From: Eric Watt Forste (arkuat@idiom.com)
Date: Tue Nov 23 1999 - 16:11:33 MST
Robert Bradbury writes:
> On Sat, 20 Nov 1999 Spudboy100@aol.com wrote:
> > Unless we gain positve evidence, Fermi and Tipler are right.
> > We have met 'intelligent-life' and they is us.
>
> Distinctly possible, but saying it doesn't explain *why* that is
> the case.
Perhaps Universe is so young that we are the first kids on the
block.
I have been looking around for data on what a chemist would call
the reaction rate of the production of metals in this and other
galaxies. Lighter metals such as carbon and oxygen are produced in
medium-size stars and distributed by planetary nebula ejection and
by white-dwarf novas. The heaviest elements are made and distributed
only in supernovas. There are several different kinds of events,
and each has a different characteristic rate, and the rate for each
event varies from galaxy to galaxy. As a result, I haven't been
able to find any good numbers yet. I did find a textbook of
cosmochemistry that looked promising, but if the numbers I'm looking
for were in there, they were deeply buried in mathematics that I
have yet to do the homework for.
Another way to estimate these rates is to look for a metallicity
gradient in redshift. How much richer in metal are nearby (older)
galaxies with respect to distant (younger) galaxies? I haven't yet
done any research in this direction. I don't even know whether it
is more difficult to measure metallicity from distant-galaxy spectra
than from individual star spectra.
In the absence of this information, I'm comfortable with the
assumption that most planetary systems formed before 4.6 gigayears
ago (when ours formed) were below the metallicity threshold required
for the spontaneous development of self-reproducing molecules.
If any astrophysicists or cosmochemists on the list know where to
get to the data that bear on this question (Amara?), I'd love to
learn more about it.
-- arkuat
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