One can of course put such beacons at the planet's poles or in long period orbits in space. Whatever is done to minimize Doppler and oscillator drifts, the frequency proportionality remains. The absolute achievable bandwidth at any frequency may be reduced by improved technology but the frequency of the minima in figures 3 and 4 is unaffected.

THE WATER HOLE

Even though minimum receiver noise per channel can be achieved at about 1.5 GHz, as shown in figure 2, the increase is not very rapid on either side. We are left with at least a 2-GHz-wide region of the spectrum with very little technical reason to prefer one part over another. Clearly, this band is too wide to reserve for interstellar search purposes, being needed for other services (see Sections III-8 and III-9). It was here that the Cyclops team, seeking to economize on search time and spectrum occupancy, observed that the hydrogen and hydroxyl lines are right at the optimum spectral region and, between them, define a rather conspicuous band. As stated in conclusion 6 of the Cyclops report:

Nature has provided us with a rather narrow band in this best part of the spectrum
that seems especially marked for interstellar contact. It lies between the spectral lines
of hydrogen (1420 MHz) and the hydroxyl radical (1662 MHz). Standing like the Om
and the Um on either side of a gate, these two emissions of the disassociation products
of water beckon all water-based life to search for its kind at the age-old meeting place
of all species: the water hole.

It is easy to dismiss this as romantic, chauvinistic nonsense, but is it? We suggest that it is chauvinistic and romantic but that it may not be nonsense.

It is certainly chauvinistic to water-based life, but how restrictive is such chauvinism? Water is certain to be outgassed from the crusts of all terrestrial planets that have appreciable vulcanism and, therefore, a primitive atmosphere capable of producing the chemical precursors to life. We can expect seas to be a common feature of habitable planets. Exobiologists are becoming increasingly disenchanted with ammonia and silicon chemistries as bases for life. Water-based life is almost certainly the most common form and well may be the only (naturally occurring) form.

Romantic? Certainly. But is not romance itself a quality peculiar to intelligence? Should we not expect advanced beings elsewhere to show such perceptions? By the dead reckoning of physics we have narrowed all the decades of the electromagnetic spectrum down to a single octave where conditions are best for interstellar contact. There, right in the middle, stand two sign posts that taken together symbolize the medium in which all life we know began. Is it sensible not to heed such sign posts? To say, in effect: I do not trust your message, it is too good to be true!

In the absence of any more cogent reason to prefer another frequency band, we suggest that the water hole be considered the primary preferred frequency band for interstellar search. This does not mean that other frequencies should be ignored. Harmonics of the hydrogen line deserve some attention. In space, the waterline itself, at 22 GHz, may have merit. It does mean, however, that the water hole deserves the greatest attention for protection against interference.

It is always possible to dismiss the above argument on the grounds that we do not know everything yet and that there may be some more compelling reason to choose another frequency band or even some as-yet-to-us unknown method of communication. These assertions are undeniable and also unacceptable since they logically, lead to never doing anything. If we are to make progress we must proceed on the basis of what we know, and not forever wait for something now unknown to be discovered.

REFERENCES

1. Nature 183, 844, 1959.

2. Oliver, B. M., and Billingham, J.: Project Cyclops, A Design Study of a System for Detecting Extraterrestrial Intelligent Life. NASA CR 114445, 1972.

3. Heffner, H.: The Fundamental Noise Limit of Linear Amplifiers. Proc. IRE, vol. 50, no. 7, July 1962, pp. 1604–1608.

5. SEARCH STRATEGIES

INTRODUCTION

The objective of SETI strategies is the detection of the existence of other intelligent species in the Universe by examining the spectrum of electromagnetic radiations in the vicinity of the Earth. Communication, one-way or two-way, does not directly concern us now, only detection.

Another characteristic of this objective is that out of all the intelligent species that may exist, we are seeking another member of the subset to which we belong. The essential property of our subset is that its members radiate electromagnetic energy into interstellar space in such large amounts and in such a distinctive fashion that at interstellar distances we can recognize it as an artifact against the background of "natural" radiations.

In the past several decades we have developed a trenchant technology appropriate to searching in the spectral range of the free-space microwave window. No comparable technology has yet been generated for shorter wavelengths, so the remarks below are directed in the main toward strategies suited to an initial search in the microwave window and give special attention to the water hole (see Section II-4). However, our suggested strategies do encompass the spectrum above the microwave region, for relevant physical knowledge and suitable technologies are burgeoning (e.g., in the infrared region of the spectrum).

A final distinction search strategies as discussed here, though closely related, are not equivalent to search programs. The latter are a topic unto themselves since many factors not touched on here enter their design.

GENERAL CONSIDERATIONS

We cannot afford to search for all kinds of radiation at all frequencies from all directions at the lowest detectable flux levels. It is necessary to put some bounds on the volume of our multidimensional search space. At the same time it is important not to narrow the space too much: to put all our eggs into one basket. We submit that a rational approach is to assess all strategies and to attempt to assign relative a priori probabilities of success per unit cost. If only one strategy can be pursued at a time, one chooses the most likely and continues until success is achieved or until the accumulated negative results have depressed the probability/cost ratio below that of some other strategy, in which case the other strategy would then be pursued. If several strategies appear to have comparable probability/cost ratios they may all be pursued in proportion to these ratios.

The case for preferring electromagnetic to any other form of radiation seems compelling (see Sections II-4 and III-1). The case for preferring the microwave window (approximately 1-102 GHz) seems very strong but not necessarily compelling. The case for preferring the low frequency end of the window to the high seems strong but not so strong that no attention should be paid to higher frequencies. The case for the water hole is very appealing (as a starting place) if one has already decided on the low end of the microwave window.

The case for searching nearby main sequence F, G, and K stars at ever-increasing range seems very natural; the only life we know lives on a planet around a G2 dwarf star. To adopt this strategy takes us only as far into space as necessary to find perhaps our closest neighbor. Communication with a close neighbor would permit more two-way exchanges than with a civilization in the Andromeda nebula, for example.

On the other hand, cultures only slightly older than ours may be able to exploit enormously greater communicative capabilities. As is true for stars, the nearest transmitters may not be the strongest. The strongest signals may come from more advanced cultures at great distances. For these reasons it would be a mistake to pursue only one search strategy, such as that suggested in the Cyclops report. One should in addition examine other options. To cover other possibilities, it seems prudent to conduct a complete search of the sky over as wide a frequency band as practicable (see Section III-3). To be significant, such a search should extend to frequencies and down to flux levels not reached by existing radio astronomy surveys.

Our great uncertainty as to the likely flux levels of extraterrestrial signals argues that all search strategies should assume at the outset the strongest signals not likely to have been detected yet, and that the sensitivity should be increased with time until success is achieved or until the strategy is no longer thought to be sufficiently promising.

PARTICULARS

The fund of relevant physical knowledge and available technologies has a profound effect on estimates of a priori probability of success per unit cost of a proposed strategy. This is particularly true with respect to individual subelements. Furthermore, certain concepts may be unattractive simply because they promise significant returns too far in the future even though the total costs might be relatively low. Human time scales and human patience are important factors in the present context.

The Satellite Problem

It is difficult to survey the entire sky over the entire 1-22 GHz spectral range from the surface of the Earth. The glare of satellite transmissions over more than half this band makes it difficult to reach attractive sensitivities in the satellite bands (see fig. 1).