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... :-)