With the recent discoveries from planet searches–Kepler, etc.–it is clear that habitable planets are fairly common. The parameters of the Drake equation are filling in, and making it look likely there are many planets with life in the galaxy. So how to resolve the Fermi Paradox?
One possibility is that while life is common, intelligent life or technological civilization is rare. Certainly, there are no good estimates for this. But let’s assume that this is not the barrier, that say, 1:1000 planets with life develop a technological civilization.
Going past the existence of intelligent life, space is quite big. Likely FTL is impossible. Slower than light travel is expensive, slow, and difficult. So let’s assume everyone stays close to their home star.
How difficult is communication? Reception is fairly easy, but how expensive is transmission? How strong does a signal have to be to get received at 1000 light years, 100k light years? How much energy does it take? Also, the only stars that ‘count’ as communicating are those that can keep it up for a long time–100k, 1M years. Long enough for extended back and forth messaging.
The only potential communication partners we have–stars we can find by searching for messages (SETI)–are those with an active Messaging Extraterrestrial Intelligence (METI) program. That is, technological civilizations that have a long term program sending messages to all the nearby stars
You can run the Fermi numbers and get a reasonable chance there is a communication partner within 10k light years, but the conversation would still be slow, so the effort required is great.
Quid pro quo with a turn around time of 2X light years is very slow. So what form of communication is the most reasonable strategy? I doubt star ‘A’ wants to send a short message, and wait for a reply, leaving the channel closed 99.99%+ of the time. And yet if the star ‘B’ on the other end stops reciprocating, you don’t know for a long, long time.