Freeman Dyson was among the few scientists who defended the feasibility of interstellar travel. In 1964 Dyson, a distinguished theoretical physicist and professor at the Princeton Institute for Advanced Studies, envisioned voyages by nuclear propelled spacecraft moving at slow speeds and lasting thousands of years. The crews of such spacecraft would be frozen, placed in cold storage, and revived when needed. Travel that extended over a millennium might not appeal to humans but, as Dyson said, we have no right to impose our taste on others. Since we already have nuclear power sources, interstellar travel is more a biological than a physical problem.
fig. 8.3. A schematic drawing of a Dyson sphere fashioned from the mass of Jupiter. Its radius is one astronomical unit, the mean distance from the Earth to the Sun. (G. A. Lemar-chand, "Detectability of Extraterrestrial Activities." SETIQuest, vol. I, no. 1, 1994. Copyright 1994 Helmers Publishing, Inc.)
Despite his interest in space travel, Dyson is better known for postulating the existence of Dyson spheres in regions far from our solar system. A Dyson sphere is a huge artificial sphere built by an extraterrestrial civilization. This gigantic astroengineering project encloses the sun and planet of its builders and captures all of the sun's energy for their exclusive use (Fig. 8.3).
Dyson assumed that it was "overwhelmingly probable" that any extraterrestrial civilization we contacted would be millions of years old. The technological level of such a civilization, he imagined, far surpassed "ours by many orders of magnitude."15 Given these assumptions, Dyson concluded that most alien civilizations had "expanded to the limits set by Malthusian principles." Hence, they suffered from overpopulation and a shortage of material resources and energy, problems that plagued terrestrial civilizations. Dyson went on to explain how extraterrestrials handled these serious threats.
Planetary systems consist of a central star, or sun, and orbiting planets. These essential elements are available to advanced civilizations. It is reasonable to expect, Dyson wrote, that by the time any technological civilization is a few thousand years old, "Malthusian pressures" will force its citizens to make the most effective use of available resources. By capturing all the energy of its central star, and appropriating other planets for building materials, the hard-pressed extraterrestrials will construct "an artificial biosphere."16 This gigantic structure encloses the inhabited planet and its parent star. The proposed biosphere is not a solid shell. It is a "loose collection or swarm of objects"17 independently orbiting the star.
A Dyson sphere, built from matter taken from other planets in its solar system, captures its star's energy for use by the inhabitants of the biosphere. Dyson calculated that engineers using the matter in a Jupiter-sized planet could build a sphere with a radius of over 150 million kilometers.
The inner surface of a Dyson sphere reflects solar radiation toward its center. The conserved radiation provides ample energy for the fast-growing population living within. Although the sphere is huge, it is not visible to distant observers because it is not bright. Waste heat, however, continually escapes from the outer surface of the structure. Dyson determined that an alien-built sphere could be detected by the infrared radiation coming from its surface. Hence, he asked astronomers using sensitive detectors to scan the skies for artificial objects emitting infrared radiation within certain limits.
In the 1960 paper that Dyson prepared for the professional audience of the journal Science, he did not reveal the source of his idea of artificial spheres. However, he was more open about its origins in his memoirs published two decades later. There he acknowledged that he got the idea from an old science fiction novel, Star Maker (1937) by Olaf Stapledon. In Star Maker, Dyson read that in the future "a gauze of light traps" focused "escaping solar energy for intelligent use."18 Whole galaxies grew dim as Stapledon's aliens built light traps around stars.
In a 1984 interview, Dyson discussed the nature of extraterrestrial intelligence. He mentioned the Cocconi and Morrison paper as the source of a narrow approach to the subject. After the paper's publication, he complained, scientists tended to think that aliens were essentially like us technologically and that they communicated using radio signals.
Dyson cited his own work as an example ofa new departure. He claimed that he did not base his artificial biospheres upon any detailed assumptions about the nature of extraterrestrial life. Instead, he considered the consequences of high technology that follow from fundamental physical principles.
Dyson made explicit assumptions about alien life although he claimed he had not. In his Science article, he assumed that technological civilizations existed elsewhere in the universe, that those civilizations were similar to Western civilization except they were much older and more advanced, and that extraterrestrial civilizations faced the same pressures from population expansion that operated on Earth.
The assumptions Dyson made about the existence of intelligent life in the universe, the nature of civilization, the role of technology in intelligent life, and the part played by Malthusian principles in the growth ofcivilizations are essential to his analysis. Calculating the size ofa sphere that could be built from the matter in a large planet is easy when compared to determining the causes of the origin, growth, and collapse of civilizations, and the role played by technology in the process.
Dyson, like many other physical scientists, seems unaware of the complexity of the assumptions he brings to the topic. His ideas about extraterrestrial civilizations raise controversial historical issues that cannot be resolved by an appeal to the laws of physics. For instance, the relevance ofMalthusian principles to human populations is a contentious subject currently under discussion in the social sciences. Its extension to alleged extraterrestrial populations is highly problematical.
Dyson revisited the idea of artificial biospheres in a 1966 essay he wrote, "The Search for Extraterrestrial Technology." This essay appeared in Perspectives in Modern Physics, a volume honoring the distinguished physicist Hans C. Bethe on his sixtieth birthday. Here Dyson restated his claim that extraterrestrial civilizations would indulge in big engineering projects, such as constructing artificial biospheres, rather than concentrate upon sending radio signals to the rest of the universe. However, he reluctantly concluded that expanding technology never "really got loose in our galaxy." If it had, we would find starlight "carefully dammed and regulated" and stars "grouped and organized."19 Although he was skeptical about the existence of large-scale extraterrestrial technology projects, Dyson believed that the search for them should continue.
Dyson's grand program of infrared searches for proof of his supercivilizations found few supporters among astronomers. However, it inspired Soviet astrophysicist Nicolai S. Kardashev (1932—) to propose a hierarchical classification of the technological civilizations inhabiting the universe.
Kardashev was a student of Shklovskii and a pioneer in the Soviet Union's search for extraterrestrial intelligence. As a young man, he was deeply influenced by Flammarion and Schiaparelli's writings on Martian life. In 1964 Kardashev published a paper in Soviet Astronomy on "The Transmission of Information by Extraterrestrial Civilizations." Here he considered the amount of energy required to send coded radio signals at great distances across the universe.
Kardashev's calculation of the energy needed to transmit vast quantities of information from more highly developed civilizations to ones less developed led him to postulate the existence of three types of technological civilizations. The amount of energy it controlled determined the type of each civilization.
A Type I Kardashev civilization is similar to the modern technological societies found on Earth. It draws upon the energy falling upon a planet from its sun. Kardashev estimated the Earth's energy consumption at about 4 x 1019 ergs per second. The Earth has not quite reached Type I status because its inhabitants are unable to capture all of the radiant energy streaming down upon it. For this reason, Carl Sagan said that the Earth was more accurately called a Type .7 civilization.
Kardashev believed that given the number of stars in our Galaxy, and the number of galaxies in the universe, there must exist civilizations much older than any terrestrial civilization. Reflecting upon the Earth's progress in producing more energy over time, Kardashev argued that civilizations billions of years old must possess the ability to control enormous amounts of energy.
Kardashev's ancient and technologically superior civilizations fall into two classes. A Type II supercivilization is able to capture all the radiant energy emitted by its sun for its own technological purposes. In essence this civilization has constructed a Dyson sphere to ensure that almost no solar energy escapes into space. Energy consumption in this case is roughly 4 x 1033 ergs per second.
There is a large gap between a Type II and Type III civilization because the latter has gained control over the total energy output of its galaxy This gigantic technological achievement calls for the utilization of the power of billions of stars. Therefore, its energy consumption approaches 4 x 1044 ergs per second.
Kardashev believed that terrestrial radio astronomers had a very slight chance ofcontacting a Type I civilization. It was possible, however, to detect and receive information from Types II and III supercivilizations. He maintained that the two higher civilizations produced immense quantities of information that they broadcast continuously. Unlike Dyson, who advocated searches for radiation leaking from biospheres, Kardashev believed his supercivilizations were using the energy available to them to send coded signals throughout their galaxy and the universe. However, like Dyson, Kardashev was caught in the dilemma posed by Fermi's paradox.
Where are the billion-year-old supercivilizations who are attempting to contact intelligent life elsewhere in the universe? Is there any evidence of very powerful artificial radio sources in the universe? Kardashev claimed that such evidence might exist. Astronomers at California Institute of Technology had recently detected two sources ofradio-frequency emission; they catalogued them as CTA-21 and CTA-102. These two met Kardashev's specifications for an artificial radio source.
During a 1981 interview, Kardashev repeated his belief in the three types of technological civilizations. He also acknowledged that his mentor Shklovskii no longer believed in the existence of intelligent extraterrestrial life. Finally, he conceded that CTA-102, which he and other Soviet astronomers once publicly hailed as proofofthe existence ofa supercivilization in outer space, was a distant quasar with a very large red shift.
Despite its lack of empirical verification, Kardashev's scheme for classifying extraterrestrial civilizations joined Frank Drake's equation as a way of thinking about intelligent extraterrestrial life. Carl Sagan introduced Kardashev's ideas to American scientists in an article on the detection ofadvanced galactic civilizations in 1973. Sagan urged radio astronomers to search for Type II or III supercivi-lizations among the nearer galaxies rather than Type I or younger civilizations among the nearer stars.
The acceptance of the idea of an expanding universe, the introduction of the tools and techniques of radio astronomy, and research into the chemical basis of life helped to make interstellar space the new realm for speculation about advanced extraterrestrial civilizations. Drake's equation and Kardashev's formulation ofsupercivilizations took advantage ofthe new vistas open for speculation. The parochialism that limited advanced life to our solar system was gone. Now scientists discussed the pros and cons ofinterstellar travel, earlier visits by extraterrestrial beings to Earth, and the possibility of identifying the astroengineering projects of extraterrestrial civilizations.
The tendency to think about advanced life forms inhabiting outer space was enhanced by the late twentieth-century discovery of extrasolar planets, bodies that orbited distant stars. This discovery provided observational evidence for the second factor in Drake's equation. The Earth was not unique. Its patterns of life and culture might be duplicated on worlds located many light years in the distance.
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