Can the radio glow of a galaxy reveal alien civilizations?
For decades, humanity has peered into the cosmos in search of signs that we are not alone. From the early days of Project Ozma to the expansive Breakthrough Listen initiative, scientists have listened for artificial radio broadcasts among the stars. A recent study in the Breakthrough Listen series, led by theoretical astronomer Brian C. Lacki, asks a provocative question: could galaxies that shine brightest in the radio spectrum be rich with advanced civilizations? The answer carries implications for how we design future SETI surveys and how we interpret the radio sky itself.
What are radio-bright galaxies?
Galaxies emit radio waves for a variety of natural reasons—jets from supermassive black holes, star formation regions, and other energetic processes. Some galaxies, however, stand out as radio-bright, meaning their radio luminosity is unusually strong relative to their visible light output. These emissions often trace active galactic nuclei (AGN) powered by accreting supermassive black holes, but they can also reflect intense star-forming activity. The central question for technosignature researchers is whether a subset of these bright radio galaxies could also host large populations of intelligent, radio-using civilizations, potentially detectable through their collective broadcast power.
Two pathways to detect civilization-scale radio activity
Lacki and colleagues outline two complementary detection strategies. The first is a collective approach: if many civilizations within a galaxy emit radio signals, their transmissions could overlap and create a detectable excess in the galaxy’s total radio output. The second is a more granular approach: individual civilizations broadcasting at distinctive frequencies or patterns might stand out as anomalous sources within the galaxy’s spectrum. Each method has its own challenges, since natural processes can mimic artificial brightness, and multiple emitters can blend into a single, unresolved signal.
Key concepts: collective bounds and the Kardashev framework
The study introduces the idea of a “collective bound”—an upper limit on how many galaxies could host a population of radio-emitting civilizations based on observed radio brightness across the universe. Using a set of models, the authors examine different assumptions about how a galactic metasociety (a hub-like civilization network) might operate, how its transmissions evolve, and how bright they could be at radio wavelengths. The framework also uses the Kardashev scale, which classifies civilizations by their energy consumption: Type I harnesses planetary, Type II stellar, and Type III galactic power. The results are sobering: Type III-level radio broadcasting across a galaxy is remarkably rare. Estimates suggest fewer than 1 in 100,000 Milky Way–sized galaxies could host such a civilization, with at most about 1 in 100 large galaxies capable of a Type II.75-scale output. In other words, galaxy-wide radio civilizations are unlikely to be common, even under generous assumptions.
Implications for the search strategy
These findings don’t rule out the possibility of technosignatures in other contexts, but they do shape expectations. The collective bound gives a statistical ceiling on how prevalent galaxy-scale broadcasts could be, while the individual-broadcast approach remains essential for catching rare, bright, or distinctive signals. As Lacki notes, distinguishing artificial emission from natural sources is a fundamental hurdle; brightness alone is not proof of technology. Instead, researchers should pursue broad, multi-frequency surveys and deeper observations, aiming to detect both ensemble effects and potential outliers that could indicate artificial activity.
Beyond radio: future directions for SETI
The Breakthrough Listen program and its successors emphasize a broader technosignature landscape. The same surveys that constrain radio-bright civilizations can be extended to other wavelengths—X-rays, gamma rays, and infrared signatures that might accompany non-radio technologies. Integrating radio data with multi-mrequency observations could help discriminate natural astrophysical processes from true technosignatures and guide targeted studies of nearby galaxies like the Milky Way, Andromeda, and other bright radio sources.
Conclusion
In a universe teeming with galaxies, most radio-bright emissions arise from natural astrophysical processes. Yet by framing the problem in terms of population-level bounds and Type II/III analogies, researchers like Lacki are refining how we search the cosmos for civilizations that could rival or surpass our own. As surveys become more sensitive and expansive, the coming years may illuminate not just where to look, but how to interpret what we hear—or don’t hear—from distant galaxies.
