I had never heard of the Great Filter. Apparently, as the theory goes, since there are a statistically large number of habitable planets in our galaxy, it is statistically aberrant that we have not heard from or been visited by other space-capable species. That there must be a reason for this that either prevents intelligent beings from reaching the point were interstellar travel is possible, or or that something prevents them from evolving much beyond that … perhaps some self-cause catastrophe.
This article very briefly discusses the Great Filter … just enough to leave me wanting to learn more.
ift.tt/1noWhJu
Link: ift.tt/1noWhJu
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