Damien Broderick wrote:
>At 04:04 PM 28/10/00 -0400, Nick wrote:
>
> >the universe seems to be infinite,
>
>It does? I suppose you mean the alleged uncountable abundance of budded-off
>Linde bubble universes, each of which is very finite indeed (in the
>relevant sense).
No, I mean this particular chunk of spacetime which we are living in.
> Even if open universes like ours continue to expand
>literally forever and to [a modest class of] infinite distance, they won't
>have any meaningful structure after quite a short time (considerably less
>than 10^200 years, say).
No need to wait 10^200 years. The universe is probably already infinite. I
know there's a common misconception on this point, so let me try to nail
that ghost to the coffin once and for all. A simply connected hyperbolic
Big Bang universe is spatially infinite at every point in time, containing
infinitely many planets etc. Recent evidence indicates that our universe is
hyperbolic. Whether it is simply connected is a somewhat open question, but
it is the simplest alternative and most cosmologists tend to implicitly
assume that it is. So far there is no sign that it is multiply connected,
but some astrophysicists keep looking.
> >so there are probably infinitely many ETIs.
>
>If that is your intended meaning, I don't think it has much bearing on
>detectable ET numbers.
Correct. Owing to the expansion of spacetime, most of it will never be
accessible from our starting point. (Consider a distant point x. By the
time you have travelled half the distance to x, the remaining distance will
have increased by more than a factor 2, if x is sufficiently far away.)
This assumes we can't go faster than light.
Nick Bostrom
Department of Philosophy
Yale University
Homepage: http://www.nickbostrom.com
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