jef@LBL-RTSG.ARPA (03/02/86)
Date: 1 Mar 86 04:17:48 GMT
From: brahms!desj@ucbvax.berkeley.edu (David desJardins)
Dyson spheres are science fiction (no offense). Not that they aren't
theoretically possible (if you're willing to live without gravity),
but the technology required is incomparably greater than that needed
for interstellar ships.
The Dyson spheres that have appeared in science fiction are indeed science
fiction (big surprise), and are not even theoretically possible, but they
are not what Dyson was talking about. All his paper said was that older
intelligent species would eventually need all the solar energy their sun
produces, and so they would enclose it with a sphere of solar power
generators, and if we want to find them we should look for infrared stars.
But note well: the sphere does *not* have to be solid! The easy (!) way
to build a Dyson sphere is as billions of solar power satellites in
solar orbit.
---
JefKFL@AI.AI.MIT.EDU ("Keith F. Lynch") (03/29/86)
From: Paul Dietz <dietz%slb-doll.csnet@CSNET-RELAY.ARPA>
How about getting mass from the sun? It's about 2% "metals" (elements
heavier than helium) by mass, or about 6000 times the mass of the earth
in heavy elements. Granted, there are engineering problems (!), but
we're in fantasy mode anyway and have lots of energy to play with.
This is kind of hard to imagine. What would they collect it with
that wouldn't burn up? Of course they might simply mine it out of the
solar wind. But I think getting it from other solar systems would be
easier, especially if they are in no hurry.
Another possibility would be to turn the sunlight into matter. This
would create equal amounts of antimatter, but the antimatter could be
gradually dropped into the sun to increase its brightness slightly.
In fact they would get back just the amount of light that they used to
build the thing in the first place!
>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.
...Keithdietz@SLB-DOLL.CSNET (Paul Dietz) (03/30/86)
> >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.
KFL@AI.AI.MIT.EDU ("Keith F. Lynch") (03/30/86)
From: Paul Dietz <dietz%slb-doll.csnet@CSNET-RELAY.ARPA>
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.
Sure you can. Don't think of it as a sphere, think of it as
individual small objects. Each object receives light from the sun,
and radiates heat equally in all directions. The fact that the heat
generated in some directions will be intercepted by the sun or by
other of the small objects is not relevant.
The heat must be radiated from the *exterior* of the sphere, which is
visible to interstellar space.
Ultimately, it all is.
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.
This is a more difficult way of thinking of it, but yes.
...Keith