How the ‘Great Filter’ could explain why we haven’t found intelligent aliens

Science: “Where is everybody?” This famous quote, uttered over lunch by the renowned physicist Enrico Fermi, perfectly expresses what became known at the time as the Fermi Paradox: If life happened on Earth and things don’t just happen once in a while in the universe, then life should happen elsewhere. In fact, the universe should be full of advanced spacefaring civilizations.

Even when you take into account every technological obstacle – rocket technology, limits on speed and life span, as well as disease, war and anything else that might slow progress – the logic remains the same. Our galaxy is over 13 billion years old, which is more than enough time for spacefaring civilizations to spread throughout the Milky Way.

In fact, we should see advanced civilizations everywhere – Dyson spheres, stellar engineering, artifacts scattered throughout the solar system. But we don’t. So where is everybody? Hence the paradox: something has to give in this logic. We’ve got one, if not many or all, of these statements wrong. Which one?
First elaborated in 1996 by economist Robin Hanson. The gist of the Great Filter argument is simple: very few, if any, civilizations in the universe reach the advanced spacefaring stage.
That’s it. Fermi’s paradox breaks down into the assumption that intelligent spacefaring civilizations are common.
Given that humanity is on the verge of achieving routine spacefaring status, the Great Filter might seem a little ominous. But it doesn’t have to be. The point is that we don’t know when or where the filter actually occurs. There are a lot of steps between “random planets with the right elements for life” and “vast interstellar empire.”
Hansen breaks this down into nine distinct steps needed to move life from the smallest to the largest: the right star system, reproductive molecules, prokaryotic life, eukaryotic life, sexual reproduction, multicellular life, some vague category of intelligence (such as using tools), an advanced civilization with the ability to colonize, and finally, when all the pieces are in place, a massive galaxy-wide explosion of life.