Re: Fermi Paradox in the news

From: Nick Bostrom (
Date: Fri Oct 27 2000 - 22:08:33 MDT

Jason Thompson wrote:

>What I find sort of puzzling is the concept that somehow the variables
>required for reality have the capacity to be selected for. Scientists say:
>"Well, if this fundamental number was even -slightly- different, life never
>would have come into being! How extraordinarily unlikely!" as if the
>variable in question were being spit out by a mystical random number
>generator somewhere-- as if it had the opportunity to be many different
>numbers and by pure chance settled on this one.
>I have my thoughts on this, but let me ask: why is this an appealing way of
>looking at reality?

Suppose we know that the true theory of everything is of the form T, and
that it has one free parameter, a physical constant k that, on theoretical
grounds, can take on any integer value between 1 and 1,000,000. But there
is no a priori ground, let's assume, why k should have one of these values
rather than another. So by a principle of indifference, we assign each of
these possibilities an equal probability. Now consider the specific
hypotheses Tn:="T and k=n" for n=1, ..., 1,000,000. Probability calculus
then implies that the probability of Tn <= 1/1,000,000. In other words,
each of these specific hypotheses must be highly a priori improbable since
they are mutually exclusive and their combined probability cannot exceed 1.

Note that we are not talking about physical chances here. (There might well
not be a nontrivial physical chance that a physical constant should have
had a different value than it actually has; that's an open empirical
question.) Rather, we are talking rational epistemic probabilities -
degrees of belief. Even if it was in some sense physically necessary that
k=42925, so that the physical chance of k=42925 is 1, this is compatible
with our prior epistemic probability of k=42925 being very small.

Nick Bostrom
Department of Philosophy
Yale University

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