By Greg Taylor of The Daily Grail
The most prevalent strategy used in SETI to this point has been the scanning of the microwave radio spectrum for narrow-band signals. The band of frequencies from 1 to 10 gigahertz has the least amount of interference from both cosmic background noise and our own atmosphere, and hence has been the focus of search strategies (Horowitz et al. 1986, p. 526). Narrow-band signals are searched for because they are characteristic of an artificial transmission: the narrowest naturally occurring microwave frequency is about 300 hertz, produced by interstellar masers. Therefore searching with a higher resolution than this is the best way to detect a signal from an extraterrestrial civilisation. Also, the narrower a signal is, the more efficient it is to send, hence narrow-band signals would seem the obvious choice for broadcasting (Billingham and Tarter 1993, p. 262). However, searching for such narrow frequency bands amongst a 10-gigahertz spectrum (which, it must be remembered, is also only a small portion of the overall microwave spectrum) is a huge task. To search the whole spectrum in 1 hertz bands would, for example, take 10 billion channels per point source observed. For this reason, many searches have centred on certain 'magic frequencies'. The most predominant is the area around 1.42 gigahertz, the emission line for hydrogen - considered by many to be the most probable 'standard interstellar frequency' (MacRobert & LePage 1999). As search technologies have improved the spectrum covered has increased. The area from 1.42 up to 1.64 gigahertz (the emission line for Hydroxyl, OH), known as 'the waterhole' due to it's boundaries (H and OH = H2O), has recently been the focus of some searches (Hoang-Binh 1985, p. 493).
Even with the advances in technology, it is obvious that large parts of the spectrum remain unchartered. Unfortunately, scanning the whole spectrum at this point in time can only be achieved through a wide-band survey, which offers a lower chance of success. Only through further improvements in technology will a narrow-band search of the entire spectrum be remotely feasible. For this reason searches are separated into sky surveys and targeted searches (Cullers et al 1985, p. 38). Sky surveys are methodical sweeps of the entire sky visible from each telescope location. If a signal is strong enough and in the correct frequency band, this search should pick it up. Targeted searches concentrate on certain points in the sky - the 'good suns', some 1000 stars similar to our own sun (type F, G and K) which lie within 100 light-years of Earth (Billingham and Tarter 1993, p. 264). Obviously, the targeted searches can be conducted much more thoroughly than sky searches, however, they rely on signals coming only from these points. Detected signals themselves could be of two different types. Either they have been intentionally sent to us by an extraterrestrial civilisation, expressly designed to get our attention, or they could be 'leaked' (that is, we have eavesdropped on them). The Earth is currently leaking signals itself, the most powerful being military radar (Papagiannis 1985, pp. 269-270) - some radio and television transmitters are also broadcasting strong signals (thankfully this is the carrier signal, not the content). SETI aims to pick up either of these two types of signal originating from an extraterrestrial civilisation by listening to as many frequency bands as is possible. Overall however, microwave scans still constitute the proverbial 'needle in a haystack' search, although continual advances in technology and processing may soon make this less of an issue.
Other methods of detecting extraterrestrial civilisations have been proposed. In the 1960's Freeman Dyson raised the possibility of advanced civilisations harnessing enormous amounts of power through the use of so-called 'Dyson Spheres' (Dyson 1963, pp. 111-113). He hypothesised that the mass of a planet such as Jupiter could be distributed in a spherical shell revolving around the sun at 2 AU, which would be capable of exploiting the solar radiation striking its inner surface. Working upon this hypothesis Dyson called for SETI to be extended to the infrared range in an attempt to detect such a construction, a proposal which the Soviet Union acted upon in the 1960's (with no success). There are some sound arguments against the feasibility of Dyson Spheres though (Papagiannis 1985, pp. 268-269). However, an important aspect of Dyson's work is that it opens our eyes to possibilities outside of our limited experience. Another more accepted alternative search method is termed 'Optical SETI'. Only a couple of years after Cocconi and Morrison's initial paper on microwave communication, an alternative method of using lasers was suggested by Schwartz and Townes (1961) in their paper 'Interstellar and Interplanetary Communication by Optical Masers'. At the time lasers were in their infancy, as opposed to the relative 'maturity' of microwave communication, so the area remained unexplored. In recent years, however, a number of projects have been initiated which are grouped as 'Optical SETI'. The idea is that a high-intensity pulsed laser and a moderately sized telescope can form an efficient interstellar mode of communication (Optical SETI Homepage 1999). Although radio waves travel further, lasers have been recognised as the superior mode of direct interstellar communication (Kingsley 1999) - this method assumes that we are the target of an intentional attempt at signalling. Thus, searches for these kinds of pulses from extraterrestrial civilisations utilise modified telescopes, fitted with a device to recognise such pulses. The search is generally conducted through the visible part of the spectrum, however some have concentrated on the infrared and ultraviolet sections. Optical SETI is one of the most exciting new directions amongst the various SETI programs currently in progress.
Tarter (1985, pp. 271- 280) classifies SETI programs under three headings; directed, shared or dedicated. Dedicated programs are those in which the sole function of the equipment is SETI: obviously the favoured choice but in reality very rare. Directed programs are those that control the acquisition of data (eg. pointing the telescope): not as ideal as a dedicated program, but more realistic. Shared programs are those in which the data acquisition is shared with another user, or which re-uses existing data collected for another purpose. The most comprehensive directed search so far has been 'Project Phoenix', a privately funded search which literally rose from the ashes of a NASA program (committed to both a sky survey and targeted search) which lost funding in the early 1990's. The program utilised a truck trailer filled with custom-built equipment and travelled to whichever radio telescopes it could secure time on (MacRobert & LePage 1999) - at times being lucky enough to get time on such massive installations as the Arecibo Telescope in Puerto Rico. Project Phoenix purchased the NASA program's equipment and discontinued the wide-band sky survey element of the project, concentrating on the targeted search instead. It was able to listen to more than two billion channels between 1.2 and 3.0 gigahertz at a resolution of 0.7 hertz per channel, easily the most impressive search by any existing SETI program (Project Phoenix General Information 1999). Its greatest weakness was that due to it being a directed search, it was only running for 5 percent of the year (assuming perfect conditions). Project Phoenix has just concluded, but unfortunately found no signals from extraterrestrial sources.