An Awful Waste of Space?

It's been 50 years since Frank Drake (of the famous Drake equation) started project Ozma -- humanity's first search for signals from alien intelligent life. I thought it might be fun to post on this topic, even though I have absolutely no expertise in it.

In 1950, ten years prior to project Ozma, Enrico Fermi posed a question that might have inspired Drake: if intelligent aliens exist, why haven't we found them (or they us) yet? This question is actually worth thinking about, for the following reasons:

1. Earth is probably not the only planet in the entire universe on which intelligent life evolved. It's likely that (the building blocks of) life can form in many diverse conditions. There's probably even more evidence for these claims now than there was in 1950.
2. Once there's life, evolution should take care of producing intelligent life (at least in some cases).
3. Intelligent life would probably start explore the universe and expand at, perhaps, an exponential rate (hey, the universe is pretty big).
4. So much intelligent life, going all about the universe, you'd think they would have run into us by now! (Or at least sent us some messages)

But of course, despite looking, we haven't yet found anything.  So, why not?

Here's a list of all the (remotely plausible) reasons I can think of, from least to most likely. I realize the events are not all mutually exclusive.

• [very unlikely] Intelligent life is all over the place. But once aliens invent true virtual reality or something else that really floats their boat (and they always do), they have no good reason to go exploring the universe.
• [very unlikely] Intelligent life is everywhere, and different aliens often run into each other. However, whenever this occurs, the more advanced aliens wipe out the less advanced ones. By the anthropic principle, we'll have to wait to be the more advanced ones.
• [unlikely] Intelligent life has already found us on Earth but we don't know it. Either it is successfully hiding from us (more likely) or the government is successfully hiding its signals from us (much less likely).
• [unlikely] Intelligent life occasionally appears, but always (or usually) manages to wipe itself out with the weapons it invents before getting the chance to meet us humans.
• [possible] We are alone and very special. Intelligent life, has not appeared anywhere else. The reason we are even around to ask this question is the anthropic principle, God's will, or plain old extreme luck.
• [reasonable chance] There is life all over the universe, but either it doesn't expand exponentially or the distances are just too great to cover, both for both intelligent beings and their signals.
• [reasonable chance (most likely)] There are intelligent beings emitting signals that reach us from afar, but using methods (or languages) we haven't though of. Eventually, we'll figure it out and our world will change forever.

I'm sure I missed something, so I welcome your comments.

The image of the Very Large Array at Socorro, New Mexico, United States is under a Creative Commons Attribution-Share Alike 2.0 Generic license. Its author is here.

After writing this post, I have found a similar list on Wikipedia's article on the Fermi Paradox.