. Gravitational waves might be used to bind together elements of a galactic network. How exactly might ETI modify any of these?
It’s natural to ask whether X-ray astronomy has implications for SETI. Bursts of emission using X-rays for communication, exploiting less diffraction and the ability to produce tighter beams, might be detected if aimed specifically at us, making something like a flash at these frequencies from a nearby star an anomalous event worth studying. Or consider signals more general in nature:
Non-directional X-ray communication can be effected by dropping an asteroid onto a neutron star [4]. When it hits, it releases a burst of energy detectable at interstellar distances. The cosmos also has a number of compact high-energy “signal lamps”. X-ray binaries (XRBs) are systems with a neutron star or black hole accreting from a donor star, having luminosities of up to 105 suns. Even a kilometer-scale object passing in front of the hotspots of an XRB can easily modulate its luminosity, serving as a technosignature [4, 16]. A subplanetary-scale lens is potentially capable of creating a brief flash visible even in nearby galaxies without any power input of its own. Credit: NASA/CXC/UMass/D. Wang et al.
We don’t have a handle on how to use neutrinos for communication, although there have been experiments along these lines given the ability of neutrinos to pass right through obstacles and thus probe, for example, the oceans of icy moons. But perhaps we can home in on industrial activities, which as the authors point out, could involve not just energy collection to power scientific experiments but interstellar propulsion through antimatter rockets. The interactions between a relativistic spacecraft and the interstellar medium could become apparent through gamma rays, while X-ray binaries might show oddities in their proper motion indicative of their use as stellar engines.
This possibility, studied at some length by Clément Vidal under his ‘stellivore’ concept, stands as a particularly detectable phenomenon:
What are the limits of life, broadly defined? At the very least, complex processes require a thermodynamic gradient to feed them. In his reflections on the future of the cosmos, Dyson suggested that this is the only absolute requirement, and that long after the stars have gone out, life could still thrive in the chilly atmospheres of cooled compact objects [7]. A contemporary test of this admittedly extreme idea might be found with today’s compact objects. The accretion hotspots of XRBs have some of the greatest sustained power densities around in the contemporary universe. If thermodynamics really is the only prerequisite factor for complexity and ETIs can withstand the incredibly hostile environments, they may find the energy gradients in XRBs attractive [29].
If we look not at the stellar but the galactic level, the actual lack of X-ray binaries could be a marker, with the deficiency being a sign their energies are being exploited to some purpose. For that matter, high-energy flare activity from an individual star or the source of a gamma ray burst may point us at locations where an advanced civilization can use its technologies to deflect these energies to avoid the threat. If we push speculation to the extreme, we’re talking once again about Robert Forward territory, wondering whether environments like neutron stars can sustain their own kinds of life.
Image: HEAO-1 All-Sky X-ray Catalog: Beginning in 1977, NASA launched a series of very large scientific payloads called High Energy Astronomy Observatories (HEAO). The first of these missions, HEAO-1, carried NRL’s Large Area Sky Survey Experiment (LASS), consisting of 7 detectors. It surveyed the X-ray sky almost three times over the 0.2 keV – 10 MeV energy band and provided nearly constant monitoring of X-ray sources near the ecliptic poles. We’ve been examining high-energy targets for quite a while now and have numerous datasets to consult. Image credit: NASA.
Several things to keep in mind as we consider ideas that are on the face of things fantastic. First, the very practical fact that high-energy SETI need not be expensive, given our growing sophistication at using machine intelligence to analyze existing astronomical data (I’ve always nursed the wonderful idea that some day we’ll make a SETI detection and it will be corroborated by a century-old astronomical plate taken at Mt. Wilson Observatory). Second, existing facilities monitoring things like gamma ray bursts and detecting neutrinos are capable of full-sky monitoring and are doing good science. Our search for high-energy anomalies, then, takes a free ride on existing equipment.
So while it’s completely natural to find this approach well outside our normal ideas of astrobiology, their improbable nature should elicit a willingness to keep our eyes open. It would be absurd to miss something that has been in our data all along. And filtering incoming data as an add-on investigation into astrophysical processes may turn up anomalies that advance high-energy physics even if they never do resolve themselves into a SETI detection.
The paper is Lacki & DiKerby, “Possibilities for SETI at High Energy,” submitted for 2025 NASA DARES [Decadal Astrobiology Research and Exploration Strategy] RFI and available as a preprint.
