poly: Re: polymath digest for 06/12/97

Re: the expansion mathematics of Carl F. I suggest looking up the work of
Erik Jones and Ben Finney, re: Galactic Migration, that was done back in
the early eighties. I forget the reference, but you can find it in my own
paper on Galactic civilization, in the Quarterly Journal of the Royal
Astronomical Society, fall 1983. (It's still just about the only 'review
paper' that SETI ever had, since most other papers push a particular
theory.) Frank Tipler also did math analyzing self-replicating
interstellar probes.

Before we assume any part of space will 'crystallize,' be sure and include
two factors.

(1) galactic differential rotation 'smears' any spherical expansion shell.
(Inner parts of the galaxy rotate faster). This increases the settled
zone's surface area and rate of access to virgin territory. It also means
neighbors during one epoch will not be neighbors during the next. (Our
local orbit period is approx 250 M years)

(2) Bear in mind that complex ecosystems act to prevent prairie fire
reproduction orgies. But such ecosystems are not in place at the beginning.
When a virgin enthalpy source begins being exploited for the first time,
the opportunistic species may breed spasmodically, use everyting up, and
leave a wasteland behind, Wild swings of population may occur until
predation and other limiters talke hold. This is one possible explanation
for the local 'Great Silence.' (I'm discussing it in a coming novel.)

Robin's own discussion of the 'Great Filter' is thought provoking. Each
time humans face a crisis, we must ask ourselves -- 'Is this it? Is this
the filter that prevents sapiency from filling the universe?'

Interesting stuff.

D. Brin
Received on Sat Dec 6 22:04:29 1997