The search for extraterrestrial intelligent life, SETI, began in the 1950s when Cocconi and Morrison (1959) suggested that microwave radio signals might be used to communicate between the stars. As already mentioned in Chap. 5, this suggestion was taken up by F. Drake, at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, who over four months in project Ozma directed a 25 m radio telescope for six hours every day toward the two G-stars t Ceti and e Eridani, to search for regularly patterned pulses indicating intelligent civilizations. While Drake did not detect any signal from extraterrestrials, project Ozma spurred on the interest of others in the astronomical community, most immediately Russian colleagues such as I.S. Shklovskii and N. Kardashev. Initiated by the summer study project Cyclops at the NASA Ames Research Center, which looked for the best way to detect radio signals from extraterrestrials, the META project of Harvard University (now continued as BETA and run by the Planetary Society), the University of California's SERENDIP project, and an observing program at the Ohio State University were developed in the 1970s. The latter now cancelled program became famous because of the "wow!" signal, detected in 1977, which had the appearance of an extraterrestrial signal, but was only seen briefly and did never repeat.
META I (the Million-channel Extraterrestrial Assay) was a search program which, from 1985 to 1995, used the 26 m steerable radio telescope at Harvard, Massachusetts. Its counterpart, META II, with a 30 m antenna, at the Argentinian Institute for Radio Astronomy near Buenos Aires, provided coverage of the southern sky. META I and II monitored 8.4 million radio channels at once with a spectral resolution of 0.05 Hz, and reached a combined sky coverage of 93%. After five years of observations from the Northern Hemisphere and the recording of 6 x 1013 different signals, META I found 34 "alerts". Unfortunately, the data were insufficient to determine their real origin. Interestingly, the observed signals seemed to cluster near the galactic plane, where the largest numbers of Milky Way stars are located. After three years of observations, META II found 19 signals with similar characteristics.
BETA (the Billion-channel Extraterrestrial Assay), the follow-up program of META I (see SETI 2005), began observations in October 1995. It broke down in 1999, but is scheduled to resume operation in 2006. It employs a new strategy of rapid and automatic observation of candidate events, a better discrimination of terrestrial interference, and a much greater frequency coverage. With a 240-million-channel spectrometer, the output of which feeds an array of programmable "feature recognizers", BETA searches the full "water hole" of 1.4-1.7 GHz. The antenna incorporates two (east west) feed horns and a third low-gain terrestrial one. When, as a consequence of the Earth's rotation, a suspicious celestial signal is first seen in the east, then in the west, but not in the terrestrial horn (detection there is attributed to terrestrial sources), it triggers the antenna to jump to the west, forcing the source to move through the detection sequence again. If the signal is confirmed, the antenna will break off its general survey and start detailed tracking of the newly discovered source.
SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) is the University of California at Berkeley SETI Program (see SETI 2005). The idea of the project is to "piggyback" alongside simultaneously conducted conventional radio astronomy observations. It uses the 300 m dish at the Arecibo Observatory in Puerto Rico (see Fig. 10.5), the largest radio telescope in the world. From 1992 to 1996 the spectrum analyzer, SERENDIP III, working essentially on a full-time basis, examined 4.2 million channels every 1.7 seconds in a 12 MHz-wide band centered at 1.429 GHz. This is only a small piece of the electromagnetic spectrum, but it is by far the largest segment ever examined so comprehensively. SERENDIP IV, the next-generation instrument, examines 168 million chan-
nels every 1.7 seconds in a 100 MHz band centered at 1.42 GHz. SERENDIP signals that peak significantly above the background noise are run through a series of algorithms designed to reject terrestrial sources.
Since 1998, the Southern SERENDIP project at the University of Western Sydney, Macarthur, Australia, has operated on the same principle, by piggybacking onto the conventional radio astronomy observations at CSIRO's 64 m Parkes radio telescope, the largest radio astronomy telescope in the Southern Hemisphere. Southern SERENDIP currently scans 8 million radio channels every 1.7 seconds, and this will be increased to 58 million channels in the near future. Figure 10.6a shows a typical recording of 2.5 MHz bandwidth, from 1.4180 to 1.4205 GHz. Because of its appearance, this diagram is called a waterfall plot. On the horizontal axis one plots the 4.2 million channels that cover the 2.5 MHz, and on the vertical axis the observation time. Each horizontal line represents a momentary reading of the 4.2 million channels. A dot represents a "hit" where the signal strength is at least 12 times greater than the mean of the output of the 8000 adjacent channels. Most of these "hits" are receiver noise. The 2200 lines of observation represent about one hour of observation. This waterfall mode of recording is seen better in the older plot from the early 1970s shown in Fig. 10.6b, where the slanted track is due to a pulsar. Its frequency behavior is due to the fact that both the observer and the pulsar are moving in a nonuniform manner. The vertical tracks of Fig. 10.6a are due to interference by terrestrial radio sources. The SETI observers look for slanted tracks which indicate a celestial radio source. Comparison of the two panels of Fig. 10.6 shows the large increase in frequency and time resolution in 25 years.
Fig. 10.6. Waterfall recordings. a. 1970s with a pulsar (see SETI 2005)
SERENDIP. b. Older observation from the
Fig. 10.6. Waterfall recordings. a. 1970s with a pulsar (see SETI 2005)
In 1992, NASA started its own SETI program, called the High-Resolution Microwave Survey (HRMS). Unfortunately, after a year, as a result of intense lobbying by scientists with a strong conviction that there can be no close-by extraterrestrial intelligent societies, Congress cancelled funding. Fortunately, however, private means were found and the venture, renamed Project Phoenix, operated by the SETI Institute (see SETI 2005) from 1995 until its termination in 2004. It searched the frequency range between 1.0 and 3.2 GHz with 1 Hz resolution, using 28 million channels at a time. Rather than trying to scan the entire sky, the project targeted approximately 1000 nearby Sun-like stars. After its start at the Parkes radio telescope, it later moved to the newly upgraded Arecibo Observatory. After completing more than half of their targets, they have not yet found evidence of extraterrestrial transmissions.
Due to the large computational effort, to sift through millions of frequencies and events to isolate a few "alerts", a new strategy launched in 1999 is to tap the unused computer power of thousands of home PCs by inviting their owners to participate in the project [email protected] (see SETI 2005). In this project, instead of running a screen saver, the PC analyses SETI data.
Here computer users from around the world are able to participate in a major scientific experiment.
Because of the limited time available on the Arecibo Observatory it was long felt desirable to have a radio telescope that would dedicate its entire time to SETI. This dream could be realized by generous private donations by Paul Allen (co-founder of Microsoft) and Nathan Myhrvold (Chief Technologist of Microsoft) that allowed the construction of a new instrument called the Allen Telescope Array. This telescope of which the first test runs are expected in 2005-2006 envisions an array of 350 commercial 6-m radio dishes with liquid air-cooled feeds, built on the grounds of the existing Hat Creek Observatory, located in the Cascades near Lassen Peak, 467 km north of San Francisco, and run by the Radio Astronomy Lab of the University of California, Berkeley and the SETI-Institute. With a collecting area comparable to the Arecibo Observatory this instrument will be simultaneously used for both SETI and radio astronomy research.
Finally, based on the idea that extraterrestrial societies may communicate with light pulses and that with present technology it is possible to produce laser pulses that outshine the visible spectrum of the Sun by 4 orders of magnitude there are several new optical SETI Projects at Harvard, Princeton, Berkeley, and elsewhere (see SETI 2005). Using both dedicated and piggyback instruments, the aim is to detect laser pulses with time scales of a few nanoseconds (billionth of a second) emitted by extraterrestrial societies. The Harvard detectors, for example, were directed at some 13 000 Sun-like stars, and made some 16 000 observations totaling nearly 2400 h during five years of operation (Howard et al. 2004).
However, from the null result in all these intense efforts one must conclude that the attempt to identify extraterrestrial intelligent civilizations in our galactic neighborhood by their radio or light emissions have so far failed. This should not deter us from working hard to improve our technical equipment and search strategies because, as Giuseppe Cocconi and Philip Morrison put it in their 1959 article: "if we never search, the chance of success is zero".
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