Re: Doomsday vs Diaspora

From: Hal Finney (hal@finney.org)
Date: Wed Apr 23 2003 - 22:13:14 MDT

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    Mitchell Porter writes:

    > Validity of Cosmic Doomsday Argument (CDA) implies that
    > Frequency of Doomsday civs >= N * Frequency of Diaspora civs,
    > where N = (Typical Diaspora pop/Typical Doomsday pop)

    That's an interesting point. I think I may have seen it before although
    perhaps not expressed in such clear mathematical terms. We discussed a
    paper a few months ago, http://xxx.lanl.gov/pdf/physics/0302071, which
    expressed a similar idea in a more qualitative way:

       First, it is clear that there is some nonzero probability for a
       civilization to survive early threats to its existence (nuclear war,
       asteroid impact, etc.). Such a civilization might go on to spread
       across its galaxy. It could endure for millions of years and contain
       a huge number of individuals. We will refer to such civilizations
       as long-lived. On the other hand, some civilizations will succumb to
       existential threats and so be short-lived. What will be the fraction
       of each?

       Unless the fraction of long-lived civilizations is tiny, nearly all
       individuals will belong to them, and furthermore will live late
       in their civilizations when most of the individuals live. That,
       however, is not the circumstance in which we find ourselves. Instead,
       we find that we live either in a short-lived civilization or very
       early in a long-lived one. While we do not have a clear idea of how
       long to expect civilizations to last, when we take into account our
       circumstances, we should clearly update our ideas in favor of a much
       larger chance for civilizations to be short-lived (Carter unpublished;
       Leslie 1996 p. 231). Thus unless we previously thought that long-lived
       civilizations were much more likely, we should now think that almost
       all civilizations will be short-lived - a sort of universal doomsday.

    I'm not sure if it has been mentioned here, but fans of the Doomsday
    Argument might want to know that Nick Bostrom has published a book on
    that argument and the many related concepts, Anthropic Bias: Observation
    Selection Effects in Science and Philosophy, available from
    http://www.amazon.com/exec/obidos/ASIN/0415938589/103-4781209-8504658.

    Hal Finney



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