* S.E.T.I.: búsqueda de inteligencia extraterrestre

Our galaxy is very large. So big that many people believe that it is very unlikely that we are alone in it. In 1959 a new field in Astronomy arose when Giuseppe Cocconi and Philip Morrison published their theory that, given that our newly built radio telescopes are powerful enough to send and receive communications over interstellar distances, we can use them to see if anyone else is out there in the darkness of space.

Frank Drake developed this hypothesis further and in 1960 began the first listening sessions of two nearby stars similar to our Sun: Epsilon Eridani and Tau Ceti.

Drake also became famous for theorizing the factors on which the success of such a program, which became known as S.E.T.I., the acronym for Search for Extra-Terrestrial Intelligence, depends. According to him, the number of civilizations capable of interstellar communication would depend on the rate of formation of suitable stars, the fraction of those stars with planets, the number of planets with "appropriate" conditions per planetary system, the fraction of those planets where life actually develops, of these planets, the fraction where intelligence develops, of these planets, the fraction where detectable technologies are developed, and the lifespan of the civilizations capable of interstellar communication. In 1961, only the first factor was more or less known. Today we have some numbers on the second factor. The rest remains completely unknown, but we know that it is not zero, since we are here. The question is whether it is greater than 1.


Probably the only method we now have to unravel this mystery are our systems of long-distance communication, read: our radio equipment attached to huge concentrator dishes. A significant feat.

The first problem is that despite there are at least 400 000 million stars in our galaxy, and that about 20 000 million are of the same type as the Sun, the vast majority are very far away; in fact, we can only capture the most powerful transmissions, i.e., we could not listen to TV broadcasts or cell phones of "them". Probably we can only hear transmissions made expressly for interstellar communication.

Here comes the second problem: the communications we perform on Earth are in all types of frequencies for all types of users. What is more often used for interstellar communication? We expect "them" to be using the same frequencies we use for our spacecraft, i.e., microwave, but still, the range is too wide. We are forced to continue on with assumptions.

Suppose, firstly, there is someone out there sending signals intending to come into contact with other beings from other stars. We believe that these signals are to be sent at a frequency that suffers little interference: in our case, the lower frequencies become confused amid the radio signals that occur naturally in stars and clouds of gas in space, and the higher radio frequencies fail to penetrate our atmosphere. The frequencies that are better heard from Earth (or a planet like the Earth, another assumption) are those between 1000 MHz and 10 000 MHz. But that is still a very wide range for our Technology.

A further assumption is that those who are more likely to pick up a signal from heaven are the people who spend a lot of time looking up: astronomers. One interesting thing about this Universe is that, to the best of our knowledge, all chemical and physical phenomena are the same everywhere. Hydrogen is the most abundant element, and the frequency in Radio Astronomy where this element is best detected is at 1420 MHz. In fact, it is so important that by international agreement no one is allowed to send transmissions on this frequency. So we assume that "they" are going to be trying to get the attention of radio astronomers by interfering with their work, i.e. broadcasting at 1420 MHz. Based on these assumptions, Frank Drake and his followers have been listening at this frequency ever since that pioneering project, made from Green Bank, USA.


Some recent projects and others current are the ones for which people like Wielebinski and Seiradakis (Max Planck Institute for Radio Astronomy, in Effelsberg, Germany) are responsible; Gulkis (Deep Space Station 43, NASA's Deep Space Network, Canberra, Australia); Bowyer, Werthimer and Lampton (SERENDIP II, National Radio Astronomy Observatory, Green Bank); Colomb, Martin and Lemarchand (Argentine Institute for Radio Astronomy); another by Lemarchand (META II, Argentine Institute for Radio Astronomy); Bowyer, Werthimer and Donnelly (SERENDIP III, Arecibo); Horowitz and colleagues (BETA, Oak Ridge Observatory, Harvard University); SETI League (Project Argus, amateur-run, at some point with 1257 members at 105 sites in 19 countries); Werthimer and colleagues (SERENDIP IV and SERENDIP V, Arecibo); SETI Australia (Southern SERENDIP, Parkes radio telescope); SETI Institute (Phoenix Project, in Arecibo and at the Lovell Telescope in Jodrell Bank, United Kingdom); Werthimer and Anderson (SETI@home, Arecibo); Montebugnoli (SETItalia, Medicina Radiotelescopes, Institute of Radio Astronomy of the National Institute for Astrophysics, Italy); Shirai, Oyama, Imai and Abe (Very Large Array, National Radio Astronomy Observatory, New Mexico). The SETI Institute and the University of California at Berkeley also operate the Allen Telescope Array. Furthermore there is also the multinational Dorothy Project which includes the observatories of Green Bank, Harvard, Cardona, the stations RS503 (Buinen) and UK608 (Chilbolton) of the Low Frequency Array (LOFAR), the ones of Nançay, Medicina, Gwacheon, Nishi-Harima, Yamaguchi University, Tokai University, Takahashi Radio Station, University of New South Wales, the Giant Metrewave Radio Telescope (GMRT), the KAT-7 station (Karoo) of the 1 Square Kilometre Array (SKA), the Argentine Institute of Radio Astronomy, the Allen Telescope Array and Arecibo.

Beside microwaves, in recent years it is appearing the new trend of considering laser as a means of interstellar communication, especially after a few tests with the Galileo interplanetary spacecraft. In this case, the detectors are directly coupled to an optical telescope. Currently, the ones who work or have worked with this are Betz and Townes (in the infrared, from the Mount Wilson Observatory, California); Horowitz and colleagues (Harvard Optical SETI, Oak Ridge Observatory); Marcy, Reines, Butler, Vogt (in visible light, Lick Observatory, Keck Observatory); Werthimer (visible light, Leuschner Observatory, University of California at Berkeley); Bathal and Darcy (Australian Optical SETI [OZ SETI], Campbelltown Rotary Observatory, University of New South Wales, Australia); Drake et al. (visible light, Lick Observatory, University of California, Santa Cruz), and the Columbus amateur observatory.


Probably the most striking project at this time, and the most efficient one, is SETI@home, by the prestigious University of California at Berkeley.

From the technical standpoint, the two biggest problems for detecting extraterrestrial intelligent signals are getting big enough antennas to capture even a weak signal, and interpreting the data so gathered to verify that they are artificial and not from us.

This project attacked the first problem by turning to the world's largest radio telescope, the Arecibo radio telescope, in Puerto Rico, then managed by Cornell University, USA. Because telescope time is precious and S.E.T.I. projects, so uncertain, are classified in the scientific community as low-priority, systems have been developed that piggyback special receptors to the antenna and record, in parallel to the work of other projects, all signals in the frequency of choice that unwillingly enter the antenna. Although with this solution SETI researchers do not have control of where to point the antenna, over time a considerable portion of the sky gets swept.

The second point involves looking for a signal that is clearly identifiable from the background of natural radio noise. Natural signals are usually mixtures of many frequencies, while artificial signals are usually almost pure tones. It is alike identifying the sound of a flute or a violin in the bustle of a street: the pure notes, sharp and clear, stand out against the background confusion. At SETI frequencies, because the signals are not audible to the human ear, the identification is made by a computer. Other tests determine if the signal comes from other stars or if it is generated closer to Earth.

A related problem is that the data that enter this system is so large that it was not possible to get a computer powerful enough to analyze them with the desired accuracy. Technicians calculated that they would need a minimum of 100 000 personal computers to "chew" these data.

Then, in 1995, David Geyde devised an ingenious solution: to ask for help on the Internet. He imagined that if they could get a large enough number of volunteers to "lend" their computers to the project, the objectives could be achieved. The system consists of three stages: first, a computer receives and records data from Arecibo. Every few weeks the stored data are sent to California, where other computers divide these data into small "packets" of information. These packages, containing approximately two minutes of recording extracted each from a very limited frequency range, extracted in turn from a multitude of frequency ranges slightly above and below 1420 MHz, are small enough to be processed by a personal computer at the home or office of the volunteer. Once processed, the data is sent back to computers in California where the puzzle, already greatly solved, is reassembled.

The key to the system is that volunteers be disturbed as little as possible. For that, the inventor thought of a relatively small program, to be installed on the computer of the volunteer, that works only when the user is not using her or his computer: in this case, a "screen saver"-like program. All that the volunteer has to do is to install the program in her or his computer, and it does the rest: it asks for a package to California, via the Internet, then processes this packet each time the volunteer pauses using her or his computer for a few minutes (with decorative graphics included) and stops when she or he returns. After some days the package has been fully processed and is ready to be returned to California: new Internet communication, shipping back the result and ordering another package for analysis. The Internet connection is done automatically and is very brief (a few minutes), happening every few days.

Launched in 1999 through the website http://setiathome.berkeley.edu, about 150 000 people worldwide currently participate in SETI@home with their personal computers, without being interfered in their daily work. And the system works so well that it was copied by other scientists in the most varied fields.


No one really knows whether these efforts are sufficient to detect extraterrestrial intelligence, if "they" exist. It's almost like playing the lottery hoping that it is written in our destiny to be the winners. Although it is also true that he who never plays, never wins. S.E.T.I. is a marginal community within Astronomy today, but still it is trying to maintain seriousness and a high scientific standard. For example, institutions and individuals participating in the search for extraterrestrial intelligence have thought of a protocol to be followed in case we really discover aliens. It can be applied even if "they" are discovered by other methods than those described in this article.


1) Check if it is aliens, before disclosing the discovery.

2) Check by third parties before disclosing.

3) Notify the International Astronomical Union and the Secretary General of the United Nations, and the International Academy of Astronautics, the International Telecommunication Union, the International Council of Scientific Unions, the International Institute of Space Law, the International Astronautical Federation and the International Union of Radio Science.

4) Disclose quickly, openly, widely.

5) Pass the data to the international scientific community.

6) Monitor and record everything.

7) If it is an electromagnetic signal, minimize interference.

8) Do not reply until this has been consulted internationally.

9) Assemble an international committee to study the phenomenon.

Aldo Loup.

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Based on a lecture given at the USP, on 11 March 2000. Originally published in ABC Color, on 8 October 2006. Photograph: The Arecibo radio telescope, with its 305 meters in diameter, is the world's largest. The University of California at Berkeley collects the signals coming from the sky in this antenna and distributes them to over a hundred thousand computers of volunteers around the world, for analysis in search for extraterrestrial intelligence. Credit: Photo courtesy of the National Astronomy and Ionosphere Center - Arecibo Observatory, Puerto Rico, a facility of the National Science Foundation of the United States of America, designed and formerly operated by Cornell University, currently operated by SRI International (ex - Stanford Research Institute), the Universities Space Research Association and the Metropolitan University, Puerto Rico, under a cooperative agreement with the National Science Foundation of the USA. Photo courtesy of NAIC - Arecibo Observatory, a facility of the NSF. With permission of the U.S. National Astronomy and Ionosphere Center - Arecibo Observatory, a facility of the U.S. National Science Foundation.