# Re: Doom2k

From: Jacques M. Mallah <jqm1584.domain.name.hidden>
Date: Sun, 12 Dec 1999 19:26:59 -0500 (EST)

On Tue, 7 Dec 1999 hal.domain.name.hidden 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
>
> 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 (jqm1584.domain.name.hidden)