Dyson sphere
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Since then, other variant designs involving building an artificial structure or series of structures to encompass a star have been proposed in exploratory engineering or described in science fiction under the name "Dyson sphere". These later proposals have not been limited to solar-power stations. Many involve habitation or industrial elements. Most fictional depictions describe a solid shell of matter enclosing a star, which is considered the least plausible variant of the idea (see below). In May 2013, at the Starship Century Symposium in San Diego, Dyson repeated his comments that he wished the concept had not been named after him.[1]
Origin of concept
The concept of the Dyson sphere was the result of a thought experiment by physicist and mathematician Freeman Dyson, when he theorized that all technological civilizations constantly increased their demand for energy. He reasoned that if our civilization expanded energy demands long enough, there would come a time when it demanded the total energy output of the Sun. He proposed a system of orbiting structures (which he referred to initially as a shell) 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 is credited with being the first to formalize the concept of the Dyson sphere in his 1960 paper "Search for Artificial Stellar Sources of Infra-Red Radiation", published in the journal Science.[2] However, Dyson was not the first to advance this idea. He was inspired by the mention of the concept in the 1937 science fiction novel Star Maker,[3] by Olaf Stapledon, and possibly by the works of J. D. Bernal, Raymond Z. Gallun, and Edgar Rice Burroughs who seem to have explored similar concepts in their work.[4]Feasibility
While such megastructures may be theoretically possible, all plans to build a fixed-in-place Dyson sphere are currently far beyond humanity's engineering capacity. However, parts of the technology, like orbiting satellites and solar sails, have already been developed. Deployment of spacecraft and satellites using photovoltaics might be seen as the first small steps towards building a Dyson swarm.[5] However, the number of craft required to obtain, transmit, and maintain a complete Dyson sphere far exceeds our present-day industrial capabilities.Variants
In fictional accounts, the Dyson-sphere concept is often interpreted as an artificial hollow sphere of matter around a star. This perception is based on a literal interpretation of Dyson's original short paper introducing the concept. In response to letters prompted by this paper, Dyson replied, "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."[6]Dyson swarm
A Dyson ring — the simplest form of the Dyson swarm — to scale. Orbit is 1 AU in radius, collectors are 1.0×107 km in diameter (~25× the Earth–Moon distance), spaced 3 degrees from center to center around the orbital circle.
A relatively simple arrangement of multiple Dyson rings of the type
pictured above, to form a more complex Dyson swarm. Rings' orbital radii
are spaced 1.5×107 km with regard to one another, but average orbital radius is still 1 AU. Rings are rotated 15 degrees relative to one another, around a common axis of rotation.
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. This construction approach has advantages: components could be sized appropriately, and it can be constructed incrementally.[5] Various forms of wireless energy transfer could be used to transfer energy between components and Earth.
Disadvantages: the nature of orbital mechanics would make the arrangement of the orbits of the swarm extremely complex. The simplest such arrangement is the Dyson ring in which all such structures share the same orbit. 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.[7] Another potential problem is the increasing loss of orbital stability when adding more elements increases the probability of orbital perturbations.
As noted below, such a cloud of collectors would alter the light emitted by the star system. However, the disruption compared to a star's overall natural emitted spectrum would most likely be too small to be noticed on Earth.[2]
Dyson bubble
A Dyson bubble: an arrangement of statites
around a star, in a non-orbital pattern. As long as a statite has an
unobstructed line-of-sight to its star, it can hover at any point in
space near its star. This relatively simple arrangement is only one of
an infinite number of possible statite configurations, and is meant as a
contrast for a Dyson swarm only. Statites are pictured as the same size
as the collectors pictured above, and arranged at a uniform 1 AU distance from the star.
A second 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 is constant regardless of the distance (provided the statite has an unobstructed line-of-sight to the surface of its star[8]), 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.[9] To illustrate the low mass of the required materials, consider that the total mass of a bubble of such material 1 AU in radius would be about 2.17×1020 kg, which is about the same mass as the asteroid Pallas.[10]
Such a material is currently beyond humanity's ability to produce. The lightest carbon-fiber light sail material currently produced has a density – without payload – of 3 g/m², or about four times as heavy as would be needed to construct a solar statite.[11] A single sheet of graphene, the two-dimensional form of carbon, has a density of only 0.77 mg per square meter,[12] but has not been fabricated in large sheets and has transparency of 97.7%, making such a single sheet of graphene very ineffective as a solar sail.
However, this could change thanks to the recent creation of ultra light carbon nanotubes meshed through molecular manufacturing techniques whose densities range from 1.3g/m² to 1.4g/m². By the time a civilization is ready to use this technology, the carbon nanotube's manufacturing might be optimised enough for them to have a density lower than the 0.7g/m² mark, and the average sail density with rigging might be kept to 0.3 g/m² (a "spin stabilized" light sail requires minimal additional mass in rigging). 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, massing 3×106 tons – could be supported by a circular light sail 3,000 km in diameter, with a combined sail/habitat mass of 5.4×109 kg.[13] For comparison, this is just slightly smaller than the diameter of Jupiter's moon Europa (although the sail is a flat disc, not a sphere), or the distance between San Francisco and Kansas City. Such a structure would, however, have a mass quite a lot less than many asteroids. While the construction of such a massive inhabitable statite would be a gigantic undertaking, and the required material science behind it is as yet uncertain, its technical challenges are negligible compared to other engineering feats and required materials proposed in other Dyson sphere variants.
In theory, if enough statites were created and deployed around their star, they would compose a non-rigid version of the Dyson shell mentioned below. Such a shell would not suffer from the drawbacks of massive compressive pressure, nor are the mass requirements of such a shell as high as the rigid form. Such a shell would, however, have the same optical and thermal properties as the rigid form, and would be detected by searchers in a similar fashion (see below).
Dyson shell
A cut-away diagram of an idealized Dyson shell, a variant on Dyson's original concept, with a radius of 1 AU.
The variant of the Dyson sphere most often depicted in fiction is the "Dyson shell": a uniform solid shell of matter around the star.[14] Such a structure would completely alter 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 would be used for habitation, if the surface could be made habitable.
A spherical shell Dyson sphere in the Solar System with a radius of one astronomical unit, so that the interior surface would receive the same amount of sunlight as Earth does per unit solid angle, would have a surface area of approximately 28.1 Eha (Exa Hectare), or about 550 million times the surface area of Earth. This would intercept the full 384.6 yottawatts (3.846 × 1026 watts)[15] of the Sun's output; other variant designs would intercept less, but the shell variant represents the maximum possible energy captured for the Solar System at this point of the Sun's evolution.[14] This is approximately 33 trillion times the power consumption of humanity in 1998, which was 12 terawatts.[16]
There are several serious theoretical difficulties with the solid shell variant of the Dyson sphere:
Such a shell would have no net gravitational interaction with its englobed star (see shell theorem), and could drift in relation to the central star. If such movements went uncorrected, they could eventually result in a collision between the sphere and the star—most likely with disastrous results. Such structures would need either some form of propulsion to counteract any drift, or some way to repel the surface of the sphere away from the star.[9]
For the same reason, such a shell would have no net gravitational interaction with anything else inside it. The contents of any biosphere placed on the inner surface of a Dyson shell would not be attracted to the sphere's surface and would simply fall into the star. It has been proposed that a biosphere could be contained between two concentric spheres, placed on the interior of a rotating sphere (in which case, the force of artificial "gravity" is perpendicular to the axis of rotation, causing all matter placed on the interior of the sphere to pool around the equator, effectively rendering the sphere a Niven ring for purposes of habitation, but still fully effective as a radiant-energy collector) or placed on the outside of the sphere where it would be held in place by the star's gravity.[17][18] In such cases, some form of illumination would have to be devised, or the sphere made at least partly transparent, as the star's light would otherwise be completely hidden.[19]
If assuming a radius of one AU, then the compressive strength of the material forming the sphere would have to be immense to prevent implosion due to the star's gravity. Any arbitrarily selected point on the surface of the sphere can be viewed as being under the pressure of the base of a dome 1 AU in height under the Sun's gravity at that distance. Indeed it can be viewed as being at the base of an infinite number of arbitrarily selected domes, but as much of the force from any one arbitrary dome is counteracted by those of another, the net force on that point is immense, but finite. No known or theorized material is strong enough to withstand this pressure, and form a rigid, static sphere around a star.[20] It has been proposed by Paul Birch (in relation to smaller "Supra-Jupiter" constructions 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.[21] Masses travelling in circular tracks on the inside of the sphere, at velocities significantly greater than orbital velocity, would press outwards on magnetic bearings due to centrifugal force. For a Dyson shell of 1-AU radius around a star with the same mass as the Sun, a mass travelling ten times the orbital velocity (297.9 km/s) would support 99 (a=v2/r) times its own mass in additional shell structure.
Also if assuming a radius of one AU, then there may not be sufficient building material in the Solar System to construct a Dyson shell. Anders Sandberg estimates that there is 1.82×1026 kg of easily usable building material in the Solar System, enough for a 1-AU shell with a mass of 600 kg/m²—about 8–20 cm thick on average, depending on the density of the material. This includes the hard-to-access cores of the gas giants; the inner planets alone provide only 11.79×1024 kg, enough for a 1-AU shell with a mass of just 42 kg/m².[10]
The shell would be vulnerable to impacts from interstellar bodies, such as comets, meteoroids, and material in interstellar space that is currently being deflected by the Sun's bow shock. The heliosphere, and any protection it theoretically provides, would cease to exist.
Other types
Dyson Net
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.Bubbleworld
A bubbleworld is an artificial construct that consists of a shell of living space around a sphere of hydrogen gas. The shell contains air, people, houses, furniture, etc. It was invented to answer the question, "What is the largest space colony that can be built?"[22] However, most of the volume is not habitable and there is no power source.Theoretically, any gas giant could be enclosed in a solid shell; at a certain radius the surface gravity would be terrestrial, and energy could be provided by tapping the thermal energy of the planet.[22] This concept is explored peripherally in the novel Accelerando (and the short story Curator which is incorporated into the novel as a chapter) by Charles Stross, in which Saturn is converted into a human-habitable world.
Stellar Engine
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.[23]A black hole could be the power source instead of a star in order to increase energy-to-matter conversion efficiency. A black hole would also be smaller than a star. This would decrease communication distances which would be important for computer-based societies as those described above.[22]
Search for extra-terrestrial intelligence
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.[24]The existence of such a system of collectors would alter the light emitted from the star system. Collectors would absorb and reradiate energy from the star.[2] The wavelength(s) of radiation emitted by the collectors would be determined by the emission spectra of the substances making them up, and the temperature of the collectors. Since it seems most likely that these collectors would be made up of heavy elements not normally found in the emission spectra of their central star–or at least not radiating light at such relatively "low" energies as compared to that which they would be emitting as energetic free nuclei in the stellar atmosphere–there would be atypical wavelengths of light for the star's spectral type in the light spectrum emitted by the star system. If the percentage of the star's output thus filtered or transformed by this absorption and reradiation was significant, it could be detected at interstellar distances.[2]
Given the amount of energy available per square meter at a distance of 1 AU from the Sun, it is possible to calculate that most known substances would be reradiating energy in the infrared part of the electromagnetic spectrum. Thus, a Dyson Sphere, constructed by life forms not dissimilar to humans, who dwelled in proximity to a Sun-like star, made with materials similar to those available to humans, would most likely cause an increase in the amount of infrared radiation in the star system's emitted spectrum. Hence, Dyson selected the title "Search for Artificial Stellar Sources of Infrared Radiation" for his published paper.[2]
SETI has adopted these assumptions in their search, looking for such "infrared heavy" spectra from solar analogs. As of 2005 Fermilab has an ongoing survey for such spectra by analyzing data from the Infrared Astronomical Satellite (IRAS).[25][26] Identifying one of the many infra-red sources as a Dyson Sphere would require improved techniques for discriminating between a Dyson Sphere and natural sources.[27] Fermilab discovered 17 potential "ambiguous" candidates of which four have been named "amusing but still questionable".[28] Other searches also resulted in several candidates, which are however unconfirmed.[29]
