Let's examine these briefly.
(1) We are alone.
This doesn't mean that there can be no other
civilization -- just means we're the first.
The only explanation for this, in light
of the principle of mediocrity, is that there
is some natural reason why civilizations were
more or less prevented until "recently."
Then came us. There could easily be
others who follow us. This does seem
to contradict the principle of mediocrity
though and the idea that the Universe
is more or less the same everywhere
(and, by implication, over time).
(2) Civilizations are common, but there
is no colonizing. Why?
Maybe it's hard.
Maybe they don't feel like. Or, less
snidely, maybe the way to become a truly
advanced civilization is to lose your
drive for colonization.
Not very encouraging, but certainly possible.
(3) There are colonizing civilizations
that have chosen to leave us alone.
Intriguing. Either they are not interested
in us (we are too boring), or they are
purposefully leaving us alone (or protecting
us). This is sometimes called the
zoo hypothesis, and Star Trek fans
will recognize this as the often-broken
Prime Directive. Lastly, civilizations
might be leaving us alone until they or
we recognize that we are "ready" to join
the truly advanced community.
http://www.palantir.net/2001/index.html
Why bother with SETI in light of these solutions
to the Fermi Paradox?
What do you think?