According to some scientists, intelligent extraterrestrial (ET) civilizations in the galaxy might be conveniently characterized as falling into one of several basic levels of societal development or technology use. In SETI
activities, one of the most widely used scales is the three types of ET civilizations that were introduced in 1964 by the Russian astronomer Nikolai Semenovich Kardashev (1932- ). While examining the issue of interstellar information transmission by extraterrestrial civilizations, Kardashev postulated that there were just three basic types of technologically developed civilizations in the galaxy. He structured his proposed characterization scheme on the basis of how much total energy each type of ET civilization was able to harness and manipulate. He further suggested that the more energy a particular alien civilization controlled, the easier it would be for them to communicate across vast interstellar distances.
A Kardashev Type I civilization represents a planetary civilization similar to the technology level on Earth in the late 20th century. A typical planetary civilization would command the use of somewhere between 1012 and 1016 watts (J/s) of energy. Here, the upper limit of energy available is the total amount of radiant energy being intercepted by Earth (or an Earth-like, extrasolar planet in the continuously habitable zone) as the planet travels in orbit around the parent star.
With respect to Earth, scientists define the solar constant as the total amount of the Sun's radiant energy that normally crosses perpendicular to a unit area at the top of the planet's atmosphere. At one astronomical unit from the Sun, scientists have measured the value of the solar constant and it is approximately 127 watts/ft2 (1,371W/m2). Of course, an alien civilization might live on a planet that orbits a little closer to a less radiant and slightly cooler K-spectral class or M-spectral class parent star, so the total amount of energy available to a Type I alien civilization could vary somewhat from the suggested maximum value of 1016 watts (J/s).
A Kardashev Type II civilization would engage in feats of planetary engineering, emerge from its native planet through advances in space technology, and extend its resource base throughout the local star system. The eventual upper limit of a Type II civilization could be taken as the creation of a Dyson sphere. A Dyson sphere is a postulated shell-like cluster of habitats and structures placed entirely around a star by an advanced interplanetary civilization to intercept and to use basically all the radiant energy from the parent star. What the British-American physicist Freeman J. Dyson (1923- ) suggested in 1960 was that an advanced extraterrestrial civilization might eventually develop the space technologies that were necessary to rearrange the raw materials of all the planets in its solar system, creating a more efficient composite ecosphere around the parent star. Using the radiant energy output of the Sun as the reference, a Kardashev Type II civilization would command between 1026 and 1027 watts (J/s) of energy. Once this size solar-system civilization is achieved, the search for additional resources and the pressures of continued growth could encourage the alien society to pursue interstellar migrations. Initiation of the process of interstellar migration would mark the start of a Kardashev Type III extraterrestrial civilization.
At maturity, a Type III civilization would be capable of harnessing the material and energy resources of an entire galaxy (typically containing some 1011 to 1012 stars). Energy resources on the order of 1037 to 1038 watts (J/s) or more would be involved.
Command of energy resources is an important figure of merit in comparing (hypothetical) extraterrestrial civilizations. Within Kardashev's scheme, a Type II civilization controls about 1012 times the energy resources of a Type I civilization, and a Type III civilization controls approximately 1012 times as much energy as a Type II civilization.
What else can scientists speculate about such civilizations? Again, starting with Earth as a model (at present, the one and only "scientific" data point), scientists can reasonably postulate that a Type I civilization would probably exhibit the following characteristics: (1) an understanding of the laws of physics; (2) a planetary society, including a global communication network and interwoven food and materials resource networks; (3) intentional or unintentional emission of electromagnetic radiations (especially radio frequency) into the galaxy; (4) the development of space technology and rocket propulsion-based interplanetary travel—the vital tools necessary to leave the home planet; (5) (possibly) the development of nuclear energy technology, both power supplies and weapons; and (6) (possibly) a desire to search for and communicate with other intelligent life-forms beyond the home planet.
Many uncertainties, of course, are present in such characterizations. For example, given the development of the space technology, will the planetary civilization decide to create a solar-system civilization? Do the planet's inhabitants develop a long-range planning perspective that supports the eventual creation of artificial habitats and structures throughout their star system? Or do the majority of Type I civilizations end up destroying themselves with their own advanced technologies before they can emerge from a planetary civilization into a more stable Type II solarsystem civilization? Does the exploration imperative encourage intelligent alien creatures to go out from their comfortable planetary niche into an initially hostile but resource-rich solar system? If this "cosmic birthing" does not occur frequently, perhaps the Milky Way galaxy is indeed populated with intelligent life but at a level of stagnant planetary (Type I) civilizations that have neither the technology nor the societal motivation to create an extraterrestrial civilization. Such introvert Type I civilizations may not even to try to communicate with any other intelligent life-forms across interstellar distances.
Assuming that an extraterrestrial civilization does, however, emerge from its native planet and create an interplanetary society, several additional characteristics could become evident. The construction of space habitats and structures (leading ultimately to a Dyson sphere around the parent star) would portray feats of planetary engineering. Something as large as a Dyson sphere could possibly be detected by telltale thermal infrared emissions, as the parent star's radiant energy (predominantly in the visible spectrum) was intercepted and converted to other more useful forms of energy and as the residual energy (as determined by the universal laws of thermodynamics) was rejected to space as waste heat at perhaps 80°F (300K).
Type II civilizations might also decide to search in earnest for other forms of intelligent life beyond their solar system. Alien scientists in a Type II civilization would probably use portions of the electromagnetic spectrum (radio frequency and perhaps X-rays or gamma rays) as information carriers between the stars. Remembering that Type II civilizations would control 1012 times as much energy as Type I civilizations, such techniques as electromagnetic beacons or feats of astroengineering that yielded characteristic X-ray or gamma-ray signatures could lie well within their technical capabilities. Assuming their understanding of the physical universe is far more sophisticated than ours, Type II civilizations might also use gravity waves or other physical phenomena that are perhaps unknown at present by terrestrial scientists. In this case, the advanced alien civilization might now be sending such "exotic signals" through humans' solar system, but no equipment on Earth is currently capable of receiving and analyzing their efforts to communicate across vast interstellar distances. One currently difficult-to-detect signal would be a modulated beam of neutrinos. The field of neutrino astronomy here on Earth is in its infancy.
Type II civilizations might also decide to make initial attempts at interstellar matter transfer. Fully autonomous robotic explorers would be sent forth on one-way scouting missions to nearby stars. Even if the mode of propulsion involved devices that achieved only a small fraction of the speed of light, Type II societies should have developed the much longer-term strategic planning perspective necessary to support such sophisticated, expensive, and lengthy missions. The Type II civilization might also use a form of directed panspermia (the intentional diffusion of spores or molecular precursors through space). The alien civilization might employ robotic interstellar probes to disperse microscopically encoded viruses through the interstellar void, hoping that if such "seeds of life" found a suitable ecosphere in some neighboring or distant star system, they would initiate the chain of life perhaps leading ultimately to the replication (suitably tempered by local ecological conditions) of intelligent life itself.
Finally, as the Dyson sphere was eventually completed, some of the inhabitants of this (hypothetical) Type II civilization might respond to a cosmic wanderlust and initiate the first "intelligently crewed" interstellar missions. Complex space habitats could become space arks and carry portions of the advanced alien civilization to neighboring star systems.
Such scenarios are intriguing, but exobiologists also ask: What is the lifetime of a Type II civilization? It would appear from an extrapolation of contemporary terrestrial engineering practices that perhaps a minimum of 500 to 1,000 years would be required for an advanced interplanetary civilization to complete a Dyson sphere.
Throughout the entire galaxy, if just one Type II civilization embarks on a successful interstellar migration program, then—at least in principle—that alien race would eventually (in perhaps 108 to 109 years) sweep through the galaxy in a leapfrogging wave of exploration, establishing a Type III civilization in its wake.
This Type III civilization could eventually control the energy and material resources of up to 1012 stars—or the entire Milky Way galaxy. Communication or matter transfer would be accomplished by techniques that can now only politely be called "exotic." Perhaps very precisely manipulated beams of neutrinos or (hypothesized) faster-than-light particles (such as tachyons) would serve as the standard information carriers for this galactic society. Or, they might use tunneling through black holes as their transportation network. Perhaps, they might develop some kind of thought-transference or telepathic skills that permitted efficient and instantaneous communications across the vast regions of interstellar space. In any event, a Type III civilization should be readily evident since it would be galactic in extent and easily recognizable by its incredible feats of astroengineering and technical "magic."
In all likelihood, the Milky Way galaxy at present does not contain a Type III civilization, or else the solar system is being ignored—that is, intentionally being kept isolated—perhaps as a game preserve or zoo, as some scientists have speculated. Then again, the solar system may be simply one of the very last regions to be "filled in." These are some of the more popular speculations associated with the Fermi paradox.
There is also another important perspective: If the human race is really alone or is the most advanced civilization in the galaxy, then the men and women of Earth now stand at the technological threshold of creating the galaxy's first Type II civilization. Should the human race succeed in that task, our descendents would have the opportunity of becoming the first interplanetary (Type II) civilization to travel into interstellar space, founding a Type III civilization within the Milky Way galaxy.
Messages to the Stars: Arecibo Message, Pioneer Plaques, Voyager Records
While the human race started to leak its presence out into the galaxy unintentionally in the form of radio and (later) television signals early in the 20th century, there have also been three deliberate attempts to send messages to alien civilizations that may reside among the stars. The first attempt involved the broadcast of a powerful radio message, known as the Arecibo Interstellar Message. The other two deliberate attempts at interstellar communication involved artifacts, two special plaques and two records, placed aboard four NASA spacecraft (Pioneer 10 and 11 and Voyager 1 and 2), whose missions to the outer planets ultimately placed the far-traveling robots on interstellar trajectories. This chapter describes these efforts.
Was this article helpful?