Ben Bova has a short article in the April Analog about how “Isaac was right”, that is there is no intelligent life anywhere else but here. Stan Schmidt’s editorial in the same issue also talks about this and the original article from last year taking the alternate stance, but he comes down rather wishy-washy right in the middle. Personally, I find the arguments of people like Frank Tipler (whom Bova also mentions) fairly compelling, essentially boiling down to if there were intelligent aliens out there somewhere we would’ve found them by now, or more likely they would have found us. There are a couple of counter-arguments, though, that I’m sure other people have thought of before but aren’t mentioned by either Bova or Schmidt. One is the reliance on radio waves (including the entire broadcast spectrum for all communications, not just that used for standard radio) as the final arbiter of intelligence. There’s certainly a case to be made for aliens to have never developed radio. Also, as was pointed out at the Boskone panel on first contact, humans have only been transmitting radio for less than 100 years. It could be extrapolated that in the next fifty or a hundred years technology will have advanced such that something will render radio communication obsolete. That means not only are you searching for a needle in a haystack in terms of the amount of sky there is to point the radio telescopes at, the window of opportunity for any given intelligence to be caught during their “radio period” makes it even more unlikely. Another sort of cynical explanation would be that something in the assumption of how far those kind of radio waves travel before dissipating is wrong. We know pulsars can transmit across vast distances, but what if the kind of radio waves we produce don’t really go out all that far before they cease to be recognizable from the background chatter? That would mean there could be a zillion alien radio stations or whatever out there, broadcasting away, but they’re too far away for us to hear, and vice versa.