From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Thu Nov 25 1999 - 04:31:54 MST
On Wed, 24 Nov 1999, Robin Hanson wrote:
> Amara Graps wrote:
> >... every once in a while I bump ... questions about
> >whether stars can really produce all of the dust that we observe.
>
> Interesting. Unexplained excess of dust production seems the
> opposite of what one would expect from aliens greedily eating up
> all the metals they can find.
>
I'm ahead of you on that one Robin... I've discussed this
with Robert Freitas, and he seems to feel that harvesting
materials (esp. H or presumably dust) from the interstellar
media is tremendously unproductive, for density reasons. On
the other hand an abundance of dust, is what you would
expect if SIs are throwing away the most "useless" mass
as thrust material. H you want to keep because it can
produce energy, C you want to keep because it is a great
construction material, O, Si or Fe on the other hand are
of less value. So, if Amara would say the element abundance
in interstellar grains seems to favor O, Si, or Fe over
what would be expected for H/C, then I'd be willing to bet
on propulsion exhaust.
Now, strictly speaking you can use photons for propulsion
so there might be no exhaust "trail". Some of our rocket engineers
might want to offer some comments -- In an nanotechnological
based environment, where energy is relatively cheap compared to
mass (but some mass is "relatively" worthless), how would you design
rockets to provide course corrections to use the least valuable
combination of matter & energy?
Of course, Amara, I hope when you are having these lectures you
do point out the potential for missing factors, such as SI
exhaust trails... :-)
Robert
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