Decades of searching for extraterrestrial intelligence through SETI have not yielded a single convincing signal. Radio telescopes listen to the cosmos, laser detectors scan the skies, and infrared observatories search for the thermal signatures of megastructures—yet all remains silent. A new theoretical study offers a fresh perspective: perhaps signals have already reached Earth, but we simply failed to notice them.

Astronomer Claudio Grimaldi of the École Polytechnique Fédérale de Lausanne calculated how many such signals would have had to pass by Earth over the past 60 years for the probability of detection today to be high. The paper was published in The Astronomical Journal.

Two Conditions for Any Contact

To detect a technosignature—a radio transmission, laser pulse, or other technological trace—two conditions must be met:

  • The signal must physically reach Earth at the very moment we are listening.
  • Our instruments must be pointed in the right direction, sufficiently sensitive, and operating at that time.

A signal may pass through our Solar System unnoticed—too weak, too brief, or drowned out by cosmic noise. Grimaldi asked: if such signals have indeed crossed Earth’s path during the 60 years of SETI’s existence, how many would there need to have been for us to have at least one solid candidate by now?

A Bayesian Estimate: How Many Signals Must Be “Missed”?

The researcher applied Bayesian statistics, linking three key parameters:

  • The number of past “contacts” (signals that passed through Earth’s vicinity);
  • The average duration of each signal;
  • The sensitivity and sky coverage of current and near-future instruments.

The calculations showed that for there to be a high probability of detection in the coming years—within several hundred or several thousand light-years—an enormous number of signals would have had to cross Earth. In some scenarios, their number would exceed the number of potentially habitable planets within that volume of the Galaxy. The author considers such scenarios highly unlikely.

A more plausible picture emerges only when searches extend to distances of several thousand light-years and beyond—if technosignatures are long-lived and spread throughout the Galaxy. Even then, however, only a handful of detectable signals could be expected at any given time.

What This Means for SETI

Grimaldi emphasizes that the absence of detected signals does not prove the absence of civilizations. It merely suggests that if they exist, they are either extremely rare, very distant, or their signals are exceptionally long-lasting and distributed across vast scales. In that case, the search becomes not a matter of luck, but a long-term strategy: increasing sensitivity, expanding sky coverage, and extending observation time are essential.

“The fact that signals may have passed by unnoticed does not mean a discovery is not around the corner. If extraterrestrial technologies exist, they are likely rare, distant, or long-lived. This makes the search not a matter of luck, but of long-term strategy.”

Conclusions and Prospects

The study does not close the question “Where is everybody?” but clarifies the framework: even if civilizations do exist, the probability of catching their signal in the coming decades remains low without a substantial expansion of SETI’s capabilities. New radio telescopes such as Square Kilometre Array and Five-hundred-meter Aperture Spherical Telescope, optical searches like Breakthrough Listen, and future missions targeting infrared technosignatures could shift the statistics. For now, however, the silence appears entirely natural.

In Brief

Claudio Grimaldi calculated that for us to have a high probability today of detecting an alien signal within a few thousand light-years, an enormous number of technosignatures would have had to cross Earth during the 60 years of SETI— in some scenarios, more than the number of potentially habitable planets within that region of the Galaxy. This seems unlikely. A more realistic assumption is that civilizations are rare, distant, or that their signals are extremely long-lived. The lack of detections does not yet prove we are alone—it simply suggests that the search will require decades and more advanced instruments.