Re: Why Would Aliens Hide?

Robert J. Bradbury (
Fri, 26 Nov 1999 06:38:09 -0800 (PST)

On Fri, 26 Nov 1999, Amara Graps wrote:

> The ISO observations of the amount of dust in our Galaxy being off by
> about a factor 100 from what the scientists count (currently) as dust
> sources is a problem.

Amara, I presume the ISO observations to which you refer are the measurements of IR from diffuse background dust. A few questions:

Also, could you provide a little interpretation of the gas/dust ratio value? Is this this an atom[molecule]/dust grain # abundance or is it a mass ratio?

It is worth noting that none of the [primary] papers that I have seen regarding interstellar colonization and/or the Fermi Paradox seems to have been concerned with the hazards posed by interstellar gas & dust. I would expect that the gas/dust ratio in the galaxy should over time decline. The gas, if dominated by uncharged molecules, could not easily be repelled by magnetic methods. On the other hand if much of the dust carries a charge it might be more easily pushed out of the way. [If I've got this really wrong, someone please correct me.] So it may be that interstellar travel is easy early in galactic history but gets progressively more difficult. (Of course the argument could be reversed if the gas hitting you is like raindrops, and the dust hitting you is like cannonballs).