All unorthodox suggestions warrant some consideration because future discoveries may not be obviously implied by what we know now. We should be on the alert for phenomena that might contribute to the detection of advanced civilizations elsewhere.
An advanced civilization somewhere in the Galaxy might be engaged in an effort to make its existence apparent to other civilizations (our own, for example) or, even if it is not trying, might nevertheless be manifesting its presence unintentionally. A good deal of discussion has centered on the idea that the other party might choose to advertise itself by means of a powerful radio beacon, and other specific means have also been suggested. Not so much consideration has been given to incidental manifestations such as the intense infrared radiation produced as a by-product of large-scale utilization of stellar energy. This latter idea was introduced by Freeman Dyson, who argued that in the asymptotic limit, where a civilization was approaching utilization of all the energy from its star, the full stellar luminosity would still be apparent to an outside observer, but a good part of the spectrum would be shifted to an infrared band corresponding to room temperature.
Thus two different categories exist into which search proposals for extraterrestrial civilizations might fit. Now I wish to view the problem in a different way by listing all the modes of information transfer that could conceivably transmit signs of an advanced civilization to us. The purpose of this approach is to set up an exhaustive framework for future discussion. In addition, a beginning will be made in assessing the possibilities, again for possible  future review. Each mode of information transfer must, of course, be considered under each of the two categories mentioned above. Here is the list of modes to be discussed:
- A. Electromagnetic waves
- B. Other waves
- C. Matter transfer
- D. Exotica
An apology is needed for mode D, which some readers might label science fiction, but there is a good reason for including items such as tunneling through black holes, tachyons (particles that travel faster than light), and telepathy. The reason is that the public is widely interested and that this interest is catered to by astronomers in the public eye.
Electromagnetic waves can be classified as radio, light, x, and gamma rays. In the radio spectrum, there is a universal optimum, not especially sharp, but nevertheless favoring the microwave band when the requirement is to transmit over great distances, conditions such as source power being held constant. The microwave band is defined broadly as the spectral region in which the waveguide technique is or was used-let us say from 1 mm to 1 m. Within this range and at longer wavelengths, say to a few meters, refined considerations (see Oliver, this volume) have revealed heavy handicaps for both long radio waves and light waves-in the advertising category.
The difficulty besetting long radio waves is the natural emission from the Galaxy itself. Energetic electrons executing helical orbits around magnetic field lines in interstellar space radiate radio waves with ever-increasing intensity down to frequencies below 1 MHz. This is known as synchrotron radiation because it also occurs in synchrotrons, where high-energy particles swerve through a magnetic field. At lower frequencies synchrotron selfabsorption sets in, an effect that causes the interstellar medium to become effectively opaque. There are several reasons why we do not look for beacon transmissions anywhere in the wavelength range from a few meters to infinity. First, the natural competition from the Galaxy is too strong. Second, even if transmitter power is made high enough and bandwidth narrow enough that the natural background is dominated, a serious phenomenon of refraction sets in due to the presence of free electrons in interstellar space That is, a transmission beamed in a given direction will be bent in accordance with electron density gradients, and especially so as the waves pass through planetary systems, where electron densities might be a thousand times higher than those in interstellar space. Moreover, these changes in direction vary with time. Such behavior is less than attractive from the standpoint of the designer of a point-to-point communication system. At frequencies lower than about 10 kHz, propagation cuts off in a medium with 1 electron/cm3. Synchrotron self-absorption, which is important in determining the turnover frequency, has a negligible effect on propagation because the density of the  high-energy synchrotron electrons is much less than the density of slow interstellar electrons.
The long-wavelength (low-frequency) side of the radio band is hopeless for beacons, but what about incidental radiation? When we turn to this category, considerations such as refraction lose weight because one would be perfectly happy to detect an advanced civilization anywhere in the sky, even if the apparent direction were not the true direction. Furthermore, even below 10 kHz, which is nominally cut off from propagation, there is the theoretical possibility of propagation in the presence of magnetic fields. That this suggestion is not merely academic is illustrated by the phenomenon of whistlers, radio pulses generated by lightning discharges, which in the general range of 5 kHz are able to penetrate Earth's ionosphere, whose cutoff frequency may be several megahertz. In fact, whistlers provide us with another sobering thought. Not only do they penetrate Earth's dense ionosphere with practically no attenuation, they may even gain energy as they traverse the magnetosphere, a fact revealed by the occurrence of long trains of echoes. Therefore, the detection of incidental low-frequency emissions from another civilization is by no means excluded a priori. Further study of this field is indicated.
Moving now to the other side of the radio spectrum, we reach the infrared, visible, and ultraviolet light waves. A determined case has been made favoring laser beacons, whose two distinctive features are high spectral energy density and narrow beamwidth. Special circumstances may exist in which lasers make sense. From the standpoint of a communications designer interested in detecting a weak signal against a noisy background and also in information rate, light waves suffer from concentrating a rather large amount of energy in each quantum. This means that, for given transmitter power, far fewer quanta can be transmitted as light than as microwaves. This consideration and others favoring microwaves weigh against laser beacons. Incidental light emission, however, is quite a different matter. Infrared as a by-product of energy utilization has already been mentioned, but probably no special followup is required because infrared astronomy, which is in vigorous condition, is likely to discover detectable Dyson sources in the normal course of events. Other incidental light might also turn up in ordinary astronomical observations. Of course, background light from an adjacent star works against detection of light produced by a civilization. Nevertheless, one should not forget factors facilitating detection. Examples would be characteristic time rates of change such as a nuclear explosion would exhibit, characteristic absorption spectra, and special emission spectra as from artificial auroras.
Passing now to x- and gamma rays, we see that the disadvantages of laser beacons apply even more strongly when we think about deliberate signals. Incidental signals offer more interesting grounds for speculation. There  is, of course, the possibility of emission from bomb explosions. Sustained nuclear reactions for purposes of power generation do not seem a likely source because leakage would represent waste. There are, however, conceivable efforts of astroengineering on a colossal scale that could release x- and gamma rays. Generally speaking, light emission would accompany x-ray generation, but suppose that a galactic source were so distant that light was extinguished by interstellar dust. When we begin to talk about source distances on the order of 100,000 light years, we are encompassing a galactic habitat reaching far beyond the thousand or so light years that beacon theory contemplates. Somewhere in that vaster domain of the Galaxy there could be engineering projects entailing alteration of the orbits of planets and stars and arrangements for the collision or grazing approach of celestial bodies. The fundamental purposes of such activity would be to organize natural dispositions into a desired structure, to interfere with the degradation of energy in stellar processes, and to gain access to stellar matter. Ultimately, as when galactic evolution moves under intelligent control, the maneuvering and disassembly of stars becomes a goal, and the energies involved will liberate energetic rays. Such activities, if they exist, should be sought in external galaxies as well as our own, but we may expect the orderly development of survey instruments for x- and gamma-ray astronomy to reveal them.
Having covered the whole range of electromagnetic waves, we now turn to other waves. The only ones I can think of are shock waves, Alfven waves, and gravitational waves. We have had considerable difficulty detecting gravitational waves of any kind, even from the strongest expected natural sources, galactic or extragalactic, so we may confidently forget them for now. Alfven waves, or any mixture of magnetic-material wave motion, including shock waves, do, however, offer scope for thought. Magnetic field lines permeate the Galaxy and, if shaken at one place, will respond elsewhere in time. It is true that the direction of magnetic lines in space is contorted, but there are systematic patterns too. Near Earth, the magnetic field is also disturbed by solar phenomena, making it harder to see how information coming from outside could be detected. Even so, we now have the possibility of observing the magnetic field at great distances from the Sun, well out beyond Jupiter, and the records of the magnetic field will need to be scrutinized for wave energy arriving from outside the Solar System. Of course, the first detection of this kind will refer to some natural phenomenon, perhaps the dispersed echo of some ancient supernova, so for the time being we will just bear magneto-hydrodynamic waves in mind.
Matter transfer between interstellar bodies must occur at a rather low rate, but it has not been negligible in totality, as we note from the fact that heavy elements of Earth come from the interior of stars other than the Sun. Currently, some comets may be engaged in transferring material from System  to system. Solar wind and accretion of interstellar matter are factors. None Of these modes of transport can be considered feasible for signaling. It is barely conceivable that unusual particles, molecules, or nuclides resulting from intelligent activity elsewhere in the Galaxy might filter into our Solar System and be recognizable as being of artificial origin. Of course, the source would be obscure but, one interesting thought, an extinct civilization that had flourished in our neighborhood billions of years ago might have left analyzable material traces. The idea of panspermia, the diffusion of spores or molecular precursors of life through space, originated with and continues to be entertained by distinguished people.
Deliberate transfer of matter brings us to space probes (Bracewell, 1979). The whole idea of interstellar travel by spaceship was severely criticized by Purcell, who relegated the notion firmly into the realm of science fiction. His discussion, however, assumed two-way travel to be completed within a human lifetime. With this restriction lifted, things change. I suggested in 1960 that, for distances around 100 light years, a one-way automatic space probe might be a good way for an advanced civilization, technically superior to our own, to establish first contact with us. Attention was focused on the 30-300 light year range because, for shorter distances say around 10 light years, contact could be established by radio, and for distances of the order of 1000 light years things looked gloomy because the expected average lifetime, if civilizations were so sparsely distributed, was too short to permit roundtrip communication. The idea that longevity and distance to the nearest civilization were interdependent was then new. The probe suggestion has attracted many criticisms, such as that the plan is too costly or takes too long, that probes are not durable enough, and that the plan calls for action by the other party but not by us. It is true that it is costly to launch a space probe to a neighboring star, and indeed the cumulative total, by the time success was achieved after sending out many inconclusive probes, might exceed that of a giant array of radiotelescopes such as was considered under Project Cyclops. However, the correct way to make the comparison is between alternatives open to the one party, not between actions that might be taken by different parties. Thus the cost of probes to the superior civilization might be compared with the cost to the same civilization of maintaining a beacon to achieve the same object. For example, if a 1000-MW beacon system was adopted, one item would be the cost of electricity over the time the beacon was kept going. This time might have to be comparable with a longevity of millions of years (in which case 1023 J would be radiated), or we might want to think in terms of substantial fraction of the generation time, say a billion years (1025 J).
The fact that a probe does not travel at the speed of light certainly increases the travel time spectacularly. However, when we understand that contact between civilizations is not exactly the same as contact between  individuals, we realize that travel times comparable with the longevity of the civilizations rather than with the longevity of individuals are possibly accept. able. Of course, once first contact is achieved, communication thereafter may proceed at the speed of light.
When the cost and travel time have been worked out, as a function of distance to the target, the probabilities of successful detection of technological activity must also be factored in before comparison with an alternative approach becomes feasible. Much remains to be done here because of lack of basic data such as the weight and cost of advanced propulsion systems. Nuclear electric rocketry is a familiar idea but may have been superseded by new ideas, of which laser pellet propulsion is one. In addition, the design of electronics for long-term durability in space is an unknown and in itself constitutes technology on which work needs to be done for more immediate purposes of space science. Generally speaking, it would not seem that the durability of semiconductor electronics at the low temperatures in space would present insuperable obstacles.
Finally, the probe idea presents the psychological complication that no action seems to be called for on our part. Thinking further about this, it seems to me that some principle of equipartition of effort ought to be invoked. If the launching agent goes to the trouble to select us as a target, we are more attractive to him if we make our own contribution to the contact effort. Thus at the least we need to maintain radio receivers. But we could go further and maintain a space watch that would relieve the probe of the necessity of slowing down and stationing itself within the Solar System. It was already clear that if a probe failed to detect active radio communication on approach to Earth, then it would be more economical to send a second probe to a second likely target than to wait around for technology to evolve, which might take forever. Following this reasoning, I now think that it would pay the designer of the probe to shoot for civilizations that could detect and communicate with a fast flyby rather than to send off a smaller number of the slower and more elaborate probes that would be needed to kick into orbit in the habitable zone of the target star. If the equipartition strategy makes sense, we might expect to commit a fraction of the terrestrial GNP that is comparable with the effort of the other party. At present this seems to me to imply an intercept capability not unlike what is under discussion for a rendezvous with a comet.
To complete the discussion of deliberate matter transfer, we should look at particle beams and viruses. Beams of high-energy charged particles must negotiate magnetic fields, with a net effect over interstellar distances that will resemble diffusion, which is slow; so it is sufficient to consider neutral molecular beams, neutron beams, and neutrinos. Molecules may be propelled by laser beams and may be subjected to midcourse trajectory correction and refocusing. There may be other clever ideas to be discovered.
 Neutrinos are costly to produce but may be collimated rather well, and they have the penetrating power to reach behind the dust clouds that lie in front of most of the stars of our Galaxy as seen from our neighborhood. A good many avenues need to be explored. The virus idea is that a message may be coded microscopically onto a long molecule by the methods of molecular biology. Such molecules could be propelled into space and in effect would be small probes. How to intercept and decode such viruses has not received much attention, but in principle condensing of messages to the molecular scale is possible, and there is nothing to say that an interstellar spore could not be germinated and grown to maturity. It might then make an announcement.
The first item on my list of exotica is tunneling through black holes, a subject that has clearly caught the public interest. We turn now to Sagan (1973) for the most authoritative exposition. His conjecture is that spacecraft might use black holes for space travel. One would travel to the nearest black hole, typically 20 light years away, he estimates, taking 21 years for the trip. The spacecraft would then plunge down the black hole. Sagan states that he does not know whether it is possible to reach the point of emergence faster by going down a black hole than it would take by the direct route. However, he thinks black holes may be the transportation conduits of advanced technological civilizations, the pneumatic tubes of the Galaxy. He imagines great civilizations growing up near the black holes, just as urban development clusters around rapid transit stations, and he pictures vehicles being rapidly routed through an interlaced network to the black holes nearest their destinations. In my opinion, great danger would set in while one was still at a considerable distance from a black hole, much as it would be dangerous to be in a panic-stricken crowd of football spectators all starting to rush down a manhole in the center of the 50-yard line. Furthermore, it would not be possible for a spacecraft to emerge from a black hole and negotiate its way up through the infalling debris.
Tachyon is a term which, tangible though it may look to the public, resembles the "square circle" in having no referent in the physical world. However, there has been speculation that there may be faster-than-light entities that cannot be decelerated below the speed of light, just as the particles we know of cannot be accelerated to speeds above that of light. Should such entities be discovered, it would clearly be interesting to speculate on their use as information carriers. The same applies to other phenomena that have not been discovered yet, such as telepathy.
Although there are no generally agreed facts relating to telepathy and related words such as telekinesis, there is a body of lore. For example, among believers in telepathy it is widely held that thought-transference telepathy does not suffer from attenuation by matter. If this proved to be so, there would be a serious signal-to-noise problem in separating the wanted  signal from all the other thoughts from all over the Universe simultaneously clamoring for attention. This may be why telepathy is so hard to exhibit on demand!
To my mind, all unorthodox suggestions warrant consideration, even though one might choose not to devote personal effort to following them up, because the future undoubtedly holds discoveries that are not obviously implied by the present state of knowledge, and we should be on the alert for any phenomena that might contribute to the detection of advanced civilizations elsewhere.
- Bracewell, Ronald N.: The Galactic Club: Intelligent Life in Outer Space W. W. Norton, N.Y., 1979.
- Sagan, Carl: The Cosmic Connection. Doubleday, N.Y., 1973, pp. 248 and 266.