I just watched this podcast of a lecture by Paul Davies ( http://royalsociety.org/The-Eerie-Silence/ ), which made me wonder: What's been learnt so far, from searches for radio signals, about the likelihood of an alien civilisation within, say, 10 ly, 100 ly, 1000 ly... How does the data that's been studied constrain estimates of how common radio-using civilisations are in our part of the universe? For example, it it reasonable to say that if there was a civilisation similar to our own in the Alpha Centauri system, we'd probably know about it by now? Also, what is the greatest distance at which another civilisation of exactly our technical ability could (be reasonably expected to) detect our radio signals, such as radar and radio/TV broadcasts. If we were 50 light years away from Earth, would we be picking up 50 year-old TV programmes, or letting them slip by unnoticed? Is there a practical limit beyond which distance the signals would be too weak for anyone to detect?