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 Seventeen years ago Cocconi and Morrison (ref. 1) suggested that we search at frequencies near the hydrogen line for signals emitted by advanced extraterrestrial civilizations attempting to establish contact with us. At the time, the hydrogen line was believed to be unique but, since then, dozens of other microwave emission lines from a wide variety of interstellar molecules have been discovered. In 1971 the Cyclops study (ref. 2), for reasons that are believed to be rather fundamental, identified the band between 1400 and 1727 MHz bounded at the low end by the hydrogen line (1420 MHz) and at the high end by the hydroxyl lines (1612 to 1720 MHz) as a prime region of the spectrum to be searched for interstellar signals. Because of these limiting markers the Cyclops team dubbed this region the "water hole" and suggested that different galactic species might meet there just as different terrestrial species have always met at more mundane water holes.
At present there is no serious interference in the water hole but navigational satellites and other systems are being planned that would fill the band with interfering signals such as continuous pseudo-random wide band noise. If these systems become operational as allocated, a substantial fraction if not all of the water hole may be rendered unusable for the search. The proposed services can be shifted to other frequencies without appreciable loss of effectiveness but, if the rationale for the water hole is correct, the search for intelligent extraterrestrial life cannot. It would be a bitter irony if the desire to know exactly where we were at all times on Earth were to prevent us from ever knowing where we are with respect to other life in the Galaxy. It is therefore timely to reexamine the case for the water hole in order that we do not, out of ignorance or carelessness, forever blind ourselves to the signals from advanced societies (see Sections III-8 and III-9).
The basic premise that leads us to the water hole is that any advanced society wishing to establish contact will choose the least expensive means that will nevertheless ensure success. As we shall see, one of the dominating factors is the energy that must be expended by the society to announce its existence over interstellar distances, not just to us but to all likely planetary systems. It is this consideration that leads to the radiation of electromagnetic waves rather than probes or spaceships and to the spectral region of the water hole.
In all probability we will have to examine thousands if not hundreds of thousands of targets before we succeed in detecting intelligent life. The nearest civilization is probably several tens and maybe hundreds of light years from the Sun. With such a profusion of targets, all at such enormous distances, it appears that search by actual manned space travel or by sending out swarms of probes is out of the question (see Section III-1). This approach is far too consumptive of time and energy.
 Just to send one spaceship to a nearby star and return it in twice the round trip light time, using not fusion or fission power with low yields but matter-antimatter annihilation, would require ~1024 J. This is enough energy to supply the present total U.S. electrical power for 50,000 years or to keep a 1000 MW omnidirectional beacon shining for 10 million years. Such a beacon could be received by civilizations around any of the million or more F, G, and K stars within 1000 light years, and would probably not need to be radiated by the searching society for more than 1000 years. If so, the beacon is 1010 times as effective per joule.
If we seek to reduce the energy requirements of space travel by, say, 104, we incur round trip travel times of 400 years per light year or 1600 years for the nearest star. Clearly we are not going to find other intelligent life by hurling tons of matter through space but by receiving and possibly some day sending&emdash;some form of radiation.
Regardless of what form of radiation is used, in order to detect that a signal exists and that it is of artificial origin:
1. The number of particles received must significantly exceed the natural background count
2. The signal must exhibit some property not found in natural radiations
In addition the radiation should
3. Require the least radiated power
4. Not be absorbed by the interstellar medium or planetary atmospheres
5. Not be deflected by galactic fields
6. Be readily collected over a large area
7. Permit efficient generation and detection
8. Travel at high speed
9. Normally be radiated by technological civilizations
Requirements 1, 3, and 8 taken together virtually exclude from consideration all particles except those having zero rest mass. The kinetic energy- of an electron travelling at half the speed of light is 108 times the total energy of a 150 Ghz photon. All other factors being equal the electron communication system would require 100 million times as much power. Baryons are worse. In addition, all charged particles fail requirements 4 and 5. Of the zero rest mass particles, gravitons and neutrinos fail requirements 6, 7, and 9. Of all known particles, only low energy photons meet all the listed criteria. It is almost certain that interstellar communication, if it exists, is accomplished by electromagnetic waves.
 We believe that requirement 2 is satisfied by any spatially and temporally coherent electromagnetic wave. Modulation of such a wave in order to transmit information need not destroy the coherence to such an extent that it would be mistaken for a natural signal. Further, the modulation will very likely contain regularities and repetitions of complex patterns that are inexplicable from natural sources.
Electromagnetic radiation covers a practically unlimited frequency range. Unless we can find some reason to prefer a relatively narrow portion of the spectrum, one of the dimensions of the search space remains unbounded. Here again we can appeal to energy minimization.
Let us examine critically what is implied by requirement I above (that the number of particles received exceed significantly the natural background count). First let us assume that we are operating in the "quantum" region of the spectrum (i.e., hv/kT> 1), where direct photon detection is easy, and that there is no background radiation. In principle, we require at least one received photon in the observing time in order to detect a signal at all. This requires that the received power
so, for a constant collecting area, the equivalent isotropic radiated power (EIRP) must be proportional to frequency. In practice the reception of a single photon would hardly convince us that we had detected extraterrestrial intelligence. In order to determine the coherence and other properties of the signal we would need to receive some number, n, of photons. But n depends only upon the sophistication of our data processing and upon our assigned a priori improbability and does not depend upon the operating frequency. Thus the proportionality implied by equation (1) still holds, as long as there is no background.
If the radiated signal is coherent and of constant amplitude, the photon arrivals will be Poisson distributed. If the expected number of arrivals in the observing time is n, the mean square fluctuation will also be n. The detected signal-to-noise power ratio (DSNR) is the ratio of the square of the mean to the mean square fluctuations; that is,
where W is the total energy received during the observation and P is the instantaneous power. In communication systems is the Nyquist interval and is the reciprocal of the RF bandwidth B. Then equation (2) becomes
and the photon shot noise behaves as if it had a spectral power density hv.
 The above considerations suggest that the lower the operating frequency the less will be the required EIRP and this is true until we reach the "thermal" region (i.e., hv/kT<1 ). In this region direct photon detection becomes difficult but the "radio" techniques of linear amplification, mixing, and spectral analysis are applicable; in fact, these have by now been extended into the optical region. Suppose we Fourier transform a block of the received IF signal of duration r. In effect, a constant amplitude monochromatic signal is then detected as a rectangular pulse of duration to which noise has been added. The effective selectivity curve of the Fourier transform will be the transform of the time window or
and this is the matched filter through which a CW pulse of frequency and duration should be passed to give the greatest ratio of peak output signal power to mean square fluctuation in the peak. For this case, or for any equivalent optimum detection process, the detected signal-to-noise ratio is
where is the spectral noise power density.
Three conributions to are unavoidable and are essentially the same anywhere in the Galaxy; these are shown in figure 1 (divided by k to give their equivalent noise temperature). The first is the synchrotron radiation of the Galaxy itself, given by
where the coefficient T0 varies from about 1 to 2.5 depending upon galactic latitude. This noise dominates below 1 GHz but, above this frequency, rapidly becomes less than the relict cosmic background radiation:
where T~2.76 K. Finally, for v > kT/h ~60 GHz the spontaneous emission noise
The sum of these noise contributions defines a broad quiet region extending from about 1 to 60 GHz known as the free-space microwave window. On Earth (or on any Earth-like planet) the absorption lines of the water vapor and oxygen in the atmosphere re-radiate noise with broad....
....peaks at 22 and 60 GHz that degrade the window above about 10 GHz, as shown in figure 2. Thus the terrestrial microwave window extends from ~1 to ~10 GHz, and is clearly a preferred region of the spectrum for interstellar search using ground-based low-noise receivers.
The suggestion is sometimes made that one might avoid and by going to very high frequencies and then eliminate by employing direct photon detection. At frequencies for which WO dominates, equation (5) becomes
and, since B can, in principle, be made as narrow as we find
exactly as given by equation (2). Thus, ignoring technological limitations, linear amplification is just as good as direct photon detection. Spontaneous emission noise does not make linear amplifiers noisier than photon detectors, it merely prevents them from being quieter, which would violate the principle of complementarily (ref. 3).
We may be unable, at optical frequencies, to realize bandwidths as narrow as . The point is that, even if we could, the linear amplifier, which would then be no noisier than the photon detector, would still be far noisier than a linear amplifier operating in the microwave window.
Not only is the minimum detectable received power least in the microwave window, but also the cost per unit of collecting area is less there than f or higher frequencies. This latter consideration also favors the low end of the microwave window over the high end. Additional factors favoring the low end are:
The relative motion of transmitter and receiver caused by planetary rotation and revolution produces a frequency modulation of the received signal. A frequency drifting at a rate will drift clear through a band B in a time . If the receiver will not respond fully and if the bandwidth is larger than necessary and admits more noise per channel. Setting =1/B we find B = v1/2. Since is proportional to v it follows that the optimum bandwidth increases as v1/2. The total receiver noise per channel is therefore proportional to
If the ordinates of the curves of figures 1 and 2 are multiplied by v1/2 when u is in gigahertz, we obtain figures 3 and 4, respectively. The ordinates are now proportional to the noise contributed per channel in a receiver optimized at all frequencies for the Doppler rate produced by a given radial (line-of-sight) acceleration. We see that the upper end of the free space window has become relatively noisy and that the H and OH lines are at the quietest part of the spectrum.
The doppler drift caused by motion of the receiver can be corrected by drifting the local oscillator. This is also true for the transmitter when the signal is radiated directively. It does not appear possible to correct a coherent omnidirectional beacon for all directions simultaneously.
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.
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 hand, 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.
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.