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Great Filter

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From Wikipedia, the free encyclopedia

Hypothesis of barriers to forming interstellar civilizations

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 the idea that, in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there is a barrier to development that makes detectable extraterrestrial life exceedingly rare.[1][2] The Great Filter is one possible resolution of the Fermi paradox. The main conclusion of the Great Filter is that there is an inverse correlation between the probability that other life could evolve to the present stage in which humanity is, and the chances of humanity to survive in the future.

The concept originates in Robin Hanson's argument that the failure to find any extraterrestrial civilizations in the observable universe implies that 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 with advanced civilizations actually observed (currently just one: human).[3] This probability threshold, which could lie in the past or following human extinction, might work as a barrier to the evolution of intelligent life, or as a high probability of self-destruction.[1][4] The main conclusion of this argument is that the more probable it is that other life could evolve to the present stage in which humanity is, the bleaker the future chances of humanity probably are.

The idea was first proposed in an online essay titled "The Great Filter – Are We Almost Past It?". The first version was written in August 1996 and the article was last updated on September 15, 1998[update]. Hanson's formulation has received recognition in several published sources discussing the Fermi paradox and its implications.

Main argument<br>[edit]

Fermi paradox<br>[edit]

Main article: Fermi paradox

There is no reliable evidence that aliens have visited Earth; 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:[1]

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.[5] 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."[1]

The Great Filter<br>[edit]

With no evidence of intelligent life in places other than Earth, 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:

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 intelligence

A civilization advancing toward the potential for a colonization explosion

Colonization explosion

According to the Great Filter hypothesis, at least one of these steps—if the list were complete—must be improbable. If it is not an early step (i.e., in the past), then the implication is that the improbable step lies in the future and humanity's 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 colonies. So perhaps step 9 is the unlikely one, and the only things that appear likely...