[dworth]

There are a number of questions that need to be considered re: intelligent life on other planets and whether we could talk to them:

How common are planets?
How common are planets suitable for some form of life to occur?
How often does life actually occur on suitable worlds?
How common are complex life forms compared to microbial organisms?
How often does intelligence arise?
How often does the existence of intelligent beings result in organised societies?
How often do said societies reach a level of technology capable of communicating with other worlds?
How long do civilisations last? The universe is 13.7 billion years old and humans have had the capacity to transmit information for barely more than a century. Perhaps countless civilisations have come and gone elsewhere before we reached this level.
What is the best way of communicating over interstellar distances? Is it radio, or is it something that we haven't thought of yet?

The answers to almost all of thosequestions are currently 'don't know' of course, as we only have our own example to go on, although one or two may get answered in the next few years (the data from the Kepler mission is producing some startling results and it appears that planets may be very common indeed). Maybe life is common or maybe it's rare. Maybe life itself is common but intelligence is rare. Maybe intelligence has taken 13.7 billion years to emerge and we are the first of many. We may be unique in the universe, but the truth is that we just don't know.