Q&A results: less than one alien emitter: 54%, at least one alien emitter: 33%, at least half of alien emitters: 13%

The search for extra-terrestrial intelligence (SETI) began with the advent of radio technology in the beginning of the 20th century and has been internationally active since the 1980s. In 2015, Stephen Hawking and Russian billionaire Yuri Milner announced The Breakthrough Listen project that is planned to provide the most comprehensive search of alien communications to date.

Yet despite these efforts, alien intelligence remains to be detected.  

Our chances of detecting intelligent alien communications are usually calculated based on the Drake equation, essentially a probabilistic way to estimate the number of communicative alien civilizations in the Milky Way galaxy.

Now, EPFL physicist Claudio Grimaldi proposes a complementary approach that may explain why alien contact has not been made.  In contrast to the Drake equation, Grimaldi makes no assumptions about the number of alien civilizations out there in our galaxy. Instead, he considers the volume of the galaxy that is filled with intelligent signals. The results are published in Scientific Reports.

The communication of an alien civilization is modeled by a spherical shell of electromagnetic information emanating from the alien planet. As the signal propagates through space over time, during the existence of the civilization, the signal fills up a volume in the galaxy. Long-lived civilizations have a thicker shell than short-lived civilizations. Our planet must therefore encounter these shells in order for humans to have a chance at detecting the alien signals.


Inspired by methods in his main field of research, condensed matter physics, Grimaldi built a statistical model of the domain covered by hypothetical alien signals – the electromagnetic shells ­– and derived the probability that Earth is within at least one of these shells.

The model assumes that the Milky Way is filled with a collection of independent spherical shells of different thicknesses and radii transmitted by emitters in the Milky Way. It also takes into account the possibility that a fraction of the alien civilizations could have sent beam-like signals towards random targets in the galaxy, mirroring the various interstellar radio messages that humans have sent out into the galaxy since 1962.

Grimaldi found that the average number of detectable emitters is less than one, even if half of the Milky Way is filled with signals from alien civilizations. In order to detect at least one alien emitter, the statistical model suggests that more than 60% of our galaxy would need to be filled with alien signals.

Our chances of detecting signals from alien civilizations may be small, but there is still hope: the model does not take into account alien civilizations so advanced that they can reach us directly via interstellar space travel. Then again, if an advanced civilization had developed interstellar space travel, one would suspect that they would have the technology to communicate with us first, but would they want to?