Project Cyclops, A Design... - Department of Earth and Planetary ...

ture societywith its mastery of controlled nuclear
fusion, would fall in this category.
Type H Civilizations are capable of utilizing a substantial
fraction of the radiation of their parent star so have
powers on the order of 1026 watts at their disposal.
Type III Civilizations are extended communities with
the ability to control powers comparable to the radiation
of an entire galaxy-that is, powers on the order of
1037 watts.
Kardashev reasons that any civilization could afford
to devote a small fraction, say one percent, of its energy
resources to interstellar communication, and this leads
him to postulate extremely powerful radiations from
Type II and II1 civilizations. This has the advantage of
making detection very easy for mere Type 1 civilizations,
such as ourselves, and obviates the need for expensive
receiving arrays of antennas. Alas, so far no such
powerful radiations of artificial origin haye been detected,
so Type II and II! civilizations remain hypothetical.
Nevertheless, Kardashev's classifications are useful
reference terms in discussing supercivilizations.
DYSON
CIVILIZATIONS
Freeman Dyson (refs. 2,3) has suggested that the
pressure of population growth will have forced many
advanced societies to create more living space in their
planetary systems by disassembling unfavorably situated
planets and redistributing their matter in various ways
about the parent star. Dyson points out that the mass of
Jupiter, if distributed in a spherical shell at 2 AU from
the Sun, would have a surface density of about 200
gm/cm 2 (actually 168 gm/cm 2) and, depending on the
density, would be from 2 to 3 m thick. He goes on to
say: "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." When it was pointed out that such a
shell would be dynamically unstable 2 he replied that
what he really had in mind was a swarm of independent
objects orbiting the star. In a subsequent paper (ref. 3),
he proposes that these objects be lightweight structures
up to 106 km in diameter, the limit being set by solar
tide raising forces, and notes that at 1 AU from the Sun,
2The total heavy element content of the Sun's planets would
allow a sphere at 2 AU radius around the sun to be only about 1
cm thick. If rotating, the sphere would flatten and collapse; if
stationary, the Sun's gravity would cause compressive stresses of
about 300,000 lb]in 2 in the shell, ensuring buckling and collapse.
With only one solar gravity at 2 AU (1.48× 10-3 m/s 2) no atmossphere
would remain on the eternally dark outside, while anything
on the inside would gently fall into the Sun. It is hard to see how
Dyson finds these conditions "comfortably habitable."
200,000 of these (actually 360,000) would be needed to
intercept and thus utilize all the Sun's radiation.
One consequence of this would be that a substantial
fraction of the Sun's luminosity would be reradiated
from an extended source at about 300°K rather than a
much smaller source at 5800°K. On this basis, Dyson
feels that we are more apt to detect advanced civilizations
because of the excess infrared radiation they
produce in pursuit of their own survival than as a result
of intentional beacon signals they might radiate.
Although Dyson describes an entertaining mechanism
for the disassembly of planets to obtain the material for
lightweight orbiting structures, no details are given as to
how these structures are to be made habitable. Presumably,
since these lightweight structures would not have
enough gravity to hold an external atmosphere, the
advanced beings are to live' inside. To fill 200,000
spheres each 10 6 km in diameter with air at normal
Earth atmospheric pressure would require about
1.36×1032 kg of air, or about 50,000 times the total
mass of the Sun's planets. The air would have to be
supported against contraction under its own gravity;
otherwise, only the central region would be habitable, or
(with enough air added to fill the sphere) the object
would become a massive star. Since, even with support,
the total air mass per sphere is about 100 Earth masses,
the supporting structure could hardly be the lightweight
affair Dyson describes.
We conclude that the size limit of Dyson's spheres is
more apt to be set by the amount of air available and by
the self-gravity effects it produces than by tidal forces. If
all the mass of the Sun's planets were in the form of air
at atmospheric pressure, this air would fill a spherical
shell 1 AU in radius and 7.3 km thick. Thus, instead of
360,000 spheres each 10 6 km in diameter, we would
need more than 4X l0 t s spheres each less than 10 km in
diameter to catch all the Sun's light at I AU. These
considerations cause us to be skeptical of Dyson's latest
model and to base our calculations of excess IR
radiation (given in Chap. 4) on the redistribution of the
heavy element mass of the solar system into several new
earths rather than 10' s "mobile homes" in orbit at IAU
from the Sun) Even this seems to us a formidable
enough undertaking. We note, however, that Dyson appears
so convinced of the detectability and inevitability
of the kind of astroengineering he describes that he construes
our failure to detect any such activities as evidence
for the absence of advanced intelligent life!
3The excess IR radiation from this vast number of spheres
would be indistinguishable from that produced by a lot of dust
around a star.Gaps in the orbital pattern would cause some direct
starlight to filter through, not in occasional flashes as with fewer
106 km diameter spheres, but in a fairly steady amount.
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