nature 22 February 2001
Insight
Nature 409, 1110 - 1114 (2001) © Macmillan Publishers Ltd.

The search for extraterrestrial intelligence

T. L. WILSON

Max-Planck-Institut für Radioastronomie, Auf dem Huegel 69, D 53121 Bonn, Germany, and Sub-Millimeter Telescope Observatory, Steward Observatory, The University of Arizona, Tucson, Arizona, USA (e-mail twilson@as.arizona.edu)


As far as we know, humanity is alone in the Universe: there is no definite evidence for the existence of extraterrestrial life, let alone extraterrestrial civilizations (ETCs) capable of communicating or travelling over interstellar distances. Yet popular speculation about the existence of ETCs abounds, including reports of alien visitations either now or in the past. But there is a middle way. It is now possible to put limits on the existence of ETCs of varying capabilities, within arbitrary distances from the Solar System, and conceive of real-world strategies whereby we might communicate with ETCs, or they with us.

One of the intriguing aspects of astrobiology in all its forms is that matters once confined to science fiction are increasingly being discussed seriously by scientists. SETI — the Search for Extraterrestrial Intelligence — is perhaps the best example. Knowledge of the space environment gathered over the past 40 years, as well as improvements in our own technological capabilities, can now be applied to make plausible estimates of the number of ETCs in the Galaxy with technologies comparable to or more advanced than ours. Even so, these estimates vary greatly1, 2. Recent developments and discussion could help constrain such estimates, including the discovery and characterization of planets around other stars3; reports of the discovery of possible past life on Mars4; and Ward and Brownlee's book Rare Earth5, in which they argue that life is common in the Universe, but ETCs are rare.

A history of SETI
When modern considerations of SETI began, microwave radar and radio astronomy in the centimetre wavelength range were mature technologies. Thus it was natural to believe that an ETC might transmit signals with radio telescopes similar to those then in operation6, 7. SETI began in 1960 with the targeted search of two nearby Sun-like stars, using the 25-metre-diameter radio telescope of the National Radio Astronomy Observatory7. This was Project OZMA, named after the queen in an imaginary land, "very far away and populated by strange and exotic beings"8. OZMA was soon followed by other initiatives, all using radio telescopes9, and SETI programmes continue today, despite uncertain funding. NASA's programme, the High Resolution Microwave Search (HRMS) included both a targeted search (examination of selected stars) and an all-sky survey, but was cancelled by the US Congress in the early 1990s10. Funding for SETI initiatives today comes from non-profit organizations such as the Planetary Society11 and the SETI Institute12. The SETI Institute has revived the targeted-search portion of the HRMS as 'Project Phoenix' using systems based on the NASA detectors. Meanwhile, SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) — a programme at the University of California, Berkeley — provides detectors for large radio telescopes so that ETC searches can be made while conventional astronomical investigations are carried out13.

SETI that does not involve direct physical contact with ETCs (which I rule out for the purposes of this article) is based usually around three rather different scenarios. First, the detection of electromagnetic signals from an ETC, deliberately targeted at us with the expressed intention of communication; second, the detection of signals from an ETC targeted elsewhere, but still designed for communication (for example, a space navigational beacon); and third, the detection of stray electromagnetic radiation from an ETC, not intended for interstellar communication (for example, radio or television signals). These scenarios increase in plausibility as well as difficulty, and are not mutually exclusive, given the difficulty of judging the intentions of ETCs.

A SETI practical
Before we even begin a search, we must be clear about the physical limitations of the technology available to us, both for the reception of signals and their transmission by ETCs. But within these limitations lie certain strengths, for they allow us to set limits on the existence of ETCs capable of specific technological capabilities within arbitrary distances from the Earth.

Early SETI projects concentrated on listening for electromagnetic signals in the centimetre waveband, between around 3 and 60 cm. The reasons for this choice are purely practical: it is in this region of the radio spectrum that background noise from the Galaxy, the Earth's atmosphere and the receiving equipment is lowest. Figure 1 shows the contributions from various sources of naturally occurring noise. Some of the basic concepts of the single parabolic reflectors used conventionally in radio astronomy14, and which could be used to transmit as well as receive signals, are summarized in Fig. 2. When used for receiving transmissions, large antennas have the advantage of greater collecting area, and parabolic reflectors reap the rewards of greater sensitivity when targeted to specific directions. For example, a parabol ic reflector of diameter 100 m can detect signals that are ten-millionths the strength of signals detectable to a non-directional antenna.

Figure 1 Noise from the Earth's atmosphere and the minimum noise from our Galaxy, versus wavelength.   Full legend
 
High resolution image and legend (46k)

Figure 2 How increases in effective radiated power (ERP) of a transmitting radio dish are related to wavelength and the size and distance of the target.   Full legend
 
High resolution image and legend (43k)

The same principle applies to the transmission of signals. Antennas directed at small regions are much more efficient (they have greater effective radiated power, or ERP) than antennas radiating isotropically, and transmission is better at shorter wavelengths. For example, targeted transmission at a wavelength of around 1 cm is 100 times as effective as transmission at 10 cm: power is more concentrated at shorter wavelengths, allowing more effective propagation of a signal through space. However, the optimal waveband for signal transmission does not coincide exactly with the 3–60-cm band used in early SETI projects, and it is easy to see why. The background noise generated by a receiver operating at 1 cm is tenfold that at 10 cm, and the Earth's atmosphere is more of an obstacle. The atmosphere becomes a serious problem at wavelengths shorter than 1 cm, but this could be circumvented by siting radio telescopes at high altitude or in space.

Which wavelength?
Since the days of Project OZMA, receiver sensitivity at centimetre wavelengths has improved 20-fold. In OZMA, 100-Hz 'windows' in the spectrum were searched sequentially7. Now, large wavelength regions are analysed simultaneously by parallel sets of detectors. This allows increases in the speed of searches by a factor equal to the number of channels, and soon it will be possible to analyse a billion contiguous, narrow channels simultaneously15. Expansion is possible in another direction — using an array of coupled radio antennas, rather than a single dish, to create a radio interferometer14. Such an array would be cheaper to build than a single telescope of the same size, and could also be used to receive signals from several different regions simultaneously. The Allen Telescope Array (ATA), funded by private sources and to be operated by the SETI Institute, is one such instrument. Both SETI and conventional radio a stronomy projects could be carried out with this array, which will operate in the 3–60-cm waveband16.

But the SETI searchers of the 1960s had one big advantage over contemporary scientists — they did not have to worry about the waveband they chose to examine. The 3–60-cm band was convenient, and within that band the choice was simple: in 1960, only one spectral line was known to be present in this region, namely the well known 21-cm emission line of neutral hydrogen. The hydrogen line is a prominent and universal feature, and proponents of SETI hope that ETCs would emit signals using carriers at or near this wavelength, taking advantage incidentally of the fact that it falls within the relatively radio-quiet 3–60-cm range. Since then, it has been found that the hydroxyl radical (OH) emits at 18 cm, so the 18–21-cm region — associated with the components of water — has been dubbed the 'water hole', a feature that might be exploited by ETCs seeking to communicate with life forms for whom water would be important17.

Modern SETI is spoiled for choice: tens of thousands of spectral lines from more than 115 known interstellar molecules are now known, so that the choice of a SETI wavelength has become complex. Many authors argue that the 18–21-cm 'water hole' range is best, but this in itself does not rule out other possibilities7. The intentions of ETCs are always a wild card: ETCs might choose to send messages at a wavelength obtained from various combinations of atomic constants. One such combination gives a wavelength of 11.72 cm (ref. 18), but there is no compelling argument for this wavelength either. Because agreement is lacking on which wavelength is the most likely, one approach is a systematic search through the range from 3 to 60 cm. An added advantage of using this range is that it is easier to build radio telescopes with large collecting areas. But as already noted, there are also arguments for specific frequencies outside this range. At shorter wavelength s, other choices would be 1.35 cm (a transition of water) or 1.47 mm (the analogue of the 21-cm line for a positron–electron 'atom'19, 20).

Effective power
Successful transmission and reception are limited by available power (Box 1). If the distance at which a signal can be detected by a 100-m radio telescope is plotted as a function of transmitter power, signal bandwidth and integration time (Box 1 Figure), it is clear that the most effective radio transmission must use a narrow bandwidth. The drawback, familiar to any Internet user, is that the narrower the bandwidth, the slower the transmission of information14. Of course, we cannot know a priori the power at the disposal of any proposed ETC. As a way of addressing this, Kardashev21 devised a scheme for the classification of ETCs according to the resources they might be able to command. A 'type I' civilization controls the resources of its home planet. Humanity is close to that point now. A 'type II' civilization can control the output of its home star, whereas a 'type III' civilization can dispose of the resources offered by its home galaxy (see Box 2). This classification can be used to set bounds on the existence of ETCs of specified technological capabilities within arbitrary distances of the Earth.

Given that we seek to receive and/or transmit signals from a specific direction, rather than isotropically, it should be possible in principle to exchange signals with a type I ETC using a 100-m radio telescope, together with a high-power transmitter to send narrow-band signals. These would be detectable with another 100-m radio telescope even out to a distance of 1,000 light years. A message from a type II civilization, on the other hand, would be detectable with a telescope of 10-m diameter from a distance of 100 million light years, which is 100,000 times further than the limit for a type I ETC. Type III ETCs, with galaxies of energy at their command, should be detectable to distances 10,000 times greater than the most distant type II ETC.

Stars are concentrated in galaxies, and there are more than 20 galaxies within 3 million light years of the Milky Way. In principle, we should be able to receive a message from type II or III ETCs in any of these with technology currently available. With an average of 10,000 million Sun-type stars per galaxy, we could detect messages from ETCs even if the product of the last five terms in the Drake equation, which computes the number of communicating civilizations in a galaxy at a given time (see Box 3), were less than one part in a 100 million. These considerations provide a rationale for all-sky, untargeted searches: with the possibility of at least modest numbers of perhaps readily detectable ETCs (especially of type II or III), the extra sensitivity conferred by targeted searches would not be an absolute requirement for success. However, the fact remains that no confirmed transmissions in the centimetre wavelength range have been received9, 22, from which it has been claimed that type II and type III ETCs do not exist at the present epoch22.

This claim is overstated: it may be valid for a sizeable part of our Galaxy, but only if the ETCs are broadcasting in the centimetre wavelength range without interruption — and if they wish their signals to be detectable. Signals in the centimetre range might be attenuated or interrupted by the interstellar medium between an ETC and the Earth23, so an otherwise constant signal might be detectable only occasionally. It is simple to imagine ways round this problem, such as redundancy, repetition or the transmission of a second signal at twice or one-half the chosen wavelength24. If the ETCs do not want to transmit signals deliberately, we may have to eavesdrop. One example of an unintended signal is the broadcast from a powerful television transmitter. But the reliable detection of unintended messages would require much larger telescopes, and in the case of television, the ETCs might use transmission via satellites or cable, in which the chanc es of finding broadcasts would decline still further.

Beyond radio
There is no reason why SETI should be restricted to radio wavelengths — it is possible that ETCs might transmit in other parts of the electromagnetic spectrum, such as the infrared or optical ranges25, 26. The advantage of the latter is simply one of energy density: with optical systems, very high ERPs can be obtained with modestly sized optical telescopes. Indeed, optical SETI (OSETI) is just now beginning with 1-m class optical telescopes (ref. 27 and Box 4). The OSETI searches for type II and III civilizations should be finished quickly, but for type I civilizations, OSETI will require much more time. If type I ETCs are common, one can expect success from a complete survey of stars to a distance of 100 light years. A targeted survey would require a few years. If, however, there is an ETC for every million Sun-like stars, the time will be much longer. Using the ATA in the centimetre wavelength range or the modern OSETI detector systems in the optical or infrared ranges, it would be possible to examine a number of stars simultaneously. Such techniques should shorten the time needed for such searches to a few decades.

ET intentions
If signals are transmitted with the intention that they are detected, we would expect that ETCs would want us to recognize their signals as artificial. Broadcasts should have characteristics incommensurate with natural signals. The problem is that the characteristics of natural radio emissions vary over an extraordinarily large range. The narrowest spectral line has a width of 600 Hz, which is also a transition of the hydroxyl radical at 18 cm (ref. 14) in the 'water hole'. This line has a rather high intensity, but we are certain that it is natural as the noise statistics are typical of random processes and the time variability is very slow. At the other extreme are signals from pulsars, which show time variations on microsecond scales. At first, there was some speculation that pulsar signals were broadcasts from ETCs, but this was dismissed when measurements showed that the pulses were present over a wide frequency range. No ETC would be so wasteful, so the emission was taken to be natur al. This emission has also been identified with rotating neutron stars14. In the near infrared, several stars were found to have large amounts of excess emission. Because such phenomena had already been discussed in terms of ETCs, this was taken as a signature of type II civilizations (ref. 28 and Box 2) in the course of shielding their parent star within a Dyson sphere. However, the radiation was found to be broadband, with only slow time variations, and these objects turned out to be dust-enshrouded stars14.

In summary, we assume that any ETCs would be aware of astronomical phenomena and would construct signals that will appear to be anything but normal. If ETCs send messages, these would be sent at specific wavelengths. They would have widths of a few hertz or pulses of nanosecond duration, but not both. How can we differentiate between terrestrial interference and ETC messages? If the signals from ETCs cover a large bandwidth, these would be subject to greater time delays at longer wavelengths in travelling through the slightly ionized medium of interstellar space14. The actual ETC messages at shorter wavelengths would arrive earlier. Terrestrial interference would show no such delay, so we would be able to differentiate between broadband ETC signals and local interference. For steady, narrowband signals, a fixed direction on the sky, modulation to transmit a message, and periodic Doppler shifts in wavelength caused by orbiting a star would be signs of extraterrestrial origin. However, short-lived, narrowband signals could be either interference from Earth or ETC signals.

What would be the content of an ETC signal? There have been many studies of the coding and content of hypothetical messages. The central problem of interpreting a message from an ETC is that we would have no idea about its language or syntax. The most reasonable communication mode would be mathematical and/or pictorial. Figure 3 shows a simple example of a possible ETC transmission, which consists of a picture from a rectangular array of '1's and 0's'2. Because the time delay would be decades or even centuries, there could be no 'conversations', but a message announcing 'we are here' and presumably 'this is what we look like'.

Figure 3 An example of a possible message from an ETC.   Full legend
 
High resolution image and legend (51k)

Where are they?
If ETCs exist, they are not making their presence obvious. This in itself suggests that type III and perhaps type II civilizations are at best extremely rare. There are, however, many possible reasons why we have not made contact with ETCs. First, there may simply be very few5. Second, there may be a number of ETCs, but these may be sending messages in optical or near-infrared ranges that we have yet to explore comprehensively25. Third, there may be ETCs, but these may not be interested in communicating29 and choose to keep themselves hidden. This is more speculative since it depends on the cultural aspects of ETCs30. From searches so far, the lack of contact shows us that transmissions, if any, are weak or intermittent signals (or both). The detection of ETC messages will take time and effort.

As the Drake equation (Box 3) shows, SETI is a business full of imponderables, of unknowable answers to impossible questions. We can, however, take comfort in some of the near-certainties of physics. For example, there seems to be no hope for faster-than-light travel, so actual visits from ETCs are unlikely. Even with the most efficient propulsion systems, the energy needed to reach stars at 10 light years in 20 years would be the equivalent of the present world consumption for 1,000 years (ref. 31). Such expenditure of energy would hardly deter a type III ETC, but even then, broadcasts make more energetic sense than personal appearances. There have been suggestions that ETCs might populate space with self-replicating machines in space probes32. This would allow colonization of large regions of space in relatively short intervals of time, but it seems vastly more complex than comm unicating by means of electromagnetic radiation6.

And what of ourselves? One can only speculate on the effect that contact with an ETC would have on humankind. Such a discussion exceeds the scope of this essay, which has been confined (deliberately) to technical matters. But the simple knowledge of the existence of ETCs would doubtless have far-reaching effects on our relationship with the Universe — as, no doubt, would be a persistent failure to detect ETCs, as this would confirm our uniqueness.

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