I came to appreciate the magnitude of the distances involved some years ago when, in the course of calculating what would constitue a “practical travel time” to Alpha Centari (the nearest star at 4.3 light years (http://en.wikipedia.org/wiki/Alpha_Centauri)), I discovered that light leaving the Sun would transit our solar system in approximately 6 HOURS.
Actually, I think Fermi was pointing to how much time is involved even for the distances involved.
13.8 billion years is a long time ( http://en.wikipedia.org/wiki/Age_of_the_universe).
Assuming (and it’s a big assumption since we have only one data point) that the evolution of life on Earth during the last 4.6 billion years (http://en.wikipedia.org/wiki/Age_of_the_Earth and http://en.wikipedia.org/wiki/Timeline_of_evolutionary_history_of_life) falls within the normal range for these events, the threshold for practical interstellar travel is somewhere beyond now. But how far? Complex, increasingly technological civilization on Earth is only... what? 5-6,000 years old with the geometric acceleration only occuring since the Scientific Revolution starting in the 1500s. Assuming we don’t kill ourselves off (either deliberately or by accident) let’s say it takes another MILLION years for our technological civilization to develop “practical” interstellar travel (however defined). How far would that civilization’s exploration and colonization spread in the next million years after that and in the next and in the next?
Fermi’s point, brilliantly made in his “Where are they?” comment, is that 13.8 billion years is a long, long, long, long, long, long, long time with no evident contact even allowing for the time needed for the rise of advanced civilizations and the incredible distances involved.
My personal view is that an advanced, interstellar, spacefaring civilization would still be constrained by consideraions of resource investment versus expected returns and would establish criteria for what systems were or were not explored. First, sticking to just our galaxy, our solar system is located towards the end of one of its spiral arms. So the Earth is in a pretty marginal location, spatially speaking. Second, from a contamination standpoint, a lifeless, mineral laden planet with a stable environment (even if pretty hostile) is far easier to deal with than a planet with a highly dynamic atmosphere that is also teaming with all sorts of biota ranging from the submicroscopic to the macroscopic, most of which is aggressively predatory. Third, by the time these explorers reach the Earth, just how interesting are we going to be to them? Ethical constraints about disturbing developing civilizations probably would apply but, practically, why bother interacting with beings miliions of years behind yourselves in technology and social development?
In short, we may be too isolated, too difficult, and too boring to be worth contacting more than once (if even then).