A couple of years ago on the old MSNBC Space BBS, someone suggested that we'd be more likely to detect "Extra Terrestrial Ignorance" than intelligence. His point was that it's unlikely any other civilization would be deliverately trying to contact us. Instead, we'd probably detect their equivalent of TV and radio broadcasts (ignorance).

We've only been transmitting radio signals for about 100 years. Much of that time, the signals were relatively low powered, low frequency AM stuff that would probably be lost in the background noise. We've only been transmitting high powered high frequency signals like radar and TV for about 60 years.

At most, our early radio signals would've traveled roughly 100 light years. If you're considering the time for someone to receive those signals and to reply in kind, you're talking about a radius of about 50 light years. If you're talking about the more likely detection of the more powerful signals, the radius is probably no more than 30 light years. There really isn't that many stars within a 30-50 light year radius for there to be a reasonable probability that one would have a technologically advanced civilization capable of detecting our signals and replying.

There's also another aspect - suppose their civilization was 100 years behind ours technically. In that case, they probably wouldn't have the technology to receive any of our signals. If they were 100 years more advanced, they may no longer be using radio signals (at least not high powered ones) for most of their communications so they may not even be listening. Perhaps there's a window of opportunity when a civilization's technology has the means and need to transmit and receive high powered radio signals. If that hypothesis is correct, then the chances of SETI succeeding become very small. Still, I encourage the hunt...