Re: Fermi Paradox

From: Eugen Leitl (
Date: Sat Apr 27 2002 - 02:28:31 MDT

On Fri, 26 Apr 2002, Dani Eder wrote:

> 2) We see no obvious evidence of any other civilization that has done
> so, even though a small number of civilizations in the universe (~10)
> starting at random times in the past (0-15 billion years ago) should

According to recent data, universe is 13 billion years old, give or take
100 megayears. You need high abundancies of heavier elements to generate
life, and life is vulnerable enough so that it couldn't have arisen at
high absolute luminance. Also, it wouldn't be able to evolve to higher
life if there were high-luminosity flashes. The global environment has
been more or less favourable to life only in the last few billion years.
Plus, at least in one instance life needed several billion years to arrive
at a useful level of intelligence, and it is not at all obvious that this
must always be the case, trend in co-evolution driven cerebralization

So the bound on the aperture of an average civilization's lightcone is a
lot lower than your 0..15 GYr figure.

> B) Conditions only became favorable to civilizations recently. (i.e.
> 2nd generation stars required to create suitable planets).

Yes, but that's only part of the problem. Nearby supernova are out, as
well as other high-energy events, including those on galactic scale (a
quasar flareup in the center of ours would kill us). Giant impact are out
(impact of a few 100 km large rock would wipe all higher life). Extreme
climatic excursions driven by intrinsic nonlinearities, minor changes in
solar constant, etc. etc.
> C) Civilizations don't survive at a high tech level
> long enough to fill the universe.

The vulnerability window for that has hardly opened, and is about to close
in a few decades, when we start colonizing the local solar system, and go
beyond. As soon as the pioneers are on the cruise you can't really revoke
them, nor kill them.

As soon as you're outside of your gravity well you're essentially
unkillable by a rather large number of event classes.
> D) Civilizations don't stay in a form we can observe.

All of them? Probabilistically rather improbable, unless the total number
of them is very low.
> E) Civilizations have been observed, but we don't
> recognize them as such.

People have been looking at natural phenomena, but all of them appear
> F) The fraction of civilizations that is expansionary
> is low.

An interesting question is whether there is a common pattern for all
cultures resulting in a single bottleneck, or each developmental path is
> There may be additional hypotheses I haven't listed, or some
> combination of them may be the right answer.

The question flares up every few months on diverse lists. The online
archives are lousy with such discussions.

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