dialogue with an advanced ETI who could solve many of our problems and invite us to join the Galactic Club, while pessimists warn of invasion and slavery. But what if no signal is encountered? Demands will inevitably be made for larger and more extensive searches with radio telescopes located on the Moon at a cost exceeding $100 billion, for there can be no conclusive proof that the search has failed. Either there is a pot of gold awaiting the SETI teams or a journey down an endless road.

Despite the advanced technology, radio searches face tremendous problems. The terrestrial use of microwaves is on the increase and the consequent background noise has become a matter for serious concern. Radio frequency interference (RFI) is increasing from the use of cellular phones, microwave data transmissions, police radar and communications satellites. Over 100 new  satellites are launched each year, many of which leak signals into the spectrum reserved for radio astronomy. One serious example of radio interference was the former Soviet GLONASS navigation satellite system which, for a period after 1982, was a considerable source of interference for radio astronomy, especially for receivers on the hydroxyl line. During the 1980s transmissions from this system affected 1612 MHz OH observations all over the world. This particular problem was resolved after talks between the GLONASS administration and the Inter-Union Commission on Frequency Allocation for Radio Astronomy and Space Science (see McNally, 1994). In July 1999, astronomers presented a document seeking protection for radio astronomy to the UN Committee for the Peaceful Uses of Outer Space. It requested that light sources, radio transmissions and various space junk, should be classified as global pollutants, and that any planned space activity likely to affect astronomy should undergo prior evaluation. Unless urgent curbs are introduced to protect against RFI and other forms of space pollution, it may well be the case that our technological civilization has paradoxically foreclosed on radio communication within a little over the one hundred years it has been available.

SETI researchers are not merely looking for a needle in a haystack; they are looking for a very special needle in a haystack which is already full of needles. Or, to use another image: it is like trying to hear a pin drop in a crowded ballroom with more dancers arriving all the time. Some pessimistic forecasts suggest that the present rate of increase of human-made radio noise will put ground-based radio astronomy out of business within twenty years. As powerful interests are involved there is no strong political will to curb RFI. The future of radio astronomy in general, and SETI in particular, lies in operations conducted in outer space or on the far side of the Moon.


Optical signals: an alternative

Questions have been raised with regard to SETI’s reliance upon radio signals as a means of communicating with ETs. As early as 1961 it was proposed that optical signals may be an alternative to radio waves. The search for evidence of










extraterrestrial communication by optical masers was first proposed by R.N. Schwartz and C.H. Townes (1961) in the journal, Nature. They suggested that an advanced civilization might have become very sophisticated in the use of optical masers rather than in the techniques of short radio waves. Nigel Henbest (1992b) revived the debate over optical signals when he argued that radio communication in space is very much a 1950s’ creation, when the idea of interstellar signals was associated with powerful radio telescopes. But communicative technology has moved on during the past forty years and a more advanced civilization would have moved on even further. The future for interstellar communication, argued Henbest, lies in lasers. It is likely that a more advanced society will be thinking of laser rather than radio.

In 1974 there was a small-scale search of three stars which used the Copernicus satellite to look for ultraviolet laser emissions of intelligent origin. Nothing was found. Nevertheless, the case for an optical search rests on an appeal to the technical development in lasers during the past forty years together with the possibility of setting up telescopes beyond the Earth’s atmosphere. Lasers are amplifiers of radiation which send out extremely ‘clean’ light signals of a very small bandwidth. Their light is not diffused or fanned out like a searchlight.

J.D.G. Rather (1988: 386) has provided four arguments in support of an optical search. First, lasers are ‘a versatile technology for producing highly directional beams that deliver maximum power on target’. Second, ‘by targeting the power, rather than broadcasting it, vastly better use is made of the energy available’. Third, ‘the availability of laser power at any chosen wavelength makes possible the choice of wavelengths that show the intelligent signal in maximum contrast to its natural environment’. Fourth, the potentially enormous brightness (watts/steradian) of a well-designed laser system makes possible delivery of high signal-to-noise signals having tremendous data rates anywhere in the Galaxy. There is no need to worry about photon noise when dealing with truly advanced civilizations.

The instruments are available for an optical search, but until fairly recently it has been assumed that those who transmit would opt for microwave transmitters which require less power than optical wavelengths. A search for optical lasers was considered during the 1970s and 1980s, but was rejected by many SETI scientists, who favoured microwave searches on the grounds that microwaves offered a maximum detectable range because they contain more photons per unit of energy. Laser searches also faced problems with the construction of receivers, which were said to require highly polished antenna surfaces that are susceptible to weather changes. Interstellar dust may also interfere with optical signals. The Cyclops study made a careful comparison of lasers versus micro-waves and came down in favour of the latter. Primarily, the reasons cited were that lasers would consume more energy and were prone to interference from




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