SETI: Listening to the Silence — Are We Really Alone in the Galaxy?

Some astronomy questions fascinate us because of their sheer scale. How are stars born? What lies beyond a black hole’s horizon? And then there is another, more intimate and more vertiginous question: are we alone?

This article grew out of a talk I had the chance to give to my astronomy club in Belgium. It was part of SETI Constellations, a SETI Institute initiative designed to train and support people like me — without a specific scientific background, but with a genuine interest in the subject — so they could become local SETI ambassadors. The program is unfortunately on hold for now, which is a real shame.

The goal was not to convince a room that extraterrestrials exist. Quite the opposite: start from what we know, what we do not know, and see how far scientific reasoning can take us. As the evening went on, skepticism remained, but certainty slowly gave way to doubt.

SETI is not about little green men

SETI stands for Search for Extraterrestrial Intelligence. Modern SETI mostly looks for technosignatures: observable phenomena that might reveal technology — unusual radio signals, laser emissions, artificial atmospheric chemistry or other clues. A biosignature might say “life could be there.” A technosignature goes further: “something there may be using technology.”

Since Frank Drake, everything has changed

When Frank Drake launched Project Ozma in 1960, we did not yet know of a single planet orbiting another Sun-like star. Today, exoplanets number in the thousands. The question is no longer whether other worlds exist, but how many might host life, intelligence, and detectable technology.

This is where the famous Drake Equation enters the story:

Its purpose is not to give us the Klingons’ address. It breaks one huge philosophical question into smaller scientific ones: star formation, planets, potentially habitable worlds, life, intelligence, detectable technology, and the length of time a civilization remains detectable.

When the Drake Equation enters an astronomy club

During the talk, I turned the equation into a small workshop. We could change the assumptions: optimistic about life, cautious about intelligence, pessimistic about how long a civilization stays detectable. Nobody proved that extraterrestrials exist, but it became much harder to say with certainty: “There is nobody out there.” SETI did not ask us to believe. It asked us to accept our ignorance.

“Maybe they are listening to us…”

My daughter was there during the presentation. At the end of the discussion, she simply said: “Maybe they are listening to us… maybe they are observing us.”

I later mentioned that remark in a feedback email to Tom Dearduff from the SETI Institute. His answer made me smile: “It sounds like your daughter could be the next Jill Tarter!” Jill Tarter is one of the great figures of modern SETI, and that little exchange perfectly captured what I had hoped the evening would do: not impose a belief, but spark real questions.

So… where are they?

That is the famous Fermi paradox. If stars are numerous, planets are common and the Galaxy is ancient, why do we see no technological civilization? The problem is that we are searching an immense cosmic haystack. A civilization has to be detectable at the right time, in the right place, at the right frequency and with a technology we know how to recognize.

Modern SETI is very much alive

The Allen Telescope Array in California still plays a major role. Modern searches use huge data volumes, algorithms and GPUs to filter the countless interferences produced by… our own civilization.

That is one of the things I like most about SETI: it searches for something extraordinary while methodically trying to prove that every strange signal probably has an ordinary origin. That is exactly what makes it science.

Belief is not the real question

People often ask: “Do you believe in extraterrestrials?” I find the question almost misleading. I do not need belief. I can observe that our Galaxy contains hundreds of billions of stars, that planets are common, and that a species capable of building radio telescopes has appeared at least once in the Universe: here.

What we do not know is how often life appears elsewhere, becomes complex and technological, and how long a civilization remains detectable. Perhaps we are extraordinarily rare. Perhaps not. We do not know. And that is exactly why the search matters.

Sources and further reading

Books

Nathalie A. Cabrol, The Secret Life of the Universe: An Astrobiologist’s Search for the Origins and Frontiers of Life, 2024.
Carl Sagan, Cosmos.
Sarah Scoles, Making Contact: Jill Tarter and the Search for Extraterrestrial Intelligence.

No one left that evening having proved the existence of E.T. But skepticism had stopped being certainty. Doubt had entered the room. Maybe that is all SETI asks of us today: listen to the silence… without forgetting that one day it might answer.