I'm just trying to play Devil's Advocate here.
Milky Way has been having the ability to nurture life for ~6 BILLION YEARS. We're the first species to develop intelligence/civilization/technology? Why is intelligent life so scarce?
I did previously mention that there is the possibility that life, especially intelligent, may not be evenly spread throughout the universe. There's nothing in evolution stating that a civilization building type intelligent species is a definite avenue of evolution, so even if life is spread evenly, highly socially and technologically advanced species may not be a very guarantee at all wherever life appears.
It may be possible that humans are in the galaxy with among the lowest concentrations of life in the universe. There are countless galaxies in the universe, and it wouldn't surprise me if there were some places that were nearly barren even at large areas.
I know that my argument is really just arguing from probability and statistical ideas, but the possibility can't be discounted. Hopefully its not a complete load of rambling.
There are a few possibilities:
We're the only civilization in the Milky way.
In this case, as I said, why is life so scarce? It shouldn't be - according to what we know about how the universe works.
There are a few civilizations in the Milky way, but they're all all recent - a few thousand/tens of thousand of years old.
The question is - Why are there no older civilizations? What changed in recent galactic history?
Or there is one/a few older alien species - 1 million years old or more.
Why are this/these alien species not here? They should be, by now.
I like to think of it like finding a dozen cats in a forest as large as the surface area of the earth. Scatter those cats in there randomly and then take a few steps into the forest. Will you be shocked if you don't find them after a few steps in? Take five or six more? No cats? Hmmmm...must be a paradox. We know they are in there, so why aren't we finding them? Scale this up by a magnitude of a few billion.
, you forgot to give these cats an evolutionary advantage that ensures their survival and prosperity (equivalent of intelligence/technology) and to wait a few thousand years. Fulfill those two requirements and, when you walk into the forest, you'll find the cats everywhere.
For a more detailed explanation, read my previous post from this thread.
Well, that's one possibility. Nuclear weapons are not the only weapon of mass-destruction, chemical and biological also qualify. All could technically wipe out a civilization. WMD aren't the only technological development that could cause a civilization to destroy itself, four such examples I could readily think of could include: Internal combustion engines; Certain industrial chemicals, both of which could cause severe pollution resulting in massive loss of life or even extinction (An extreme case, but possible); Artificial intelligence that ends up destroying the civilization that created it; Self-replicating nano-technology that ends up consuming all life and anything consumable on the planet that could feed them.
Other possibilities include society dying off for reasons that are not technological, such as plagues and famine, and insufficient reproductivity.
There is a possibility that we're simply the first planet to evolve life as evolved as primates and humans.
Technological alien species, as proven by our Cold War, could destroy themselves. My problem with this is - ALL of them?
Let's say Milky way fathered a few thousand VERY DIFFERENT (in psychology/motivations) intelligent alien species. ALL self destructred? Without exception? Highly improbable - and that would also imply that we'll do the same thing, at one point down the road.
A few observations about your methods for self-annihilation:
Internal combustion engines; Certain industrial chemicals - the chances these have of eradicating a global intelligent species (via pollution) are practically 0. They're just not destructive enough/fast enoough.
Artificial intelligence - this could destroy the organic alien species that fathered it (of course, normally, it should have no such motivations); however, AI would replace the destroyed species, creating its own civilization. From a "fermi paradox" POV, the civilization endures.
Self-replicating nano-technology - in order to eat all life/everything on the planet, this nano-tech must fulfill some very specific conditions - some of which are, apparently, impossible to fulfill (from a technological POV).