Great Filter

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Concept and Motivation

The Great Filter is a very improbable step leading to an interstellar civilization, its existence is a proposed explanation for the Fermi Paradox. Robin Hanson coined the term in his 1998 essay The Great Filter - Are We Almost Past It?.

Given our actual knowledge of the universe, it is uncontroversial to assume that many other habitable planets - fulfilling the conditions for life to flourish until it develops into a interstellar civilization - are bound to exist. At the same time, there is a complete lack of evidence for the existence of other civilizations. Those two facts implies the existence of a extremely improbable step, a Great Filter, in the path leading from the basics conditions for originating life to the emergence of interstellar civilizations.

Should we worry?

The Great Filter might be the transition from prokaryotes to eukaryotes, or from unicellular to multicellular life forms, or yet the appearance of photosynthesis. Should this be the case, we have already passed through the improbable region, the filter is behind us and we shouldn’t worry. But the hard step might also be ahead of us: to survive the creation of nuclear bombs, AGI biotechnology, nanotechnology or an asteroid impact [1]. In this scenario, we are passing or will pass through the improbable region and we should worry. Estimating the location of the Great Filter is a crucial task for dealing with Existential risk. Many efforts have been made in that direction, but much remains uncertain.

Traces of life in other planets are an evidence for a later Great Filter[2]. Any indication, anywhere in the universe, that life evolved until a period anterior to ours it’s an indication that evolving until such anterior period isn’t so improbable. Hence, the Great Filter probable location shifts in the future direction, enhancing the chances he is just ahead of us.

The study of past mass extinctions and astrobiology can provide ideas for the estimation of the Great Filter probabilistic distribution. However, there are many difficulties involved. For instance, one can not estimate the improbability of a step based on its completion time. Robin Hanson gives the following example in his seminal paper: “…say you have one hour to pick five locks by trial and error, locks with 1,2,3,4, and 5 dials of ten numbers, so that the expected time to pick each lock is .01,.1, 1, 10, and 100 hours respectively. Then just looking at those rare cases when you do pick all five locks in the hour, the average time to pick the first two locks would be .0096 and .075 hours respectively, close to the usual expected times of .01 and .1 hours. The average time to pick the third lock, however, would be .20 hours, and the average time for the other two locks, and the average time left over at the end, would be .24 hours. That is, conditional on success, all the hard steps, no matter how hard, take about the same time, while easy steps take about their usual time”


On a subsequent paper, Hanson constructs a simulation of the hard steps distribution given earth habitable time, and concludes that there is a total of 4 to 7 hard steps uniformly distributed . It also shows that since hominid evolved there has been at least one hard step and that the best extinction model that fits all these requirements is William Schopf’ model. Taking evolutionary arguments for AGI and observational selection effects together, Bostrom and Shulman argue that Hanson’s results can narrow the bounds on where AGI engineering difficulty can be found.

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