The Drake Equation proposes to predict the number of technological civilizations whose radio emissions are detectable. It is the basis of SETI.

But the universe is silent.

The great filter is proposed as a possible reason, some great mechanism that filters out most possible civilizations before they get the chance. A variation of the They all destroy themselves theory.

Or there is the von Neumann probe theory: that we wouldn't know it if we saw it, and we wouldn't know where to look theory. Not eactly why the universe is silent, but in the same general line of thinking.

Many years ago, reading a science fiction author, I think Stanislaw Lem, I came across a much simpler theory, called the butterfly hypothesis. Simply put, it states that the stage of radio technology, the period in which a civilization actually wishes to communicate with another is relatively short. And the period of desire to communicate not longer.

Very quickly, the civilization enters the butterfly stage, and flys away.

I suppose this aligns with the notion of the singularity, though I had never heard of it until I started here on Everything. And I know that I hadn't read about it when I read about the butterfly hypothesis. It could certainly be one reason for a civilization turning inward.

We are still in the birth of a radio civilization. We have no idea what things will be like, even in 15 short years (if that is the date of the expected singularity).

A purely pedantic response to Rook:

By silent, I mean devoid of intelligent communication, either directly to us, or that we can overhear

The Drake equation is filled with wonderfully well known numbers, like the probability for life, then intelligent life to evolve. Also with quite well known statistics such as how many stars have earth-sized planets with similar enviornments (which we know very well because the science of planet formation is understood so well)

No, wait a minute, we don't know any of those thinks, Doh!. I guess the Drake equation might just be wrong then eh!

On a purely pedantic note, I would like to point out that the universe itself is not silent--in fact, thanks to all the very small things whacking into other things out there in the blackness, the universe makes a great deal of "background" noise that radio astronomers are quite familiar with.

Another possibility not discussed here is that we are alone in the universe. It's not a popular theory, and personally I don't believe it, but logical rigor demands that we consider it.

Why is the universe silent? Why can't we find any aliens? Is there nobody out there? SETI has searched the skies very thoroughly by now. We should have found somebody. Many theories have been proposed as answers to this question, which is called Fermi's Paradox. In this writeup I will try to give an overview of the various explanations for the great silence.

One possibility is that we, the human race, are the only sentient species that will ever evolve. That theory, however, is rather improbable, considering the size and life span of the universe. Unless of course, you believe that there is a god who created humanity, and the cosmos is our playground.

Another possibility, the most probable one, is that there are no other sentient life-forms around at the moment, but there will be other intelligent life in the future, or there has been in the past. We've had radio for only a few decades, while the universe's been around for billions of years. How big are the chances that two sentient beings would coexist at the same time close enough to each other, each possessing radio technology?

That of course implies that a species goes through a very short time-span during which it can contact others by radio. This can be due to two reasons: Either a sentient species transcends shortly after getting radio (see butterfly hypothesis in the above writeup). Or, for a bleaker theory, all sentient species either exterminate themselves shortly after, if not before, getting radio technology. Or, if they don't, they'll colonize every solar system in this galaxy within a few millennia (an eyeblink on galactic time-scale). This would leave no space for other sentient beings to develop - the whole galaxy would be so crowded with that one species. Think population explosion.

This way we would have to be the first: If any other sentient species had evolved first, Earth would be an alien colony, and humanity would never have had the opportunity to evolve into sentient beings.

There are many other less probable theories, some of which I will now describe. Though improbable, they cannot be discounted, for one of them might just turn out to be true.

There is the theory that we're using a completely wrong medium of communication. Perhaps we're the only species as unreasonable as to use radio. Maybe all the aliens are happily chatting away telepathically, and haven't even noticed us poor telepathy-impaired beings. Maybe they use highly focused laser beams for communication, or some physical concept we don't even know of yet.

It might also be that the alien lifeforms are well aware of us, but are hiding. Why would they do that? For one, they might have heard our radio and TV transmissions and decided that they don't want contact with such an irrational species. Maybe they're afraid of us. Considering the pictures of violence we continually shout out into the void, this theory might just be true.

Or possibly they don't want to interfere with our development. That might be because they know they'd harm us if they would interfere with our development at this stage. Because they adhere to the Prime Directive from Star Trek. Or perhaps they're a bunch of darwinists who want to see whether we manage to survive on ourselves before they make themselves known.

Either way they'd probably reveal themselves to us at some point. Perhaps when we secure world peace and government. Or when our first space probe reaches another solar system. Or perhaps they're only waiting for the abolishment of the death penalty.

I'd love to attribute all these theories, but unfortunately I have no idea whatsoever about who first proposed them. /msg me if you know.

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