Insights into the search for extraterrestrial life are uncovered by silence.

A search for radio signals from extraterrestrial civilizations has yielded no evidence of alien technological activity. Research conducted at EPFL indicates that we continue to research while optimizing the use of available resources.

For more than sixty years, amateur and professional astronomers have been watching the skies for extraterrestrial intelligence (SETI). So far, to no avail. But how should we read the absence of satellite radio signals? Is it time to stop searching? Or should we double down and look harder, looking deeper into our galaxy than ever before? A recent statistical analysis of the sixty years of silence suggests a simple, optimistic explanation and urges the SETI community to continue searching, but be patient, because the chances of detecting the signals in the next sixty years are slim.

The prevailing explanations for the absence of electromagnetic signals from extraterrestrial communities fall into two extreme categories, says Claudio Grimaldi of the EPFL Laboratory of Statistical Biophysics. The “optimist” camp argues that we have used detectors that are not sensitive enough or have missed incoming signals because we have been pointing our radio telescopes in the wrong direction. On the other hand, the “pessimistic” camp interprets the silence as indicating that there is no alien life in our galaxy.

According to Grimaldi’s study published in The Astronomical Journal, there is a third explanation. “We’ve only been looking for 60 years,” he says. “Earth could simply be in a bubble that happens to be devoid of radio waves emitted by extraterrestrial life.”

Modeling the Milky Way as a sponge

Grimaldi’s study builds on a statistical model initially developed to model porous materials such as sponges, which he sees as an apt analogy for the question at hand: “You can imagine the solid matter of a sponge to represent electromagnetic signals radiating out spherically from a planet harboring extraterrestrial life in space.” In this analogy, the sponge’s holes—its pores—represent areas where signals are absent.

By reusing mathematical tools to study porous materials and using Bayesian statistics, Grimaldi was able to draw quantitative conclusions from sixty years of observed silence. Its results are conditioned by the assumptions that there is at least one electromagnetic signal of technological origin in the galaxy at any time and that the Earth has been in a silent bubble or “pore” for at least 60 years.

“If it is true that we have been in a void zone for sixty years, then our model indicates that there are less than one in five electromagnetic emissions per century anywhere in our galaxy. That would make them as rare as supernovae in the Milky Way. Way,” Grimaldi says. In the most optimistic scenario, we will have to wait more than 60 years for one of these signals to reach our planet. In the least optimistic scenario, this number will increase to about 2,000 years. Whether we detect signals when they cross our path is another question. Either way, our radio telescopes would have to be pointed in the right direction to see them.

Identify best practices for further research

The search for extraterrestrial intelligence currently carries the wind in its sails, buoyed by the discovery, some 20 years ago, of the first planets outside our solar system. Today, researchers hypothesize that there could be as many as 10 billion Earth-like planets out there – rocky, the right size, and located at the right distance from the sun to harbor life. Their sheer number increases the possibility that technological life evolved on one of them.

This has led to new initiatives across the SETI community. The privately funded “Breakthrough Listen” project, the largest of its kind, has committed nearly $100 million to allocate radio telescope time to search for technical signals from extraterrestrial civilizations. With the initiative ending in two years, Grimaldi says it’s a good time to think about how to pursue the search for extraterrestrial intelligence in the future.

“The dream of the SETI community is to look for the signals all the time, across the entire sky. Even today’s largest telescopes can only see a small part of the sky. Today, there are telescope arrays, such as the Allen Telescope Array (ATA) in California, that point in different directions and can Pointing them at specific areas to get more detailed information when necessary. The same is true for optical telescopes.”

But the truth is, Grimaldi says, “we don’t know where to look, and at what frequencies and wavelengths. We’re currently looking at other phenomena with our telescopes, so the best strategy might be to adopt a past SETI community approach using data from other astrophysical studies — revealing radio emissions from other stars or galaxies – to see if they contain any technical signals, and make that a standard practice.”

Ineffective or unlucky?

When asked whether he considered his conclusions encouraging or not, Grimaldi laughed and said, “This is something we need to think about. Perhaps we were lucky because we figured out how to use radio telescopes just as we were traversing a part of space in which electromagnetic signals from other civilizations were absent.” To me, this hypothesis seems less extreme than the supposition that we are constantly bombarded with signals from all sides but are, for some reason, unable to detect them.”