> if life were abundant in the universe, the naïve expectation is that we'd see it everywhere, but we don't
No, that's my point: We don't see it because we can't; we don't have the technology. The fact that we see no aliens has nothing to do with the number of aliens, and everything to do with the fact that are more or less incapable of looking.
It would be like saying, "if butterflies were abundant in my garden, I'd see them everywhere, but I don't, so what gives?" when you are blind, and also stuck inside your house. There is nothing to explain. We can talk about seeing (or not seeing) butterflies when you are able to see at all.
You do realize that what you're proposing isn't in direct contradiction to the Fermi paradox, but is one of the most popular solutions to it?
The Fermi paradox is the observation that, based on the size of the universe, we'd naïvely expect to observe several alien lifeforms. That's it. It doesn't imply a "Great Filter", or any other solution. It's just the observation.
The "Great Filter" theory is a commonly mentioned one, because it's pretty easy to imagine humanity, our only reference to intelligent life, killing human civilization. Your hypothesis about not having the technology is also very, very popular, and as such I don't understand why you have such a big apprehension to the Fermi Paradox in general.
No, I am not proposing that the universe is full of life, nor am I proposing that it is empty of life. I am objecting to the notion of "drawing conclusions about how much life there is based on how much life see," because we are blind.
A universe empty of life is just as compatible with our observations as one that is full of it. We should not "expect" anything at this point.
> We don't see it because we can't; we don't have the technology.
Why are radio telescopes insuffient?
Given what we know, humans building a Dyson sphere would be pretty obvious to anyone looking, simply by the waste heat. We'd have to take specific measures to actively avoid detection. It's not even clear how that's possible, given the second law of thermodynamics.
Positing that civilizations are everywhere but hiding seems the more extraordinary claim. It just takes one "person" sending one message to Earth for the veil to be broken.
The key difference between a technological civilization like ours, and butterflies, is the butterflies have predators to limit their numbers. What is limiting technological civilizations? The most unnerving answer might be "predators".
Butterflies don't fly very fast, but if there were no butterfly predators, they would be everywhere. Not just in your garden, but absolutely everywhere. With exponential population growth and a slow flight speed - or even flightless varieties - they would have conquered Earth millions or billions of years before mankind arrived.
We don't need to be able to see a single butterfly to know they are everywhere. Everything we know about the physics of the universe tells us energy would eventually be the limiting factor for EVERYTHING AND ANYTHING. Whether we're talking about butterflies or technological civilizations, the easiest thing to see from a great distance is their energy demand. You could have a planet full of butterflies on the other side of the solar system, and I guarantee you any cheap telescope could detect their presence simply by looking at the spectra the planet emits. No image forming capability necessary. Not a single butterfly has been seen. But we would know they are there.
In fact, I'd bet my left pinky toe if that butterfly planet were anywhere else in the galaxy where we could see it, within a few decades from now, or perhaps a single century at most, we would notice it is not at all like any of the other planets. Something is consuming energy on that planet, converting it to different forms, changing the atmosphere, and making it completely impossible for the presence of butterflies to go unnoticed.
The same is true for technological civilizations. But, unlike butterflies, technological civilizations are fully capable of changing the spectrum of not just a single planet, not just a single star, and perhaps not even just a single galaxy. If there were no predators to limit population growth, every galaxy in the universe would be the color of butterflies.
So where the fuck is everybody?!
You want to take a closer look at that butterfly planet with a fine new telescope that can photograph a single butterfly in perfect color. But you look, and you don't see any butterflies. Not one. You see leaves. Lots of leaves. That seems nice too, at first, but then you realize those aren't all truly leaves. Some of them are butterflies, and they look like leaves. How interesting. Why would beautiful butterflies want to look like ordinary leaves? Because they are afraid of something out there.
Should we be worried about predators?
For now, there are no leaf-shaped butterfly planets, but one thing is certain. We will all agree about this, I guarantee it. This universe is violent, and it will never stop trying to kill us. We have to get up and move, NOW. We have one chance to do this right. ONLY ONE. We decide to fight and survive NOW, together, or we die here. In the nursery where we were born. Daydreaming about butterflies in the garden.
No, that's my point: We don't see it because we can't; we don't have the technology. The fact that we see no aliens has nothing to do with the number of aliens, and everything to do with the fact that are more or less incapable of looking.
It would be like saying, "if butterflies were abundant in my garden, I'd see them everywhere, but I don't, so what gives?" when you are blind, and also stuck inside your house. There is nothing to explain. We can talk about seeing (or not seeing) butterflies when you are able to see at all.