Why We See No Alien Civilizations: The Great Filter

The Great Filter isn't space: it's the bomb that hasn't exploded yet

Madrid, Barcelona, and Lisbon turned into smoking craters. In Huesca, with 50,000 inhabitants, not a single artifact would fall: the province would survive the day of judgment and, according to optimists, so would the human species. This scene summarizes the Fermi Paradox, the question left floating by the Italian physicist: if there are hundreds of billions of stars in the universe and intelligent civilizations should exist, why do we see none?

The answer that underpins the discussion is called the Great Filter. A barrier, a bottleneck, something that prevents a technological civilization from taking the final leap and expanding across the galaxy.

What is the Great Filter and why does it have two versions?

The initial approach distinguishes two scenarios. If the filter has already passed—a catastrophe almost no one survives—then humanity's future is expansion. If it lies ahead, it is extinction. And there is a specific thesis about what it is: nuclear energy and chemical and bacteriological weaponry.

The underlying reasoning is almost economic. Maintaining the level of well-being requires increasingly powerful forms of energy, and those forms of energy come with the capacity for total destruction of the planet that develops them. There is no technological race without fuel, and there is no fuel on that scale without existential risk.

Three generations with the red button and a hundred more to go

We have been living for three generations since nuclear weapons appeared. There are still people alive born before them, including, according to the premise opening the issue, the last two US presidents. Since then, there have been several escalations, and this analysis argues that the probability of use tends toward 1 as generations renew.

A hundred generations are about 3,000 years. A blink. The argument points less at current leaders than at arithmetic: it suffices that someone willing to press the button reaches command of a nuclear power at some point. The most cited historical objection is that those who had chemical weapons during the siege of Berlin in March 1945 did not use them. Another participant recalls the contrary precedent: nuclear artifacts have already been dropped on civilian populations, and it was not the Axis powers who launched them.

A Chernobyl in Madrid doesn't kill Huesca

The most repeated counter-argument against the end of the world is geographical. One Chernobyl in Barcelona, another in Madrid, and another in Lisbon would spike cancer cases in the rest of the country but would not wipe out everyone. In the worst-case scenario of the Cold War, thousands of people would have survived in areas outside the conflict, with time to prepare: the Southern Cone, remote Pacific islands.

Paradoxically, that catastrophe would function as a vaccine and force the move to other planets in the solar system before repeating the mistake. The brake would not be technological but cultural: seven people died in the Challenger disaster, and the space agency dragged through a crisis for years. The more a society advances, the less willing it is to accept casualties in an expedition. A planetary-scale Segurolas (a Spanish reference to cautious driving campaigns).

If the filter is much earlier: supernovae, eukaryotes, and coal

Another line of thought holds that the bottleneck has nothing to do with us. In the galaxy's past, supernova explosions were much more frequent and emitted X-ray jets capable of wiping out primordial life. Only now, in a period of calm, would conditions for intelligence arise.

There are more candidates. The fusion of two cells that gave rise to eukaryotic organisms, as far as we know, peine only once. And then there are fossil fuels: without oil or coal, you cannot get past the first industrial revolution. Coal exists because for millions of years no organism was able to degrade wood, and oil exists because no one digested algae deposited on the seabed. Without that geological accident, there is no industrial civilization. Added to this is the hypothesis that we are the first: the sun would be a fifth-generation star, and heavy elements require several previous generations of stars.

Biolabs, Venus, and theories without proof

Biological weapons appear as a threat superior to nuclear ones for one reason: they are within anyone's reach. In this area, claims circulate with no backing whatsoever, such as the alleged laboratory origin of cobi19 or the existence of a network of biolabs in Ukraine. The same lack of evidence accompanies the thesis that nuclear weapons do not exist and everything is a setup.

At the other extreme, some recall that serious signs have already been detected in the atmosphere of Venus and that it would be strange if there were no life. Also the natural reserve hypothesis: we would be inside the territory of a much more advanced civilization that doesn't even look at us, just as an ant does not perceive a highway.

The filter nobody wants to look at

Remains the most uncomfortable reading: the filter is human consciousness. Leaving the planet forces us to relativize local problems and set a greater goal, and a short-sighted and warlike species will not pass beyond the solar system. It is not a physical limit, it is one of character.

And a warning about haste. The diplodocus lived 150 million years ago; the tyrannosaurus, 66 million years ago. A tyrannosaurus is closer in time to us than to a diplodocus. If an industrial civilization had flourished in the mid-Cretaceous, about 120 million years ago, its remains would be indistinguishable from nature.