A new approach to the Search for Extraterrestrial Intelligence (SETI) suggests that astronomers might be looking for alien life on too small a scale. Instead of eavesdropping on individual star systems, a recent study proposes scanning the total radio output of entire galaxies. The research posits that a galaxy containing a large number of technologically advanced societies, all broadcasting radio signals, would collectively shine with an unusually high radio luminosity, creating a detectable cosmic signature.
This novel method, outlined in a paper by Brian C. Lacki of the Breakthrough Listen Initiative, reframes the search for alien life by treating galaxies themselves as potential beacons. If a civilization’s radio transmissions, whether for communication, navigation, or other purposes, are powerful and numerous enough, they would contribute to their home galaxy’s overall radio brightness. While current technology cannot distinguish these artificial signals from natural cosmic radio waves based on brightness alone, the study provides a new framework for analyzing galactic populations and placing upper limits on how common such highly advanced, radio-emitting societies might be.
A New Scale for SETI
For more than 60 years, SETI efforts have largely focused on nearby targets within our own Milky Way galaxy. The endeavor began in 1960 with Project Ozma, where astronomers used radio antennas to listen for deliberate transmissions from specific star systems. This foundational approach has guided the field ever since, leading to massive modern projects like Breakthrough Listen, which surveys millions of individual stars with powerful radio telescopes like the Green Bank Observatory and Parkes Observatory. These searches operate on the assumption that a civilization would either intentionally beam a message toward Earth or that we could intercept their local radio “leakage,” such as television or radar signals.
Lacki’s research, presented in the paper “Artificial Broadcasts as Galactic Populations,” takes a significant step back from this star-centric view. Rather than hoping to find a single needle in a cosmic haystack, it proposes a method to spot haystacks that might be full of needles. The study explores the statistical probability that galaxies appearing exceptionally bright in the radio spectrum could be home to what the paper terms “artificial radio galaxies.” This approach does not depend on detecting a single, structured message but instead looks for the cumulative, brute-force energy output of one or more widespread technological cultures. It shifts the search from a targeted hunt for a specific broadcast to a broad survey for the unintended energy signature of civilizations on a galactic scale.
The ‘Artificial Radio Galaxy’ Hypothesis
The central idea behind the study is that any population of artificial radio broadcasts within a galaxy will inevitably add to its integrated radio luminosity. If a galaxy hosts a sufficiently advanced civilization—or, more likely, many independent civilizations—their combined radio output could make the entire galaxy stand out from its peers. As Lacki explained, “If you have some subset that has a lot of radio transmissions, they will appear radio-bright.” Such a galaxy would become an “artificial radio galaxy,” a celestial object whose radio emissions are significantly inflated by technological activity.
This concept broadens the definition of a technosignature. Traditionally, a technosignature is a specific, narrowband signal that is unambiguously artificial. In this new model, the technosignature is the galaxy’s excessive radio glow. This could be produced by a single, globe-spanning civilization (often categorized as a Type I civilization on the Kardashev scale) or many smaller, independent technological species scattered throughout the galaxy. The hypothesis is particularly relevant for searching for extremely advanced, galaxy-spanning civilizations, known as Kardashev Type III, which could harness the energy output of their entire galaxy.
The study uses models to estimate the potential abundance of these hypothetical civilizations. By comparing the known population of radio-bright galaxies with theoretical limits, Lacki’s work begins to quantify the search parameters. It provides a baseline for future surveys to determine how many such artificial radio galaxies might exist, even if we cannot yet definitively identify one.
Distinguishing Artificial from Natural Signals
The greatest challenge in this galactic-scale search is differentiating potential artificial signals from the universe’s abundant natural radio sources. Galaxies are inherently noisy places in the radio spectrum. Supermassive black holes at the center of most large galaxies, like Sagittarius A* in the Milky Way, are powerful radio emitters. Intense star-forming regions, supernova remnants, and pulsars also contribute to a galaxy’s overall radio luminosity. These natural phenomena can easily create the “radio-bright” appearance that the study proposes as a potential technosignature.
Lacki acknowledges this fundamental ambiguity. “The trouble is that you can’t tell whether that emission is natural or artificial just from knowing how bright it is in the radio,” he noted. For any individual galaxy observed, astronomers can currently only establish an upper limit on its potential artificial radio emissions based on its total output. This is what the paper refers to as the “collective bound,” which is the maximum amount of radio energy that could be artificial without exceeding the total observed emission. Separating the two sources would require identifying a unique characteristic in the artificial signal, a task beyond current observational capabilities for distant galaxies.
Because of this limitation, the research is not about pointing to a specific galaxy and declaring it inhabited. Instead, it is a statistical exercise to constrain possibilities. By analyzing large catalogs of galaxies and their radio emissions, scientists can begin to ask how many civilizations could exist before their collective signal would become obviously anomalous in cosmic surveys. This provides a valuable, albeit indirect, tool for SETI research.
Modeling the Search
To put a quantitative foundation under the hypothesis, Lacki’s paper developed models to set bounds on the prevalence of artificial radio galaxies. These models considered different scenarios, including a baseline case where every galaxy hosts one “metasociety” whose broadcasts remain constant over time. By comparing the energy output in these models to actual observational data from radio astronomy, the study could establish statistical limits on how common galaxy-spanning civilizations might be.
The results provide a sobering but important reality check. Based on the analysis, the abundance of Kardashev Type III civilizations—those capable of using all the energy of their host galaxy—is extremely low. The research suggests that such civilizations exist in fewer than one in a million large galaxies. This finding aligns with the general lack of success in SETI searches to date and reinforces the idea that hyper-advanced, galaxy-wide empires are exceptionally rare, if they exist at all. The models help quantify the famous “Fermi Paradox,” which asks why we see no evidence of intelligent life despite the high probability of it existing elsewhere.
These calculations serve as a guide for future SETI strategies. They can help astronomers prioritize which types of galaxies might be the most promising targets for follow-up observations. For instance, a galaxy that is unusually radio-loud for its size, mass, and star-formation rate could be flagged as an object of interest, even if the source of its excess emission remains unknown.
Implications for Future Searches
This new galactic-scale perspective complements, rather than replaces, traditional SETI methods. The search for directed messages or signal leakage from nearby stars remains a critical component of the quest for extraterrestrial intelligence. As Lacki’s paper notes, humanity does not yet know if even our own Milky Way contains other radio-broadcasting civilizations, which is why local surveys are indispensable. However, the “artificial radio galaxy” concept opens a second front in the search, one that leverages vast amounts of existing and future radio survey data in a novel way.
Future radio observatories, with their increasing sensitivity and ability to scan huge swaths of the sky, are well-suited to this type of statistical search. By analyzing the properties of thousands or even millions of galaxies, astronomers can refine the constraints on the prevalence of technologically active galaxies. While the definitive identification of an artificial signal remains the ultimate goal, this approach allows for meaningful scientific conclusions to be drawn even in the absence of a “contact” event. It represents a subtle but significant evolution in SETI, moving from a search for a message to a search for the subtle, energetic footprint of civilization itself.