Dyson sphere
A Dyson sphere [ ˈdaɪ̯sn̩ ˌsfɛːrə ], named after Freeman Dyson , is a hypothetical construct that absorbs or redirects the energy of a star in order to be able to use it optimally.
background
The structure of a Dyson sphere was first described by physicist Freeman Dyson in the June 1960 issue of Science magazine . In the article, Dyson discussed the search for advanced extraterrestrial intelligences using infrared sources ; because the energy of the respective central star must also be released again after its full use for the purposes of a civilization (see the law of conservation of energy ). However, after the energy of the short-wave light has been used to reduce the entropy of the system, this would happen in a longer-wave form and therefore in the infrared range.
Dyson's original proposal did not go into the details of the construction of such an object, but focused more on the more fundamental issue of how an advanced civilization could expand its energy production to the maximum achievable for a planetary system . Such a civilization would be classified as Type II on the Kardaschow scale developed by the astronomer Nikolai Kardaschow .
Although Dyson is considered the "inventor" of the Dyson sphere, he claims to have been inspired by similar ideas in Olaf Stapledon's 1937 science fiction novel Star Maker . An even earlier possible suggestion for both Stapledon and Dyson is the Bernal Sphere , first described by John Desmond Bernal in 1929 . Dyson himself later referred to his theory as "joke" (joke).
properties
The star within a Dyson sphere would not be directly visible, but this would itself emit an amount of energy in the form of infrared radiation corresponding to the star's energy output . Dyson suggested that astronomers look for such anomalous "stars" in order to discover advanced alien cultures.
The symmetrical construction around the central star enables a driveless operation of the Dyson sphere, only course corrections are necessary.
Types
There are several types of Dyson "spheres" that have been suggested.
The swarm
From today's point of view, the most realistic form that corresponds most closely to Dyson's original ideas is the Dyson swarm. It consists of a large number of independent solar collectors that orbit the star. They could differ in size and shape and possibly form independent habitats . A variety of suggestions for possible distribution patterns have been made, each with its own advantages and disadvantages. For example, a disk-shaped Dyson swarm makes best use of the kinetic energy of its raw material, which consists primarily of asteroids that orbit near the plane of the ecliptic in roughly the same orbital direction. In any case, some collectors would spend part of their orbit in the shadow of others and thus reduce the efficiency of the swarm somewhat. The ratio of the Earth's orbit radius of 149,600,000 km to the solar diameter of 1,392,700 km is around 107.4. This means that with the radius of the earth's orbit, the umbra of a solar collector assumed to be circular disk-shaped is around 107.4 times as long as the diameter of the solar collector. At a distance of 1074 times the diameter of the solar collector, the maximum shading can therefore only be 1% because, viewed from there, the apparent diameter of the solar collector is 10 times smaller than the apparent diameter of the sun.
The shell
Another shape is the solid shell that completely encloses the star. This variant is very popular in science fiction (as an example, the episode Visit to the old Enterprise of the Star Trek series called Spaceship Enterprise: The Next Century ) and is also often described with an atmosphere on the inside that is formidable Forms habitat for biological organisms. With the physical laws known today, however, such an atmosphere cannot be realized, since a symmetrical, hollow sphere does not have its own gravitational field in its interior and the gravitation of the sun would cause the atmosphere and all moving objects to plunge into the sun. An atmosphere on the outside would be possible, but there would have to be no direct sunlight. In addition, the gravitation of the sun would be only 5.93 · 10 −3 m / s 2 at the earth's orbit radius . Due to the occurrence of enormous tangential forces, a purely static implementation with materials available today (e.g. steel ) is not feasible due to insufficient compressive strength . It is unclear whether the minimum required compressive strength of around 10 MN / mm² can ever be achieved using novel materials (e.g. nanoporous metal foams ). It would also be conceivable to let parts of the bowl rotate around the sun; the resulting centrifugal forces could relieve the shell and reduce the required compressive strength.
About the Star Trek episode mentioned, Dyson said, “Actually it was sort of fun to watch it. It's all nonsense, but it's quite a good piece of cinema. "(For example: That was actually quite entertaining. It's all nonsense, but pretty good cinema. )
The bubble
A third form is the "Dyson bubble", which consists of very little mass and is kept stable by the radiation pressure of the sun and the solar wind . A support frame is not necessary. In the illustration opposite, the central star can be seen for better understanding. In fact, the material of the bubble would absorb most of the visible light for the purpose of generating energy and thus cover the star.
Calculation of the mass supported by the radiation pressure
The radiation pressure depends on the absorbed or radiated power per area. The wavelength of the radiation does not matter.
For example, at a radius of 149,600,000 km (corresponds to the radius of the earth's orbit) the solar constant is 1367 W / m 2 and the resulting radiation pressure (with absorption) is 4.56 · 10 −6 N / m 2 . The counterweight is the gravitation of the sun with 5.93 · 10 −3 m / s 2 . In order to keep a segment of the bladder in suspension, both opposing forces must cancel each other out. This would be the case for an area of 7.69 · 10 −4 kg / m 2 . The radiation pressure supports the bladder segment against gravity. This mass per area also applies to all other distances to the sun, because the radiation pressure and gravity decrease equally towards the outside (with the reciprocal of the square of the sun distance). Individual objects that float due to the radiation pressure of the sun without orbiting them fast enough are called statites (in contrast to satellites).
For a Dyson bubble with the radius of the earth's orbit, a mass of 2.16 · 10 20 kg results from a total area of 2.81 · 10 23 m 2 . This corresponds roughly to the mass of a larger planetoid .
If the density of the material used were 1 g / cm 3 (about the density of a plastic film), the layer thickness of the Dyson bubble would be only 769 nm. This corresponds to the wavelength of red light near the infrared . The resulting reduced absorption capacity of this thin layer would also reduce the supporting radiation pressure.
Reinforcement of the radiation pressure
.PNG)
The entire outer surface of the bubble emits exactly the same radiation power as the sun generates, in short 1 P Sol . A radiation equilibrium is established. This also applies to a changed spectrum.
In the case of a double-sided black Dyson bubble, this means that its entire inner surface also emits 1 P Sol , because if the layer is thin, the material has the same temperature inside and outside. The pressure effects of the radiation emitted outside and inside cancel each other out.
The radiant power emitted inwards is ultimately absorbed again by the (opposite side of the) bubble itself. The inner surface absorbs 1 P Sol from the sun and an additional 1 P Sol from its own opposite inner surface, i.e. 2 P Sol in total . The radiation pressure is therefore twice as large as with a single solar sail and can therefore support twice as large a mass.
An additional radiation pressure is created by the radiation reflected on the inside. In the end, all the radiation emitted by the sun is absorbed on the inside of the bubble, but in the meantime parts of this radiation can be reflected back and forth a few times. Each reflection increases the total radiation pressure and thus also the supportable mass. With a well-reflective interior coating, for example with aluminum , the radiation density inside the Dyson bladder would take on very high values as a result of multiple reflections. However, this would result in the outer layers of the sun heating up and expanding.
The ring
The ring surrounds a star, e.g. B. with a radius of about one astronomical unit . The ring thus represents an incomplete shell. Because of the enormous tangential forces, a realistic construction is only possible with a balance between centrifugal force and gravitational force, which means that weightlessness prevails on the surface of the ring . At the edge of the ring world, the centrifugal force and gravity of the sun do not point exactly in the opposite direction, resulting in a force that tries to reduce the width of the ring world. Assuming a compressive strength of 100 N / mm 2 and a density of 1 g / cm 3 (this ratio of compressive strength and density is already achieved by many materials today), then this ring world can be 4,500,000 km wide, which is 3% of the Corresponds to the radius of the earth's orbit. At the edge of the ring world, an acceleration of 9 · 10 −5 m / s 2 would act in the direction of reducing the width of the ring world, which corresponds to 1.5% of the sun's gravity at the Earth's orbit radius. If the ring world were cylindrical, then the centrifugal force at its edge would be greater than the gravitation of the sun, which is dependent on 1 / radius 2 . If the ring world were a section of a spherical surface, then the gravitation of the sun would be greater at its edge than the centrifugal force, which depends linearly on the radius. The optimal shape of the ring world is between these two shapes, the direction of the compressive force at each location being parallel to the surface of the ring world, so that no bending moment occurs. An example from the science fiction is the Ringworld by Larry Niven , which, however, rotates much faster to generate artificial gravity by centrifugal force, and is only 1.6 million km wide.
Matryoshka brain
A Matrjoschka -Brain (engl. Matrioshka brain ), in turn, is an onion-like accumulation of Dyson spheres whose objective is not to maximize the habitable surface, but maximum energy yield, with which then a huge computer is operated. The innermost sphere would be placed as close to the star as possible and the outermost as far outside as it is still possible to generate energy from the temperature difference between the next inner space and the empty space.
The concept was designed by computer scientist Robert Bradbury in the late 1990s. In science fiction literature it was made famous by Charles Stross in his novel Accelerando .
observation
The observation of the 1450 light years distant star KIC 8462852 revealed irregular drops in brightness in the years 2009, 2011 and 2013. In extreme cases the star lost up to a fifth of its luminosity. In addition to several other explanations, it is speculated that the star could be surrounded by a Dyson swarm that could partially absorb the luminosity of the star. However, new data revealed that the cause of the darkening is extremely fine dust. This absorbs the light of different spectral ranges to different degrees. That would not be the case with an artificial construct or with planets.
literature
- Freeman J. Dyson: Search for Artificial Stellar Sources of Infrared Radiation . In: Science . tape 131 , no. 3414 , May 3, 1960, pp. 1667–1668 , doi : 10.1126 / science.131.3414.1667 .
- Richard A. Carrigan: IRAS-based whole-sky upper limit on Dyson Spheres . In: The Astrophysical Journal . tape 698 , no. 2 , 2009, p. 2075-2086 , doi : 10.1088 / 0004-637X / 698/2/2075 , arxiv : 0811.2376 .
Web links
- Freeman J. Dyson and Richard Carrigan (2009) Dyson sphere . Scholarpedia , 4 (5): 6647
- Dyson Sphere in the Star Trek Wiki Memory Alpha
- www.heise.de: A brief future history of Freeman Dyson's technology
- Dyson Spheres around White Dwarfs arxiv.org
Individual evidence
- ^ Freeman J. Dyson: Search for Artificial Stellar Sources of Infrared Radiation . In: Science . tape 131 , no. 3414 , May 3, 1960, pp. 1667–1668 , doi : 10.1126 / science.131.3414.1667 .
- ↑ Dyson FAQ: What Dyson First?
- ↑ a b Video interview with Freeman Dyson
- ^ Robert J. Bradbury: Matrioshka Brains . August 16, 2004 ( online ( memento of September 18, 2008 in the Internet Archive ) [accessed February 25, 2007]). Matrioshka Brains ( Memento of the original from September 18, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ KIC 8462852: Enigmatic Star Arouses Alien Fantasy - Süddeutsche Zeitung , October 15, 2015
- ↑ Martin Scheufens: Enigmatic Star: Are Aliens Working on the Energy Transition ? - Spiegel online , October 16, 2015
