Dyson sphere
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Author: Dr. Freeman J. Dyson, Institute for Advanced Study, Princeton, NJ
A Dyson sphere is a hypothesized artificial habitat built around a star by a civilization with sufficient technology. It is intended to capture as much as possible of the power output of the star, and should be a distinctive characteristic of a civilization able to control the resources of a planetary system.
Search for alien civilizations
Giuseppe Cocconi and Philip Morrison, who were at the time professors of physics at Cornell University, proposed in 1959 that radio-astronomers should search for artificial radio-signals transmitted by alien civilizations (Cocconi et al., 1959). Such signals might be aimed at us by the aliens, or might be sent by one alien civilization to another and overheard by us. Cocconi and Morrison pointed out that narrow-band radio signals transmitted by existing radars and received by existing telescopes could be detected over interstellar distances, so that a search for such signals was reasonable. Frank Drake quickly began a search which he called "Project Ozma", using the 85-foot-diameter radio-telescope at Green Bank (Drake, 1959). He wrote, "It is named for the queen of the imaginary land of Oz, a place very far away, difficult to reach, and populated by exotic beings". He did not hear any signals. Searches for alien radio signals have continued ever since, with increasing sensitivity and increasing coverage of the sky, but always with negative results. It appears that alien civilizations, if they exist in our neighborhood, do not wish to communicate by radio.
Dyson in 1960 asked the question, whether there was some way to detect alien civilizations who did not wish to communicate, and answered it in the affirmative (Dyson, 1960). He observed that any civilization using a large flow of energy, whether or not it wished to communicate, would be forced by the laws of thermodynamics to get rid of waste heat, which must be radiated into space in the form of infra-red radiation. Uncommunicative civilizations could therefore be detected as sources of infra-red radiation, provided that they used a flow of energy large compared with natural infra-red sources in the same part of the sky. Dyson proposed that a search be made for point sources of radiation in the ten-micron band. Ten-micron radiation would be emitted from the outer surface of any habitat containing creatures at a temperature around 300 degrees Kelvin at which water is liquid. Since the earth's atmosphere radiates strongly in this band, a search for extraterrestrial sources requires a telescope with an infra-red detector orbiting in space.
After Dyson's proposal, twenty years went by before a spacecraft carrying a liquid-helium-cooled detector of long-wave infra-red radiation was launched. In 1983 the Infra-red Astronomy Satellite (IRAS), an international mission funded jointly by the United States, the United Kingdom and the Netherlands, did the first sky-survey in the long-wave infra-red band, discovering 245389 sources (Beichman, 1987). The mission was spectacularly successful. It discovered that the sky is infested with infra-red sources. Unfortunately, the overwhelming success of the mission made it useless for the purpose of detecting alien civilizations. The vast majority of the sources are undoubtedly natural objects, mostly new-born stars which are still embedded in the dense dust-clouds out of which they recently condensed. The dust-clouds are heated by the new-born stars inside them, and radiate away the energy of the stars in the form of infra-red radiation. As seen by our infra-red detectors, these natural "cocoon stars" look just like alien civilizations. If any alien civilizations exist in our neighborhood, they are hidden among the huge population of natural infra-red sources, and we have no way to tell them apart. After IRAS, other spacecraft have been launched, carrying more sensitive detectors and discovering an even larger number of sources. But we have still not solved the problem of discriminating artificial from natural sources. Until we have excellent discrimination with few false alarms, increasing the sensitivity of detectors will not help.
According to the Russian astrophysicist Nikolai Kardashev (Kardashev, 1962), long-lived civilizations are likely to belong to three types which he calls I, II and III. Type I controls the resources of a planet, type II controls the resources of a star, and type III controls the resources of a galaxy. Our own civilization is not yet type I but will be type I within a few hundred years at most. Kardashev observed that the transitions from birth of a civilization to type I, from type I to type II, and from type II to type III, will be rapid compared with the probable life-times of civilizations that remain in one of the three types. A type I civilization at interstellar distance will be undetectable with existing instruments, and a type III in our own galaxy would change the appearance of the sky so drastically that it could hardly have escaped our attention. So any alien civilization that we discover is likely to be a type II civilization controlling the resources of a star. The civilization would occupy a habitat surrounding the star and using for its own purposes the star's entire output of starlight. The flux of energy carried by the starlight would then be radiated as waste heat from the outer surface of the habitat.
In his paper (Dyson, 1960) proposing the search for artificial infra-red sources, Dyson wrote, "Taking our own solar system as the model, we shall reach at least a possible picture of what may be expected to happen elsewhere. I do not argue that this is what will happen in our system; I only say that this is what may have happened in other systems". He remarked that the mass of Jupiter, if distributed in a spherical shell revolving around the sun at twice the earth's distance from it, would have a thickness of 2 tons per square meter of surface area. "A shell of this thickness", he wrote, "could be made comfortably habitable, and could contain all the machinery required for exploiting the solar radiation falling onto it from the inside". This remark gave to readers the misleading impression that the habitat of an alien civilization would be a big round ball with a star at the center. Various science-fiction writers adopted this notion of a big round ball inhabited by aliens and gave it the name "Dyson Sphere". Dyson used the phrase "artificial biosphere" to describe the habitat of an alien civilization. He was well aware that the artificial biosphere could not be a big round ball. A big round ball, whether rotating or not, would be mechanically too weak to support its own weight against the gravity of the star. He imagined the artificial biosphere to be a cloud of inhabited objects orbiting a star, surrounding the star densely enough to absorb all the starlight, but with the orbits carefully arranged so as to avoid collisions.
Literary origin
The true progenitor of the idea of an artificial biosphere was the science-fiction writer Olaf Stapledon. In his novel, "Star Maker" (Stapledon, 1987, page 179), he describes the appearance of a galaxy in the far future occupied by a type III civilization and seen from a great distance. "Not only was every solar system now surrounded by a gauze of light traps, which focused the escaping solar energy for intelligent use, so that the whole galaxy was dimmed, but many stars that were not suited to be suns were disintegrated, and rifled of their prodigious stores of sub-atomic energy". The novel was published in 1937 and Dyson read it in 1945. Unfortunately he neglected to refer to Stapledon when he published his paper (Dyson, 1960). If Dyson had given proper credit to Stapledon in that paper, then the artificial biospheres would have been called "Stapledon clouds", and would never have been given the misleading name "Dyson spheres".
References
- C. Beichman, "The IRAS View of the Galaxy and the Solar System", Annual Reviews of Astronomy and Astrophysics, 25, 521-563 (1987).
- G. Cocconi and P. Morrison, "Searching for Interstellar Communications", Nature, 184, p. 844 (1959) doi:10.1038/184844a0.
- F. Drake, "How can we Detect Radio Transmissions from Distant Planetary Systems?", Sky and Telescope, 19, 140 (1959).
- F. Dyson, "Search for Artificial Stellar Sources of Infrared Radiation", Science, 131, 1667 (1960).
- N. Kardashev, "Transmission of Information by Extraterrestrial Civilizations" (in Russian), Astronomicheskii Zhurnal, 41, 282 (1962), English translation, Soviet Astronomy AJ, 8, 217 (1964).
- O. Stapledon, Star Maker, (London, Methuen, 1937), 50th Anniversary Edition, (Los Angeles, Jeremy Tarcher, 1987).
| Author: Dr. Freeman J. Dyson, Institute for Advanced Study, Princeton, NJ |
