Reginald Smith of Bouchet-Franklin Institute in Rochester, NY says that there is a limit to how far a signal from ET can travel before it becomes too faint to hear, and when you factor that in, everything changes.

The paper develops a detailed quantitative model that uses the Drake equation and an assumption of an average maximum radio broadcasting distance by an communicative civilization to derive a minimum civilization density for contact between two civilizations to be probable in a given volume of space under certain conditions, the amount of time it would take for a first contact, and whether reciprocal contact is possible.

Results show that under certain assumptions, a galaxy can be teeming with civilizations yet not have a guarantee of communication between any of them, given either short lifetimes or small maximum distances for communication.

Other factors could include the range of usable and probable frequencies, modalities, and modulations monitored and the range of signal-processing and pattern-recognition algorithms employed (in the case of Earth, these are negligible) – Ed.