For decades, astronomers have searched for signs of extraterrestrial technology – radio signals, lasers or excess heat from hypothetical megastructures. However, a new theoretical study raises another question: If the signals reached Earth undetected, how likely are we to detect them now? Claudio Grimaldi from École Polytechnique Fédérale de Lausanne has estimated how many alien signals would have to pass by Earth for us to catch them today. The work was published in AstroJournal.
Since 1960, when the first SETI experiment was launched, scientists have been scanning the Milky Way for technological signatures—measurable signs of advanced extraterrestrial technologies. These can be artificial radio transmissions, laser beacons or heat signals of large engineering structures.
To detect a signal, two conditions must be met: it must physically reach Earth, and our instruments must be sensitive enough and pointed in the right direction to detect it. A signal can pass through our space but go unnoticed – too weak, short-lived, or “drowned” in the background noise.
Grimaldi wondered: If such signals had indeed passed through Earth in the past 60 years, how many of them would have to be present for the probability of detection to be as high as it is today?
In his model, he considered technological signatures as sources of emissions from hypothetical technological civilizations somewhere in the Galaxy. Signals travel at the speed of light and can last from a few days to thousands of years. Using Bayesian statistics, the scientist linked three parameters: the number of past “contacts,” the average duration of the signal, and the distance at which modern or nearby devices could record that signal.
Calculations show that to have a high probability of detection within a range of several hundred or thousand light years, a large number of signals would have to pass through Earth today. In some cases, this number is so large that it exceeds the number of potentially habitable planets in the Galaxy. Such options are considered extremely unlikely.
The images only become more realistic when searching at distances of a few thousand light years or more – if technological signatures are persistent and spread throughout the Galaxy. However, only discrete signals can be detected at any given time.
The fact that signals can pass unnoticed does not mean that discovery is “just around the corner,” the scientist emphasized. If extraterrestrial technology exists, it could be rare, remote, or long-lived. This makes the search less about luck and more about long-term strategy.
