Steve Willner <swillner@cfa.harvard.edu>

One of the problems that the Drake Equation produces is that if you take
reasonable (some would say optimistic) numbers for everything up to the
average duration of technological civilizations, then you are left with
three possibilities:

1. If such civilizations last a long time, "They" should be _here_
(leading either the the Flying Saucer hypothesis---they are here and
we are seeing them, or the Zoo Hypothesis---they are here and are
hiding in obedience to the Prime Directive, which they observe with
far greater fiqdelity than Captain Kirk could ever muster). -or-

2. If such civilizations last a long time, and "They" are not "here"
then it becomes necessary to explain why each and every technological
civilization has consistently chosen not to build starships.  The
first civilization to build starships would spread across the entire
Galaxy on a timescale that is short relative to the age of the Galaxy.
Perhaps they lose interest in space flight and building starships
because they are spending all their time surfing the net. (Think about
it---the whole point of space flight is the proposition that there are
privileged spatial locations, and the whole point of the net is that
physical location is more or less irrelevant.) -or-

3. Such civilizations do not last a long time, and blow themselves up
or otherwise fall apart pretty quickly (... film at 11).

Thus the Drake Equation produces what is called the Fermi Paradox
(i.e., "Where are They?"), in that the implications of #3 and #2 are
not terribly encouraging to some folks, but the two flavors of #1 are
kinda hard to come to grips with.


An alternate version of 2 is that interstellar travel is far more
difficult than we think it is.  Right now, it doesn't seem much beyond
the boundaries of current technology to launch "generation ships," which
amount to an O'Neill colony plus propulsion and power systems.  An
alternative is robot probes with artificial intelligence; these don't
seem so difficult either.  The Milky Way galaxy is well under 10^5 light
years in diameter and over 10^9 years old, so even travel beginning
fairly recently in Galactic history and proceeding well under the speed
of light ought to have filled the Galaxy by now.  (Travel very near the
speed of light still seems very hard, but such high speed isn't
necessary to fill the Galaxy with life.)  The paradox, then, is that we
don't observe evidence of anybody besides us.