On Tue, 7 Dec 1999 [EMAIL PROTECTED] wrote:
> Suppose there are two possibilities: you live in a universe where there
> will be 100 billion people total, or in a universe where there will be
> 100 trillion people total, and a priori you think there is a 50-50 chance
> which one is the case.  You check your birth order and find that you are
> about number 50 billion.
> Now, that would be pretty likely if you were in the 100-billion universe,
> but it would be very unlikely if you were in the 100-trillion universe.
> Hence by Bayesian reasoning you find you are more likely to be in the
> 100-billion universe, and therefore the human race is likely to end
> relatively soon.  This is the Doomsday argument.
> However introducing the all-universe model and the self-selection
> assumption (that you are a random individual from among all individuals in
> all universes) then a priori the chances that you are in the 100-trillion
> universe are ten times greater than that you are in the 100-billion
> universe.  This exactly counters the shift which you made in the Doomsday
> argument, based on your birth order, which made you think you were more
> likely to be in the 100-billion universe.

        The Doomsday argument still works.  The uncertainty is not which
"universe" you're in; as you say, if both universes exist and you know
that, there's no Doomsday argument.  But the thing is, you don't know
that.  Suppose there are N "universes" that all exist.  Some X of them
have 10^11 people, (N-X) have 10^14, but you don't know what fraction X/N
is.  If your number is 5*10^10, this suggests X/N is large: Doomsday.  Of
course, if you could calculate X/N from first principles, there would be
no argument.  The one-world case is just N=1; again, if you could
calculate whether X=0 or X=1 in this case, there would be no argument.

                         - - - - - - -
              Jacques Mallah ([EMAIL PROTECTED])
       Graduate Student / Many Worlder / Devil's Advocate
"I know what no one else knows" - 'Runaway Train', Soul Asylum
            My URL: http://pages.nyu.edu/~jqm1584/

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