What exactly is the “Great Filter”?

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According to the Fermi paradox, there are multiple filters and a great filter that civilizations have to pass to keep evolving. What are these barriers?

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A planet capable of harboring life must form in a star’s habitable zone.

Life itself must develop on that planet.

Those lifeforms must be able to reproduce, using such molecules as DNA and RNA.

Simple cells (prokaryotes) must evolve into more complex cells (eukaryotes).

Multicellular organisms must develop.

Sexual reproduction, which greatly increases genetic diversity, must take hold.

Complex organisms capable of using tools must evolve.

Those organisms must create advanced technology needed for space colonization. (This is roughly where humans are today.)

The spacefaring species must go on to colonize other worlds and star systems, while avoiding destroying itself.

There have been many “great filters” that have been proposed in an attempt to explain the Fermi Paradox. Some examples include:

* Life emerging on a planet
* The transition from prokaryotic/single-celled organisms to eukaryotic/multi-celled organisms
* Rapid development of weapon technology

There are natural and physical barriers, like, the conditions on the planet have to be right for life to form in the first place. If it’s just a big toxic mess of nasty gases and nothing to sustain life, that’s a filter.

Then there’s the biological part, like stuff has to *evolve* properly for life to even get to advanced stages. If you have a colony of like 100 bacteria just chilling somewhere for a gazillion years and they never get the right combination of DNA to start evolving into fancier life forms, then that’s a great filter.

And another natural part, like even if stuff *does* evolve there’s no guarantee it doesn’t get erased when an asteroid decimates the planet or the star in its system explodes and there’s no more energy left for that life to grow from.

And then the technical aspect, even when all the conditions are totally perfect and things are progressing, there’s always the chance we fudge it up ourselves as “advanced beings” – could be a nuclear war destroys life as we know it so we go back a few hundred years and people never trust technology again to continue advancing, could be we destroy the environment and cause irreversible climate change.

The Drake equation lets us hypothesize how many planets exist that can have life (or even intelligent life).

The Fermi paradox says “if there are potentially so many of these planets why haven’t we seen any evidence of life outside Earth?”

The answer to that is The Great Barrier. We haven’t seen evidence of life outside Earth because of reasons. We can only guess at the reasons.

Perhaps life from non-life (abiogenesis), which is how we think life may have happened on Earth is unbelievably rare. We haven’t been able to reproduce it.

Perhaps going from single-cell to multi-cell to intelligent life is so rare we may be the first to have done it. Not that it’s impossible but maybe it’s not out there yet.

Perhaps life that becomes sentient and intelligent always destroys itself before it can become a type 2 or 3 civiization.

There can be more. The main concept is that there must be barriers to becoming noticed by us, and we don’t know if we have passed our barrier, or maybe our barrier is out there to destroy us in the future.

Quick note: these aren’t filters that we know for sure exist, they’re just *possible* filters that stop species from becoming intelligent, spacefaring civilizations, and there’s no definitive list, iirc. They’re broadly split into two groups, those bring ones humanity had already passed and those we have yet to pass.

The already passed filters include:
– planets with conditions for life are just rarer than we think
– life (or intelligent life) is just really rare
– development of modern-level technology is rare either because species can’t hit a certain necessary bar (like aquatic life being unable to use fire or the possibility that maybe humans are just really good inventors even among all intelligent life) or they kill themselves before

The to be passed filters include:
– development of nuclear technology usually leads to a nuclear apocalypse before a species can colonize other plants
– industrial ages run a species home planet into the ground before they can set up independent colonies and most species can’t cooperate enough to stop it
– another spacefaring alien specie kills of other species once they hit a certain technology level
– there’s some technological barrier or challenge we don’t yet know about that kills of or prevents development past a certain point

I’d have to look em up to find all the big ones, but does that give you an idea?