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). Figure 1.- Frequency bands in the free-space microwave window, allocated to certain transmitting AVIATION METEOROLOGICAL AIDS RADIO LOCATION RADIO ASTRONOMY SETI Radio Astronomy Surveys Radio astronomers have carried out few surveys of the sky to what are believed to be flux levels of interest here. It is even possible that one or more of the "point" sources appearing in a survey in one frequency band but not in another survey in a different band, could be an unexpectedly powerful artificial signal. Radio astronomical surveys of large areas of the sky can be divided into two classes: broadband continuum surveys (1 MHz to 5 GHz bandwidth), and spectral line surveys (102 to 106 Hz bandwidth). Large bandwidths, absence of adequate spectral resolution, customary on-line instrumental data compaction, and absence of specific interest in the existence of extraterrestrial artifacts, all have reduced enormously the chances of recognizing an artificial extraterrestrial signal. In fact, a signal so strong as to be noticed, generally would have been ascribed to man-made radio frequency interference (RFI). To illustrate these points, we describe briefly two surveys reaching low flux levels by radio astronomical standards. The Parkes 2700-MHz source survey of the sky visible from Australia employed a bandwidth of 200 MHz and a sensitivity such that a signal flux level of about 1020 W/m2 would have produced unity signal-to-noise ratio (SNR = 1). No procedures were in effect to recognize a narrow band signal at this or much higher levels, unless the absence of this "radio source" on other surveys at adjacent frequencies caught an astronomer's interest and the source was then reexamined with suitable care. Westerhout used the NRAO meridian transit 300-ft antenna to survey a region along the Galactic equator 225° in longitude by 4° in latitude. Frequency resolution was 9.5 kHz in a band about 1 MHz wide. Unity SNR corresponded to about 3X1023 W/m2, but automatic data handling algorithms and RFI subtraction procedures would generally have prevented output registration of a coherent signal orders of magnitude stronger. The radio astronomy situation (and parallel situations in other areas) can be summarized this way. Procedures in use in high data volume observations tend to discriminate against the discovery of unexpected phenomena. Scientists generally do not explore a range of phenomena merely because it is not forbidden by any known laws. They usually have more immediate objectives in mind. The polarization of starlight was observed half a century after the work of Maxwell and Hertz; and the discovery of pulsars was a fortuitous accident due to the presence and curiosity of an astute graduate student in a favorable observational situation. Analog pen and paper recording of data was in use. For a third and final example, proof of the polarization of the diffuse background radio radiations was delayed at least 5 years by an unwillingness to test for polarization just because it was an unexplored degree of freedom in nature. Signal Classes Postulating the characteristics of signals we might be able to detect from another species has enlivened many a casual moment. We have only our experience over the past 30 years or so, our electromagnetic technology in which we take some pride, and our projections of how in the near future we might exploit this technology to suit our manifold desires. Above some tens of megahertz most of the power from all our surface transmissions is dissipated in the endless reaches of outer space. For over a quarter century and with increasing intensity, we have been generating an expanding aura of fairly powerful signals about the Earth, one we could detect if we had a sufficiently sensitive radio telescope situated 20 light years from Earth. As has been suggested many times, this may be a transient phase in our development, one that may be over in a time period microscopic on the cosmic scale. Nevertheless, a reversal of the historical trend toward higher transmitter powers is not yet in evidence. It does seem likely that some of our present transmissions will vanish from our scene in favor of more efficient, more directive procedures; cable and low power satellite transmissions may replace high power TV broadcasting, for instance. The future of high power radar is perhaps less clear. Table 1 is a fairly complete tabulation of published current high power radar signals in the 1 GHz to 2.5 GHz frequency range. They generally sweep out the sky continuously in regular patterns both in time and in direction. One class of signal for which we might search is the intraspecies type of transmission. We may be fortunate in having a relatively close neighbor on whom we can eavesdrop. From our own experience, this would require not only proximity or exceptionally great sensitivity because of the modest power levels we may perhaps expect, but also a complex pattern recognition capability in the time, frequency, and modulation domains. Most of our transmissions have a strong carrier component. If this holds with our nearest neighbor (and it may because it is a simple way to aThis table is a summary of published radar installations. bEquivalent Isotropic Radiated Power in watts. CUHF color TV (460-890 MHz provides many carriers in the 105-106 W range. They are radiated with high stability and the toroidal dSee table 3 in Section III-2 for further details. maintain coherence), it eases the pattern recognition problem since stable, monochromatic signals are relatively easy to detect. Finally, we are in total ignorance of their spectrum utilization and have little idea even how they might utilize transmitters in "space activities." We can only assume that bandwidth and propagation requirements dictate some frequency allocation scheme there as here. We have, so far, no reason to search any frequency band outside the low noise, free-space microwave window; and since signal stability so directly affects our achievable carrier search sensitivity, the water hole seems a likely place to start an eavesdropping search. Eavesdropping can be classified as a non-cooperative search situation. Alternatively, "they" may be transmitting in a cooperative, beacon mode, carefully arranged to assist discovery by others, particularly newcomers like ourselves. Any concern on their part for range would surely highlight the advantages of the low end of the free-space microwave window. Since we have conceived it and it is feasible for us today, it is quite possible (but who knows how likely) that other species are curious as are we, and indulge in their allotted share of an obvious cooperative search strategy. |