Great Filter

This article is about an implication of the Fermi paradox. For the music album by Tub Ring, see The Great Filter (album).

The Great Filter is a possible implication of the failure to find any extraterrestrial civilizations in the observable universe, despite hundreds of years of increasing sophisticated astronomy. As proposed by Robin Hanson, a "Great Filter" might act to reduce the great number of potential sites to the tiny number of intelligent species actually observed (currently just one: ours). This probability barrier, which could lie behind us or in front of us, could be either a low likelihood of the evolution of intelligent life, or a high probability of self-destruction. The main counter-intuitive conclusion of this observation is that the easier it was for life to evolve to our stage, the bleaker our future chances probably are.

Main argument

We have not yet observed evidence of intelligent extraterrestrial life, though we have observed a great number of stars. Therefore, the whole process of starting with a star, and ending with communicating intelligent life, must be unlikely (the Great Filter). This implies that at least one step in this process must be improbable. Robin Hanson listed the likely steps as:

1. The right star system (including organics & potentially habitable planets)
2. Reproductive molecules (e.g. RNA)
3. Simple (prokaryotic) single-cell life
4. Complex (archaeatic & eukaryotic) single-cell life
5. Sexual reproduction
6. Multi-cell life
7. Tool-using animals with big brains
8. Colonization explosion

Hanson considers this list to be incomplete.

At least one of these steps must be improbable. If it's not 1 through 7, then the implication is that our future (step 8) is bleak. If 1–7 are likely, then many civilizations would have developed to the current level of the human race. However, none appear to have made it to step 8, or the Milky Way would be full of colonies. So perhaps step 8 is the unlikely one, and the only thing that appears likely to keep us from step 8 is some sort of catastrophe. So by this argument, finding multicellular life on Mars (provided it evolved independently) would be bad news, since it would imply steps 2–6 are easy, and hence only 1, 7 or 8 could be the big problem.

Although steps 1–7 have occurred on Earth any one of these may be unlikely. If the first seven steps are necessary preconditions to calculating the likelihood (using the local environment) then an anthropically biased observer can infer nothing about the general probabilities from its (pre-determined) surroundings.

See also

• Big Bounce
• Black swan theory
• Doomsday argument
• Final anthropic principle
• Goldilocks Principle
• Inverse gambler's fallacy
• Metaphysical naturalism
• Neocatastrophism
• Principle of mediocrity
• Rare Earth hypothesis
• Selection bias