The Great Filter is a concept originated by economist Robin Hanson with regards to the Fermi Paradox, to wit: why there is no evidence of advanced civilizations in our corner of the universe. This implies that at least one step in the process of starting with a star and ending with at least level 2 Kardeshev civilization must be unlikely (since that is presumably observable by current technology). Hanson's list describes the following nine steps in an evolutionary sequence that results in the colonization of the observable universe:
According to the Great Filter hypothesis, at least one of these steps must be improbable. If it's not an early step (i.e., in our past), then the improbable step must logically lie in our future and our prospects of reaching step 9 (interstellar colonization) are still bleak. If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none appear to have made it to step 9, or the Milky Way would be full of observable interstellar activity by these civilizations. By this argument, finding multicellular life on Mars (for example) would be bad news, as it would imply the early steps are easily reproducible and the potentially insurmountable hurdles to expansion (and by extension, continued survival of our species) lie ahead.
Either we are a rare first, or doomed.
- The right star system (including organics and potentially habitable planets)
- Reproductive molecules (e.g. RNA)
- Simple (prokaryotic) single-cell life
- Complex (eukaryotic) single-cell life
- Sexual reproduction
- Multi-cell life
- Tool-using animals with big brains
- Planetary civilization
- Interstellar colonization explosion
According to the Great Filter hypothesis, at least one of these steps must be improbable. If it's not an early step (i.e., in our past), then the improbable step must logically lie in our future and our prospects of reaching step 9 (interstellar colonization) are still bleak. If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none appear to have made it to step 9, or the Milky Way would be full of observable interstellar activity by these civilizations. By this argument, finding multicellular life on Mars (for example) would be bad news, as it would imply the early steps are easily reproducible and the potentially insurmountable hurdles to expansion (and by extension, continued survival of our species) lie ahead.
Either we are a rare first, or doomed.