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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We don’t know if great filters are really a thing and what form they take if they are.
The problem is we are working from a sample size of one. Everything that happened to get us where we are now did happen to us and in retrospect it is quite hard to tell how likely any of it was.
If you win a lottery or get hit by lightening you know that this was pretty unlikely simply by looking at all the people it didn’t happen to.
We don’t really have anyone else to compare things to.
We don’t know if for example the evolution of multicellularism was something that was unlikely or if this is happen that happens in some form every place life evolves. We aren’t even sure if multicellularism is a necessary step to reach the goal of building advanced civilizations that are noticeable from distant stars.
We do know that it took quite a while to happen here, so it seems like it was a big hurdle, but that is just basing things on how they went here.
We only know that we have come as far as we did, that if we only go a bit further we will have reached a point where we would be noticeable by any other civilization on our level in the area and that we can’t see any other civilization that has reached such a level.
We can only assume that reaching that level must be very hard if no one else has done it before us.
We can make guesses about which events in our past would could have been what stopped others from reaching that goal. And we can speculate about hurdles in the future.
We know some things by now like planets being pretty common and the building blocks for life being common too.
We can look at some parts that might make us special.
We look at our history and an pick out some developments were all the ingredients were there for a long time without it happening and consider those rare.
We can also look at things that might do us in in the future. The idea of any sufficiently advanced civilization killing itself of as it develops technology to dangerous to handle was certainly popular during the cold war, but that seems unlikely as most of the candidates would just be a set-back not an end.
We really can’t know for sure until we meet some alien civilizations and compare notes or at least find some alien ruins and figure out what did them in before they could colonize the stars.
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