One of the many problems with the idea that we'll one day encounter another technological species is that the probability of a technological species evolving anywhere appears to be very, very small. Now add the problem of time: we've been around about 400,000 years as a species and only in the last 100 have we had anything remotely approaching suitable technologies. The overwhelming probability is that we'll exterminate ourselves within the next 2,000 years. Assume, for good evolutionary reasons, that nearly all technological alien species would suffer the same fate. So now we have the time problem: the probability of overlap within a 5 billion year window for a relatively short-lived species is as close to zero as one cares to calculate.