models of the external world and the ability to use these skills to build things and, in some cases, predict the future (Gribbin, 1991). A related definition of intelligence has served as a guideline for the Harvard-based SETI project: ‘life that has acquired a technology required to carry out interstellar communication’ (Horowitz and Alschuter, 1990: 261).
What kind of signal is likely to be transmitted by intelligent ETs operating with similar technology to our own? One of NASA’s radio astronomy experts in SETI, Jill Tarter, has specified the requirement that:
the candidate signal be narrowband or concentrated in frequency, and perhaps also concentrated in time. If the latter is the case, then the signal must be no more extended in frequency than is demanded by its time duration and the laws of physics as we now understand them.
(1991: 191)
What frequency, or range of frequencies, should be searched? A meaningful signal cannot be identified against a noisy background. This eliminates long radio wavelengths. At long wavelengths (approximately one metre) noise back-ground is too great, owing to a process known as synchrotron radiation. On the other hand, very short (higher) frequencies (near one millimetre) exhibit too much noise from the remains of the big bang that created the universe. In the infrared region, at wavelengths of the order of one micron, there is a strong background noise from the glow of warm dust clouds. At the shortest wave-lengths of all, X-ray and Gamma-ray emissions create background noise.
The quietest space is in frequency units known as the microwave portion of the electromagnetic spectrum. In frequency units this extends from about 1GHz to 10GHz which includes the hydrogen frequency and the Waterhole. For SETI researchers the range of frequencies relatively free from cosmic noise is known as the Free Space Microwave Window, representing a relatively clear channel for any species, anywhere in the galaxy, that can operate a radio transmitter.
Project Cyclops
In 1971 a NASA team headed by Bernard M. Oliver, Vice President of the Hewlett Packard Computer Company, proposed a sophisticated system for listening for an ETI radio signal. It was published under the title Project Cyclops: A Design Study of a System for Detecting Extraterrestrial Intelligent Life (Oliver and Billingham, 1973). The Cyclops proposal would have involved a system of 1,000 antennae connected by information tunnels to a central information-gathering building. It was to be highly automated, requiring human attention only if an anomaly was found. Staff and their families would live in Cyclopolis, a town to be constructed with schools, housing and stores. Although Cyclops was designed as a receiving system, there was nothing in its design to prevent it from being used as a beacon. It was considered a good idea to alternate between listening
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and transmitting; if the proposed target search of 1,000 stars produced no signal, then a brief message could be broadcast for a year or so before moving the search into deeper space.
Project Cyclops recommended the microwave window as the most suitable part of the spectrum for the search and outlined the design of a data-processing system capable of a simultaneous search of the most likely area within this window. A blind search was ruled out and Cyclops envisaged initial target lists of stars which would expand and be subject to re-evaluation as construction of the complete system developed towards a total fully automated search. Some places, however, were deemed to be better to search than others: for example, if interstellar communication is a reality, then there may be a special place in the galaxy where the beacon is likely to be located. Thus, held the Cyclops team, it is likely that the galactic culture has constructed a powerful beacon in the centre of the galaxy, or maybe members of the galactic culture have adopted a convention of radiating beams in a specific direction, either from or towards the centre.
The Cyclops team expressed a preference for a radio search at the lower end of the microwave window, between 1–3 GHz. Their reasons were that there are smaller Doppler shifts; less stringent frequency stability requirements; a greater collecting area for the narrowest usable beam; reduced cost per unit area of collecting surfaces; smaller power densities in transmitter tubes, waveguides, feeds and/or radiators, thus allowing higher powers per unit, greater freedom from 02 and H20 absorption, which may be more on some planets having our life-forms.
What kind of radio signal did Cyclops envisage? The underlying assumption was that the extraterrestrials would go to great lengths to make the message as decipherable and as obvious as possible. The beacons would have the following properties: there would be continuous transmission; they would be aware that those listening may ration time for each star, so that they would signal continuously to avoid being missed; they would be monochromatic; they would probably be circularly posed; their radio waves would arrive with either right- or left-hand polarization, thus reducing the need for extra equipment by the searching race. Finally, they would be information-bearing, not just a signal awaiting a reply, which would waste too much time; they would start with information, including information on how best to reply, explaining which frequency or modulation rate was preferable. The authors of Project Cyclops also recognized the problem of communicating with beings who may not share similarities in natural history or culture; they recognized, from terrestrial examples, that there is a great difference among races as to what is considered obvious. Mathematics, they held, has a universality which would have a meaning for any society capable of radio communication, and the Cyclops team suggested that ETIs might attempt a pictorial message in the form of a binary code of sequences having particular lengths that could be arranged into a picture. There would, however, be a major problem of interpreting the picture without a common set of rules or conventions for recognizing and reading pictures.
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