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A Dyson sphere (or "shell" as it appeared in the original paper) is a hypothetical megastructure that was originally described as a system of orbiting solar power satellites meant to completely englobe a star and capture its entire energy output, although other variants on this idea have been proposed — most notedly the solid shell concept pictured at right.Contents
Origin of concept
The concept of the Dyson sphere was the result of a thought experiment by physicist and mathematician Freeman Dyson, where he noted that every human technological civilization has constantly increased its demand for energy. He reasoned that if human civilization were to survive long enough, there would come a time when it required the total energy output of the sun. Thus, he proposed a system of orbiting structures designed to intercept and collect all energy produced by the sun. Dyson's proposal did not detail how such a system would be constructed, but focused only on issues of energy collection.
Although Dyson is credited with being the first to formalize the concept of the Dyson sphere in his 1959 paper "Search for Artificial Stellar Sources of Infra-Red Radiation" — published in the journal Science — Dyson himself was inspired by the mention of the concept in the 1937 science fiction novel Star Maker, by Olaf Stapledon.
While building a Dyson shell is impractical – even considered by many to be impossible (see below) – other proposed design variants of the sphere based on orbiting satellites or solar sails do not require any major theoretical breakthroughs in our scientific understanding. However, such constructs are well beyond our present-day industrial capabilities, or those of the foreseeable future. It is also likely that there are unforeseen industrial scaling difficulties in such a construction project, and that our current understanding of industrial automation is insufficient to build the self-maintaining systems needed for the sphere's upkeep.
Dyson spheres and SETI
In Dyson's original paper, he speculated that sufficiently advanced extraterrestrial civilizations would likely follow a similar power consumption pattern as humans, and would eventually build their own "sphere of collectors". Constructing such a system would make such a civilization a Type II Kardashev civilization. Since such a system would (at least partly) block the normal emissions of a star, and radiate blackbody radiation (most probably with a strong infrared component) rather than the emission spectrum of a stellar atmosphere, he reasoned that it may be possible to detect advanced civilizations by examining the light from stars, looking for such atypical spectra. There have been attempts by SETI to search for Dyson spheres, and as of 2005 Fermilab has an ongoing survey for such spectra by analyzing data from the Infrared Astronomical Satellite (IRAS).
There are several variants on Dyson's original concept that have been proposed over the years, which differ based on their composition and method of construction.
The variant closest to Dyson's original conception is the "Dyson swarm". It consists of a large number of independent constructs (usually solar power satellites and space habitats) orbiting in a dense formation around the star. These constructs could range widely in individual size and design, and could be constructed over a long period of time, making the construction of such a swarm an incremental process.
Such a swarm is not without drawbacks. The nature of orbital mechanics would make the "orbital formation" of such a swarm extremely complex. The simplest such pattern is the Dyson ring in which all such structures share the same orbit, although this pattern would intercept very little of the star's output. More complex orbital patterns add more rings, offset the "axis of rotation" of the rings' orbits with regards to one another, change the eccentricity of the orbits, and add rings at varying distances from the central star. More complex patterns with more rings would intercept more of the star's output, but would result in some constructs eclipsing others periodically when their orbits overlap. There is a definite trade-off between the complexity of the orbital formation and the efficiency of the sphere collecting the star's output. That is, as the number of units in the swarm increases, the increase in collected energy slows because the amount gathered per unit decreases. Because of this, there would come a point when the cost of building another unit was no longer exceeded by the amount of energy the unit would gather. Thus, the Dyson swarm is unlikely ever to completely engulf the star so it may be worthwhile to search instead for large bodies whose outputs range from infrared to the full spectrum. Another drawback would be a loss in stability as more rings increase the chance of orbital perturbations.
The variant of the Dyson sphere most often depicted in fiction is the "Dyson shell": a uniform solid shell of matter around the star. Such a structure would completely conceal the emissions of the central star, and would intercept 100% of the star's energy output. Such a structure would also provide an immense surface which many envision being used for habitation, if the surface could be made habitable.
A third type of Dyson sphere is the "Dyson bubble". It would be similar to a Dyson swarm, composed of many independent constructs (usually solar power satellites and space habitats) and likewise could be constructed incrementally.
Unlike the Dyson swarm, the constructs making it up are not in orbit around the star, but would be statites — satellites suspended by use of enormous light sails using radiation pressure to counteract the star's pull of gravity. Such constructs would not be in danger of collision or of eclipsing one another; they would be totally stationary with regard to the star, and independent of one another. As the ratio of radiation pressure and the force of gravity from a star are constant regardless of the distance (provided the statite has an unobstructed line-of-sight to the surface of its star), such statites could also vary their distance from their central star.
Another possibility is the "Dyson net", a web of cables strung about the star which could have power or heat collection units strung between the cables. The Dyson net reduces to a special case of Dyson shell or bubble, however, depending on how the cables are supported against the sun's gravity.
A Niven ring, could be considered a particular kind of Dyson sphere could perhaps be described as a slice of a Dyson Sphere (taken through its equator), spun for artificial gravity, and used mainly for habitation as opposed to energy collection.
Stellar engines are a class of hypothetical megastructures, whose purpose is to extract useful energy from a star, sometimes for specific purposes. For example, Matrioshka brains extract energy for purposes of computation; Shkadov thrusters extract energy for purposes of propulsion. Some of the proposed stellar engine designs are based on the Dyson sphere. A third kind of shell would be very thin and non-rotating, held up by the radiation pressure of the sun. It would consist of statites – a solar sail that hovers without orbiting above the sun. Essentially it is a "dyson bubble", where reflecting sails reflect light onto collectors for use in external habitats. Its mass would be very small, on the order of a small moon or large asteroid.