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/