Re your last comment, the transmitting civilization would certainly select a frequency at which the interstellar medium was transparent and quiet. Certainly not 21cm.

Did you read Henry Cordova's essay on this bbs at

SETI: Reality Check - ljk - 5/8/2006 12:41:18 PM

It covers many of these issues.

You mean this one? It's not often that I'll open an instance of IE just to use the "earlier" button, which still doesn't appear in Firefox (~ahem~, Robert...).

But I did, because I had not read it before now (though I had read other posts in the same thread). Henry's ideas about a completely different SETI search strategy make a lot of sense -- in context. And there's the rub: I wasn't thinking about SETI.

No matter how many there are, if all potential players in the SETI game are waiting for someone else to say something, no one ever hears anything. They can calculate their local version of Drake Equation probablilties untli the cows come home, without ever reaching N = 1.

The kind of detection I think of most, when I contemplate such things, is of non-deliberate -- shall I say "collateral"? -- artifacts of technological sentience: unnatural, omnidirectional brightness at certain wavelengths (like those used for radar) is one of my favorites*. And of course, there is that sphere of radio and TV emanating from Earth, approaching 100 ly in radius, used to such good effect in the opening sequence to the film version of Contact.

My speculation at the end of my previous post was meant to address such collateral artifacts, rather than deliberate SETI-like behavior, and to wonder out loud if a listener "above" or "below" the intervening gas and dust, with a broader view of the galactic disk than ours, might be more able to detect them.

To invoke archaeology as a metaphor: We learn a lot more about life in other civilizations from their middens than we do from their monuments. Unfortunately, given where we live, we'll never find the trash heaps of other civ's in this galaxy... and somebody has to have built a monument for one to be found.

[* - I can think of one instance where a civ' might employ one of those quiet places in the microwave bands for mundane (non-SETI) purposes: if they had achieved interstellar settlement, and needed the low-noise bandwidth for communication with their colony, or with ships in flight. But then the signals would be tightly beamed.]