The Great Filter

Concept

The Great Filter is the idea that somewhere on the Kardashev Scale, without making any kind of further assumption about where it might be, there is an event that generally occurs in the advancement of every intelligent species and wipes all or atleast most of them out.

The Great Filter is one of many possible explanations for the Fermi Paradox and a very elegant one at that, because it does not try to explain the permanent absence of alien life but instead breaks it down into nothing more than an asynchrony of them.

If you had issues following the last paragraph, let me explain it a bit more practically: There may be a lot of promising planets for intelligent life to emerge, but the universe has also been around for an insanely long time. If there was a great filter that wiped out every alien civilization after say 5,000 years, you wouldn't be able to find any cosmic traces of them and after just half a million years, a blink of an eye on a cosmic timescale, you wouldn't even be able to tell they have ever been around if you stood on their very home planet.

So, if there is a great filter, maybe right in front of us, despite it being pretty depressing, it would atleast explain the absence of Type 2 civilization monuments around our galaxy, because no civilization came far enough to build them. Let us explore the possible filters we might already have or might still have to overcome.

The probability for life

It might just be that life as we know it is much more improbable than we think it is. This could mean a lot of different things: Simple prokaryotic single-cell life, complex eukaryotic life or reproductive molecules at all. Even if that takes place on many planets, the emergence of intelligent life might be very rare still.

Actually, I imagine this to be the most probable scenario, since it happened once on Earth, many thousands of years ago, and it never happened again. As the biologist Ernst Mayr already said: There have perhaps been as many as 50 billion species since the origin of life and only one of these achieved the kind of intelligence needed to establish a civilization.

Nuclear Wars

As we all sadly had to experience not too long ago, humans have not entirely passed the stage of nuclear threats yet, indicating other species may have struggled with them, either. After all, extinction by nuclear war is a very tricky potential filter because it would only require two big countries with a lot of nuclear material - e.g. russia and the US - to attack each others with them, which would probably take just a couple of hours for both to be reduced to inhabitable wastelands while additionally threatening life in surrounding countries.

If many species make it up to this point, I myself am very convinced we have lost lots of intelligent species to nuclear wars. I have to admit, this thought is pretty depressing.

Artificial intelligence

Another topic that has had it's hot takes and breakthroughs in the last couple of years and will certainly continue to deliver in the near future: Artificial Intelligence. Of all the possible great filters, this luckily is the one being discussed most often and the most popularized - thanks to Keanu Reeves' breathtaking performance in "The Matrix".

For real, though, Artificial Intelligence systems built to solve any given problem could just rightfully come to the conclusion that humanity is a contributing factor to it and erase them from the surface of the Earth. The possible threat through an intelligence explosion in AI and the emergence of a rogue AI is taken seriously enough by some influencial people for them to have signed an open letter urging a pause on AI (opens in a new tab), among them being Elon Musk, Steve Wozniak, Stuart Russell and Max Tegmark.

Due to AI being an intriguingly promising way to increase our quality of life however, the open letter doesn't ask for a permanent halt but just a 6 month timeframe of no further progress being made for sufficient regulations to be put into place. So, after all... This filter might still come into play. Not trying to jinx it.

Watcher civilization(s)

As mentioned earlier, one possible explanation for us to see no other prospering intelligent life in our galaxy might be watcher civilizations being around. The concept of a watcher civilization revolves around a hypothetical Type 3+ civilization that - for reasons beyond our comprehension - does not want other intelligent lifeforms to emerge in their territory, which is why they wipe them out as soon as they advance far enough.

In the context of a hypthetical watcher civilization being around, it would also make sense for us to not find any traces of alien life even if they didn't manage to wipe out all of their competition: Any surviving species would have every reason to hide themselves from other's radars, which would obviously make it hard for us to get in touch with alien life without being reduced to atoms.

Summary

Wherever the great filters lie, it's save to say we made it pretty far already. After all, we already shot satellites, probes and space stations into space so even in the eventual case of sudden death, we leave some traces behind for others to find. Now that we know what alien life would be up to and what could make them unlikely to be around, we are able to come up with lots of possible explanations for the Fermi Paradox!