A great filter is anything that prevents nonliving matter from becoming living matter that undergoes evolution until it is smart enough and capable enough to colonize the entire galaxy, and possibly further. Presumably, life is subject to one or multiple of these filters, since otherwise we would expect the galaxy to be teeming with life.
The presumption is that the filter lays either in our past, in some improbably event that led to life as we know it, or else it lays in the future, where all advanced civilizations converge towards some self-annihilation like malevolent AI or grey goo, or nuclear weapons etc.
I personally don’t think great filters in our future are particularly likely to be the answer. That’s not to say I don’t think we should take existential risks seriously, I just think it’s unlikely that there were millions of civilizations like ours out there that all, (without exception) annihilated themselves somehow before doing anything that would reveal their existence to us. It’s much more likely that we’re alone in the universe, and our own evolution involves several very improbably steps that require the anthropic principle to resolve.
In fact, I think we passed through several great filters on our way to the present. Here I’m going to catalogue things that we don’t currently have an answer for but that are critical components of allowing us to even have gotten to our current level of advancement.
Note: My assumed explanation for us having passed all of these filters is an anthropic argument coupled with multiple universes.
In universe filters
These filters are things that are possible within the current laws of physics, and are possibly just very improbable.
- Abiogenesis - the first replicator to arise from non-living matter.
- Evolution of eukaryotic cells - All complex life is eukaryotic
- Evolution of sexual reproduction - All higher life forms are sexually reproducing
For these filters, depending on how unlikely each step is, the number of planets supporting habitable life in the universe may be enough to explain our existence.
So, for example, if abiogenesis is hard, say 1 out of 40 billion earth-like planets experience it, but the other filters are actually inevitable and “easy” to evolve once life exists, then we might say it’s reasonable to find ourselves alone in this galaxy.
If each of these has a conditional probability of 1 in 100 billion for each filter, then it may be that we don’t expect to see even one advanced life form in the universe, and we need to postulate multiple universes in order for an anthropic argument to make sense.
Out of universe filters
These are filters based on the way the universe is. In order to make an anthropic argument to explain why we exist if any of these filters are applicable, we need to postulate multiple universes.
- Fine-tuned physical constants - Numerous dimensionless physical constants seem to be fine-tuned to make the building blocks of life possible or abundant
- Carbon seems chemically unique - No other element supports such long branched chains that bond with so many other elements to produce such an array of possible compounds and complex macromolecules that are a prerequisite of complex life (specifically, DNA is super dependent on these long chains to encode enough information for complex replicators). There is one such element in the periodic table (as far as we know). What if there were zero such elements?
- Orderly universe - The universe seems to allow order to arise from chaos at multiple scales. (This is, admittedly, not a fully developed idea). Why should that be so? If I create a simulation with random initial conditions and random transition functions, I should imagine the simulation to produce nothing more than randomness. But in our universe, stars fuse, supernovas form, rocks exist, galaxies exist, an array of elements are produced from stellar processes. None of this is subject to the ratchet of evolution by natural selection, it’s some property of the universe itself.