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From: dietz@SLB-DOLL.CSNET (Paul Dietz)
Newsgroups: net.space
Subject: Dyson spheres
Message-ID: <8603292312.AA03434@s1-b.arpa>
Date: Sat, 29-Mar-86 17:14:17 EST
Article-I.D.: s1-b.8603292312.AA03434
Posted: Sat Mar 29 17:14:17 1986
Date-Received: Tue, 1-Apr-86 05:09:40 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 19

>    >If you make the foil just the right thickness, the Sun's gravity and the
>    >light pressure exactly balance, and the foil will remain stationary.
>
>    Not if you absorb the sunlight and radiate it as waste heat.  The energy
>    flowing out must equal the energy flowing in, so the radiation pressures
>    will balance.
>
>  No.  Each part of the sphere radiates heat equally in all
>directions, so there is no net pressure from that.

But you can't dump heat back into the inside of the sphere, since all you
can see there is other radiating surfaces or the sun.  The heat must be
radiated from the *exterior* of the sphere, which is visible to
interstellar space.

I think I see your point, though.  Even if there is no net radiation
from the interior sufaces the waste heat will still exert pressure
there (when it is emitted and reabsorbed).  A gas of photons in thermal
equilibrium with the radiators, if you will.