Sixty-five years of searching for extraterrestrial intelligence, and still no confirmed result. The first SETI experiment took place back in 1960, and since then radio telescopes and optical observatories have scanned a substantial share of the sky — with nothing to show for it.
One of the most popular explanations sounds reassuring: the search space is so vast that we've sampled only a tiny fraction of it. If true, technosignatures could already have reached Earth — nobody just happened to be listening at the right moment in the right direction. Swiss physicist Claudio Grimaldi decided to test this idea not intuitively, but mathematically.
A Bayesian count of missed contacts
Grimaldi applied a Bayesian framework — a method for updating probability estimates as new data comes in. Here, the "data" is 65 years of failed observations.
The logic is straightforward: if signals from other civilizations really have passed by Earth before, how many such missed contacts would need to have occurred for today's detection probability to be meaningfully high? The researcher calculated the minimum number n — undetected contacts over the past six and a half decades — required to be consistent with the ongoing lack of results.
The calculation was run separately for different types of technosignatures: omnidirectional ones, such as emissions from Dyson-sphere-like megastructures, and directional ones, such as intentional messages aimed at a specific point in the sky.
Why the numbers don't add up
The result is inconvenient for anyone favoring the "we just weren't looking hard enough" explanation. To achieve high detectability for signals within a few hundred light-years of Earth, the number of past missed contacts would have to be so large that it exceeds the total number of habitable planets within that same volume of space.
In other words, even assuming every single habitable planet in that radius had at some point sent out a signal, that still wouldn't be enough to account for the required n. The problem is even worse for short-lived technosignatures — ones that exist for a limited time and vanish before they can repeat.
The pattern holds for both omnidirectional and directional signals: in both cases, the required number of emitters turns out to be implausibly large relative to the actual population of planets.
Two conditions under which the math works
The author points to two scenarios in which the estimates become more reasonable.
The first is that signal sources are tightly clustered near the Solar System rather than evenly distributed across the Milky Way. The second is that the population of technological civilizations in the galaxy has undergone a very recent and sudden surge, meaning most potential emitters appeared relatively recently on a cosmic timescale.
Without these assumptions, the "we just haven't looked enough" explanation loses its mathematical footing. The researcher concludes that even if past undetected contacts did occur, the best prospects for future detection lie not within a few hundred light-years, but at distances of several thousand light-years — though even there, the expected number of detectable technoemissions remains small.
The study, published in The Astronomical Journal, doesn't rule out the existence of other civilizations. It challenges one specific argument: that incomplete sampling alone explains the silence of the universe.
