Indeed, for all we know, the air might already be thick with signals from our interstellar neighbours, waiting for us to develop the technology to receive them. A second assumption underlying the equation is the idea that technological civilizations each represent a single potential source of radio transmissions. In other words, it is assumed that each civilization is sedentary, remaining on its home planet , or at least within the system of its home star . This may not be the case. Even from the pessimistic calculations above, there would be about 4,000,000,000 planets suitable for life in the Galaxy , ample incentive for a technologically advanced species to investigate actual travel between the stars .
Interstellar travel is certainly a theoretical possibility.
For species that evolve in denser regions of the Galaxy than ours, it might even be quite practical, since the stars in such a region would be much nearer to one another. This is an important consideration for the Drake Equation, since each new star system 'colonised' represents another potential radio signal. Several civilizations expanding like this over a period of millions of years might increase the number of signals exponentially. There is simply no way of knowing. This might not necessarily be the case.