As the topic came up in the Corona Virus discussion, I thought it was worthwhile to have a place to discuss it.
The Great Filter started with the Fermi Paradox, which is itself addressing the Drake equation.
The Drake equation starts
N= R∗ x fp x ne x fl x fi x fc x L
R∗ = the average rate of star formation in our Galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of planets that could support life that actually develop life at some point
fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
what all this breaks down to is that given a value for each variable in the equation based on our best evidence, we should have a sense of how many civilizations of intelligent beings exist in the universe.
The Fermi Paradox is essentially looking at the real world numbers for how many Sol type stars there are in this galaxy alone, and how long they have existed, and the likelihood they might have planets around them, (and it starts to get more estimation here) and how many planets may be able to support life, and the chances of that life leading to Sapient beings, who develop technology and ultimately leave their planet... given our best estimates on these factors there should have been enough spacefareing civilizations that have been around long enough that we should have been visited or at least seen evidence of their existence by now.
The Paradox is that all calculations don't add up to what we actually see in the galaxy.
The great filter is a theoretical additional variable that's meant to resolve the paradox. It basically says that there must be some barrier that comes into the whole thing that gets us the result of lack of visitation or evidence of advanced sapient life existing in the galaxy, despite the huge number of opportunities for it to arise on planets in the habitable zone around stars most like our own that lead to us.
Basically something is filtering out lifeforms by some unknown filtering standard before they can reach the levels where they could have gotten here or left evidence of their existence. The question is also asked if we have already passed that filter of if it still lies ahead for us?
Theories of the filter include the splitting of the atom, where reaching that technology filters out those that would use this tech for constructive ends or destroy themselves through destructive applications. It might also be things like chemical or biological warfare, failed eugenics. Meteor impacts survival (the dinosaurs perhaps could have continued toward civilization) or maybe even the survival of the switching to oxygen atmosphere.
Some of those filters are behind us, some are ongoing with uncertain outcomes, and some are ahead of us. Of the ones ahead of us some are things we can see ahead, some we have vague idea to watch out for, and some we don't even know could be ahead of us.
In one sense these can seem to imply that there might be many complex and independent filters, and not every civilization will have to overcome the same ones. Maybe like a choose your own adventure book with multiple win scenarios the filters one passes earlier on sort civilizations toward filters later that are specific to their paths.
But it is still possible that the whole tangle of choices amounts to one great filter. If we step back and decrease the resolution of the image we are looking at, perhaps instead of nuclear power, or biological weapons, we look at it instead as intra-species conflict. Instead of climate change, or depletion of natural resources, it's sustainable technological advancement. Maybe instead of intra-species conflict, or sustainable technological advancement, it's sustainable intra-species globalized social and technological advancement... and half a dozen other things in there too.
It is possible to look around and see ongoing struggle and failures that look to have such big consequences, and wonder if this is the filter, or a part of the filter. and maybe that is the case.
but increasingly think that instead of one clear linear path that all pass the same tests to emerge into the wider universe as civilizations, that it's entirely possible for one civilization to have gotten there with Feudalism and Eugenic as it's core values, while another Capitalistic Theocracy made it, and a Sociocratic adhocrasy made it as well. They each had to pass through 'the great filter' but the things they had to overcome were different for each based on the starting positions and core values of each.
and it begs the question: what happens to inter-civilization relations if the things that one had to double down on to successfully make it out of the great filter were the same things another had to purge in order for them to make it through their own great filter. And what does that mean for advanced civilizations that might wish to help or subjugate species in the midst of their own great filters, the guidance of one group to follow it's path may be the destruction of the fledgling group.