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Extraterrestrial

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Publication year: 2014

Postulated entities beyond with a level of intelligence and comprehension at least equal to that of present-day . While extraterrestrial intelligence is usually envisioned as an advanced , populated by creatures that have evolved via Darwinian on a vaguely similar to Earth, it could conceivably be artificial intelligence initially created by biological beings. Extraterrestrial intelligence is a subset of , which encompasses all aspects of the existence of, and search for, . Astrobiology is sometimes referred to as exobiology or bioastronomy. See also: ; 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, ) 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 of the type that might spawn intelligence, with thick 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, have detected many hundreds of planets around other, -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 consisting of at least one orbiting body. The major discovery techniques so far have been to measure the small motions of the 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 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 ( in particular, but also Callisto and Ganymede). , a large moon of Saturn swathed in a thick, hydrocarbon-laced and dotted with lakes filled with liquid methane and ethane (natural gas), is considered a possible

(albeit unlikely) for simple life. And geysers have been seen spewing water vapor and other material from cracks in the surface of , 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 , 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 . See also: MICROWAVE ; RADIO .

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 , 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 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 ; 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 are actually visiting Earth. Most scientists are skeptical of such claims, and one of their principal objections is the lack of convincing physical evidence, despite more than a half-century of publicized sightings.

In addition to the expected improvements in search technology, advances in astronomy will aid SETI efforts. In particular, it is possible to construct space-based telescopes that could directly image planets around other stars.

By analyzing the light reflected from these planets’ atmospheres, one could search for biomarkers such as oxygen AccessScience from McGraw-Hill Education Page 4 of 6 www.accessscience.com

and methane that would indicate the presence of biology. Future SETI experiments will benefit from being able to concentrate their efforts on star systems where worlds with life are known to exist.

Drake equation

In 1961, Drake devised a simple formula to calculate N , the number of broadcasting societies in the

Galaxy. The computation multiplies the rate at which such societies arise, R , by the average length of time they survive, L . This method is akin to computing the number of students at a university as the product of the number admitted each year times the number of years (4) before the average student graduates. The rate R is composed of terms that estimate the birth rate of stars, the fraction that have habitable planets, the chances that biology will appear, and the fraction of the worlds with life that develop intelligent, technically competent beings. While many of the terms in the remain very speculative (particularly L , which depends on sociological factors), the formula remains a highly useful framework for discussing the subject of extraterrestrial life.

Fermi paradox

Enrico Fermi is said to have posed the question “Where is everybody?” in 1950. His remark was intended to point out that, while humans are in no position to colonize other star systems, advanced extraterrestrials—if they exist—might do so. Even if they require thousands of years to travel from one star to the next, an ambitious society could spread itself throughout the entire Milky Way Galaxy in only a few tens of millions of years. Since this interval is far less than the age of the Galaxy (about 12 billion years), it suggests that, if sophisticated and ambitious societies arose in the past, evidence of their presence should now be everywhere. Since that evidence is lacking, the implication is that the Galaxy is inhabited solely by humans.

There are many suggestions of how this paradox might be resolved; in other words, how the failure to see local evidence of alien activity could be reconciled with a Galaxy that we think might house many sophisticated societies. For example, it could be that interstellar colonization is so daunting that no one ever undertakes it.

Perhaps humans are incapable of recognizing the widespread presence of intelligence. Or the extraterrestrials could know about humans, but have arranged for a “one-way mirror,” whereby they can watch, but humans cannot detect them (the so-called “”). The , while intriguing, remains a point of discussion rather than a key to new knowledge.

Consequences of discovery

If a signal or other verifiable proof of extraterrestrial intelligence is discovered, what would be the effects on society? Clearly, this result depends on how distant the intelligence is, and whether (in the case of a signal) an embedded message can be deciphered. It is possible that any intelligence that is found will be hundreds or even thousands of light-years distant, in which case two-way conversation or direct, physical would be exceedingly difficult. AccessScience from McGraw-Hill Education Page 5 of 6 www.accessscience.com

In the case of a SETI detection, it is quite likely that the transmitting intelligence will be far more advanced than our own. This is because there is a good chance of detection only if L is large (at least thousands of years), and it is unlikely that the first signal received would be from a society that is only a few hundred years beyond us. (Of course, signals will not be found from societies that are not at least as accomplished as ours.) If such advanced beings wish to transmit useful information, human society would be greatly altered. On the other hand, it may be that humans would never be able to decode the transmissions from such a sophisticated civilization. Even in that case, merely learning that intelligence has developed elsewhere in the cosmos would be a profound event.

Seth Shostak

Keywords extraterrestrial intelligence; SETI: aliens

Bibliography

P. Davies, The Eerie Silence: Renewing Our Search for Alien Intelligence , Houghton Mifflin Harcourt, Boston,

2010

I. Shklovskii and C. Sagan, Intelligent Life in the Universe , Holden-Day, New York, 1966, reissue edition,

Emerson-Adams Press, Boca Raton, FL, 1998

S. Shostak, Confessions of an Alien Hunter: A Scientist’s Search for Extraterrestrial Intelligence , National

Geographic, Washington, D.C., 2009

S. Webb, If the Universe Is Teeming with Aliens, Where Is Everybody? Fifty Solutions to Fermi’s Paradox and the Problem of Extraterrestrial Life , Copernicus Books, New York, 2002

Additional Readings

I. Alm´ ar and J. Tarter, The discovery of ETI as a high-consequence, low- event, Astronaut. Acta ,

68(3):358–361, 2011 DOI: http://doi.org/10.1016/j.actaastro.2009.07.007

L. Billings, Astrobiology in culture: The search for extraterrestrial life as "," Astrobiology , 12(1):966–975,

2012 DOI: http://doi.org/10.1089/ast.2011.0788

C. B. Cosmovici, S. Bowyer, and D. Werthimer (eds.), Astronomical and Biochemical Origins and the Search for Life in the Universe , Editrice Compositori, Bologna, 1997 AccessScience from McGraw-Hill Education Page 6 of 6 www.accessscience.com

R. D. Ekers and D. K. Cullers (eds.), SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence , SETI

Press, Mountain View, CA, 2003

A. A. Harrison, After Contact , Plenum Press, New York, 1997

B. McConnell, Beyond Contact , O’Reilly and Associates, Sebastopol, CA, 2001

H. P. Shuch, Searching for Extraterrestrial Intelligence: SETI Past, Present, and Future , Springer, Little Ferry,

NJ, 2011

P. D. Ward and D. Brownlee, , Copernicus Books, New York, 2000, paper, 2003

Berkeley SETI Program

Harvard SETI Program

Listing of All Extrasolar Planetary Discoveries

SETI Institute

SETI Italia