Thread: "The Great Silence"

So, I know that most AI threads deal with social/political issues or vague philosophical questions instead of science, and maybe we don't have many posters who are interested in scientific questions in general, but I hope at least a few people find this topic intriguing even though it may seem a bit speculative.

"The Great Silence" is a term used to refer to the fact that we (humans) don't seem to be picking up any radio transmissions, or other forms of communication, from any extraterrestrial civilizations. This is probably something most people take for granted, thinking that it would be far more bizarre if we WERE to receive such transmissions, but it does make some scientists wonder why this is the case. Is it because life itself is very rare in the universe? Or because intelligent (tool-using, technological) life rarely arises even though life itself may be common? Or because such intelligent life as does exist in the universe tends to keep quiet for some reason? If so, why? This question is related to the Fermi Paradox (http://en.wikipedia.org/wiki/Fermi_paradox, which in its basic form asks why we haven't encountered alien spaceships/probes if extraterrestrial civilizations are as probable as has sometimes been estimated), but is perhaps more basic; before you can ponder the Fermi Paradox you have to take for granted that technologically sophisticated extraterrestrial civilizations are indeed probable.

Also of related interest is the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation), a mathematical formula used to guesstimate the number of communicating civilizations in the Milky Way galaxy at any one moment. This equation has been called meaningless, but it was never supposed to actually "tell us" how many such civilizations exist or to yield a certain predictively correct result. Anyone who likes can plug in their own values, at least some of which are necessarily pulled out of thin air, and see what result they get. It's value is thus to simply tell us the hypothetical implications of different combinations of values for those terms, allowing us to see how many communicating civilizations the galaxy might contain IF a certain set of values is accurate, and from there to provoke thought and perhaps guide research. Frank Drake never claimed that the particular set of values he plugged into his equation were definitely correct, and nor should anyone else working with the equation. Think of it as a visualization tool for examining where certain assumptions lead to.

Anyhow, there are a lot of possible explanations for the "Great Silence", many of which are discussed on the Wikipedia page for the Fermi Paradox I linked to above. The major ones are probably that other civilizations are rare (and thus unlikely to be nearby, or extant at all, at the same time we're listening) or nonexistent for some reason; technological civilizations tend to somehow destroy or radically transform themselves comparatively soon after achieving the ability to communicate; we're not looking for them in the right way; or they aren't interested in communicating with us for some reason. All of these broad explanations can break down into many different specific ones. Again, this question may seem speculative, but I think it's an interesting one, and it's answer has potentially deep significance to many fields of science as well as to religion, humanity's view of itself, and so forth. It also also probably raises many more questions, ethical and moral and religious (and perhaps practical) as well as the obvious scientific ones. In fact the existence of any extraterrestrial life at all, communicating or otherwise and intelligent or otherwise, raises those questions. Speculative as it may seem, there are also serious scientists still involved today in searching for evidence of extraterrestrial civilizations. The recent explosion in the rate at which planets are being discovered in other solar systems--increasing Earthlike ones, too--also makes this question a bit more interesting and maybe a bit less pie-in-the-sky.

I know none of us here are scientists and I suppose I'm mainly just interested in hearing opinions and talking them over. What do you think? What explanation(s) for the "Great Silence" strike you as reasonable/likely or unreasonable/unlikely? Why? In general, what do you make of the idea of extraterrestrial life itself, extraterrestrial life that's "intelligent" (tool-using), or extraterrestrial life that might possess the ability to communicate over interstellar differences? Hopefully I'm not the only one here who finds these questions intriguing.