I sometimes think that (as I think an earlier poster mentioned), the reason there seems to be such an "emptiness" and "great silence" out there
may be because once a civilization reaches a level of technology comparable to ours that the civilization very shortly afterwards becomes something
that's no longer remotely recognizable as a civilization.
I know the above is not very clear ... so let me try this ...
Humans have had a civilization of sorts for in excess of 5000 years (Sumeria, Mesopotamia, Egypt, Rome, etc). For the majority of that time period,
technology (as we recognize it) has been virtually non-existant. It's only in the last 200 years or so that we've made any significant progress
Think about how incredibly rapidly we as a technological society and technology users have developed since the 1st major technological leap occured
with the introduction of steam power. That one simple development transformed us from an agricultural society to a mechanical society and began the
complete and utter transformation of society and the planet itself.
So, for thousands of years it was basically status quo with minor improvements. Then 200 years ago around the late 1700's / early 1800's) in came
steam power. From that point, it took us less than 100 years to discover electricity ... it's now approx 1900.
Around the same time (late 1800's) came the 1st experiments in flight and in less than 50 years after those 1st tentative flights, we became a
society that took global air travel for granted ... it's now around 1940 - 1950.
The 1st serious rocketry experiments began to take shape in the 1930's and in less than 30 years (late '60's), we had gone from no space travel
capabilities to actually landing men on the moon (let's assume it happened !
The 30's saw the 1st experiments in nuclear energy and within 10 years we had the capability to wipe out entire cities.
The '50's saw the discovery of the genetic code and less than 30 years after that we could alter the very nature of organisms.
I could go on and on ... tv, radio, communications, lasers, satellites, internet, mobile phones, cd's, dvd's, computers, etc, etc, etc
The point I'm making (finally ... I hear you all saying !) is that in only 200 very short years, we've remade our entire world and civilization. If
you picture the preceeding 5000 years of human civilization, it's basically a flat line going from left (the past) to right (the future). But just
200 years ago, that essentially flat line began to rise upwards very quickly. As each year goes by, more and more technology is being developed, more
and more knowledge is being acquired that in turn results in even more technology and more changes to our world, our lives and our civilization.
This upward trend shows no sign of slowing down, in fact, estimates are that it's becoming exponential. This means that it only takes a very few
short years for major changes in our technology and civilization. Extrapolating that exponential upwards curve suggests that in an incredibly short
time (perhaps less than 200 more years), that civilization will no longer be even remotely recognizable by today's standards and may have needs and
priorities totally alien to those of us currently alive.
This scenario may in fact be the end result of the headlong rush into a technological future and may even be a natural end result for ANY civilization
that manages to achieve the minimum starting technological capability.
So, as a "human society", our remaining time may indeed be very limited
before we PERHAPS follow in the steps of other technologically advanced alien civilizations and effectively become "invisible" to other less
developed technological civilizations.