Dyson sphere

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File:Dysonspherediagram.gif
Diagram of an idealized Dyson shell of 1 AU radius

A Dyson sphere is a hypothetical megastructure first described in 1960 by the physicist Freeman Dyson. The idea was established in a short paper published in the journal Science, entitled "Search for Artificial Stellar Sources of Infra-Red Radiation". Simply put, a Dyson sphere is a structure of enormous scale which is designed to capture the entire energy output of a star. A Dyson sphere is far beyond the capabilities of present-day technology, and would require a technological step change in order to construct.

It is often interpreted as an artificial hollow sphere of matter around a star (as the diagram depicts). This perception, which is often used in science fiction, television shows such as Star Trek, games like Freelancer and so forth, is strictly speaking a misinterpretation of Dyson's original concept, which envisaged a loose sphere made of smaller objects (Dyson used the term "shell", not "sphere", in his original paper). Dyson also clearly stated in his comments to the letters resulting from his original article, "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." The term Dyson swarm has recently come into use as a way to make this distinction clearer. The primary motivation Dyson proposed for constructing a Dyson swarm is to allow a civilization to capture nearly all of a star's energy - by contrast, less than one part per billion of the Sun's energy even hits Earth.

Although Dyson is credited with being the first to formalize and popularize the concept of the Dyson sphere, Dyson himself got the idea in 1945 from a science fiction novel titled Star Maker by Olaf Stapledon. The original proposal by Dyson did not go into much detail about how a Dyson sphere would be constructed, focusing instead on the more fundamental issue of how an advanced civilization could expand its energy production to the maximum possible for a given solar system. Such a civilization would be classified as a Type II civilization under the Kardashev classification scheme developed by the astronomer Nikolai Kardashev. It is useful to understand that all KT-II civilizations (i.e. all civilizations which may have constructed a Dyson swarm) are unlikely to emit light in the visibile spectrum (because they are harvesting it for energy production). So attempts at SETI involving pointing receivers at visibile stars are inherently targeting civilizations which have not reached the KT-II level.

Properties

A star contained within a Dyson sphere would not be directly visible to the outside universe, but the Dyson sphere itself would radiate an equivalent amount of energy in the form of infrared light due to solar heating from within. In addition, since Dyson spheres are composed of solid matter instead of heated gas, the emission spectrum of the Dyson sphere would more closely resemble a black body spectrum than the typical emission spectrum of a star, which has absorption features introduced in the stellar atmosphere. Dyson proposed that astronomers search for such giant anomalous "stars" in order to detect advanced alien civilizations, but none have been recorded. Attempts to detect Dyson spheres using the IRAS (Infrared Astronomical Satellite) sky survey data are currently underway.

However, if the creators of a Dyson sphere wanted to increase the temperature gradient (and thereby increase the efficiency of the energy gathering process), they could reflect the light from the inner surface of the sphere onto certain areas on the surface of the sphere. The concentration of light would be restricted by the maximum allowable temperature of the materials involved in the energy conversion process. As a result the radiation toward the outside world not have to be omnidirectional, so Dyson spheres may well be invisible.

Shell-type Dyson spheres are neutrally stable, having no apparent overall gravitational attraction toward the enclosed star (in contrast to a ringworld, which is much more unstable). Thus the sphere would only need propulsion to counteract any unusual forces moving it out of place relative to its star.

Varieties

There are several basic varieties of hypothetical design for a Dyson sphere.

Swarm

The most realistic variety, and the closest to Dyson's original conception, is the "Dyson swarm". It consists of a large number of independent solar collectors orbiting in a dense formation around the star. The solar collectors could range widely in individual size and design, and possibly include space habitats for biological creatures to live in, but as a group they would collectively intercept almost all of the star's total light output. A number of different orbital patterns for the collectors have been proposed, each with different benefits and drawbacks; whatever pattern is chosen, some solar collectors will spend part of their orbits in the shadows of other solar collectors, reducing the sphere's efficiency somewhat. Since the collectors operate largely independently of each other, a Dyson swarm can be constructed incrementally over a long period of time and provide useful output throughout. The ultimate extension of Dyson's concepts involves a number of nested levels of swarms constructed from different materials and operating at widely different temperatures. It was invented by Robert Bradbury and is known as a Matrioshka Brain. Most of the "life" within Matrioshka Brains is in the form of uploaded human intelligences or artificial intelligences.

Shell

A second type of Dyson sphere is a uniform solid shell around the star, sometimes called a "Dyson shell", often with a layer of atmosphere and soil on the inner surface to provide an astronomically large living space for organic life forms. This form of Dyson sphere is much more popular in science fiction, but is not physically feasible for a variety of reasons. One is the immense strength that would be required to hold such an enormous structure together.

Another is the fact that the net gravitational force exerted by a uniform hollow sphere on anything inside is zero; there would be nothing holding the atmosphere to the sphere's surface, and it would fall into the star. In fact, whatever the mass distribution, a spherically symmetric gravitational field is never outward, see also the divergence theorem applied to gravity. This is a characteristic of the fundamental inverse square falloff nature of gravitational force combined with the fact that this force is always attractive. Thus it would still apply even if hypothetical "gravity generators" were used (devices that can produce the gravitational field of a non-existent mass-distribution), unless they could simulate negative masses.

Though the sphere could be rotated to produce centrifugal pseudogravity around its equator, this would require the shell material to be even stronger.

An alternative design would be to put the atmosphere on the outside of the sphere. Though this area would not be directly lit by the interior star, the gravity generated by the star would keep the atmosphere (and the inhabitants) from drifting away.

Yet another design possibility would be to use two concentric spheres, with the atmosphere trapped between them. While keeping an atmosphere secure, this would have the drawbacks of difficulty in keeping the spheres separated, and that the slight net gravitational pull would tend to make "down" be in the direction of the illuminated inner sphere.

Construction

A Dyson sphere is a neutrally stable design, but during construction — before the sphere's domes are closed — it would act as an unstable ringworld and would need to be kept in place with thrusters to keep it centered around the star. One possible way to avoid this is to construct a Hoberman sphere with a far greater radius around the star. Because of the increased radius the sphere would have much less effect from the gravity of the star. Once the Hoberman sphere has been completed it can be folded (collapsed) onto the star until the shell of the sphere is within the habitable zone. From then on it is a matter of filling in the gaps of the Hoberman sphere until it forms a Dyson sphere.

While no known or theorized material is strong enough to form a rigid, static sphere around a star like this, it may be possible to support a Dyson shell by dynamic means similar to those used in a space fountain. Masses travelling at significantly greater than orbital velocity could be contained in enormous circular tracks around the sphere's circumference in various orientations, their outward centrifugal force counterbalancing the inward weight of the shell. The velocities required for most shell designs would be large, however; orbital velocity at 1 AU from the Sun is roughly 30 km/s, so a mass would need to be travelling at 300 km/s to support nine times its weight in shell structure at this distance.

Bubble

A third type of Dyson sphere called a "Dyson bubble" is occasionally considered, composed of beans that hover motionless relative to the englobed sun using light pressure; this form of Dyson sphere has such low mass requirements that it could potentially be built from the material contained in a single small moon or large asteroid. However, a Dyson bubble has few practical applications (harvesting energy would be difficult due to its low mass and dependence on high reflectivity) and so it is not often discussed.

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, a toroidal structure encircling a star, could be considered a particular kind of Dyson sphere. However, Larry Niven, who first developed the ringworld 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 used mainly for habitation as opposed to energy collection.

Dyson spheres in fiction

Most fiction features the shell variant; unless otherwise noted, that is the type of Dyson sphere in the instances below.

Television

Written

Comics

  • The sci-fi web-comic Schlock Mercenary by Howard Tayler features an alien race known as the Gatekeepers, who live in habitats hanging from the interior surfaces of Dyson Bubbles they call Buuthandi (an abbreviation of the Gatekeeper phrase "Buut go buut-buut nnaa-nnaa cho handi", which translates to "This was expensive to build", or more literally as as "Expensive and expensive-expensive [expletive] we built").[1] They use these Dyson bubbles to collect power with which to operate a galaxy-wide network of transportation wormholes.
  • The comic BLAME! by Tsutomu Nihei. In the artbook, BLAME! And So On it is revealed by the artist that the "city" the characters keep referring to and are currently wandering in is actually a Dyson Sphere extending to the orbit of Jupiter.
  • In the Marvel comic series New Mutants (original series) the rock star Lila Cheney, a mutant with the power to teleport across interstellar distances, had a home on an abandoned Dyson Sphere.

Games

Websites

  • The collaborative worldbuilding website Orion's Arm describes several fictional planetary systems which incorporate Dyson Swarms, which orbit the local star and collect energy to use for life support, industry and computation.[2]

References

See also

External links

de:Dyson-Sphäre fi:Dysonin kehä es:Esfera de Dyson fr:Sphère de Dyson it:Sfera di Dyson ja:ダイソン球 he:כדור דייסון pl:Sfera Dysona