Re: Fermi Paradox in the news

From: Nick Bostrom (
Date: Tue Oct 24 2000 - 19:40:40 MDT

Jason Thompson wrote:

>From: <>
> > But until then, a single observation that intelligent life exists does
> > not really shed light on the probability that other such life exists.
>Sure it does. It doesn't shed as much light as TWO observations that
>intelligent life exists, but it sheds a helluva lot more than ZERO
>observations that intelligent life exists, which is why it's got a lot more
>going for it as a theory than belief in a supernatural creator.
>And to come at this from the other direction, if you don't believe that ONE
>observation that intelligent life exists is useful data, then the
>observation of, for instance, one -other- intelligent life won't help you
>out either (towards predicting the existence of additional intelligent

Oh yes, the second data point would make a helluva difference! The problem
with the one data point we have is that because of observational selection
effects it doesn't tell us much. No matter how small a fraction of all
planets in our infinite universe develop life, we would by necessity find
that we originate from one of the exceptional ones that did. So this data
point is predicted equally well (i.e. has a conditional probability of one)
given any theory that says that intelligent life would develop somewhere.
But finding a second source of intelligent life, in our relative vicinity,
would dramatically boost the support for those theories which say that
intelligent life is common. (My PhD thesis at has more on this and many other
related topics.)

Regarding Hal's comment: While it is true that the mere fact that
intelligent life evolved here doesn't give us any useful information, we
might know some things about the manner in which it developed that may
enable us to draw some interesting conclusions. For example, we can compare
the time it actually took for intelligent life to evolve and compare that
to the time that was available (an upper bound on which is given by the
time when the sun engulfs the earth in about five billion years). This can
tell us something about the number of critical steps in our evolutionary
past. If there were very many critical steps (defined as successive
transitions which were both necessary and highly improbable) then we should
have expected to have evolved closer to the cut-off time. Brandon Carter
pioneered this line of reasoning (which is also summarized in my thesis).
We can also look at our evolutionary record and try to identify plausible
candidates for critical steps. For example, the step from prokaryotic to
eukaryotic life seems to have been very difficult. By contrast, the
evolution of humanoid apes into humans does not seem to have been very
difficult, because it happened very quickly. The bottom line, however, is
that the present poverty of empirical data makes it difficult to say
anything conclusive about these things, except that all the evidence we
have is perfectly consistent with the hypothesis that it is extremely
difficult for intelligent life to evolve. This is also consistent with (and
in my opinion is the most probable explanation of) the absence of any signs
of extraterrestrial intelligence (the Fermi paradox).

Dr. Nick Bostrom
Department of Philosophy
Yale University

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