From: Jeff Davis (jdavis@socketscience.com)
Date: Wed Feb 21 2001 - 01:43:11 MST
Extropes,
Back in December, I began a thread entitled "Dyson sphere stability". (I
just tried to find it, and the rest of the posts in the thread, so I could
include direct links, but the 4th quarter 2000 posts are not yet accessible
in the archives.) I liked the elegance of the Dyson shell, and sought to
rescue it from predictions of unworkability.
At the time, Eugene gave a lengthy response, the relevant bits of which,
from my own email archive, I cut and paste below.
One of the problems is the structural stability of the shell. Gravity from
the enclosed star is pulling on it tending to make it collapse. Someone
has run some numbers that predict compressive stresses in excess of the
strengths of available construction materials. Thus Gene's use below of the
term 'Unobtainium'.
Gene said:
>Even if you could construct it, what about self gravitation
>(collapse is autocatalytic)...
>... you can't build a sphere of any relevant size
>without infinitely strong material (Unobtainium still isn't in the PSE
>last time I looked).
Now, you can spin the sphere, and thereby counter the gravitational forces
in the equatorial region, but the caps of the sphere, the portions in the
high lattitudes north and south, bear down in compression on the equatorial
region.
This is the problem, those end caps bearing down, too heavy for any known
structural material to support.
Now hold that thought.
Further on in the discussion Gene addressed a point I was interested in.
The shell completely encloses the star, intercepting the entire radiated
output. Absorbtion, reflection, secondary absorbtion and reflection, by
both the shell and the star, and finally transmission through the shell,
and radiation from the exterior of the shell, constitutes a whole new
regime of energy transmission compared to the old star-and-planets model.
What will the equilibrium situation look like when it all settles down?
Gene weighs in on this and says:
>Have you considered that you're heating the star up, by reflecting the
>bulk of radiation back in in form of shortwave photons? (Instead of
>making the orbiting objects as black as possible). Whatever the star
>is going to do, this is not going to be sustainable. You're going to
>heat the star, which will bloat and emit bluer and bluer. You'll
>likely can boost mass loss that way, by making the surface hotter.
Ok. So, without going too deeply into the machinations of the unconscious
mind, I find myself tootlin' along some months later when--blink!--on goes
the little light. Perhaps it was Robert Bradbury's piece on
"Planet Disassembly", where he says, in the section entitled 'Planet
Vaporization',
"Future technologies might produce giant reflectors to focus IR, visible
and UV photons produced by the sun onto the planet. Materials making up the
surface of solid planets have boiling points from 2500 to 3800 degrees K.
Applying sufficient energy would heat the surface material of the planet to
the boiling point. If enough energy were applied, a significant fraction
of the gas molecules would achieve escape velocity and the planet would
literally evaporate."
Whatever it was, I'm thinkin', "If you heat up the star--which is after all
just a ball of hydrogen gas--and it bloats up, and it bloats up some more,
and you trap the energy from the star in the intervening space between the
star and the shell, won't that space fill up with heated gas, until finally
it bears up against the interior of the shell, supporting it against the
force of gravity? Won't the Dyson shell become a stellar hot-hydrogen
balloon? And what of the temperature profile of the newly configured
stellar atmosphere? Cooler in the upper regions, perhaps? Does the star
have enough hydrogen to provide the appropriate
volume and density of atmosphere? Can the size of the Dyson shell be
chosen so as to make it work?
Once more into the breach!
Best, Jeff Davis
"Everything's hard till you know how to do it."
Ray Charles
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