A Dyson sphere is an immense artificial structure proposed by Freeman Dyson in 1959^[1,2] for the purpose of intercepting vast amounts of solar energy. Dyson calculated that, if mankind's current Malthusian (exponential) growth rate in energy consumption were to continue, the human race would reach a crisis point within the next two to three millennia (see Malthus, Thomas Robert). At this point, all the non-renewable sources, such as fossil and nuclear fuels, would be exhausted, and even renewable sources exploited on a planet-wide scale would be unable to cope with further demand. The problem is that the Earth intercepts less than one billionth of the total radiant output of the Sun. To provide for future growth, the human race will need to capture much more of the Sun's light. It could do this with a Dyson sphere: a spherical shell, 2 to 3 meters thick, centered on the Sun and rotating around it at about twice the orbital distance of Earth (see Note 1 below). The material for building the sphere, Dyson suggested, could come from disassembling the planet Jupiter and utilizing the minerals and metals from its deep interior. Dyson acknowledged that the inspiration for his scheme came from Olaf Stapledon's Star Maker:

 

As the eons advanced, hundreds of thousands of worlds were constructed, all of this type, but gradually increasing in size and complexity. Many a star without natural planets came to be surrounded by concentric rings of artificial worlds. In some cases the inner rings contained scores, the outer rings thousands of globes adapted to life at some particular distance from the Sun.

 

Stapledon, in turn, may have borrowed the idea from J. D. Bernal, who also influenced Dyson directly.

 

Following the publication of Dyson's proposal, several letters appeared in Science pointing out that a solid circumstellar sphere would be subject to intolerably high stresses. Dyson replied that what he actually envisaged was a loose collection of over 100,000 objects traveling on independent orbits in a shell about 1 million kilometers thick. Dyson's intended (fragmented) arrangement may be referred to as a Type I Dyson sphere and a solid shell as a Type II. It is the latter, in fact, which subsequently received most publicity through a number of science fiction stories woven around the theme, including Robert Silverberg's Across a Billion Years (1969), Bob Shaw's Orbitsville (1975), and Frederick Pohl and Jack Williamson's Wall Around a Star (1983). A Type II Dyson sphere also features in the Star Trek: New Generation episode "Relics". In order to produce the artificial gravity to allow such a structure to be habitable on its inner surface, the sphere would have to rotate. But the required rate of rotation (see Note 2) would set up such extreme stresses in the shell (tending to deform it into an oblate spheroid) that no known material would be able to withstand them. Even if the structural integrity of the shell were not compromised, only the equatorial regions would be suitable for permanent habitation, for two reasons: the air of the inner biosphere would tend to collect at the equator, and it would only be here that the maximum level of artificial gravity would be achieved. The "pull" would decrease as the distance from the equator increased, until it became zero at the poles. Additionally, a Type II sphere would be unstable since even the slightest collision, say with an asteroid or comet, would cause the structure to drift and eventually collide with the Sun.

 

A Type I Dyson sphere, on the other hand, appears to be such an inevitable and achievable piece of astroengineering that we might expect not only the human race eventually to build one but for other civilizations, more advanced than our own, already to have theirs in place. Dyson pointed this out and suggested how such structures would appear over interstellar distances. A Dyson sphere would absorb most of the visible and shorter wavelength radiation from its host star and re-emit a portion of it in the form of infrared radiation.^[3] Its infrared output, in fact, would be similar to that of a protostar. The artificial nature of the object might be revealed in other ways. Said Dyson: "One would look in particular for irregular light variations due to starlight shining through chinks in the curtain, and for stray electromagnetic fields and radio noise produced by large-scale electrical operations ..." Several searches have been carried out.^{[4, 5, 6, 7, 8]}

 

A third type of Dyson sphere – a Dyson "bubble" – has been proposed in which the shell would be complete, very thin, and non-rotating. It would consist of statites, or stationary solar sails, which reflected light onto collectors for use in external habitats. The entire mass of the structure would be approximately that of the Moon or a large asteroid.

 

Notes

1. The surface area of a sphere of radius r is 4πr². The area of a circle of radius r is πr². The interior surface area of a Dyson sphere with a radius of 150 million kilometers (the Earth-Sun distance) is 4 × 3.14 × (150 × 10⁶)² or 2.83 × 10¹⁷ square kilometers (283,000 trillion square kilometers). The cross-sectional area of a sphere the size of Earth (equatorial radius 6,378 kilometers) is 3.14 × (6.38 × 10³)² or 1.28 × 10⁸ square kilometers (128 million square kilometers). Hence, the amount of solar radiation captured is greater in the case of the Dyson sphere by a factor of (2.83 × 10¹⁷)/(1.28 × 10⁸) or 2.2 billion. The interior surface area of the Dyson sphere exceeds the total surface area of Earth by a factor of just over half a billion.

 

2. To produce Earth-normal gravity at the equator of a Dyson sphere, the sphere would have to rotate so that its angular acceleration matched the acceleration due to gravity at Earth's surface (approximately 9.8 meters per second per second). Angular acceleration is given by v²/r, where v is the tangential velocity and r the distance to the center of rotation. Therefore, we require
v²/r = 9.8
or v = √(9.8r)
Since r = 150 million km, this gives
v = √((9.8 × 150 × 10⁶)/1000) kilometers per second
or v = 1,212 kilometers per second

 

References

1. Dyson, F. J. "Search for Artificial Sources of Infrared Radiation," Science, 131, 1667-1668 (1959).
2. Dyson, Freeman J. "The Search for Extraterrestrial Technology." In R. E. Marshak, ed., Perspectives in Modern Physics: Essays in Honor of Hans Bethe. New York: John Wiley & Sons (1966).
3. Sagan, C., and Walker, R. G. "The Infrared Detectability of Dyson Civilizations," Astrophysical Journal, 144, 1216-1217 (1966).
4. 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, pp. 295-298. Berlin: Springer-Verlag (1991).
5. Jugaku, J., and Nishimura, S. "A Search for Dyson Spheres Around Late-Type Stars in the Solar Neighborhood," vol. 74 of ASP Conference Series, pp. 381-385. San Francisco: Astronomical Society of the Pacific (1995).
6. Jugaku, J., and Nishimura, S. "A Search for Dyson Spheres Around Late-Type Stars in the Solar Neighborhood. II," No. 161 in IAU Colloquium, pp. 707-709. Bologna: Editrice Compositori (1997).
7. Jugaku, J., and Nishimura, S. "A Search for Dyson Spheres Around Late-Type Stars in the Solar Neighborhood. III," in G. Lemarchand and K. Meech, eds. A New Era in Bioastronomy, vol. 213 of ASP Conference Series. San Francisco: Astronomical Society of the Pacific (2000).
8. Slysh, V. I. "Search in the Infrared to Microwave for Astroengineering Activity," in The Search for Extraterrestrial Life: Recent Developments, M. D. Papagiannis (Editor). Boston, Mass.: Reidel Pub. Co. (1985).