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My personal addition: there is no great filter but life evolves towards embracing Buddhist principles of contemplation and ceases to pursue growth
Underrated explanation not represented here is that life goes extinct before it gets technology. There have been multiple mass extinction events in earths history and worse versions of these events are quite conceivable. Supernovae, gamma ray bursts, meteors, protracted volcanism, solar instability, runway greenhouse effects, runway cooling from glaciation increasing planetary albedo, oxygenation or any other toxic byproduct of life building up faster than life can adapt, exhaustion of resources without tectonic recycling... all of these resemble actual things that happened on Earth and could have destroyed life completely if things were different. Even if events like this don't completely wipe out life, they could set back the complexity of life periodically.
I don't know if it's my preferred Great Filter explanation but it's not covered by any of the options here and it seems at least single-digit percentage plausible.
@C3POtheDragonSlayer I'd say that "Emergence of complex animal-like life with brains" is the option that best fits this, since what you described as a filter applies mostly to this category.
@ScipioFabius That option doesn't fit that nearly, several of the mass extinction events (e.g. Siberian traps, Deccan traps, the meteor at the end of the Cretaceous) where worse versions could have completely killed life off or pushed evolutionary complexity back, happened after the Cambrian explosion at which point brains were well established.
@C3POtheDragonSlayer Yes, you are correct, I for some reason equated "emergence" with "ability to survive/remain" in my head whilst writing that. This definetely deserves an option of its own, what would you suggest as a conscise one?
@VanessaKosoy It will resolve if there is a scientific consensus about this question of the order comparable or higher than the scientific consensus about the existence of the black holes right before (1 year) the images of the black holes were obtained. Also, see the comments below for the specific qualifying percentages if we find a lot of dead civilizations.
@TheAllMemeingEye perhaps life is physically possible on almost all celestial bodies. But the chance of molecules randomly assembling into a functioning, self-replicating nanomachine is simply unbelievably small.
@Jan53274 I was under the impression that the Drake Equation and Great Filter were built on the model that only terrestrial planets with liquid water are considered, otherwise we'd have to count the vast interstellar medium, and then difficulty of life emergence being the main filter becomes a no brainer
@TheAllMemeingEye The interstellar medium isnt accounted for because it is (as far as we know) very unfriendly to life as we know it, minus possibly a short time period after the big bang when CMB was just right for life to be possible everywhere. So as @Jan53274 said, it could be suitable almost everywhere but could be just extremely unlikely to start off, hence being the great filter.
@Khazar_Man_From_Turan For example, life evolves with no obstacles to intelligent civilisations, and there are a lot of them around us, but we do not see them for some reason.
Does the aspect of the Dark Forest Hypothesis where there are numerous extraterrestrial civilisations but they are hiding because they are afraid of destruction rather than actually getting destroyed resolve to the third party destruction Dark Forest Hypothesis option or the no filter option?
@IhorKendiukhov suggestion: edit the no filter option to mention the hiding dark forest and early grabby aliens scenarios in parentheses so people don't bet up the wrong option
What are the numerical requirements for a given stage to count as a great filter? Can multiple resolve yes? For example, we might find the habitable planets of the galaxy are 99% procaryotic equivalent, with the remainder 99% single cellular eukaryotic equivalent, with the further remainder 99% multicellular but non-complex brainless life. In this case, do all 3 resolve yes, or do they resolve to equally split percentage, or does only the first one resolve yes, or do none of them because no single one was sufficient alone etc?
@TheAllMemeingEye Multiple can resolve yes. If there are several options that significantly “filter” biospheres, all of them will be considered filters. I would refrain for now from giving specific percentages, but it looks like 99% is a lower bound on the percentage that qualifies as the definition of the Great Filter. In ambiguous cases, I will resort to the best judgement of exobiologists (or similar relevant experts) available to me.
@IhorKendiukhov this is a tangent but I've been wondering it for ages, is exobiology considered a subset focusing on the biological aspects of a larger field about extraterrestrial civilisations in general (perhaps called exology?), or is that larger field still called exobiology even when discussing the potential political systems and technology of such civilisations?
@robm yeah that's what I was getting at, though what would you say is the correct blanket term for the speculative biology, technology, politics, culture etc? Is it exology (made up off the top of my head)?