With the Drake equation we can use this to calculate how many civilizations could exist in our Milky Way (and by extension, in the observable universe). But if there are other civilizations out there, why haven’t we seen a trace of them?
As Enrico Fermi put it, the belief that there are numerous intelligent civilizations in the Universe is paradoxical when compared to what our observations suggest.
Therefore, either our belief is wrong, or our observations are flawed and / or incomplete (today we are well aware that our tools to detect intelligent life on other planets is extremely limited).
This (at least apparent) absence of extraterrestrial civilizations suggests that, of necessity, there has to be an extremely unlikely step on the way from a humble planet to an interstellar civilization.
The absence could well be because intelligent life is extremely rare, or because intelligent life has a tendency to self-destruct (it is important to mention here that Fermi formulated this paradox back in 1950, when the fear that Humanity would destroy itself it was more than founded).
We call this bottleneck that prevents alien civilizations, from any of the billions of planets out there, from becoming interstellar civilizations, we call the Great Filter.
That the number of habitable exoplanets increases may be bad news for us …
The idea of the Great Filter was proposed by Professor Robin Hanson in 1996, in an attempt to respond to the Fermi paradox, and it is one of the most popular options to explain the paradox (other than the intelligence necessary to achieve to create technology advanced may be very rare, or that Earth is an extremely rare planet on which many things have coincided).
If Professor Hanson’s idea is correct (there must be some kind of barrier that makes it incredibly difficult to go from uniplanetary civilization to interstellar civilization), every discovery of a potentially habitable exoplanet means that our civilization may not have reached that point. barrier, and further detracts from the possibility that Earth is an extremely rare planet.
What if the great filter is ahead of us?
Although it is possible that the appearance of intelligent life is rare (there are no reasons that make us think that this cannot be an explanation), the silence that we obtain from the galaxy may be the result of the intelligent life that develops does not manage to survive during enough time. Is it possible that all advanced civilizations end up reaching an unsustainable point for their own existence, such as suicidal technology or depletion of your planet’s resources?
Of course. If we start from the basis that this Great Filter exists as an impediment to the appearance of interstellar civilizations, we are unable to determine if Humanity has overcome it (we were certainly close during the Cold War) or if it awaits us at some point in our future.
Humanity has survived supervolcanoes, asteroid impacts, pandemics … but our survival with nuclear weapons is limited to just a few decades.
What if a planet with intelligent life appeared?
The answer is complicated, and it would depend on many factors: is it the only planet of that civilization, and therefore are they in a state of development similar to ours? Or is it simply one of many colonized planets, and therefore its technological development is infinitely superior to ours?
If the answer is the first one (its stage of development is similar to ours) then it may be a point in favor of the fact that the Great Filter is still to come for our civilization. But it is also possible that the Great Filter is in our distant past (and that it is, for example, if a civilization develops the use of tools).
There are many other theories / explanations for the fermi paradox
The Universe is very large, and all theories may have something to do with it. But it is equally likely that, although very common, that life is not close enough to our Solar System, or even, that they have not yet developed enough, or that they are already extinct (it is not simply a matter of space, if not also of coexistence at the same time).
In one way or another, the explanation to try to answer why we have not found other intelligent civilizations is fascinating (in both ways).