Solution They Are Signaling But We Do Not Know at Which Frequency to Listen

57 channels and nothing on.

Bruce Springsteen

If ETCs do indeed use EM radiation to communicate with each other or to notify their presence to less advanced civilizations, then there are several different types of signal we might search for.

The easiest type of signal to detect would be one that an ETC has deliberately targeted at us. It is not too arrogant of us to suppose a nearby ETC would beam signals toward the Sun. Advanced civilizations would classify the Sun as a good candidate for possessing life-bearing planets, and they could probably detect the existence of Earth over interstellar distances. With our present level of technology we can detect Saturn-sized planets around other stars, so advanced ETCs will be able to do much better. If they beam signals to target stars in the hope of making contact, then our Sun would be on their list. (Upon re-reading this paragraph, some of the statements sound too definite. We are in the realm here of trying to second-guess the motives and intentions of putative aliens — an enterprise fraught with risks. But we have to begin somewhere.)

A second type of signal would be one meant for communication but targeted elsewhere, a signal we might nevertheless overhear. Yet another type of signal would be one not intended for communication at all, but instead leakage from other activities —just as EM signals leak out from Earth due to our radio and television transmissions, and our use of military radar. (Such signals have been leaking from Earth for several decades, but developments in cable and satellite telecommunications systems suggest they may soon cease. Perhaps the same will be true for ETCs, and the period over which a technological civilization is "radio-bright" can be measured in decades, in which case we have essentially no chance of discovering this type of signal. On the other hand, maybe future technological developments — solar satellites that beam energy back to the home planet in the form of microwaves, perhaps, or navigational beacons for steering through a crowded planetary system — would leak EM radiation into space.)

With our present level of technology, it makes little sense to look for leakage radiation. We should do the easy things before attempting harder projects, and it is easier to detect radiation intended for communication. But at which wavelength will ETCs choose to transmit? In other words: at what frequency should we listen?

The EM spectrum is extremely broad. Visible light, which reaches from 7.5 x 1014 Hz (deep violet) to 4.3 x 1014 Hz (red) forms a minuscule part of the spectrum. Ultraviolet, X-rays and gamma-rays have progressively higher frequencies, reaching up to 3 x 1019 Hz or higher. Infrared, microwaves and radio waves have progressively lower frequencies, reaching down to 108 Hz. Our technology employs all these wavelengths for a variety of purposes, ranging from medical applications (X-ray frequencies) to household devices (garage door openers work at 40 MHz, for example, and baby monitors at 49 MHz). There seems to be a frequency for everything. So which frequency is best for interstellar communication?

In the late 1950s, Philip Morrison and his colleague Giuseppe Cocconi were among the first to consider this question. Astronomers had by then developed radio telescopes and were using them to make significant discoveries. It was against this background that Morrison investigated the possibility of using gamma-rays as a different window on the Universe. As part of this work he showed how gamma-rays, unlike visible starlight, could travel across the dusty plane of the Galaxy. He told Cocconi of this result, and his colleague pointed out that particle physicists already generated gamma-ray beams in their synchrotrons; why not send the beam into space and see if an ETC could detect it? It was a fascinating question, and it got Morrison to thinking about the prospects for interstellar communication. He replied that they should consider not just gamma-rays but the whole EM spectrum — from radio waves all the way up to gamma-rays — and choose the most effective band for signaling.

figure 32 The wavelengths and frequencies of the electromagnetic spectrum. The horizontal lines appear on a logarithmic scale: each "tick" corresponds to a factor often. It is clear from this diagram that visible light corresponds to only a small fraction of the electromagnetic spectrum.

They quickly concluded that visible light was a poor choice for signaling, since the signals would have to compete with starlight; gamma-ray telescopes were not feasible at that time; the radio band seemed the best bet. The Arecibo radio dish in Puerto Rico was the appropriate instrument with which to search for signals: they calculated that if an ETC had its own Arecibo and used it to transmit a directed beam at a sharply tuned frequency, then our Arecibo could detect the alien dish from halfway across the Galaxy.113

Narrowing down the search to the radio band was a major advance, but it still left many possible frequencies. Radio waves can be anywhere between about 1 MHz and about 300 GHz.114 This is bad news, for the following reason. If an ETC wishes to transmit a signal over large distances, then it needs to send a narrowband signal — a signal at a precise frequency — since broadband signals are too easily mistaken for background noise. (When you twiddle the dial on a radio, you hear the background hiss of broadband noise between the narrowband signals of the radio stations.) The narrowest frequency generated naturally is by an interstellar maser. (Masers, which amplify microwaves, act very much like lasers.) It has a width of about 300 Hz; anything much narrower than this is a candidate for an artificial signal. Suppose, then, that ETCs transmit signals with a bandwidth of 0.1 Hz. (It makes little sense to transmit over interstellar distances with a bandwidth less than 0.1 Hz, since electrons in interstellar clouds will tend to disperse the signal.) This means that we have a huge number of radio channels to search through. Unless we narrow the search even further (or we get extremely lucky) we could be searching for a long time.

figure 33 The Arecibo radio telescope, located in Puerto Rico, is a huge structure. The dish itself is 305 m in diameter, 51 m deep, and covers an area erf about 8 hectares. This telescope could detect an alien transmission from the other side of the Galaxy.

Cocconi and Morrison pointed out that at frequencies less than about 1 GHz the Galaxy is noisy. It makes little sense to send a signal at a frequency lower than 1 GHz because background noise would drown it. On the other hand, at frequencies higher than about 30 GHz Earth's atmosphere becomes noisy. If an ETC were broadcasting at frequencies higher than 30 GHz, we would be unlikely to detect the signal because of atmospheric interference. In fact, the quietest region is between about 1 GHz and 10 GHz. Cocconi and Morrison suggested that it makes most sense to search for radio signals in that region, where an artificial signal would really stand out.

They refined the frequency range even further. They pointed out that clouds of neutral hydrogen — the simplest and most common element in the Universe — strongly emit radiation at 1.42 GHz. Every intelligent observer in the Universe will know about the hydrogen line. It makes sense to look there. Soon after, it was discovered that the hydroxyl radical radiates prominently at 1.64 GHz. Hydrogen, H, and hydroxyl, OH, together make up the compound water: HOH — or H2O. Now water, as far as we know, is absolutely necessary for the existence of life. Find water, and you have a chance of finding life. And since the region between 1.42 and 1.64 GHz is about the quietest part of the radio spectrum, it seems a logical place for a civilization to broadcast if it wants to attract attention. This band has been dubbed the waterhole. It is a beautiful name, conjuring up visions of many different species coming together at a life-giving source of water.

At about the same time that Cocconi and Morrison presented theoretical reasons for listening in the long-wavelength region near the hydrogen line, Frank Drake was doing exactly that: listening for signals near the hydrogen line. Drake had built equipment to study this part of the radio spectrum for mainstream astronomical purposes, but he had an abiding interest in the possibility of extraterrestrial life. He used the radio telescope at Green Bank to listen to two stars — Tau Ceti and Epsilon Eridani — for signals. His Project Ozma was the first time mankind had searched for an ETC. Although the results were negative, Drake's observations — along with the Cocconi-Morrison paper — proved to be a watershed for SETI.

The situation now seems much more complicated than it did four decades ago for Drake, Cocconi, and Morrison. They knew only of one spectral line, the hydrogen line, so the choice of where to search seemed quite clear. Modern astronomers, however, are aware of tens of thousands of spectral lines emanating from more than 100 types of molecule in interstellar space. There are very good arguments why we should study other frequencies. (Important examples include 22.2 GHz, which corresponds to a transition of the water molecule, and simple multiples of the hydrogen-line frequency — twice the hydrogen-line frequency, n times the hydrogen-line frequency, and so on. There is a particularly attractive "natural" frequency for intergalactic communication, which I discuss in a later section.)115 Although many authors maintain that the waterhole is the "natural" place to search for signals from within our Galaxy, we may eventually be forced to search through the whole window from 1 to 30 GHz.

figure 34 Frank Drake is a towering figure in the seti field. In addition to the eponymous Drake equation, he is known for carrying out the first radio search for an etc.

figure 35 The famous "Wow" signal. The Ohio State University Big Ear Observatory scanned 50 channels and recorded the observations on a printout sheet. For each channel a list ofletters and numbers appeared on the printout. In the Big Ear system, numerals 1 to 9 represented a signal level above background noise. For strong signals, letters were used (with Z being stronger than A). On the night of 15 August 1977, Jerry Ehman spotted the characters "6EQUJ5" on channel 2. This signal started from roughly background level, rose to level U, then decreased back to background level in 37 seconds. This was exactly what an extraterrestrial signal might look like; Ehman circled the characters and wrote "Wow!" in the margin.

In over 40 years of listening, none of the radio searches has found an extraterrestrial signal that is clearly artificial in origin. That is not to say that no signals have been found, of course. (Drake himself detected a signal emanating from the general direction of Epsilon Eridani, just a few hours after the commencement of Project Ozma. However, further investigation showed that the signal was clearly terrestrial in origin.) The radio searches have detected lots of signals, many of them rather intriguing. The famous "Wow!" signal is typical of the best signals found so far. It was a powerful narrowband spike, with characteristics indicating that it almost certainly came from space, but when Big Ear listened again to that part of the sky the signal had gone. Several attempts to relocate the "Wow!" signal have failed. Recently, for example, searches with the Very Large Array enabled astronomers to investigate two hypotheses regarding the signal. First, perhaps it came from a weak yet steady transmission, which momentarily increased in strength due to scintillation (like the twinkling of a star). Second, perhaps the signal was a powerful pulse, designed to attract attention to a much weaker continuous signal. Both possibilities seem to have been eliminated. Nothing interesting was found, down to a level that was 1000 times weaker than the original signal.

The "Wow!" signal may have emanated from a distant civilization, a beam that happened to sweep across Earth's path one August night and then moved on. But it seems much more likely that the signal came from a man-made satellite.116

SETI Projects

Since Project Ozma there have been more than 60 SETI projects, most of which have searched the waterhole region. In recent years, the projects have become increasingly sophisticated. Project META (Million-channel Extra-Terrestrial Array), developed in 1985 by Paul Horowitz,117 could study a million channels at once in the waterhole region. In 1990, META II started searching the southern sky, monitoring 8 million extremely narrow 0.05-Hz channels near the hydrogen line at 1.42 GHz, and also at twice this frequency, 2.84 GHz. In 1995, Horowitz initiated Project BETA (Billionchannel Extra-Terrestrial Array), which scans the waterhole region at a resolution of 0.5 Hz. From META to BETA in just ten years is significant progress! Project SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) piggybacks on radio telescopes being used for other astronomical purposes. The drawback with this approach is that there is no choice over where to listen; it can look for signals only where the telescope happens to be pointing. On the other hand, since it does not interfere with the normal functioning of the telescope, the project can be run continuously.118 The present incarnation of the project piggybacks on the Arecibo telescope and searches 168 million channels, each 0.6 Hz wide, near 1.42 GHz. Southern SERENDIP piggybacks on the Parkes Observatory in Australia to search the southern sky, also at the hydrogen line. Project Phoenix, which began in February 1995, is halfway through a search for signals within the range of 1.2 to 3.0 GHz in channels that are just 0.7 Hz wide.

Despite the increasing sophistication of radio SETI, sorting through billions of channels in the hope of finding a signal remains a laborious task. Is there really no alternative to the microwave/radio part of the electromagnetic spectrum? It happens that there is.

At about the same time that Cocconi and Morrison suggested listening for radio transmissions, Arthur Schawlow and Charles Townes outlined the working principles of lasers. Early devices were feeble, but just as computing power has increased geometrically, so has the power of lasers. It now seems clear an advanced ETC could communicate its presence using laser pulses and might prefer this method over radio. Not only would a short pulse of laser light stand out even over interstellar distances, it would plainly be artificial. Furthermore, an ETC could send beacon signals to millions of stars each day. Perhaps we should not be listening for radio signals alone; we should also be looking for signals in the visible spectrum.119

figure 36 The Very Large Array in Socorro, New Mexico. The array consists of 27 dishes, each of which is 25 m in diameter. Despite their appearance in the movie Contact, the telescope only rarely listens for broadcasts from extraterrestrials. Recently, though, it tried to relocate the "Wow!" signal — but unfortunately found nothing unusual.

figure 36 The Very Large Array in Socorro, New Mexico. The array consists of 27 dishes, each of which is 25 m in diameter. Despite their appearance in the movie Contact, the telescope only rarely listens for broadcasts from extraterrestrials. Recently, though, it tried to relocate the "Wow!" signal — but unfortunately found nothing unusual.

Optical SETI is not as advanced as traditional radio seti, but this is changing thanks mainly to the efforts of Stuart Kingsley. Kingsley uses his COSETI (Columbus Optical SETI) Observatory to look for narrowband laser signals from a list of target stars. It is encouraging that the equipment required for such a search is relatively simple and within the range of the dedicated amateur astronomer.120 Professional SETI scientists have caught on, however, and are beginning to develop large-scale projects.121

Even gamma-rays have been suggested as a communications channel for civilizations in contact over intergalactic distances. John Ball hypothesizes that gamma-ray bursters are messages sent by ETCs. However, although the detailed origin of these events is still being debated, nearly all astronomers believe bursters are a natural phenomenon. We have to employ Occam's razor once more: if we can explain bursters as a natural phenomenon, then Ball's hypothesis is simply unnecessary.

In 40 years of searching — mainly in the radio, but occasionally in the infrared and increasingly in the visible — astronomers have detected no signal. To rephrase Fermi's question: where are the signals? The lack of signals means that we can now start to place limits on the number and type of ETCs in our neighborhood. Some authors claim that this null result means that we can rule out the presence of K2 and K3 civilizations not only in our Galaxy, but beyond even our Local Group of galaxies.122 This claim is overstated, since it rests on several assumptions that may not be valid. Nevertheless, taking a conservative viewpoint, we can probably rule out the existence of a K3 civilization anywhere in our Galaxy and a K2 civilization in our particular part of the Galaxy: if they were there, we would surely have heard from them. In a few years, if the null result continues, we will be able rule out the existence of K1 civilizations out to 100 light years.

Billions of channels and — so far — nothing on.

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