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The Great Filter: why the universe is silent

Is there an almost impossible step between inert matter and a galactic civilization? Where it might lie, and why the answer matters.

by L.R. · November 11, 2026 · 6 min read

In brief

The “Great Filter” is an idea proposed in 1996 by the economist Robin Hanson to explain the Fermi paradox.
It says that between inert matter and a civilization that colonizes the galaxy there is at least one step that is almost impossible to get past.
The crucial question is where that step lies: behind us or ahead of us.
That is why, paradoxically, finding life on Mars could be bad news.

A staircase with a missing step

Picture the history of a civilization as a staircase. On the first step there is a planet with the right chemistry. Then come the origin of life, the complex cell, multicellular organisms, intelligence, technology, and finally expansion into space.

Suitable stars and planets are very numerous. If every step were easy, the galaxy should be full of visible civilizations. It is not. So, Hanson reasons, at least one of the steps must be enormously difficult: a filter that almost no one gets through.

The candidates

The origin of life

Perhaps the filter is the first step: the passage from chemistry to biology. No one has ever reproduced it in a laboratory, as we explain in the article on how life began. On Earth, however, life appeared very early, which suggests it was not that difficult.

The complex cell

The cells of plants, fungi and animals (the eukaryotes) have a nucleus and contain mitochondria, the descendants of bacteria engulfed by another cell about two billion years ago. This merger seems to have happened only once in the whole history of the Earth, after more than a billion years of nothing but microbes. It is one of the most serious candidates.

Technological intelligence

Multicellular life has appeared several times independently. Many animals are intelligent: octopuses, crows, dolphins, apes. But only one species, in about 600 million years of animals, has built tools capable of leaving the atmosphere.

Self-destruction

The last candidate is the most unsettling: perhaps technological civilizations tend to destroy themselves, through weapons, through the collapse of their own environment or through technologies that get out of hand, shortly after becoming able to make themselves noticed.

Why finding life on Mars would be bad news

The philosopher Nick Bostrom made a counterintuitive argument famous. If we discovered that life had also arisen independently on Mars, we would know that arising is easy. The filter would then not be behind us, at the origin of life, but further ahead. And the more evolved forms of life we found in the cosmos, without any civilization, the greater the risk that the filter lies ahead of us.

Conversely, a sterile universe would be reassuring news: it would mean that we have already passed the difficult step.

Where science stops

The Great Filter is an argument, not a discovery. It says that there must be an obstacle somewhere, but it does not know which one. And as with the Fermi paradox as a whole, it rests on a single example of life, our own, from which it is hard to tell which steps were likely and which were not.

From here on we are no longer talking about what science has established, but about a hypothesis: the one proposed in the book Lux Lucis. It should be read as such.

Not one filter, but many

Chapter 8 of Lux Lucis, “Universal life,” places the filter at a precise point. According to the book, the first step is not an obstacle: life “switches on” almost inevitably when a planet offers water, organic chemistry and the stable light of a star. Simple life, therefore, should be widespread.

The difficult steps come later, and there are four of them: from the simple cell to the complex cell, from simple organisms to organisms with tissues and nervous systems, from complexity to intelligence, from intelligence to technology. None of them, on its own, is an impassable wall. But if each were passed, say, one time in ten, only one evolutionary line in ten thousand would get as far as building radio telescopes. And the odds are probably lower still.

The result is a universe in which life is common, but technological intelligence is rare and scattered across enormous distances. The silence, in this reading, is not a threat: it is a sign of how precious the road that brought us here is.

What remains open

The book's reading is more optimistic than Bostrom's: if simple life is easy but the later steps are hard, finding microbes on Mars would not point to a filter ahead of us, but behind us. It remains a hypothesis, though, one that future observations will be able to put to the test. We cannot rule out that some filter still awaits us.

Perhaps the most useful message of the Great Filter is precisely this: having come this far is not something to take for granted, and what we have is worth treating with care.

Go deeper in the book

This topic is developed in chapter 8 of Lux Lucis – Children of the Light.

Read the first chapter for free · English edition in preparation; available now in Italian and Spanish