14.Other Dyson Sphere-Like Concepts
Ringworlds were introduced by Larry Niven in Ringworld. A ringworld is essentially a band encircling a star, rotating to create gravity and covered with an ecosphere. The atmosphere is held in by gravity and 1000-kilometer mountain walls at the edges. Day and light are provided by a second ring of "shadow squares", black rectangles covering the sun during "night". Cooling systems keep the climate stable and an automated meteor defense system shoots down meteors.
Just as with a type II Dyson sphere the internal stresses would require an immensely strong material (Niven uses the invented material scrith, a greyish translucent material with strength on the order of the nuclear binding strength). The stress is
F= r rho g [N/m]
where rho is the weight of the ringworld per square meter (kg/m^2) and g is the surface acceleration and r is the radius. For the ringworld g was close to earthly, a radius of around 1 AU and there was at least a kilometre of surface material of approximately earthlike density. This would provide a stress on the order of 1e18-1e19 N/m.
The ringworlds instability is also (in)famous. It is not neutrally stable like a dyson sphere, but dynamically unstable - a small disturbance (such as the inhomogenities in the solar wind or meteor strikes) will grow gradually, and the ringworld would gradually loose its centeredness until it runs into its sun (the ringworld is transversely stable, if the ring is perturbed along its axis it will oscillate around the equilibrium position). Niven solves this problem in Ringworld Engineers by placing ramjets along the edges, forming an active stabilization system.
A related idea to ringworlds is Ian Bank's orbitals. An orbital is a small ringworld orbiting the sun (instead of encircling it; this circumvents the instability), with a rotation period of 24 hours and earthlike gravity due to the spin. Its size would be
r=g T^2 / (4 pi^2)
which gives a radius of around 2 million kilometers. If the orbital is tilted and sufficiently broad, it could shade itself to provide day and night. Note that it still requires superstrong materials, although less extreme than scrith.
Bubbleworld, by Dani Eder < firstname.lastname@example.org>
A bubbleworld is an artificial construct that consists of a shell of living space around a sphere of hydrogen gas. It was invented to answer the question "what is the largest space colony that can be built". The answer is a non-rotating bubbleworld can be as large as 480,000 km in diameter (about 3 times the diameter of Jupiter), if you make certain assumptions. A rotating one is too hard for me to analyze.
Assume that you wish to have a large living volume in the form of a shell. The shell contains air, people, houses, furniture, etc. that averages 10 kg/m^3. If you fill the interior of the shell with hydrogen (the lightest gas) at room temperature, it will assume a distribution based on self-gravity. If the pressure at the inner shell boundary, where the hydrogen and living space meet, is 1 atmosphere, then there is a largest size you can build such a structure before the self-gravity of the hydrogen starts to make it smaller.
The living space has a thickness of 2400 km if you assume that the outer surface is at a pressure equal to that at 3000 m (10,000 ft) above sea-level on Earth. Such a bubbleworld would have about 5 million times the useable living volume of the Earth. The atmosphere is held in by a cap of material (such as 500 meters of iron-nickel) so as to balance the gas pressure from below. The entire structure is in pressure equilibrium, so it requires no particular structural strength.
A rotating bubbleworld would be a flattened ellipsoid and could be several times larger, but determining the shape is more complicated than the non-rotating spherical case.
The living volume of the Bubbleworld would be in a 0.001 to 0.01 gee environment, making unusual architecture and human-powered flight possible. The entire bubbleworld would mass about 3 Earths in mass.
Submerged Dyson Spheres
Nick Szabo proposed that since communications delays are rather long in a normal-size dyson sphere and energy densities grow as it becomes smaller, it would be advantageous to build spheres closer and closer to the star for advanced "solid state civilizations". The logical conclusion would be a shell around the core of the star, through which all energy would be filtered.
The problem with this is that the amount of energy that can be extracted from the radiation depends on the difference in temperature on the two sides of the shell, and inside the star this will be rather low, while outside the star the difference will essentially be between the shell temperature and the cosmic background radiation. But it should be noted that if neutrinos can be captured, they would provide a kind of temperature differential that could be used (since the sun is almost transparent to them).
15.Have any Dyson spheres been observed?
I have found the following three searches for Dyson spheres at http://www.seti-inst.edu/searches/searches-list.html:
SITE: NASA - U OF A, MT. LEMMON
INSTR. SIZE (M): 1.5
SEARCH FREQ.(MHz): 8.5 microns - 13.5 microns
FREQUENCY RESOL.(Hz): 1 micron
OBJECTS: 20 STARS
FLUX LIMITS (W/m**2): N MAGNITUDE EXCESS < 1.7
TOTAL HOURS: 50
COMMENTS: Search for IR excess due to Dyson spheres around solar
type stars. Target stars were
chosen because too faint for spectral type.
INSTR. SIZE (M): RADIOMETER
SEARCH FREQ.(MHz): 37x10**3
FREQUENCY RESOL.(Hz): 4x10**8
OBJECTS: ALL SKY 3K BB
FLUX LIMITS (W/m**2): T/T =< .01
TOTAL HOURS: 6000
COMMENTS: Lack of fluctuations in 3K background radiation on
angular scales of 10**-2 Strd. rules
out optically thick Dyson spheres radiating more than 1 solar
luminosity within 100 pc.
OBSERVER(S): TARTER, KARDASHEV & SLYSH
INSTR. SIZE (M): 26 (9 ANTENNAS)
SEARCH FREQ.(MHz): 1612.231
FREQUENCY RESOL.(Hz): 6105
FLUX LIMITS (W/m**2):
TOTAL HOURS: 1
COMMENTS: Remote observation (by VLA staff) of IRAS source near
galactic center to determine if
source could be nearby Dyson sphere. Source confirmed as OH/IR
In short, none have been observed yet.
Slysh, V. I., Search in the Infrared to Microwave for Astro- engineering Activity, in The Search for Extraterrestrial Life: Recent Developments, M. D. Papagiannis (Editor), Reidel Pub. Co., Boston, Massachusetts, 1985
Kardashev, N. S., and Zhuravlev, V. I., SETI in Russia, paper presented at the IAA/COSPAR/IAF/NASA/AIAA symposium on SETI: A New Endeavor for Humankind, The World Space Congress, Washington, D.C., August 30, 1992. To appear in a special issue of Acta Astronautica.
Jugaku, J., and Nishimura, S., A Search for Dyson Spheres Around Late-Type Stars in the IRAS Catalog, in Bioastronomy: The Search for Extraterrestrial Life, J. Heidemann and M. J. Klein (Eds.), Lectures Notes in Physics 390, Springer-Verlag, 1991
16.What has been written about dyson Spheres?
The original papers:
Dyson, F. J., Search for Artificial Stellar Sources of Infrared Radiation, Science, vol. 131, pp. 1667-1668, 1959
Dyson, F. J., The Search for Extraterrestrial Technology, in Perspectives in Modern Physics (Essays in Honor of Hans Bethe), R. E. Marshak (Editor), John Wiley & Sons, New York, 1966
Larry Niven: "Bigger than Worlds" in A Hole In Space (1974) and Playgrounds of the Mind. Deals with all kinds of megaengineering structures.
Marshall T. Savage: The Millennial Project (ISBN 0-316-77163-5). Describes a plausible space-colonization scenario, involving the construction of a type I dyson sphere.
Fiction that involves dyson spheres or linked concepts:
(sources gathered from Usenet discussions and "Megastructures in Science Fiction" by Ross Smith, http://www.algonet.se/~aleph/Trans/Tech/Megascale/megastruct.txt)
Star Maker (1937) by Olaf Stapledon (An enthusiastic review)
The World is Round by Rothman
Larry Niven: Ringworld, Ringworld Engineers and Ringworld Throne
Lord Kalvan of Otherwhen by H. Beam Piper
"Relics" episode of Star Trek The New Generation (regarded as very bad by many sf lovers)
Cageworld 1: Search for the Sun, Cageworld 2: The Lost Worlds of Cronus, Cageworld 3: The Tyrant of Hades and Cageworld 4: Star-Search by Colin Kapp.
Orbitsville (1975), Orbitsville Departure (1983) and Orbitsville Judgement (1990) by Bob Shaw
Across a Billion Years by Robert Silverberg
Farthest Star (1975), Wall Around a Star (1983) by Frederik Pohl & Jack Williamson
Dyson spheres need great big walls
To keep the world from spilling out
They make them out of buckyballs
And use gravitons
Mister Skin < email@example.com>