I was sitting staring at the sun today when I had a revelation.
You know how we're always sitting with our gigantic radio telescopes trying to hear alien life? Well, I pondered about it and this is what I thought.
First, if the alien civilization was too primitive to broadcast their existence across the cosmos, then we would never hear them. Obviously enough.
Second, if they were advanced enough, the signals they give off might be faint enough to be mistaken for background noise. Also, there's the chance that we might not recognize any kind of pattern that could be emitted. I look at the kind of diversity we encounter on ol' Earth here and the multitude of communication employed, I think it's likely that we might overlook something not of this planet if the method of communication is sufficiently complex or simple or just 'out there'.
The counter argument is that any kind of pattern would be enough to detect alien life. After all, communication would be worthless if it changed constantly in the random manner of, say, a star or a giant ball of gas. However, signals get distorted. Light itself can get warped and bent by gravity, so there is no guarantee that we'd be able to pick up what would be a 'strange' pattern in the first place traversing past stars, black holes, and other assorted things in the universe. And I personally think that an alien language would follow some if any of our concepts of communication is a failure of imagination. Maybe I'm wrong, but that would be boring.
Finally, I thought about how we're going about trying to find this kind of thing. I'm going to use Earth as an example here. We don't exactly power our signals to shoot across space, pretty much we're just aiming for whatever is nearby and about the farthest objects I can see a signal reaching for our purposes is satellite probes such as the Voyagers. Even their signal strength is quickly fading and they're barely outside of the Sun's backyard. All this while we're trying to parse alien radio signals.
Think about it. Do we really think aliens of our level or even significantly beyond would pump enough power to shout across the universe? You have to remember that a light year is the distance that light, the fastest thing in the universe, travels in a year. A year! 186,000 miles or so in a second (the speed of light) adds up to 5,865,000,000,000 miles! The star closest to us, Alpha Centauri (or one of those, I always forget which) is about 4.4 light years away. Voyager is about 9,820,000,000 miles away and we're losing signals with it. Our poor radio signals dissipating into the void.
If aliens were the same as us, their radio signals wouldn't radiate far (or at least, remain intelligible far enough). And if they too choose to listen instead of reach out, then we have a situation in which all sides are listening for the other. I don't think such an arrangement would result in anyone hearing much. I don't think radio telescopes radiate any kind of energy that would be visible at the distances an alien listening post would pick up.
Finally, consider the possibility that we might not want visitors. For every bunch of Vulcans, there's a Borg cube just waiting to enslave the world. If someone mastered the nontrivial task of managing to transverse the universe fast enough (as in within a decade as opposed to millions of years), I don't think they would have neglected warfare and certainly our weapons may well be useless against what they've got. Or maybe it takes a large punch, like a nuclear missile, to really get to them. Then we run the risk of nuclear fallout from the resulting debris of the craft and that's assuming that they just get hit and crumple and won't explode with disastrous consequences for the environment.
All in all, aliens are hard to find.