Re: PhD-thesis on Observational Selection Effects

From: Nick Bostrom <>
Date: Tue, 31 Oct 2000 21:14:56 -0400

Matthew Donald wrote:

>First of all, I would be unhappy with the requirement to specify a
>unique hyperplane of simultaneity.

As an aside, the people who think of the world as generated by a Turing
machine could presumably define a "metaphysical time" axis consisting of
the discrete sequential operations of this world-generating Turing machine.
This time would not necessarily be related to any internal time coordinates
in the universes it creates. (E.g. you could have the Turing Machine create
a finite universe from back to front, beginning by specifying the final
conditions and working its way backward from there; the observers in such a
world would not notice anything unusual.

>This means that I do not view the ``birth rank'' problem as being
>well-posed for my theory. Nevertheless, the broader problem of the
>extent to which we should expect to see ourselves as being
>``typical'' observers is one which I think is both absolutely crucial
>and very hard to deal with.

Agreed. Birth ranks are relevant only if (1) they are well-defined, and (2)
knowable. Neither seems to be the case in the actual world. (Or perhaps one
should rather say that the evidence we have suggests that our birth ranks
are infinite? I wonder if that is a well-defined claim to make. There are
probably not infinitely many observers in our backward light cone, but if
the universe is spatially infinite then there were infinitely many
observers "a moment" after big bang. One could say, for instance, that
there are infinitely many observers living in the sort of climate that on
each world like characterizes the universe a short time after the big bang.)

> I appreciate Bostrom's work because I
>think that he is helping to clarify some of the issues involved in
>that problem.
>I also think that whether we are typical observers and whether
>time is observer-dependent are questions which are relevant for
>any ``theory of everything''.

I would agree with that. Especially the former is an absolutely key
question for any theory on which most possible types of observations are
actually made.

>I wrote
> > As I see it, *the question* is ``Given what I have learned
> > from my life up to now, how should I predict my future?''
>Bostrom replied
> > I'm not sure whether we are really disagreeing on this point,
> > although I would insist that one sort of question we may
> > meaningfully ask is not just what my own future will be but also
> > what the world is like (and was like in the past) in general.
>I am afraid that we are disagreeing here, because I do not agree that
>``the world'' exists in anything like the way that Bostrom implies.

I'm not sure what you think I was implying, but it seems that you are
committed to the existence of objective facts about the world. We can
define "the world" as the set of all propositions that are true for all
observers, and your theory of everything is, I take it, an attempt to
describe the world thus defined. In other words, a theory of everything
specifies the overall structure of the world. Each observer would want to
complement a theory of everything with a "local theory" describing their
own position within this overall structure.

Dr. Nick Bostrom
Department of Philosophy
Yale University
Received on Tue Oct 31 2000 - 18:19:48 PST

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