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Extraterrestrial Intelligence AccessScience from McGraw-Hill Education Page 1 of 6 www.accessscience.com Extraterrestrial intelligence Contributed by: Seth Shostak Publication year: 2014 Postulated entities beyond Earth with a level of intelligence and comprehension at least equal to that of present-day humans. While extraterrestrial intelligence is usually envisioned as an advanced civilization, populated by creatures that have evolved via Darwinian evolution on a planet vaguely similar to Earth, it could conceivably be artificial intelligence initially created by biological beings. Extraterrestrial intelligence is a subset of astrobiology, which encompasses all aspects of the existence of, and search for, extraterrestrial life. Astrobiology is sometimes referred to as exobiology or bioastronomy. See also: ARTIFICIAL INTELLIGENCE ; ASTROBIOLOGY . Scientific rationale The expectation that extraterrestrial intelligence exists derives from two facts and one assumption: (1) The 11 22 universe is vast, with approximately 10, galaxies (a total of about 10, stars) within the reach of telescopes. This number is so large that even if the emergence of intelligence is improbable, such intelligence could still have arisen frequently. (2) The physics and chemistry of the universe are everywhere the same. This fact is known from astronomical observation. (3) Habitable, Earth-like planets of the type that might spawn intelligence, with thick atmospheres and liquid water on their surface, are not extraordinarily rare. This is a hypothesis, sometimes called the principle of mediocrity. According to this principle, our planet is not extraordinary in any of its important properties. The principle dates, in its modern form, to Nicolaus Copernicus (1473–1543), who dethroned the Aristotelian idea of an Earth-centered cosmos. See also: UNIVERSE . In addition to these general arguments, research gives support to the idea that extraterrestrial biology (not necessarily intelligent) might be plentiful. Since 1995, astronomers have detected many hundreds of planets around other, Sun-like stars. On the basis of the systems discovered so far, these researchers estimate that as many as half or more of all stars could have a solar system consisting of at least one orbiting body. The major discovery techniques so far have been to measure the small motions of the star induced by the planet, a scheme that is most sensitive to massive worlds in tight orbits, or to detect drops in intensity from planetary transits. It is still unknown what fraction of stars has small, rocky planets similar to Earth, but the Kepler and Darwin space-based telescopes will soon decide this question. See also: EXTRASOLAR PLANETS . The possibility that some other worlds in our solar system could have spawned life has also increased. There is good photographic evidence that Mars once had lakes and possibly oceans, and may still harbor liquid water hundreds of meters below its surface. Direct evidence for ancient martian life, claimed to have been found in a meteorite that is known to have come from the Red Planet, is still highly controversial. A surprising discovery has AccessScience from McGraw-Hill Education Page 2 of 6 www.accessscience.com been the growing indication for enormous oceans beneath the surface crust of several of Jupiter’s moons (Europa in particular, but also Callisto and Ganymede). Titan, a large moon of Saturn swathed in a thick, hydrocarbon-laced atmosphere and dotted with lakes filled with liquid methane and ethane (natural gas), is considered a possible (albeit unlikely) habitat for simple life. And geysers have been seen spewing water vapor and other material from cracks in the surface of Enceladus, another moon of Saturn. These geysers suggest that underground aquifers exist beneath Enceladus’s frigid exterior. Whereas Earth was once thought to be the only solar system body that could support biology, we now find several other candidates. If any of these has spawned indigenous life, that would demonstrate that biology is a commonplace occurrence. See also: JUPITER ; MARS ; SATURN . But even if life is widespread, can intelligent life be expected to evolve frequently? This question will probably remain unanswered until we either detect extraterrestrial intelligence or learn what drove the emergence of intelligence on Earth. Search schemes 13 Given the enormous distances between the stars (the nearest is 4.4 light-years distant, or 4.1 × 10, km or 2.6 × 13 10, mi), it is beyond our current capability to send robotic probes to search directly for intelligence elsewhere. A more promising approach is to look for signals that are either deliberately or inadvertently transmitted from their world to Earth. While there are many possible ways to signal, the most promising is probably electromagnetic radiation, and more specifically, light and radio waves. In the twentieth century, radio was recognized as an effective way to send information across space; and as early as 1900, Nikola Tesla mistakenly thought he had picked up transmissions from Mars. See also: ELECTROMAGNETIC RADIATION . Radio waves travel at the speed of light, which according to current understanding of physics, is the fastest possible. Stars produce relatively little radio emission, and the universe is very “quiet” at radio frequencies, particularly in what is called the microwave part of the spectrum (approximately 1000–100,000 MHz), thereby making communication easier. Microwave signals also pass unperturbed through the gas and dust clouds that float between the stars. In 1959, Philip Morrison and Giuseppe Cocconi made calculations regarding the equipment and power necessary to signal over interstellar distances. It turned out that the requirements were not much beyond the type of equipment we could build now. Consequently, the two physicists urged that a search be made for signals broadcast by other societies that had reached or surpassed our own level of technology. They also noted the advantages of the microwave band, and suggested that 1420 MHz (21-cm wavelength), which is the frequency at which interstellar hydrogen naturally radiates, was the best part of this band to monitor. Since hydrogen is the most abundant element in the universe, this frequency will be known to all technologically sophisticated civilizations. See also: MICROWAVE ; RADIO ASTRONOMY . Frank Drake had independently reached the same conclusions, and in April 1960 he searched for artificial radio emissions from two nearby Sun-like stars, Tau Ceti and Epsilon Eridani. Drake used an antenna at the National Radio Astronomy Observatory in Green Bank, West Virginia, that was 26 m (85 ft) in diameter and tuned his AccessScience from McGraw-Hill Education Page 3 of 6 www.accessscience.com receiver near the hydrogen frequency. His search, whimsically named Project Ozma, became the prototype for today’s more comprehensive experiments, known as SETI (Search for Extraterrestrial Intelligence). In the 1970s, the National Aeronautics and Space Administration (NASA) began a modest SETI program to build equipment and develop search strategies. In late 1992, the NASA program initiated its search using the 305-m (1000-ft) Arecibo Radio Telescope in Puerto Rico, and the 34-m (111-ft) Goldstone antenna in California. However, action by the U.S. Congress stopped the program within a year. Today, radio SETI bears a strong likeness to the halted NASA effort, although advances in digital technology have improved the equipment. The largest current experiments are those conducted by the SETI Institute using the Allen Telescope Array; Project SERENDIP, which uses the Arecibo antenna and is run by a group at the University of California, Berkeley; and a search pursued at the Medicina radio observatory run by the University of Bologna, in Italy. Some of the data from SERENDIP are freely distributed for analysis by home computers, a project known as SETIhome. See also: RADIO TELESCOPE . So far, no confirmed extraterrestrial signals have been found. However, as technology improves, radio SETI will speed and extend its search for artificially produced transmissions. A major development was the first use, in 2007, of the partially completed Allen Telescope Array, which can be devoted to SETI nearly full-time. It is planned to eventually consist of hundreds of 6-m-diameter (20-ft) antennas located at the Hat Creek Observatory in northern California. A second search method is to monitor star systems for brief flashes of light, signals that might be deliberately beamed in the direction of Earth with powerful lasers. By using mirrors to focus the lasers, it is straightforward to −9 produce flashes lasting a nanosecond (10, s) or less that can greatly outshine the light from the transmitting civilization’s home star. Optical SETI, as searches for such light pulses are called, has already examined several thousand star systems. The research is being conducted at observatories in California and at Harvard University. The Harvard experiment uses a sky-scan strategy, rather than concentrating on individual star systems. See also: LASER . While searching for signals is generally regarded as the most promising scheme for proving the existence of extraterrestrial intelligence, other approaches have been suggested. One might look for examples of astroengineering by very advanced societies, or possibly the radiation produced by high-powered, interstellar spacecraft. In much of the public’s mind, the many thousand sightings of unidentified flying objects (UFOs) each year are proof that some extraterrestrials
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