Kardashev scale

From Wikipedia, the free encyclopedia

The Kardashev scale is a method of measuring a civilization's level of technological advancement, based on the amount of energy a civilization is able to use for communication, proposed by Russian astrophysicist Nikolai Kardashev. The scale has three designated categories:

• A Type I civilization—also called a planetary civilization—can use and store all of the energy which reaches its planet from its parent star.
• A Type II civilization—also called a stellar civilization—can harness the total energy of its planet's parent star (the most popular hypothetical concept being the Dyson sphere—a device which would encompass the entire star and transfer its energy to the planet(s)).
• A Type III civilization—also called a galactic civilization—can control energy on the scale of its entire host galaxy.

The scale is hypothetical, and regards energy consumption on a cosmic scale. It was proposed in 1964 by the Soviet astronomer Nikolai Kardashev. Various extensions of the scale have since been proposed, including a wider range of power levels (types 0, IV and V) and the use of metrics other than pure power.

Definition

In 1964, Kardashev defined three levels of civilizations, based on the order of magnitude of power available to them:

Type I

Technological level of a civilization that can harness all the energy that falls on a planet from its parent star (for Earth-Sun system, this value is close to 7x10¹⁷ watts), which is more than five orders of magnitude higher than the amount presently attained on earth, with energy consumption at ≈4×10¹⁹ erg/sec (4 × 10¹² watts).^[1] The astronomer Guillermo A. Lemarchand stated this as a level near contemporary terrestrial civilization with an energy capability equivalent to the solar insolation on Earth, between 10¹⁶ and 10¹⁷ watts.^[3]

Type II

A civilization capable of harnessing the energy radiated by its own star—for example, the stage of successful construction of a Dyson sphere—with energy consumption at ≈4×10³³ erg/sec.^[1] Lemarchand stated this as a civilization capable of utilizing and channeling the entire radiation output of its star. The energy utilization would then be comparable to the luminosity of our Sun, about 4×10³³ erg/sec (4×10²⁶ watts).^[3]

Type III

A civilization in possession of energy on the scale of its own galaxy, with energy consumption at ≈4×10⁴⁴ erg/sec.^[1] Lemarchand stated this as a civilization with access to the power comparable to the luminosity of the entire Milky Way galaxy, about 4×10⁴⁴ erg/sec (4×10³⁷ watts).^[3]

Energy consumption estimated in three types of

civilizations defined by Kardashev scale

Current status of human civilization

Total World, Annual Primary Energy Consumption.

 

According to the astronomer Carl Sagan, humanity is going through a phase of technical adolescence, "typical of a civilization about to integrate the type I Kardashev scale."

 

At the current time, Humans have not yet reached Type 1 civilization. Physicist and futurist Michio Kaku suggested that humans may attain Type I status in 100–200 years, Type II status in a few thousand years, and Type III status in 100,000 to a million years.^[4]
Carl Sagan suggested defining intermediate values (not considered in Kardashev's original scale) by interpolating and extrapolating the values given above for types I (10¹⁶ W), II (10²⁶ W) and III (10³⁶ W), which would produce the formula

${\displaystyle K={\frac {\log _{10}P-6}{10}}}$,

where value K is a civilization's Kardashev rating and P is the power it uses, in watts. Using this extrapolation, a "Type 0" civilization, not defined by Kardashev, would control about 1 MW of power, and humanity's civilization type as of 1973 was about 0.7 (apparently using 10 terawatt (TW) as the value for 1970s humanity).^[5]

In 2012, total world energy consumption was 553 exajoules (553×10¹⁸ J=153,611 TWh),^[6] equivalent to an average power consumption of 17.54 TW (or 0.7244 on Sagan's Kardashev scale).

Observational evidence

In 2015, a study of galactic mid-infrared emissions came to the conclusion that "Kardashev Type-III civilizations are either very rare or do not exist in the local Universe".^[7]

On October 14, 2015, the detection of an unusual light curve for star KIC 8462852 raised speculation that a Dyson Sphere (Type II civilization) may have been discovered. The SETI Institute's initial radio reconnaissance of KIC 8462852, however, found no evidence of technology-related radio signals from the star.

In 2016, Paul Glister, author of the Centauri Dreams website, described a signal apparently from the star HD 164595 as requiring the power of a Type I or Type II civilization, if produced by extraterrestrial lifeform.^[16] However, in August 2016 it was discovered that the signal's origin was most likely a military satellite orbiting the Earth.^[17]

Energy development

Type I civilization methods

• Large-scale application of fusion power. According to mass–energy equivalence, Type I implies the conversion of about 2 kg of matter to energy per second. An equivalent energy release could theoretically be achieved by fusing approximately 280 kg of hydrogen into helium per second,^[18] a rate roughly equivalent to 8.9×10⁹ kg/year. A cubic km of water contains about 10¹¹ kg of hydrogen, and the Earth's oceans contain about 1.3×10⁹ cubic km of water, meaning that humans on Earth could sustain this rate of consumption over geological time-scales, in terms of available hydrogen.
  
• Antimatter in large quantities would have a mechanism to produce power on a scale several magnitudes above our current level of technology.^{[citation needed]} In antimatter-matter collisions, the entire rest mass of the particles is converted to radiant energy. Their energy density (energy released per mass) is about four orders of magnitude greater than that from using nuclear fission, and about two orders of magnitude greater than the best possible yield from fusion.^[19] The reaction of 1 kg of anti-matter with 1 kg of matter would produce 1.8×10¹⁷ J (180 petajoules) of energy.^[20] Although antimatter is sometimes proposed as a source of energy, this does not appear feasible. Artificially producing antimatter—according to current understanding of the laws of physics—involves first converting energy into mass, which yields no net energy. Artificially created antimatter is only usable as a medium of energy storage, not as an energy source, unless future technological developments (contrary to the conservation of the baryon number, such as a CP violation in favour of antimatter) allow the conversion of ordinary matter into anti-matter. Theoretically, humans may in the future have the capability to cultivate and harvest a number of naturally occurring sources of antimatter.
  
• Renewable energy through converting sunlight into electricity—either by using solar cells and concentrating solar power or indirectly through wind and hydroelectric power. There is no known way for human civilization to use the equivalent of the Earth's total absorbed solar energy without completely coating the surface with human-made structures, which is not feasible with current technology. However, if a civilization constructed very large space-based solar power satellites, Type I power levels might become achievable—these could convert sunlight to microwave power and beam that to collectors on Earth.

Figure of a Dyson swarm surrounding a star

Type II civilization methods

• Type II civilizations might use the same techniques employed by a Type I civilization, but applied to a large number of planets in a large number of planetary systems.
  
• A Dyson sphere or Dyson swarm and similar constructs are hypothetical megastructures originally described by Freeman Dyson as a system of orbiting solar power satellites meant to enclose a star completely and capture most or all of its energy output.
  
• Perhaps a more exotic means to generate usable energy would be to feed a stellar mass into a black hole, and collect photons emitted by the accretion disc.^[25][26] Less exotic would be simply to capture photons already escaping from the accretion disc, reducing a black hole's angular momentum; this is known as the Penrose process.
  
• Star lifting is a process where an advanced civilization could remove a substantial portion of a star's matter in a controlled manner for other uses.
  
• Antimatter is likely to be produced as an industrial byproduct of a number of megascale engineering processes (such as the aforementioned star lifting) and, therefore, could be recycled.
  
• In multiple-star systems of a sufficiently large number of stars, absorbing a small but significant fraction of the output of each individual star.

Type III civilization methods

• Type III civilizations might use the same techniques employed by a Type II civilization, but applied to all possible stars of one or more galaxies individually.
  
• They may also be able to tap into the energy released from the supermassive black holes which are believed to exist at the center of most galaxies.
  
• White holes, if they exist, theoretically could provide large amounts of energy from collecting the matter propelling outwards.
  
• Capturing the energy of gamma-ray bursts is another theoretically possible power source for a highly advanced civilization.
  
• The emissions from quasars can be readily compared to those of small active galaxies and could provide a massive power source if collectable.

Civilization implications

There are many historical examples of human civilization undergoing large-scale transitions, such as the Industrial Revolution. The transition between Kardashev scale levels could potentially represent similarly dramatic periods of social upheaval, since they entail surpassing the hard limits of the resources available in a civilization's existing territory. A common speculation^[27] suggests that the transition from Type 0 to Type I might carry a strong risk of self-destruction since, in some scenarios, there would no longer be room for further expansion on the civilization's home planet, as in a Malthusian catastrophe. Excessive use of energy without adequate disposal of heat, for example, could plausibly make the planet of a civilization approaching Type I unsuitable to the biology of the dominant life-forms and their food sources. If Earth is an example, then sea temperatures in excess of 35 °C (95 °F) would jeopardize marine life and make the cooling of mammals to temperatures suitable for their metabolism difficult if not impossible. Of course, these theoretical speculations may not become problems possibly through the applications of future engineering and technology. Also, by the time a civilization reaches Type I it may have colonized other planets or created O'Neill-type colonies, so that waste heat could be distributed throughout the planetary system.

Extensions to the original scale

Many extensions and modifications to the Kardashev scale have been proposed.

• Types 0, IV, and V Kardashev rating: The most straightforward extension of the scale to even more hypothetical Type IV beings who can control or use the entire universe or Type V who control collections of universes. This would also include Type 0 civilizations, who do not rank on the Kardashev scale. The power output of the visible universe is within a few orders of magnitude of 10⁴⁵ W. Such a civilization approaches or surpasses the limits of speculation based on current scientific understanding, and may not be possible.
  • Zoltán Galántai has argued that such a civilization could not be detected, as its activities would be indistinguishable from the workings of nature (there being nothing to compare them to).^[28]
  • In his book Parallel Worlds, Michio Kaku has discussed a Type IV civilization that could harness "extragalactic" energy sources such as dark energy.
    
• Kardashev alternative rating characteristics: Other proposed changes to the scale use different metrics such as 'mastery' of systems, amount of information used, or progress in control of the very small as opposed to the very large.
  
• Planet mastery (Robert Zubrin): Metrics other than pure power usage have also been proposed. One is 'mastery' of a planet, system or galaxy rather than considering energy alone.
  
• Information mastery (Carl Sagan): Alternatively, Carl Sagan suggested adding another dimension in addition to pure energy usage: the information available to the civilization.
  • He assigned the letter A to represent 10⁶ unique bits of information (less than any recorded human culture) and each successive letter to represent an order of magnitude increase, so that a level Z civilization would have 10³¹ bits.
  • In this classification, 1973 Earth is a 0.7 H civilization, with access to 10¹³ bits of information.
  • Sagan believed that no civilization has yet reached level Z, conjecturing that so much unique information would exceed that of all the intelligent species in a galactic supercluster and observing that the universe is not old enough to exchange information effectively over larger distances.
  • The information and energy axes are not strictly interdependent, so that even a level Z civilization would not need to be Kardashev Type III.
    
• Microdimensional mastery (John Barrow): John D. Barrow, going by the fact that humans have found it more cost-effective to extend any abilities to manipulate their environment over increasingly smaller dimensions rather than increasingly larger ones, reverses the classification downward from Type I-minus to Type Omega-minus:
  • Type I-minus is capable of manipulating objects over the scale of themselves: building structures, mining, joining and breaking solids;
  • Type II-minus is capable of manipulating genes and altering the development of living things, transplanting or replacing parts of themselves, reading and engineering their genetic code;
  • Type III-minus is capable of manipulating molecules and molecular bonds, creating new materials;
  • Type IV-minus is capable of manipulating individual atoms, creating nanotechnologies on the atomic scale and creating complex forms of artificial life;
  • Type V-minus is capable of manipulating the atomic nucleus and engineering the nucleons that compose it;
  • Type VI-minus is capable of manipulating the most elementary particles of matter (quarks and leptons) to create organized complexity among populations of elementary particles; culminating in:
  • Type Omega-minus is capable of manipulating the basic structure of space and time.
    
• According to this scale, humans, having wide expertise in various branches of chemistry and biology, have passed the stage of Type III-minus. Type IV-minus technologies (that have had practical and widespread applications) have been seen in areas like nanotechnology, semiconductors, materials science and genetic engineering, whereas Type V-minus has seen large scale application in the field and subfields of nuclear physics. Type VI-minus has had tentative research in the field of particle physics with particle colliders such as the Large Hadron Collider.
  
• Civilizational range (Robert Zubrin): Robert Zubrin adapts the Kardashev scale to refer to how widespread a civilization is in space, rather than to its energy use.
  • In his definition, a Type I civilization has spread across its planet.
  • A Type II has extensive colonies in its respective stellar system, and
  • A Type III has colonized its galaxy.^[30]

Criticism

It has been argued that, because we cannot understand advanced civilizations, we cannot predict their behavior. Thus, the Kardashev scale may not be relevant or useful for classifying extraterrestrial civilizations. This central argument is found in the book Evolving the Alien: The Science of Extraterrestrial Life.

Dyson sphere

From Wikipedia, the free encyclopedia

 

A Dyson sphere is a hypothetical megastructure that completely encompasses a star and captures a large percentage of its power output. The concept is a thought experiment that attempts to explain how a spacefaring civilization would meet its energy requirements once those requirements exceed what can be generated from the home planet's resources alone. Only a fraction of a star's energy emissions reach the surface of any orbiting planet. Building structures encircling a star would enable a civilization to harvest far more energy.

The first contemporary description of the structure was by Olaf Stapledon in his science fiction novel Star Maker (1937), in which he described "every solar system... surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use."^[1] The concept was later popularized by Freeman Dyson in his 1960 paper "Search for Artificial Stellar Sources of Infrared Radiation".^[2]

Dyson speculated that such structures would be the logical consequence of the escalating energy needs of a technological civilization and would be a necessity for its long-term survival. He proposed that searching for such structures could lead to the detection of advanced, intelligent extraterrestrial life. Different types of Dyson spheres and their energy-harvesting ability would correspond to levels of technological advancement on the Kardashev scale.

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, with many involving habitation or industrial elements. Most fictional depictions describe a solid shell of matter enclosing a star, which is considered^{[by whom?]} the least plausible variant of the idea. 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.^[3]

Origin of concept

Freeman Dyson

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 human 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, on the basis that such a structure could be distinguished by its unusual emission spectrum in comparison to a star. His 1960 paper "Search for Artificial Stellar Sources of Infra-Red Radiation", published in the journal Science, is credited with being the first to formalize the concept of the Dyson sphere.^[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,^[4] 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.^[5]

Feasibility

Although such megastructures may be theoretically possible, all plans to build a fixed-in-place Dyson sphere are currently far beyond humanity's engineering capacity. The number of craft required to obtain, transmit, and maintain a complete Dyson sphere far exceeds present-day industrial capabilities. George Dvorsky has advocated use of self-replicating robots to overcome this limitation in the relatively near term.^[6] Some have suggested that such habitats could be built around white dwarfs^[7] and even pulsars.^[8]

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 some papers, 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."^[9]

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 (10 Gm or ≈25 times 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.^[10] Various forms of wireless energy transfer could be used to transfer energy between swarm components and a planet.

Disadvantages resulting from 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.^[11] Another potential problem is that 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 for Earth-based astronomers to observe.^[2]

Dyson bubble

A Dyson bubble: an arrangement of statites around a star, in a non-orbital pattern. As long as a satellite 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 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. Because the ratio of radiation pressure to 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^[12]), 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 100% reflective statite deployed around the Sun would have an overall density of 0.78 grams per square meter of sail.^[13] 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.^[14] Another illustration: Regular printing paper has a density of around 80 g/m².

Such a material has not yet been produced in the form of a working light sail. 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.^[15]

A single sheet of graphene, the two-dimensional form of carbon, has a density of only 0.37 mg per square meter,^[16] making such a single sheet of graphene possibly effective as a solar sail. However, as of 2015 graphene has not been fabricated in large sheets, and it has a relatively high rate of radiation absorption, about 2.3% (i.e., still about 97.7% will be transmitted).^[17][18] For frequencies in the upper GHz and lower THz range, the absorption rate is as high as 50–100% due to voltage bias and/or doping.^[17][18]

Ultra-light carbon nanotubes meshed through molecular manufacturing techniques have densities between 1.3 g/m² to 1.4 g/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 necessary 0.7 g/m², 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.^[19] 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. Although the construction of such a massive habitable statite would be a gigantic undertaking, and the required material science behind it is early stage, there are other engineering feats and required materials proposed in other Dyson sphere variants.

In theory, if enough satellites 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.^[20] 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 that 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 2.8×10¹⁷ km² (1.1×10¹⁷ sq mi), or about 550 million times the surface area of Earth. This would intercept the full 384.6 yottawatts (3.846 × 10²⁶ watts)^[21] of the Sun's output. Non-shell 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.^[20] This is approximately 33 trillion times the power consumption of humanity in 1998, which was 12 terawatts.^[22]

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.^[13]

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.^[23][24] In such cases, some form of illumination would have to be devised, or the sphere made at least partly transparent, because the star's light would otherwise be completely hidden.^[25]

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 because 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.^[26] 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.^[27] 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.^[14]

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 that 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. The idea was conceived to answer the question, "What is the largest space colony that can be built?"^[28] 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.^[28] 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.^[29]

A black hole could be the power source instead of a star in order to increase the matter-to-energy conversion efficiency. A black hole would also be smaller than a star. This would decrease communication distances that would be important for computer-based societies as those described above.^[28]

Search for megastructures

In Dyson's original paper, he speculated that sufficiently advanced extraterrestrial civilizations would likely follow a similar power-consumption pattern to that of humans, and would eventually build their own sphere of collectors. Constructing such a system would make such a civilization a Type II Kardashev civilization.^[30]

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. Because 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 compared to what 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).^{[31][full citation needed][32]} Identifying one of the many infrared sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources.^[33] Fermilab discovered 17 potential "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which are, however, unconfirmed.

Postulated sightings

On 14 October 2015, the realization of a strange pattern of light from star KIC 8462852, nicknamed "Tabby's Star" after Tabetha S. Boyajian — the lead researcher who discovered the irregular light fluctuation— captured by the Kepler Space Telescope, raised speculation that a Dyson sphere may have been discovered.^[38][39] In February 2016, Boyajian gave a TED talk where she explained the story of how her research on the star quickly took a turn into the mysterious. However, she was skeptical and in the talk she reminded everyone that skepticism is the best policy whenever delving into alien territory. Her exact quote is as follows:

"Extraordinary claims require extraordinary evidence, and it is my job, my responsibility, as an astronomer to remind people that alien hypotheses should always be a last resort."^[40]

Wanting to understand the strange light pattern, Tabetha S. Boyajian put several hypotheses to the test. The first general assumption was an exoplanet transiting (eclipsing) its massive star, but the dips in light lasted between 5 and 80 days and were erratically spaced apart, thus ruling out any kind of an orbit for one celestial object.^[41] A dust cloud was proposed but the star showed no signs of being young so a dust cloud was highly improbable. Lastly, a comet shower was hypothesized. However, as Boyajian pointed out in her TED talk this was also highly improbable. "It would take hundreds of comets to reproduce what we're observing. And these are only the comets that happen to pass between us and the star. And so in reality, we're talking thousands to tens of thousands of comets."^[42]

So after all the natural explanations turned up weak, her team decided to send off their research to SETI (Search for extraterrestrial life) to rule out alien structures. After reviewing the research, the SETI Institute was so intrigued that they decided to study the star themselves and pointed their Allen Telescope Array (ATA) at the star "with hopes of catching a tell-tale signal that might reveal a technological civilization."^[43]

The SETI Institute mentioned that what caught their interest was that "the timing of the present dip (in light) suggests that whatever this material is, it is situated at just the right distance from the star to be in the habitable zone, where we believe life like ours could develop as it has on Earth."^[43]

Being skeptical as Boyajian was, she finally decided to take SETI's approach and allow herself to have a bit of fun in hypothesizing what the light pattern could have been. In her Ted Talk she joked: "Another idea that's one of my personal favorites is that we had just witnessed an interplanetary space battle and the catastrophic destruction of a planet. Now, I admit that this would produce a lot of dust that we don't observe. But if we're already invoking aliens in this explanation, then who is to say they didn't efficiently clean up all this mess for recycling purposes?"^[44] The search for answers to KIC 8462852 is still ongoing.

On August 25, 2016, a similar phenomenon was reported for another stellar object: EPIC 204278916,^[45] a young M-type pre-main-sequence star with a resolved disk. Dimmings of up to 65% for 25 consecutive days (out of 79 total observation) were observed. The variability is highly periodic and attributed to stellar rotation.^[45] The researchers hypothesize that the irregular dimmings are caused by either a warped inner-disk edge or transiting cometary-like objects in either circular or eccentric orbits.^[45]

Fiction

As noted above, the Dyson sphere originated in fiction,^[46][47] and it is a concept that has appeared often in science fiction since then. In fictional accounts, Dyson spheres are most often depicted as a Dyson shell with the gravitational and engineering difficulties of this variant noted above largely ignored.

Megastructure

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Not to be confused with Superstructure or Skyscraper.

For other uses, see Megastructure (disambiguation).

The Great Wall of China is a megastructure. This picture was taken near Beijing in February 2005.

A megastructure is a very large artificial object, although the limits of precisely how large vary considerably. Some apply the term to any especially large or tall building.^[1][2] Some sources define a megastructure as an enormous self-supporting artificial construct. The products of megascale engineering or astroengineering are megastructures.

Most megastructure designs could not be constructed with today's level of industrial technology. This makes their design examples of speculative (or exploratory) engineering. Those that could be constructed easily qualify as megaprojects.

Megastructures are also an architectural concept popularized in the 1960s where a city could be encased in a single building, or a relatively small number of buildings interconnected. Such arcology concepts are popular in science fiction. Megastructures often play a part in the plot or setting of science fiction movies and books, such as Rendezvous with Rama by Arthur C. Clarke.

In 1968, Ralph Wilcoxen defined a megastructure as any structural framework into which rooms, houses, or other small buildings can later be installed, uninstalled, and replaced; and which is capable of "unlimited" extension. This type of framework allows the structure to adapt to the individual wishes of its residents, even as those wishes change with time.^[3]

Other sources define a megastructure as "any development in which residential densities are able to support services and facilities essential for the development to become a self-contained community".^[4]

Many architects have designed such megastructures. Some of the more notable such architects and architectural groups include the Metabolist Movement, Archigram, Cedric Price, Frei Otto, Constant Nieuwenhuys, Yona Friedman, and Buckminster Fuller.^[5]

Existing

There are structures that may be considered megastructures, such as

• The Great Wall of China is a human-built megastructure, a few meters wide and 3,947 miles (6,352 km) in length, about 4,975,318 square yards (4,160,000 m²).^[6]
• The Rice Terraces of the Philippine Cordilleras, a 10,360-square-kilometer (4,000 sq mi) sprawling agricultural landscape carved in the mountains by free tribesmen of Ifugao some 6,000 to 2,000 years ago.^[7]
• Skyscrapers represent our current state-of-the-art in large structure engineering. (See the List of tallest buildings and structures in the world and list of largest buildings in the world.)
• The Large Hadron Collider consists of, among other structures, a ring 27 kilometers in circumference.
• The Expressways of China are the longest expressway system in the world.^{[citation needed]}

Networks of roads or railways, and collections of buildings (cities and associated suburbs), are usually not considered megastructures, despite frequently qualifying based on size. However, an ecumenopolis might qualify.

Proposed

• Atlantropa, a hydroelectric dam to be built across the Strait of Gibraltar, and the lowering of the surface of the Mediterranean Sea by as much as 200 metres.
• Trans-Global Highway, highway systems that would link all six of the inhabited continents on Earth. The highway would network new and existing bridges and tunnels, improving not only ground transportation but also potentially offering a conduit for utility pipelines.
• Cloud nine is Buckminster Fuller's proposal for a tensegrity sphere a mile in radius which would be large enough so that it would float in the sky if heated by only one degree above ambient temperature, creating habitats for mini cities of thousands of people in each "Cloud Nine".

Theoretical

A number of theoretical structures have been proposed which may be considered megastructures.

Stellar scale

A cut-away diagram of an idealized Dyson shell—a variant on Dyson's original concept—1 AU in radius.

Most stellar scale Megastructure proposals are designs to make use of the energy from a sun-like star while possibly still providing gravity or other attributes that would make it attractive for an advanced civilization.

• The Alderson disk is a theoretical structure in the shape of a disk, where the outer radius is equivalent to the orbit of Mars or Jupiter and the thickness is several thousand miles. A civilization could live on either side, held by the gravity of the disk and still receive sunlight from a star bobbing up and down in the middle of the disk.
• A Dyson sphere (also known as a Dyson shell) refers to a structure or mass of orbiting objects that completely surrounds a star to make full use of its solar energy.
• A Matrioshka brain is a collection of multiple Concentric Dyson Spheres which make use of different wavelengths of light.
• A Stellar engine either uses the temperature difference between a star and interstellar space to extract energy or serves as a Shkadov thruster.
• A Shkadov thruster accelerates an entire star through space by selectively reflecting or absorbing light on one side of it.
• Topopolis (also known as Cosmic Spaghetti) is a large tube that rotates to provide artificial gravity.
• A Ringworld (or Niven Ring) is an artificial ring encircling a star, rotating faster than orbital velocity to create artificial gravity on its inner surface. A non-rotating variant is a transparent ring of breathable gas, creating a continuous microgravity environment around the star, as in the eponymous Smoke Ring.

Related structures which might not be classified as individual stellar megastructures, but occur on a similar scale:

• A Dyson swarm is a Dyson sphere made up of separately orbiting elements (including large habitats) rather than a single continuous shell.
• A Dyson bubble is a Dyson sphere in which the individual elements are statites, non-orbital objects held aloft by the pressure of sunlight.

Planetary scale

• A Bishop Ring, Halo or Orbital is a space habitat similar to but much smaller than a Niven Ring. Instead of being centered on a star, it is in orbit around the star and its diameter is typically on the order of magnitude of a planet. By tilting the ring relative to its orbit, the inner surface would experience a nearly conventional day and night cycle. Due to its enormous scale, the habitat would not need to be fully enclosed like the Stanford torus, instead its atmosphere would be retained solely by centripetal gravity and side walls, allowing an open sky.
• Globus Cassus is a hypothetical proposed project for the transformation of Planet Earth into a much bigger, hollow, artificial world with the ecosphere on its inner surface. This model serves as a tool to understand the World's real functioning processes.
• Shellworlds or paraterraforming are inflated shells holding high pressure air around an otherwise airless world to create a breathable atmosphere.^[8] The pressure of the contained air supports the weight of the shell.
• Completely hollow shell worlds can also be created on a planetary or larger scale by contained gas alone, also called gravitational balloons, as long as the outward pressure from the contained gas balances the gravitational contraction of the entire structure, resulting in no net force on the shell. The scale is limited only by the mass of gas enclosed, the shell can be made of any mundane material. The shell can have an additional atmosphere on the outside.^[9][10]
• It can also refer to terraformed or artificial planets with multiple concentric layers.

Orbital structures

• An Orbital ring is a dynamically elevated ring placed around the Earth that rotates at an angular rate that is faster than orbital velocity at that altitude, stationary platforms can be supported by the excess centripetal acceleration of the super-orbiting ring (similar in principle to a Launch loop), and ground-tethers can be supporter from stationary platforms.
• The Bernal sphere is a proposal for a space colony with a maximum diameter of 16 kilometers.
• Rotating wheel space stations, such as the Stanford torus, are wheel-like space station which produce artificial gravity by rotation. Typical designs include transport spokes to a central hub used for docking and/or micro-gravity research.
• The related concepts, O'Neill and McKendree cylinders, are both pairs of counter-rotating cylinders containing habitable areas inside and creating 1g on their inner surfaces via centripetal acceleration. The scale of each concept came from estimating the largest 1g cylinder that could be build from steel (O'Neill) or carbon fibre (McKendree).^[11][12]
• Hollowed asteroids (or Bubble worlds or Terraria) are spun on their axis for simulated gravity and filled with air, allowing them to be inhabited on the inside. In some concepts, the asteroid is heated to molten rock and inflated into its final form.^[13][14]

Trans-orbital structures

One concept for the space elevator has it tethered to a mobile seagoing platform.

• A skyhook is a very long tether that hangs down from orbit.
• A space elevator is a tether that is fixed to the ground, extending beyond geostationary orbital altitude, such that centripetal force exceeds gravitational force, leaving the structure under slight outward tension.
• A space fountain is a dynamically supported structure held up by the momentum of masses which are shot up to the top at high speeds from the ground.
• A launch loop (or Lofstrom loop) is a dynamically supported 2000 km long iron loop that projects up in an arc to 80 km that is ridden by maglev cars while achieving orbital velocity.
• StarTram Generation 2 is a maglev launch track extending from the ground to above 96% of the atmosphere's mass, supported by magnetic levitation.
• A Rotovator is a rotating tether where the lower tip is moving in the opposite direction to the tether's orbital velocity, reducing the difference in velocity relative to the ground, and hence reducing the velocity of rendezvous; the upper tip is likewise moving at greater than orbital velocity, allowing propellantless transfer between orbits. Around an airless world, such as the moon, the lower tip can actually touch the ground with zero horizontal velocity.^[15] As with any momentum exchange tether, orbital energy is gained or lost in the transfer.

Fictional

A number of structures have appeared in fiction which may be considered megastructures.

Stellar scale

• The Dyson shell has appeared in many works of fiction, including the Star Trek universe.
• Larry Niven's series of novels beginning with Ringworld centered on, and originated the concept of a ringworld, or Niven ring. A ringworld is an artificial ring with a radius roughly equal to the radius of the Earth's orbit (1 AU). A star is present in the center and the ring spins to create g-forces, with inner walls to hold in the atmosphere. The structure is unstable, and required the author to include workarounds in subsequent novels set on it.
• In the manga Blame! the Megastructure is a vast and chaotic complex of metal, concrete, stone, etc., that covers the Earth and assimilates the Moon, and eventually expands to encompass a volume greater than the orbit of Jupiter.
• In White Light by William Barton and Michael Capobianco, a Topopolis is presented as taking over the entire universe.
• In the Heechee books by Frederik Pohl the race of pure energy beings called The Foe have constructed the Kugelblitz, a black hole made of energy and not matter.
• In the Xeelee series of books by Stephen Baxter, the eponymous alien race constructed the Ring, a megastructure made of cosmic strings, spanning over 10 million light years.
• In Freelancer, The Dom'Kavosh's Dyson shell that is inhabited by a drone race created by the Dom'Kavosh, Nomads. This is reached via a hyper gate, created by the same creators as the Dyson sphere.
• The Saga of Cuckoo series novel Wall Around a Star mentions a proposal to build a super dyson sphere, completely enclosing the Galactic Center.
• The title of the novel Helix by Eric Brown directly references a stellar-scale helical megastructure. Different types of environments and habitats are interspersed along the structure, while their varying distance from the central star affects the climate.
• The Quarg in the game Endless Sky are shown building a massive ring around one of their stars, which is most likely around one astronomical unit in diameter. A completed version of this can also be found in another location.
• In Space Empires 4 and 5, the player can construct sphereworlds and ringworlds around stars.
• In the Utopia expansion of Stellaris, the player can construct both ringworlds and Dyson spheres around stars.

Planetary and orbital scale

• Several structures from the fictional Halo universe:
  • The original twelve Halos, seen in Halo: Cryptum, were 30,000 kilometers in diameter; a separate array of six Halos are 10,000 kilometers in diameter, with one of the original twelve later being reduced to this size in Halo: Primordium.
  • The Ark is a 127,530 km diameter structure from which the Halo Array can be activated and capable of building 10,000 km Halos. The "greater" Ark, seen in Cryptum and Primordium, is capable of producing 30,000 km Halos.
  • Onyx is an artificial planet made entirely out of Forerunner Sentinels (advanced replicating robots). At its core is a "shield world", contained within slipstream space, that is approximately one astronomical unit in diameter. The much smaller Shield World 0459, (approximately 1,400 km in diameter), is the setting for the latter half of Halo Wars. A third shield world, Requiem, is the primary setting for Halo 4. Requiem is an artificial hollow planet encased in a kind of Dyson Sphere.
  • High Charity, the Covenant's mobile planetoid station
• Death Star from Star Wars
• In Sonic Adventure 2 and Shadow The Hedgehog, the Eclipse Cannon is a planet-destroying WMD built inside of the Space Colony Ark.
• Buster Machine III from Gunbuster.
• Culture Orbital
• In the 2013 CGI anime movie, Space Pirate Captain Harlock, the Jovian Accelerator is an ancient, Death Star-like Weapon of mass destruction that uses energy from Jupiter's atmosphere to create a large beam of intense light strong enough to destroy an entire planet.
• Sidonia, the main ship and home of millions of humans, 1000 years in the future in the Knights of Sidonia manga and anime series, created after the destruction of the earth along with other unnamed seed ships.
• Trantor, the capital of an interstellar empire in Isaac Asimov's Foundation series, is an ecumenopolis, a planet entirely covered in one huge metal clad building, with only one small green space: the Emperor's palace grounds.
• Coruscant, capital city in the Star Wars universe, entirely covers its host planet. It serves as capital of first the Republic and then later the First Galactic Empire.
• The Galaxy Gun from the Star Wars universe, a large space station designed to destroy entire planets from across the galaxy could be considered a megastructure because its size is more than seven kilometers long.
• The Centerpoint Station, from the Star Wars universe, a 350 km spherical space station at the Lagrangian point between the planets Talus and Tralus in the Corellia system. It was a gigantic and ancient hyperspace tractor beam with which an ancient race, known as Celestials, created the Corellia star system. With the help of the tractor beam whole planets could be moved through hyperspace and arranged into their actual orbits acound the central star. On the other hand, the same technology could be used as weapon to destroy even stars. On the inside of the main sphere a huge living space called Hollowtown was home to many people in similar fashion as on the inside of a dyson sphere.
• The Ori Supergate seen in a number of episodes of Stargate SG1 could be classed as a megastructure
• In The Hitchhiker's Guide to the Galaxy series, Earth, as well as other planets, were artificial megastructures. Earth was intended to function as a gigantic computer, and was built by a race of beings who made their living by manufacturing other planets.
• The Star Forge from Star Wars: Knights of the Old Republic
• Mata-Nui in the BIONICLE franchise is classifiable as a megastructre. In the story he is a massive robot as tall as a planet, and inside his body, every inhabitant of the BIONICLE Universe (Matoran, Toa, etc.) all live, unaware that they live inside a massive, space-traveling entity.
• In the Robotech Sentinels novels, Haydon IV is an artificially constructed cyber-planet with android citizens.
• In the Invader Zim episode "Planet Jackers", two aliens surround the Earth with a fake sky in order to throw it into their sun.
• Nightmare's fortress from Kirby: Right Back at Ya! can be classified as a megastructure because it's the size of a small planet.
• In several works, Arthur C. Clarke writes about a colossal hollow tube, first described in Rendezvous with Rama (1973), and inhabited by different races.
• The Citadel in the Mass Effect universe is an enormous space station constructed by an ancient race of machines called the Reapers millions of years before the games in the series. At the time of Mass Effect 2, its population is 13.2 million.
• In the game Airforce Delta Strike a large Space Elevator called the Chiron Lift is used to send supplies out into outer space.
• In the Warhammer 40,000 series, the Imperial Palace (site of the Golden Throne wherein the Emperor of Mankind is kept alive indefinitely) could be considered a megastructure. The palace is a complex of continent-wide structures with the Golden Throne being located in an area stretching across the whole of the Himalayan mountains.
• In the film Elysium, a luxury space station (a Bishop Ring) called Elysium houses the wealthy population of the human species.
• Large rotating space-stations are a staple of science fiction, including Arthur C. Clarke's novel 2001: A Space Odyssey and the eponymous Babylon 5.
• Hollowed asteroids feature in various fiction, such as Kim Stanley Robinson's novel 2312, Larry Niven's Known Space, and Golden Age SF writers like Clarke and Asimov.