synchrotron radiation, thermal noise, quantum noise and star noise. The authors of Cyclops put forward the case for a search in the free space microwave window between 1–60 GHz.

Initially, then, the SETI community were unfavourable towards optical searches, but lately the Planetary Society and the SETI Institute have adopted a more positive standpoint. The Columbus Optical SETI Observatory is committed to a laser search, which has been endorsed by Arthur C. Clarke. In 1998 the SETI Institute initiated ‘Optical SETI’ which searches for pulsed laser signals. This development was facilitated by the promise of a high-intensity pulsed laser which, working together with a modern telescope, could transmit signals beyond 1,000 light years. The argument is that if such a system is possible on Earth, then ETs with similar technological skills might already be transmitting laser signals. The objective of Optical SETI is to employ  Harvard’s 61-inch optical telescope in a radial velocity of 2,500 nearby solar-type stars. Two teams are associated with this project: Paul Horowitz and his team at Harvard, and one led by Dan Werthimer at the University of California.

Among the reasons for the revival of optical searches is that recent calculations have indicated that ionized hydrogen in the interstellar medium can cause a dispersion that degrades transmitted radio signals, and it may be the case that a more advanced extraterrestrial civilization will be aware of this fact and consequently prefer to communicate via optical signals. Moreover, it has been recognized that laser detection does not require very sophisticated equipment: primarily it requires a pair of fast photon-counting detectors working in ‘coincidence’, argues Horowitz (2000: 10). One of the problems with optical signals could be the requirement to separate the luminosity of the beam from the background light of the parent star. But even with Earth’s current level of laser technology a beam could be sent that ‘would appear about 5,000 times brighter than the background light from our Sun’, Horowitz points out (ibid.: 10). In which case an extraterrestrial society, at the same level as us or only slightly more advanced in laser technology, would have no problems in producing a beacon that would outshine its parent star.

How would an extraterrestrial society transmit a laser signal? They might either send out a series of laser pulses of considerable intensity or they might send out a continuous beam. Horowitz and the Harvard team employ a system to detect a pulsed laser signal using Harvard’s optical telescope. From October 1998 to November 1999 they observed about 2,500 stars without finding evidence of intentional laser signals. But the search has barely begun. With further collaboration these optical searches will extend in both their scope and accuracy. Plans are underway for an ‘All Sky Optical Search’, using over 500 detectors, which could cover the full northern-hemisphere sky within 150 clear nights (ibid.: 14).

Searches are also underway for a continuous laser signal. Planet finder Geoffrey Marcy is conducting a search at the Lick Observatory in California, the Keck Observatory in Hawaii and the Anglo-Australian Observatory in Australia.

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This search is examining the vicinity of 1,000 stars for evidence of intentional narrow laser-like signals.

A step-by-step approach

The main problems with both the microwave search and the proposals for an optical search is that they are high risk methods; they can only be successful if we are listening at the precise time that they are transmitting. Moreover, the longer the search continues without results – despite valid claims that only a small fraction of space has been searched – the risk of public disenchantment increases. The way forward was outlined in the late 1980s when Bruce Campbell (1990), who had been developing techniques for observing planets, suggested that radio searches should be combined with observational searches for planetary systems. He described this as a step-by-step approach. The first step, involved a search for ex-solar planetary systems by extending and improving planetary detection techniques. This meant that larger planets would be discovered first.

The second step depends upon further developments in detection techniques which may lead to information about smaller planets, such as their frequency, whether they have sufficient mass to retain a dense atmosphere, and whether they are located in a thermally habitable zone. The third step would involve a search for conditions favourable to life, by seeking signs of oxygen, whose presence may be an indicator of plant life. A step-by-step approach may not lead to direct contact with intelligent life and may only yield knowledge of primitive plant life. But this would be of enormous significance. This approach is certainly compatible with radio searches and optical searches and, as early steps are taken, with the promising discoveries of large ex-solar planets since 1995, there would be good reasons for devoting longer listening periods in the vicinity of particular stellar systems. Thus further stages in the step-by-step approach could be followed up by greater concentrations of listening zones where potentially habitable planets are predicted.

Neutrino searches

Searches for ETI have to be based on known principles of science and technology. This is one of the ground rules which separate science from fiction, and such rules are the subject matter of philosophical inquiry. According to J.G. Learned, S. Pakvasa, W.A. Simmons and X. Tata (1994: 328), a useful SETI strategy would be to search for neutrino timing signals from an advanced intergalactic civilization. They suggest that ‘an advanced civilization colonizing our Galaxy will be likely to have the need to synchronize clocks over interstellar distances’. If this civilization is to maintain communication with its colonies it will ‘require interstellar time standards at the limits allowed by physical processes’ (ibid.: 321). They propose that this civilization communicates with its colonies with the help of neutrino timing pulses, a system of ‘timing marker

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