Introduction: A fresh angle for SETI
For decades, the Search for Extraterrestrial Intelligence (SETI) has scanned the cosmos for signs that civilizations beyond Earth broadcast across the radio spectrum. From the first listening experiments in the 1960s to the Breakthrough Listen initiative that began in 2016, researchers have sought both publicized signals and subtle technosignatures. A new line of inquiry suggests that galaxies bright in radio waves could be fertile ground for studying civilizations at a population level, not just as solitary, star-bound civilizations.
The Breakthrough Listen program and the new analysis
Breakthrough Listen unites radio observations from facilities like the Green Bank Telescope and Parkes, with optical measurements from instruments such as the Automated Planet Finder. The initiative has released a series of results, culminating in a study titled “Artificial Broadcasts as Galactic Populations.” Led by theoretical astronomer Brian C. Lacki, the paper reframes the SETI search as a question about populations of technosignatures in galaxies rather than single, isolated transmissions.
The core idea is straightforward: if a subset of stars in a galaxy hosts broadcasting civilizations, the galaxy’s overall radio emission could be enhanced beyond natural levels. However, distinguishing artificial from natural radio emission remains a challenge. Lacki emphasizes that intensity alone is not a smoking gun; many galaxies produce strong radio signals naturally, driven by supermassive black holes, star formation, and other astrophysical processes.
To address this, the study introduces two complementary approaches: a collective bound based on a galaxy’s total radio output, and an individual-broadcast search for distinctive, narrow-band transmissions. Together, these methods aim to quantify how common artificial radio activity could be across the observable universe.
From individual signals to a galactic population
The paper is the third in a series examining technosignatures when considering entire galactic populations rather than isolated civilizations. Lacki notes that even if some civilizations broadcast, their signals may overlap in time and space, making the artificial component hard to disentangle from natural processes. The concept of a “collective bound” is especially important: it sets limits on how many galaxies could host large-scale radio activity given existing source counts and flux measurements across the sky.
Key results include the assessment that galaxy-spanning civilizations (Kardashev Type III) with radio luminosities equivalent to a significant fraction of a galaxy’s output are extremely rare. Specifically, the research suggests that less than one in 100,000 Milky Way-sized galaxies could host such a civilization. The team also places an upper bound on galaxies hosting a Kardashev Type II–style observer, estimating that at most about one in 100 large galaxies could harbor a civilization contributing a substantial portion of the galaxy’s radio light.
Kardashev scale, Fermi Paradox, and implications
The discussion ties into the broader framework of the Kardashev scale, which gauges a civilization by power consumption—from planetary (Type I) to stellar (Type II) to galactic (Type III). While the Fermi Paradox asks why we have not yet detected advanced life, Lacki’s results imply such civilizations are not easy to spot on galactic scales, even when their radio output appears prominent. This doesn’t close the door on SETI; it reframes the search strategy toward probabilistic constraints and broader survey designs.
Observational strategies and future surveys
Two principal strategies emerge. First, a collective approach that uses deep, wide-field radio surveys to bound how common artificial radio galaxies could be across the universe. Second, targeted searches for discreet, artificial transmissions within nearby galaxies, leveraging multi-frequency observations to separate potential technosignatures from natural radio sources. The study also points to exploring non-radio technosignatures, including X-rays, gamma rays, and infrared emissions, as complementary avenues for future surveys.
Beyond radio: a broader technosignature landscape
While radio remains a powerful probe, Lacki advocates expanding the search to frequencies and modalities beyond radio. A galaxy’s overall energy budget might reveal atypical signatures—whether in infrared heat signatures akin to Dyson-sphere concepts or other non-electromagnetic indicators. The emphasis is on developing robust, multi-wavelength strategies that can distinguish artificial activity from natural phenomena without relying on a single telltale signal.
Conclusion: A probabilistic path to discovery
By treating galaxies as potential hosts for populations of advanced civilizations, the latest Breakthrough Listen results offer a nuanced view of the likelihood of galactic-scale technosignatures. The notion that radio-bright galaxies could be home to multiple civilizations remains scientifically testable, but the evidence suggests such scenarios are rare, guiding researchers toward more precise search strategies and broader technosignature horizons.
