> One would either have to belive that intelligent life is vastly less likely than non-intelligent life
That seems like a valid belief. Getting to a technological stage such that a species would be detectable over the vast distances of space could indeed be quite rare. You have to also consider the temporal aspect: intelligent, technologically advanced species may have evolved several times but gone extinct before we could notice them. Do other technically advanced species exist in the universe? Probably, but it could be that at any given time there might only be about 1 in any given galaxy and the distances between galaxies are great enough that we'd never likely be able to make contact. (and ~1 per galaxy would still mean that there would be a whole lot of intelligent species out there - it's just that it would be extremely difficult to make contact with any of them)
Its not at all clear in general. It might be true. But it also might not. It seems quite reasonable to believe that life inevitably evolves into intelligent life if given enough time. Why some life would and some life wouldn't isn't at all clear.
> advanced species may have evolved several times but gone extinct before we could notice them.
All the potential answers to the Fermi Paradox, for sure. But it would almost definitely have to be species that never got to the "expand rapidly into other solar systems" phase.
> at any given time there's only about 1
This doesn't preclude us knowing about that 1. If it got to earth at any time in the last billion years, we might have a pretty high chance of discovering it if it existed on earth for any significant legnth of time.
> But it would almost definitely have to be species that never got to the "expand rapidly into other solar systems" phase.
It's certainly not a given that our species will ever do that or that we'll last long enough to do that.
> This doesn't preclude us knowing about that 1.
Let's say we're the 1 currently in the milky way galaxy. There could be another in the closest galaxy the Canis Major Dwarf Galaxy which is 25,000 light years away. But being able to detect a signal from 25,000 light years away... well, that's the problem. And what if they're just getting to the point where they could transmit a signal now? So maybe in 25,000 years we'd notice something... maybe? (if we're still around) As far as physically traveling 25,000 ly, well we know that even trying to go 1 ly is going to be super difficult technically. Similar problems even if there's an intelligent species on the other side of our own galaxy since it's 52K ly across.
> It seems quite reasonable to believe that life inevitably evolves into intelligent life if given enough time.
From what we have seen on Earth, it does not seem inevitable. Dinosaurs were the largest land animals for a very long time and they did not, as far as we know, evolve towards being more intelligent. The same seems to be true for many other animal groups that have been around for a long time.
They don't seem to have created any technology, but it is hard to know how intelligent they were. Their brains were small, but crows have quite small brains and they are relatively intelligent. If dinosaurs had used tools, would this show up in the fossil record?
The belief that intelligent life isn't inevitable is not inconsistent with the belief that intelligent life usually will happen from any life given enough time. Why are you positioning those things as opposed? They are not.
> It seems quite reasonable to believe that life inevitably evolves into intelligent life if given enough time.
Intelligence is an arbitrary yardstick. Compared to a yeast cell that just sits on a surface, a tree that grows toward light, strengthens its stem in response to wind forces etc, could be considered intelligent.
More useful is tool use: animals that use sticks, stones etc. to get at food unreachable without those tools.
But even that won't do: a planet full of life like that, but it never gets off that planet. Bits of life getting spread by meteor strikes etc? Possible. But chances of that drop off sharply with distance (within solar system -> interstellar -> between galaxies & up).
The missing bit? Technology itself evolving from simple -> complex much like life evolved before that. In our (1) case: simple tools -> use of mechanical devices to replace muscle power -> industrial revolution -> automation -> entering the AGI age (where biological life may not be a strict requirement for a civilization to advance further).
Quite possible that tool-using intelligent life is relatively common, but some of those extra steps to 'get off the home planet' are rare.
And even if all of that happens: well... the universe is big. And so are timescales. Blips on the radar are easily missed.
That seems like a valid belief. Getting to a technological stage such that a species would be detectable over the vast distances of space could indeed be quite rare. You have to also consider the temporal aspect: intelligent, technologically advanced species may have evolved several times but gone extinct before we could notice them. Do other technically advanced species exist in the universe? Probably, but it could be that at any given time there might only be about 1 in any given galaxy and the distances between galaxies are great enough that we'd never likely be able to make contact. (and ~1 per galaxy would still mean that there would be a whole lot of intelligent species out there - it's just that it would be extremely difficult to make contact with any of them)