Re: Why Would Aliens Hide?

Eric Watt Forste (
Tue, 23 Nov 1999 15:11:33 -0800

Robert Bradbury writes:
> On Sat, 20 Nov 1999 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.