On the detectivity of advanced galactic civilizations.

Even with slow rates of technological advance, extraterrestrial civilizations substantially in our future will have technologies and laws of nature currently inaccessible to us, and will probably have minimal interest in communicating with us. If this communication horizon is about 1000 years in our future, other crude estimates previously published imply that only about .0001 of the technical civilizations in the Galaxy are accessible to us. The mean distance to the nearest such society is then about 10,000 light years. Radio detection of extraterrestrial intelligence seems to imply either (1) much larger telescopes or antenna arrays for the detection of civilizations within our Galaxy than now exist; or (2) attention to the nearer extragalactic systems, with smaller radio telescopes, to detect the very small fraction of very advanced societies which may choose to make their presence known to emerging civilizations via antique communication modes.