Great Filter

intelligent life

The Great Filter, in the context of the Fermi paradox, is whatever prevents ‘dead matter’ from giving rise, in time, to ‘expanding lasting life.’ The concept originates in Robin Hanson’s argument that the failure to find any extraterrestrial civilizations in the observable universe implies the possibility something is wrong with one or more of the arguments from various scientific disciplines that the appearance of advanced intelligent life is probable.

This observation is conceptualized in terms of a ‘Great Filter’ which acts to reduce the great number of sites where intelligent life might arise to the tiny number of intelligent species actually observed (currently just one: human). This probability threshold, which could lie behind us or in front of us, might work as a barrier to the evolution of intelligent life, or as 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.

The idea was first proposed in an online essay titled, ‘The Great Filter – Are We Almost Past It?’ written by economist Robin Hanson in 1996. There is no evidence aliens have visited earth and we have observed no intelligent extraterrestrial life with current technology nor has SETI found any transmissions from other civilizations. The universe, apart from the Earth, seems ‘dead’; Hanson states: ‘Our planet and solar system, however, don’t look substantially colonized by advanced competitive life from the stars, and neither does anything else we see. To the contrary, we have had great success at explaining the behavior of our planet and solar system, nearby stars, our galaxy, and even other galaxies, via simple ‘dead’ physical processes, rather than the complex purposeful processes of advanced life.’

Life is expected to expand to fill all available niches. With technology such as self-replicating spacecraft, these niches would include neighboring star systems and even, on longer time scales which are still small compared to the age of the universe, other galaxies. Hanson notes, ‘If such advanced life had substantially colonized our planet, we would know it by now.’

With no evidence of intelligent life other than ourselves, it appears that the process of starting with a star and ending with ‘advanced explosive lasting life’ must be unlikely. This implies that at least one step in this process must be improbable. Hanson’s list, while incomplete, describes the following nine steps in an ‘evolutionary path’ that results in the colonization of the observable universe: 1) the right star system (including organics and potentially habitable planets), 2) reproductive molecules (e.g., RNA), 3) simple (prokaryotic) single-cell life, 4) complex (archaeatic and eukaryotic) single-cell life, 5) sexual reproduction, 6) multi-cell life, 7) tool-using animals with big brains, 8) where we are now, and finally, 9) colonization explosion.

According to the Great Filter hypothesis at least one of these steps – if the list were complete – must be improbable. If it’s not an early step (i.e. in our past), then the implication is that the improbable step lies 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 race. However, none appear to have made it to step 9, or the Milky Way would be full of colonies. So perhaps step 9 is the unlikely one, and the only thing that appears likely to keep us from step 9 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, 8 or 9 (or some unknown step) 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.

There are many alternative scenarios that might allow for the evolution of intelligent life to occur multiple times without either catastrophic self-destruction or glaringly visible evidence. These scenarios are part of the Fermi paradox argument, ‘They do exist, but we see no evidence.’ Additional arguments include it is too expensive to spread physically throughout the galaxy, they tend to experience a technological singularity, Earth is purposely isolated, it is dangerous to communicate and hence civilizations actively hide, and many others.

As one example, astronomer Seth Shostak of the SETI Institute argues that one can postulate a galaxy filled with intelligent extraterrestrial civilizations that have failed to colonize the Earth. Perhaps the aliens lacked the intent and purpose to colonize or depleted their resources, or maybe the galaxy is colonized but in a heterogeneous manner, or the Earth could be located in a ‘galactic backwater.’ But, absence of evidence is not evidence of absence. The absence of extraterrestrial megascale engineering projects, for example, might point to the Great Filter at work. Does this mean that one of the steps leading to intelligent life is unlikely?

According to Shostak: ‘This is, of course, a variant on the Fermi paradox: We don’t see clues to widespread, large-scale engineering, and consequently we must conclude that we’re alone. But the possibly flawed assumption here is when we say that highly visible construction projects are an inevitable outcome of intelligence. It could be that it’s the engineering of the small, rather than the large, that is inevitable. This follows from the laws of inertia (smaller machines are faster, and require less energy to function) as well as the speed of light (small computers have faster internal communication). It may be–and this is, of course, speculation–that advanced societies are building small technology and have little incentive or need to rearrange the stars in their neighborhoods, for instance. They may prefer to build nanobots instead. It should also be kept in mind that, as Arthur C. Clarke said, truly advanced engineering would look like magic to us–or be unrecognizable altogether. By the way, we’ve only just begun to search for things like Dyson spheres, so we can’t really rule them out.’

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