From: Frederick.A.Ringwald@dartmouth.edu (Frederick A. Ringwald)
Newsgroups: sci.space
Subject: Re: Dyson Spheres, again
Date: 23 Nov 92 02:00:45 GMT
 <ewright.722280175@convex.convex.com>
Organization: Dartmouth College, Hanover, NH
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In article <ewright.722280175@convex.convex.com>
ewright@convex.com (Edward V. Wright) writes:

> Well, Dyson himself did not actually propose a solid sphere, which would
> require materials that are not only unavailable but unreasonably strong,
> given our understanding of physics.  What he invisioned was a large number
> of habitats in orbit about a star which, together, would completely 
> encircle the star in a ball-of-twine formation.  From a distance, it would
> look like a solid shell.

Have you read Dyson's original article? I have it here in front of me.
He says:

"The material factors which ultimately limit the expansion of a
technically advanced species are the supply of matter and the supply of
energy...The reader may well ask in what sense can anyone speak of the
mass of Jupiter or the total radiation from the sun as being accessible
to exploitation...the mass if Jupiter, if distributed in a spherical
shell revolving around the sun at twice the Earth's distance from it,
would have a thickness such that the mass is 200 grams per square
centimeter of surface area (2 to 3 meters, depending on the density). A
shell of this thickness could be made comfortably habitable, and could
contain all the machinery required for exploiting the solar radiation
falling onto it from the inside."

This sounds like a solid sphere to me. He goes on to say:

"If the foregoing argument is accepted, the the search for
extraterrestrial intelligent beings should not be confined to the
neighborhood of visible stars. The most likely habitat for such bveings
would be a dark object, having a size comparable with the Earth's
orbit, and a surface temperature of 200 deg to 300 deg K. Such a dark
object would be radiating as copiously as the star which is hidden
inside it, but the radiation would be in the far infrared, around 10
microns wavelength." 

The jist of my (month-old) posts was that even if Dyson's basic idea is
right, the details probably aren't. A genuine Dyson sphere probably
would have plenty of starlight shining through it: a solid shell 2 or 3
meters in thickness simply would not be rigid enough to withstand the
pressure of the sun's gravity, unless made of material with properties
that don't look too plausible. In other words, a Dyson sphere probably
would not be a dark object. It would more probably look like a normal
F-K dwarf, with a slight IR excess that might be hard to distinguish
from a quite-natural circumstellar dust shell or ring. 

Then again, never second-guess an alien intelligence. Ideas about
completely reworking a planetary system, often involving draining
matter off the star, come to me in the night, and force me to conclude
that, like most things in SETI, there's an infinity of possibility even
within the narrow confines of the known laws of physics. We really
don't even know enough to ask intelligent questions. Jill Tarter is
taking as flexible approach as she can with the NASA HRM survey, and it
may well be that the SETI project that succeeds will be done by
astronomers with SETI initially being the farthest thing from their
minds. (Then again, watch out for fascinating false alarms, such as
Martian canals, pulsars, and gamma ray bursts.) 

By the way, the idea of a swarm of independent space habitats wasn't
Freeman Dyson's, it was Gerard O'Neil's, circa the late '60s, about ten
years after Dyson's 1960 paper, which I quote above. I know,
Tsiolkovsky, Bernal, and a legion of sci-fi writers had thought of it,
too, but the credit still should go to O'Neill for getting the
technical community to take the concept at least halfway seriously, as
a place to live and not just an orbiting laboratory, and as something
more than strictly sci-fi. After all, his 1974 paper did make the front
cover of Physics Today.

I recommend to anyone a look at Dyson's original paper; the reference
is:

Dyson, F. J. 1960, Science, vol. 131, p. 1667 (No. 3414, 1960 June 3).

You'll see immediately it's a one-pager, in the fine tradition of Fermi
(i.e., say your piece and get out). While looking up this paper, I came
across an interesting ad, in the same volume: a 1960 announcement that
Werner von Braun had just left the U.S. Army Redstone Arsenal to join
NASA, with a photo of him talking with several Mercury astronauts,
including Gordon Cooper and John Glenn - before their space flights, of
course. (Thanks goodness hairsyles have changed!) 
Newsgroups: sci.space