Re: Why Would Aliens Hide?

Robert J. Bradbury (
Mon, 29 Nov 1999 08:06:40 -0800 (PST)

On Sun, 28 Nov 1999, Robin Hanson wrote:

>>  I had commented on what metals SIs might throw away as thrust.

> This prediction doesn't seem to be confirmed by the data
> Amara pointed us to (Table 4, P. Frisch et al,
> "Dust in the Local Interstellar Wind" ApJ 1 Nov 1999.)
> If anything, Si seems to be the left out element, not C.
> (See section 6.4 of the paper.)

I'll put this on the list for my next library trip.

I think the key thing is the word "local". All this may end up suggesting is the relatively local abundance of elements from stars of various masses and the types of supernovas that have occured in our local region in recent times (i.e. the time it takes for the dust clouds to significantly dissipate). We may have local star masses that produce a lot of C but not much Si.

I'm not optimisitic in the long run about using interstellar dust as tracers for history (or astroengineering activities). One paper I glanced at lately documented a galaxy with two distinct populations of stars with an age difference of ~2 billion years. They attributed it to a galactic collision in the intermediate past. If you take a single galaxy, you are going to get a distribution of star masses based on the initial density. Then you could run a simulation and probably come up with a good model for what you should see in the interstellar dust. But it looks like you also have to factor in galactic collisions and the effects of faster/slower evolution in the colliding galaxies and gravitational effects on dust flows. I think the complexity is going to go up very fast and you are going to need a lot of data to begin to read the history.

Also of note is a recent note from an astronomer using the ISO to do infrared observations, suggesting that the current "opinion" is that the gravitational microlensing observations are telling us something about the structure of the LMC rather than the Milky Way. So it may be that the strongest piece of SI evidence is resting on a very shakey leg. We really need the SIRTF and perhaps a replacement for the WIRE mission up and operational.