Joe Strout wrote:
In article . com,
Einar wrote:

wrote:
snipsnipsnip

On the lack of observed alien civilization, we need to remember that
the universe is still pretty young. Sure, 13 billion years sounds a
real lot, but remember if we subtract 3 billion years that leaves 10
billion. The point is, it?s taken life here about that time to evolve
intelligent life. While that might take shorter time ellsewhere, one
has to remember that evolution of life from bacteria is not simple and
unlikelly to take a short time.

This is missing the point. The time from the beginning of the Universe,
to the formation of a technological civilization, should take the form
of a normal distribution (i.e. bell curve), as pretty much any other
natural process does, due to the central limit theorem. If our
civilization is average (i.e. by the Copernican principle), then the
mean of this distribution is somewhere around the present. That means
that about half of the civilizations that will ever arise, arose before
us; and half will arise after us.

Now, we don't know what the standard deviation of this distribution is,
but we can make some guesses by looking at our history. How tightly
constrained was the development of civilization just now, given our 4.5
GY history? The answer appears to be, not very. Some really pivotal
moments in evolution, like the CretaceousTertiary extinction event, were
the result of highly random processes (a major impact event in this
case) which could have just as easily happened much sooner or later. So
the standard deviation is probably hundreds of millions of years at
least.

But with a standard deviation that high, and given that there are over
200 billion stars in the galaxy, there would necessarily be some
outliers to the population who happened to evolve very much earlier than
the rest of the population -- even at 3 sigma (standard deviations) away
from the mean, you'll find 0.37% of the population, which would be 540
million civilizations, half of which evolved earlier than the mean by
three sigma. Even if most of those stars can never support life, the
numbers (of both stars and years) is so large that it's very hard to
avoid the conclusion that the first civilization must almost certainly
arise a billion years or more before the mean.

This, combined with the observation that it takes only a few hundred
million years (after the development of space colonization) to settle
the whole galaxy, presents Fermi's paradox.

There are darn few parameters you can tweak in this analysis that make
much difference. The only escape I see is to assume that planets where
civilization can arise are very, VERY rare, so that the total population
size is not in the billions but perhaps in the thousands. Of course,
even with N=1000, there should be at least one civilization that
develops at least three sigma before the mean. So we have to further
assume that we are NOT an average observer, but are one of the first
civilizations to arise, maybe even the very first. Otherwise, we would
have arisen in an already-settled galaxy, and this does not appear to be
the case.

But of course, that makes a philosopher of science uncomfortable as
well. The odds of us, as a civilization, happening to be the first are
quite low. Moreover, if there are eventually going to be many orders of
magnitude more people, spread throughout the galaxy and over millions or
billions of years, why do you and I happen to be born into this time,
when there are fewer than 10 billion of us, all cooped up on one planet,
and within a few hundred thousand years of the birth of civilization?
The odds against THAT boggle the mind.

The most logical explanation is that all civilizations, including ours,
destroy themselves (or are destroyed) before interstellar colonization
begins. But, despite the logic of it, I find I can't accept that. So,
I'm left befuddled, with no neat solution. I consider this one of the
great mysteries of our time, right up there with the nature of
consciousness.

Best,
- Joe

Following your line of thought, one might argue that on average
our solar system will be visited occasionally by one of the
early advanced civilizations. If by 'occasionally' one means every
500,000 years or so, the chances of such a visit during the last
10,000 years may be quite low. Because of the great distances
the effort to colonize a planet may not be worth while. On the
other hand, there may be places in our solar system where evidence
of such visits still exists. In the past I have suggested just
such evidence on the asteroid Eros (see IOD 5-3), and others have
found anomalies that do not appear to be made by natural processes
on Phobos (see "Phobos monolith") on Mars (see UFO crash on Mars) and
even on the moon (see the illustration on p. 33 of Ad Astra 2007 -or
ask the Ad Astra editor for a copy).