Dyson sphere

Diagram of an idealized Dyson shell — a variant on Dyson's original concept — 1 AU in radius

A Dyson sphere (or "shell" as it appeared in the original paper) is a hypothetical megastructure originally described by physicist Freeman Dyson in his 1959 paper "Search for Artificial Stellar Sources of Infra-Red Radiation", published in the journal Science. It is a construct of enormous scale which is designed to englobe a star and capture its entire energy output. Building a Dyson sphere is far beyond present-day industrial capabilities, though some proposed designs do not require technology much in advance of our own.

The concept of the Dyson sphere was the result of a thought experiment by 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 propsed 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. Dyson speculated that sufficiently advanced extraterrestruial civilizations would likely follow the same pattern and build their own "sphere of collectors" (the construction of such a system would make such a civilization Type II on the Kardashev scale). Since such a system would (at least partly) block the normal emissions of a star, and radiate blackbody radiation 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 specta [1] by analyzing data from the Infrared Astronomical Satellite (IRAS).

Although Dyson is credited with being the first to formalize the concept of the Dyson sphere, Dyson himself was inspired by the 1945 science fiction novel Star Maker, by Olaf Stapledon.

Dyson spheres in fiction

Main article: Dyson spheres in fiction

The Dyson sphere concept is often interpreted as an artificial hollow sphere of matter around a star (see diagram). This perception — often depicted in science fiction (see Dyson spheres in fiction) — is a misinterpretation of Dyson's original concept, which envisaged a loose sphere made of smaller objects: Dyson responsed to letters resulting from his original article by saying, "A solid shell or ring surrounding a star is mechanically impossible. The form of 'biosphere' which I envisaged consists of a loose collection or swarm of objects traveling on independent orbits around the star." More recently, the terms Dyson swarm and Dyson shell have come into use to make the distinction between Dyson's original concept and the popularized depctiction of a solid shell.

Variants

There are several variants on Dyson's orginal concept that have been proposed over the years, which differ based on their composition and method of construction.

Dyson swarm

The variant closest to Dyson's original conception, is the "Dyson swarm". It consists of a large number of independent contructs (usually solar collectors 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 incermental process.

Such a swarm is not without it's 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 definate trade off between the complexity of the orbital formation — and thus it's succeptibility to gravitational perturbations — and the percentage of the star's output that the swarm intercepts.

Dyson shell

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 structure would also provide an immense surface which many envision being used for habitation, if the surface could be made habitable.

There are several serious theoretical difficulties with the solid shell variant of the Dyson sphere.

Such a shell would have have no net gravitational interaction with its englobed sun (see the divergence theorem applied to gravity). Such structures would need either some form of propulsion to counteract any "drift" moving it out of place relative to its star, or some way to repel the surface of the sphere away from the star.
For the same reason, such a shell would have have no net gravitational interaction with anything else inside it. Atmosphere placed on on the inner surface to make it habitable, and any inhabitants, would not be attracted to the sphere's surface and would simply fall into the star. It has been proposed that atmosphere could be contained betwen a "main" sphere and an internal transparent sphere, though this could lead to difficuties in keeping the environment's contents from falling to the transparent sphere's surface. It has also been suggested that the atmosphere could be placed on the outside of the sphere, where it would be held in place by the star's gravity. Some form of illumination would have to be devised in this case as the star's light is otherwise completely hidden from the habitable surface.
The tensile strength or the material forming the sphere would have be be immense. No known or theorized material is strong enough to form a rigid, static sphere around a star [2]. It has been proposed by Paul Birch [3] (in relation to smaller "Supra-Jupiter" constructed around a large planet rather than a star) that it may be possible to support a Dyson shell by dynamic means similar to those used in a space fountain. Masses travelling in circular tracks on the inside of the sphere, at velocities significantly greater than orbital velocity would press outwards due to centrifugal force. For a Dyson shell of 1AU radius around a star with the same mass as the Sun, mass travelling ten times orbital velocity (300 km/s) would support nine times its own mass in additional shell structure. The arrangement of such tracks suffers from the same difficulties as arranging the orbits of a Dyson swarm, and it is unclear how much energy would be consumed ensuring the velocity of the masses was maintained.

Dyson bubble

A third type of Dyson sphere is the "Dyson bubble". It would be similar to a Dyson swarm, composed of many independant contructs — probably solar collectors 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 — suspended by use of enourmous light sails using radiation pressure to counteract the sun's pull of gravity. Such constructs would not be in danger of collision or of eclipsing one another; they would be totally stationary with regards to the star, and independant of one another. As the ratio of radiation pressure and the force of gravity from the sun are constant regardless of the distance from the sun (provided the statite has an unobstructed line-of-site to the surface of it's star)[4], such statites could also vary their distance from their central star.

The practicality of this approach is questionable with modern material science, but cannot yet be ruled out. A statite deployed around our own sun would have to have an overall density of 0.78 grams per square meter of sail[5]. In comparison, new carbon-fiber sail material has a density — without payload — of 3g/m². There has been some speculation about the creation of ultra light nanomesh sail materials created through molecular nanotechnology whose density would be below 0.1g/m². If such materials are feasible, and the average sail density with rigging might be kept to 0.3g/m² (a "spin stabilized" light sail requires minimal additional mass in rigging), a Dyson bubble could be possible. If such a sail could be constructed at this areal density, a space habitat the size of the L5 Society's proposed O'Neill cylinder (500 km², with room for over 1 million inhabitants) could be supported by a circular light sail 3,000 km in diameter, and sail and habitat together would mass 5.4 x 10⁹kg. For comparison, this is just slightly smaller that the diameter of Jupiter's moon Europa (although the sail is a flat disc not a sphere), or the distance betweenn San Francisco and Kansas City. Such a structure would, however, masse quite a lot less than many asteriods. While the construction of such a massive inhabitable statite would be a gigantic undertaking, and the required material science behind it as-of-yet uncertain, it's technical challenges are slight compared to other engineering feats and required materials proposed in other Dyson sphere variants.

Other types

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, like the one used in the book Star Trek: Voyager - The Final Fury. 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.
The ringworld, or Niven ring, could be considered a particular kind of Dyson sphere. Larry Niven, who first developed the concept, described it as "an intermediate step between Dyson Spheres and planets". The ringworld, or "Niven ring", 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.

References

Analog, March 1974- Bigger than Worlds, Larry Niven.
Science, 1959, "Search for Artificial Stellar Sources of Infra-Red Radiation", Freeman Dyson.

External links