Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979;

(c) 1979 Robert A. Freitas Jr. All Rights Reserved.




Chapter 24.  Interstellar Communication Techniques

"I know perfectly well that at this moment the whole universe is listening to us -- and that every word we say echoes to the remotest star."
          -- from The Madwoman of Chaillot, by Jean Giraudoux (1882-1944)

"The reader may seek to consign these speculations wholly to the domain of science fiction. We submit, rather, that ... if signals are present the means of detecting them is now at hand. The probability of success is difficult to estimate; but if we never search, the chance of success is zero."
          -- Giuseppe Cocconi, Philip Morrison; "Searching for Interstellar Communications" (1959)1033

"And who, in time, knows whither we may vent
The treasure of our tongue, to what strange shores
This gain of our best glory shall be sent,
T' enrich unknowingly nations with our stores?
What worlds may come refin’d with th’accents that are ours?
          -- from Musophilus, by Samuel Daniel (1562-1619)

"The Pioneer plaques are destined to be the longest-lived works of mankind. They will survive virtually unchanged for hundreds of millions, perhaps billions, of years in space. When plate tectonics has completely rearranged the continents, when all the present landforms on the earth have been ground down, when civilization has been profoundly transformed and when human beings may have evolved into some other kind of organism, these plaques will still exist. They will show that in the year we called 1973 there were organisms, portrayed on the plaques, that cared enough about their place in the hierarchy of all intelligent beings to share knowledge about themselves with others."
          -- Carl Sagan, Frank Drake (1975)3143

"So deep is the conviction that there must be life out there beyond the dark, one thinks that if they are more advanced than ourselves they may come across space at any moment, perhaps in our generation. Later, contemplating the ¥ of time, one wonders if perchance their messages came long ago, hurtling into the swamp muck of the steaming coal forests, the bright projectile clambered over by hissing rep tiles, and the delicate instruments running mindlessly down with no report."
          -- from The Immense Journey, by Loren Eiseley (1957)2110


Most xenologists today earnestly believe that millions upon millions of inhabited worlds pepper the Disk of the Milky Way. Innumerable intelligent extraterrestrial races may await contact and communication with humanity. How might scientists -- human or alien -- best bring this about?

More than a century ago the great mathematician Karl Friedrich Gauss set forth the startling suggestion that the only way to communicate with sentient beings on other worlds was by means of a "mathematical language." Gauss’ idea for contacting the inhabitants of Mars was to lay out the figure of a giant right-angle triangle in the middle of the Siberian forests. His plan called for 15-kilometer-wide strips of forest to delineate the lines and golden fields of wheat to fill the interior of the symbol. The geometric solution to the Pythagorean theorem, Gauss believed, would communicate the fact of terrestrial intelligence to any Martian astronomers who happened to be eyeing our Earth. (One writer has estimated that the scheme would have involved more than 4 million acres of forest and nearly 13 million acres of wheat, a total area roughly the size of the entire country of Ireland.45)

In Vienna, the astronomer Joseph Johann Littrow, a pioneer in spectrography, is reported to have proposed that great canals be dug in the Sahara desert in the shape of a variety of geometrical figures -- circles, squares, triangles, and so forth. Each would measure perhaps 30 kilometers on a side and would be filled with water. For signaling purposes, kerosene was to be spread over the surface of the water and ignited. The flaming symbols, it was expected, would alert observant Martians to our presence on this planet. Assuming the ditches were 1 kilometer wide and the layer of petroleum many centimeters deep to permit a burn of several hours’ duration, roughly five million cubic meters of kerosene would be required each night (at a cost of $400 million at current world prices).

Another magnificently implausible project was put forth by Charles Cros, a Frenchman, in 1869.3101 Cros apparently spent much of his life trying to persuade the French government to help him construct a giant mirror to send messages to Mars. The device was planned to have a focal length equal to the Earth-Mars separation, thus permitting Cros to focus concentrated sunbeams onto the Martian desert. This would melt the sand, and Cros could then carve various figures and numbers on the surface. Another technique using mirrors involved the construction of a large checkerboard of shiny surfaces which could be covered and uncovered in sequence to describe patterns and shapes (a kind of slow-motion semaphore). Yet another scheme was advanced by one Schmoll, who wanted to establish mirror works at Bordeaux, Cherbourg, Marseille, Stockholm, Amsterdam, Copenhagen, and upon the shores of the Gulf of Bothnia to give the appearance of the Big Bear constellation (Ursa Major) as seen from space.*

It is hard for us today to fathom how anyone could have taken any of these proposals seriously. But a century ago the state of xenological know ledge was extremely poor. Little was known of the actual conditions on the surface of Mars or any other planet in our solar system; the size and extent of the Milky Way Galaxy was unknown and undreamed of; extrasolar worlds were believed extremely rare; and radio waves had yet to be discovered. Gauss and Littrow were forced to speculate within the bounds of their limited knowledge, and arrived at conclusions which seemed moderately plausible at the time. It is sobering to consider that hundreds of years from now the people of the future may regard our own current proposals with equal astonishment and disbelief.

Xenologists today believe that there are two fundamental avenues of contact that are feasible across interstellar distances: Probes (or artifacts) and signals. As one writer puts it:

People in SETI {Search for Extraterrestrial Intelligence} can be divided into two groups: listeners and travelers. Listeners believe that interstellar travel is so difficult and costly that the only practical method of contacting an extraterrestrial civilization is by using radio messages. Travelers believe that voyages between stars are practical for advanced civilizations. Travelers are in the minority.3251

Which is the superior mode of communication? That is, are probes or signals better? The answer to this seemingly innocuous question is not at all obvious. Both talk and travel are commonplace on Earth, and we cannot use as our guide any current human technology (which elsewhere in the Galaxy may be found in a variety of states of retarded or advanced development). Xenologists most properly must seek to address the question in a manner independent of the vagaries of human technological history. Judgment criteria must be selected which spring from those basic and nigh universal laws of existence which all sentient races and alien cultures share in common.

Certainly the most basic goal of life and intelligence -- the accumulation of information and complexity -- involves judicious energy-flow management. Evolution via natural selection generally favors those living systems which manage their limited resources most efficiently. In other words, of two organisms, one of which is less wasteful than the other, the more frugal but equally effective creature has a higher probability of being favorably selected for survival.

On this basis, astronomer Frank Drake has proposed what he calls the Principle of Economy. This is the notion that economy is practiced universally or nearly so, or at least represents the ideal. Drake argues that technological civilizations everywhere in the Galaxy will attempt to choose those alternatives which are least expensive:

They, like us, will use procedures which minimize the needs for personnel, materials, and energy to achieve their ends. It may seem that economy or thrift is a peculiarity of mankind or of life on Earth, but in fact it is a principle practiced by all living things simply because the resources to support life are limited on all other planets as on the Earth. Planets have a limited surface area, limited food supplies, and limited energy sources, and this has been in fact the basic cause for evolution in the first place. The ability to practice economy with the available resources has enormous survival value and will be developed in all living things. Therefore, it is quite reasonable to believe that the concept of economy is well established in civilizations throughout the universe.3284,3123

It is quite possible that some races may not be subject (or as subject) to the same competitive rules of natural selection as are we -- recall, for example, Sneath’s "soil creatures" mentioned in Chapter 6. Still others may choose a shorter-lived, more profligate lifestyle than ordinary thrift would dictate. But general living systems theorists agree that Drake’s principle is applicable virtually at all levels of living systems from cells to societies, since it appears to be a manifestation of the well-known principle of least effort.3071

If xenologists accept Drake’s Principle of Economy, then it follows that a technical communicative civilization will choose those means of communication which cost the least to do the job. The "job," of course, is the transfer of information and complexity across interstellar distances. The quantitative units of information are bits. (See discussion, Chapter 14.) Assuming that all data must be carried on markers of matter-energy, the units of "cost" are joules. Our criterion for judging whether signals or probes are superior thus reduces to the simple query: Which mode of communication maximizes bits/joule transmitted per unit time interval?

Unfortunately, to answer this question we need to do a little physics. Readers who are not mathematically inclined may skip the next three paragraphs.

Consider signaling by photons (radio, microwave, visible, ultraviolet, x-rays). According to Shannon’s classic theory,3186 the rate of information transmission through a channel of frequency bandwidth nmax is nmax log2(1+S/N) bits/second, where S/N is the signal-to-noise power ratio. This rate is a theoretical maximum when bandwidth equals carrier frequency; that is, when nmax = n. From quantum physics we know that the energy per carrier photon is equal to hn joules, where h is Planck’s constant. Hence we calculate that the maximum theoretical photonic information transmission efficiency eg is:

eg = {log2(1+S/N)} / h bits/joule-second

Consider next the possibility of signaling with masses. Any self-contained material system of mass m is subject to a limitation in the maximum energy it can utilize as a marker for carrying information. That limit is the mass-energy of the matter, or mc2 where c is the speed of light. If we use energy levels as markers, then Heisenberg’s Uncertainty Principle defines a minimum measurement accuracy for these markers: DE Dt > h, where DE is the uncertainty in energy and Dt the duration of the measurement. According to Bremermann,3072 the optimal use of a given amount of mass-energy available occurs when (mc2 / DE) markers with values between zero and DE are used (which permits the representation of mc2 / DE bits). This means that no material mechanism of mass m using energy levels as markers can measure more than mc2Dt / h bits during the time interval Dt. Hence, the maximum rate of information transmission is mc2 / h bits/second. From special relativity we know that the total energy (mass-energy plus kinetic energy) of any material body is mc2 / (1-v2/c2)½, where v is the velocity of the mass relative to a stationary frame of reference. Hence we calculate that the maximum theoretical information transmission efficiency for matter em is:

em = (1-v2/c2)½ / h bits/joule-second

Maximum photon efficiency eg and maximum matter efficiency em for the transmission of information are compared on the graph in Figure 24.1. Photon efficiency is represented by the vertical line at the right, which shows different efficiencies depending upon the signal-to-noise ratio achieved. (Note that the effect of S/N on efficiency is negligible at the upper end.) The matter efficiency function curves downward to the right, and lies below the photonic efficiency for all S/N > 1. Strictly speaking, then, photons would appear always to give the most bits/joule.


Figure 24.1 Comparison of the Theoretical Maximum Information Transmission Efficiency of Photonic and Matter Markers


But the truly amazing feature of the two curves is their unexpected convergence. If any reasonable velocity and signal-to-noise ratio are chosen for comparison, the difference in transmission efficiency is only an order of magnitude or two. To advanced Type II and Type III technical civilizations equipped with virtually perfect photon- and matter-handling technologies, the choice of communication mode probably will depend far less upon energy economy than on other considerations (see below). With few if any techno logical limitations on their activities, stellar and galactic cultures most likely will view signals and starprobes as energetically indistinguishable alternatives for interstellar communication.

Type I societies are another matter, however. Lacking advanced technological expertise, planetary civilizations will be forced to rely upon the most primitive, simpleminded, easy-to-construct communications devices imaginable. Across interstellar distances radio waves, which require nothing more complicated than a small metal wire-mesh dish and a few volts of electricity, seem to be the best bet for technological dullards.

Xenologists may draw two specific conclusions from the above discussion. First, since interstellar travel on a large scale is virtually impossible for any Type I civilization,3078 transmission of signals is probably the method of choice for such technologically-limited planetary cultures. Second, since starprobes and signals are energetically equivalent exercises for technically proficient Type II and Type III civilizations, both probes and signals probably will be utilized in interstellar communications depending on the particular purposes and needs of the societies seeking interaction.


* A modern version of the Gauss-Littrow proposals has been suggested by Kh. Geshanov, a Bulgarian, who recommends turning all the world’s radio stations on and off in careful synchrony, providing a regular "blinking" effect in Earth’s artificial radio emissions.1331 In March of 1964, two Russian science fiction writers put forth the idea that the Great Siberian Meteorite of 1908 was caused, not by a fallen object, but rather by an attempt at communication [a powerful laser beam) by the planets orbiting the star 61 Cygni.3267



24.1  The Cosmic Miracle

Before any two sentient races may converse across the interstellar void, they first must have each other’s attention. Each civilization must know who or what to look for, where and when to look, and how, technologically, to communicate. This is the problem of "acquisition." Each party must know that the other exists.

A number of interesting acquisition techniques have been discussed in the literature, and will be dealt with in the following sections. But there is another approach -- called the "cosmic miracle" -- which bypasses entirely the need for mutual acquisition. The cosmic miracle method allows one civilization to detect another without that other necessarily being aware it has been discovered. Essentially, the idea is to try to observe a kind of "cosmic advertisement" that says, loud and clear, "Here we are! Look at us!" The advertisement may be purposeful, or merely the natural byproduct of large-scale astroengineering projects. But it should be obvious.

It is surprisingly difficult to imagine a plausible celestial signpost that is clearly artificial. At first, it seems simple enough. For instance, imagine we design a brilliant glowing "billboard" in the sky. It should be rectangular in shape with sharp edges and distinct vertices, and measure some fraction of a light-year on a side (say, 0.3 light-years). We make its color fiery crimson to attract the attention of visible-visioned creatures. At the exact center of this celestial marquee we place our premiere attraction: The star system to which we wish to call attention. The adjacent star field, as viewed from the most probable direction of contact, is nearly empty, so the star of our show has little competition for the spotlight. Surely such a setup would be "clearly artificial," a sufficiently strange cosmic miracle to warrant the conclusion of sentient extraterrestrial intervention?

Sorry not! The above is an excellent description of the so-called "Red Rectangle" (actually it is shaped more like a very fat hourglass), discovered by astronomers back in 1915. The object, located near the southern border of the constellation Monoceros, lies 1100 light-years from Earth. The central star (HD 44179) is a visual binary, and both components are believed to be late spectral class B.3102 Astronomers have attempted to explain the appearance of the rectangle as a complicated reflection of reddish starlight from a surrounding disk of dust.2076

Many unusual possibilities crop up repeatedly in the literature. One proposal is to place in orbit around the target star a cloud of material that absorbs some uncommon band of radiation in the stellar spectrum. Viewed from afar,’ the star would appear to possess an artificial spectral line of unknown origin. The creation of such a stellar marker, as discussed by Drake and several others, would require dumping perhaps 400 tons of matter into circum stellar orbit.312 This marker material ought to be of a type that is difficult to explain by natural causes -- for example technetium, a short-lived element (half-life 2 million years) which does not occur naturally on Earth or, presumably, other worlds generally.’ It is said by the proponents of this scheme that the presence of unusual markers in the spectrum of a stellar atmosphere would be difficult to explain as anything other than fairly recent technological activity by alien beings. Unfortunately, a great many natural "technetium stars,"3103 "platinum stars,"3104 and so forth are already known to astronomers.

Philip Morrison at MIT has suggested that a star could be converted into a giant signaling beacon if an opaque screen of dense particles were placed in orbit around it. The clouds would periodically cut off enough light (in certain preferred directions) so that the sun would appear to be flashing when seen from a distance. According to Morrison:

[The cloud] would have to weigh about 1017 kilograms (the mass of a comet), distributed in micron-size particles over a 50 zone of a sphere surrounding the star, and moving in an orbit like the orbit of a planet. If this could be modulated every six months or so., taken away and put back again, or changed to affect the interstellar intensity, we could make it beam a series of algebraic equations at us. Perhaps in that remote galaxy, some patient signalers have for 50 million years tried to modulate a star.1053

(In 1970 it was reported that the eclipsing binary star CV Serpentis had stopped eclipsing, for reasons unknown.1163)

Still stranger sights have been observed in the sky by astronomers. There are Cepheid stars and RR Lyrae variables, suns which beat like great cosmic hearts, expanding and contracting rhythmically over fixed intervals of time. (The surface area of such objects actually grows and shrinks by as much as 40% during each cycle.1810) Cepheids typically are 3000 times more luminous than Sol, with periods between 1-100 days. RR Lyrae variables are about 100 times brighter than Sol, with cycles from 7-16 hours. Each star in this class of objects has its own unique period of pulsation from which it never varies. For instance Polaris, Earth’s "north star," is a Cepheid variable beating with a period of exactly 3 days, 23 hours, 13 minutes, and 31 seconds.1556 (Brightness oscillates over 0.1 magnitude, too little to notice with the naked eye.) Cepheids have also been known to change color, say, from yellow-green to orange and back again, during a cycle. What a fine display of "unnatural" behavior for xenologists to attribute to the doings of sentient ETs!

There are "flash stars," cool main sequence suns which blaze up several orders of magnitude in brightness in just seconds, at irregular intervals. (They return to their normal state minutes or hours later.) Another possible candidate for "cosmic miracle" is the class of suns known as "spectrum variable stars." These suns display a periodic change in the intensity of certain spectral lines, while all the other lines remain constant. To take an example, a2 Canum Venaticorum has a spectral period of 5.5 days. During this interval the spectroscopic lines of the chemical ions EuII and CrII vary cyclically with opposite phases, while the SiII and MgII spectral lines remain unchanged.1945

Then there are the so-called "interstellar masers," regions in space where natural radio lasers emit perfectly monochromatic radiation.3134 In one case, a water vapor maser (IC 1795) was observed to "turn on" over an 8-day period, and then "turn off" again after 1 month.3106 The recently discovered objects known as Cygnus X-l and HZ Herculis are supposedly natural x-ray lasers,3140 and the Crab Nebula provides an example of a natural gamma-ray laser.3163 Recent satellite surveys of x-ray point sources in the Milky Way have uncovered transient sources which flick on, then off again in the space of a few months. It is estimated that several hundred such objects may turn on and off in our galaxy each year, but astronomers still don’t understand why.3140 And there is at least one report of a gamma-ray burster with a mysterious "gap of silence" timed to occur exactly at the peak of its burst.3162

Perhaps a beacon consisting of a rare lasing material (say, ionized deuterium) might be considered sufficiently strange to be judged artificial, but this is highly problematical. And causing a celestial maser or laser to blink doesn’t help us at all -- pulsars flash in one-second cycles, and x-ray and gamma-ray* bursts of incredible intensity are observed over periods of 10 seconds or less.3141 Other sources pulse semiregularly. Over an interval of a day and a half, source Centaurus X-3 emits x-ray flashes that slowly increase in duration from 4.84 to 4.87 seconds, then suddenly drop in intensity in about an hour, then return to normal 12 hours after that.1478


* According to the "catastrophe interpretation," gamma-ray pulses are the remnants of huge nuclear explosions, the last remaining traces of a vast war fought between distant extraterrestrial civilizations.3153


24.1.1  Eavesdropping

What if we wish to discover extraterrestrial civilizations which are not deliberately attempting to make their presence known to us by posting a cosmic signpost? Xenologists generally agree that probably one of the best ways to do this is to attempt to monitor the putative culture’s "technological garbage."80,57 (But note Sagan and Sullivan.3192) As any first-year anthropology student is well aware, that which is thrown away often bears much information about the thrower.

From the xenological standpoint there are two basic constraints on this mode of contact. First, since the effluents of technical civilization must cross interstellar distances to be detectable, said effluents must be of such character as not to suffer undue attenuation during transit. Second, the technology of the wasting culture must be sufficiently advanced and sufficiently energy-rich so that the flux of garbage into interstellar space is great enough to be detectable far from the original source. The only serendipitous effluents which satisfy both of these requirements are electromagnetic waves (radio, microwaves, etc.).

Even a backwoods emergent Type I civilization such as humanity emits enough electromagnetic garbage to be detectable over interstellar distances. Assuming for the moment that our planet represents a fairly typical primitive planetary culture, the radio spectrum of Earth indicates the presence of technological activity at the hands of presumably sentient beings. Carl Sagan explains the reasoning behind this conclusion:

Since the Earth is at a temperature of roughly 300 K, we would expect it to have an emission spectrum following the blackbody curve, except for some absorption by the atmosphere. Further, the curve should be centered at about 10 microns in the infrared and fall off inversely as the square of the wavelength to ward very long wavelengths. What we find is a truly remarkable peak in the meter band. The amount of radio emission is extremely striking. Indeed, it is so large that if one observed only at meter-band frequencies and assumed the emission to be of thermal origin, the deduced blackbody temperature of the Earth would be about 40,000,000 K. This is an example of extreme disequilibrium. That is, the Earth’s radio radiation is not characteristic of thermodynamic equilibrium. The strong disequilibrium component is due to the activities of the modicum of intelligence residing on the planet.3287

Woodruff T. Sullivan and his colleagues have completed an extensive survey of all sources of radio leakage on our planet. The video carrier in TV broadcasting is responsible for most of the radiative wastage. Even though power emitted from antennas on tall towers is deliberately concentrated downward to avoid waste, a thin sheet nevertheless escapes over the horizon as leakage radiation into space. This amounts to a total effective radiated power of about 10,000 megawatts/Hz for the TV video carrier, 400 megawatts/Hz from FM broadcasting, and about 200 megawatts/Hz from BMEWS-type military radars.

According to Sullivan, the Arecibo radio dish used by NRAO in Puerto Rico, if operated by inquisitive aliens on another world looking back at Earth, could detect a. strong UHF television station at a range of 1.8 light-years. BMEWS radar could be picked up out to 18 light-years using an Arecibo-type radiotelescope. The construction of a system similar to the proposed Cyclops SETI network (see description next section) would permit detection of video carriers out to 25 light-years (which encompasses about 300 stars) and BMEWS radar out to 250 light-years (covering about 200,000 stars). Our hypothetical extraterrestrial observers who were able to detect video carrier waves would recognize them as artificial, although an additional four orders of magnitude of antenna sensitivity would be required to pick up actual program material.

After carefully monitoring the intensity and frequency variations of terrestrial transmitters for several years, distant ET scientists could deduce an incredible number of important facts about Earth:

1. The complete orbit of our world;
2. The existence of station broadcast schedules influenced by the sun;
3. The presence of an ionosphere and perhaps a troposphere;
4. The size, rotation rate, and axis of rotation of the Earth;
5. A complete map of the stations;
6. The mass and distance to the moon;
7. The size of the radiating antennas; and
8. Various cultural inferences concerning our civilization.

Sullivan elaborates further:

The deductions that might be made from this wealth of information can only be conjectured, but certainly the extraterrestrial "humanists" and scientists would all have their favorite theories concerning (i) the purposes of these transmitters, as well as their physical structure; (ii) the nature of this planet’s relationship to the sun, as well as its geography, geology, and atmosphere; and (iii) the nature of this civilization’s biology, sociology, commerce, politics, economics, philosophy, technology, and science. We feel that far more could be deduced about our culture than one would at first think. For instance, political spheres of influence could be measured quite accurately by noting the frequencies and other technical conventions of stations. Furthermore, the varying broad cast schedules of stations (set by policies of national networks in most cases) would sharply delineate political boundaries, as distinct from spheres of influence. Further possible deductions are left to the imagination of the reader.310

As technology advances and a civilization enters the Type II stage, the waste products of an aspiring society may become even more readily apparent. For instance, if we place in Earth-orbit a series of gigawatt-capacity solar power satellites (as many proponents of space industrialization have urged), the minor lobes of the spaceborne transmission antenna radiation will make humanity highly visible on the galactic scene. Incessant radio traffic between interplanetary or interstellar ships, or between such vessels and their home planets, will contribute to the "garbage." (A gradual increase in strength of such a signal. together with a spectral blueshift might be taken to indicate the approach of a high-velocity starship towards our solar system.1381,49,3138) Powerful defensive radars probably would be detectable anywhere in the Galaxy using a Cyclops-type radiotelescope array.

Let us assume a mature Type II culture which has spread itself around the parent star in the traditional Dyson Sphere configuration. Viewed from interstellar distances, the optical characteristics of the sun itself may appear drastically altered. Independent of specific engineering details, the Second Law of Thermodynamics requires that a technically advanced society that exploits the entire energy output of its stellar primary must re-radiate every bit of that energy in some degraded form to maintain thermal equilibrium. If the shell lies at the radius of Earth and the central star is like Sol, the mean temperature of the Dyson Sphere will be 250-300 K. The waste energy will come off as heat, down in the infrared portion of the spectrum around 7-10 microns. This waste is not easily concealed from nosy interstellar neighbors.*

One writer has suggested that beings based on a hotter biochemistry than our own might construct Dyson Spheres with a higher waste-heat temperature. The infrared emissions could run as high as 2-5 microns, which "would look deceptively like red giant or supergiant stars."673 Marvin Minsky at MIT offers a different viewpoint: In his opinion, radiative emissions at any temperature above the natural 3 K background is wasteful and a squandering of scarce energy resources. According to Minsky, "the higher the civilization, the lower the infrared radiation. We should look for extended sources of 4 K radiation. There should be very few natural such sources."22 (4 K corresponds to a wavelength of about 700 microns.)

Indeed, in recent times a number of large infrared objects with solar-system-sized dimensions and temperatures below 1000 K have been discovered by astronomers,3109,3108 including object T in the constellation Taurus and object R in Monoceros.1344 But regardless of our secret hopes, says Iosef Shklovskii, a noted Soviet astrophysicist, "we must assume all astronomical phenomena natural until proven otherwise."15 Shklovskii’s well-known "Principle of Naturalness," while questioned by some,3177 is widely accepted by working xenologists.

As before, much information may be concealed in the effluents of Type II societies. Soviet xenologists have given much thought to the question of "civilization detectors" and remote cybernetic analysis of advanced extraterrestrial cultures. Kardashev has suggested a way to distinguish Dyson Spheres from dust clouds across interstellar distances -- the two have distinctly differently shaped emission spectra.22 If the Type II society is viewed as a complex cybernetic system then, according to Soviet radioastronomer B.N. Panovkin, much information about how the civilization actually works may be derived from its electromagnetic effluents:

[If we consider] the radiation associated with life activity as an output of this system, on the basis of an analysis of radiation one can draw certain conclusions as to the functional structure of the system and its internal organization. For example, based on the properties of radiation it can be established that a system belongs to a wide class of objects in which feedback is present. From this class, after a more detailed analysis, a narrower sub class of systems can be discriminated in which homeostasis is manifested. From the class of homeostatic objects one can isolate a group of objects possessing even more complicated functional properties -- for example, "systems logic," etc. Ultimately, in principle it is possible to isolate a class of objects which (regardless of their physical structure) can be recognized as equivalent, let us say, to our earth civilization in terms of their functional properties and their manifestations.25

When at last we come to an analysis of the expected appearance of the "garbage" generated by the technological activities of a Type III galactic civilization, we leap ahead at least ten orders of magnitude in available energy and complexity. Such a society should be capable (see Chapter 19) of directly altering the physical characteristics of entire galaxies. According to Freeman Dyson, the acknowledged premier speculator on the topic of galactic technology, "tame" galaxies should appear far differently than "wild" ones still in the natural state. Such taming appears largely to be absent from the Milky Way. Says Dyson:

It seems to me clear that we could turn the galaxy upside down if we wanted to, within a million years; there’s nothing in the world of physics to stop us from doing that. There may be good reasons for not doing it, and there may be good reasons why other intelligent species are not doing it.... I have the feeling that if an expanding technology had ever really got loose in our galaxy, the effects of it would be glaringly obvious. Starlight, instead of wastefully shining all over the galaxy, would be carefully dammed and regulated. Stars, instead of moving at random, would be grouped and organized. We don’t see any traces of this when we look in the sky, which is peculiar. Nothing like a complete technological takeover has occurred in our galaxy.. . .To search for evidence of technological activity in the galaxy might be like searching for evidence of technological activity on Manhattan Island. If an Indian from 400 years ago were to come paddling into New York harbor, he might not understand what he sees, but he would at least notice there is something there.1558,1450

Despite the apparently "wild" state of the Milky Way, astronomers have discovered a number of highly unusual and inexplicable phenomena in the heavens.3135 Perhaps the best-known of these is the galaxy M87, also called Virgo A. M87 is a giant elliptical galaxy located about 50 million light-years from Earth. Photographs clearly show a string of bright knots -- the "galactic jet" -- extending outward from the nuclear regions. Like a giant searchlight beam, the jet stretches 5000 light-years in length and measures 500 light-years thick. Astronomers believe that this artifact has existed only for a few millions of years, and that its total energy is approximately 1051 joules. If we consult Table 19.2 in the chapter on high technology, we find that this is sufficient energy to engage in a galactic transport operation at a speed of 40,000 meter/second, or about 0.01%c.

Other celestial oddities are known. Stephan’s Quintet is a congregation of five separate galaxies in the constellation Serpens, each connected to the others by mysterious gaseous "bridges." Many Seyfert galaxies (characterized by intense radiation emissions from their core regions) exhibit highly unusual shapes, often including jet-like structures emanating from the nucleus. Object 3C273 and several other quasars also have elliptical jets or tails extruding from the main body. (The tails are visually dim, but emit 90% of the total radio energy.1214) There are spectacular "radio galaxies" such as Cygnus A in which 0.01% of the total galactic mass has been catastrophically converted into radio wave energy by means unknown.1338 But perhaps most startling of all are the famous Ring Galaxies,3164 of which about 16 have been discovered to date.3159,3110 These monstrous halos of stars are known to be dynamically unstable stellar aggregations with astronomically ephemeral lifetimes of only 100 million years -- which implies fairly recent assembly. Scientists already have devised a number of theories of formation,3111 but xenologists continue to hope.

A few speculators have pinned their hopes on some of the recently discovered marvels of the astronomical zoo, in particular the pulsars. (See Kardashev,1320 McDonough,1384 Verschuur,1337 and Spaceflight1173.) Pulsars are objects which emit regular pulses of radio energy at extremely regular intervals.** These intervals range from 1-30 seconds for different objects. Pulsars are so regular that a variety of putative intelligent functions have been assigned to them by some xenologically-inclined writers. These range from artificial extraterrestrial acquisition beacons (the so-called LGM or "Little Green Men" theory) to waste energy from the exhaust from pulsed x-ray lasers used in some advanced starship propulsion system. The orthodox explanation is that pulsars are fast-spinning neutron stars with asymmetrical surface "hot spots" or polar fusion of infalling gas that traces round and round like a revolving searchlight.3139

While it is most probable that pulsars arise by natural causes, it is quite possible that advanced technical communities may still he exploiting them as navigational buoys or galactic chronometers. To make proper use of a spinning neutron star, a Type III civilization should have available the technology required physically to rotate the spin axis of the object. ETs will need to orient the pulsar’s flashes towards certain specific preferred directions -- say, into the plane of interstellar trade routes -- since natural pulsars may have random orientations. To shift the axis of a spinning neutron star of 1 solar mass and a period of 1 second through a full right-angle turn (90°) will require at least 3 x 1029 joules. If reorientation is accomplished in a single century, 8 x 1019 watts of power must be expended continuously over that time. If the alien engineers are in a real hurry and choose to do it in only 10 years, then 8 x 1021 watts are needed. The energies required to meet either of these schedules is well within the means of a Type II civilization, and a Type III galactic society would have no trouble at all.

Another favorite cosmological object for xenological speculation is the quasar.1557,1814 This fascinating phenomenon has a bluish starlike visual appearance and is characterized by strong ultraviolet and (usually) radio emissions. A large spectral redshift seems to indicate that quasars are very very far away -- perhaps at cosmological distances (billions of light-years) -- and their lack of proper motion against the background field of stars tends to support this conclusion.

However, quasars also vary in brightness over time periods as brief as a single day.2638 If it is true that no material disturbance can travel faster than the speed of light, then the maximum size of the primary quasar energy source should be on the order of a light-day in diameter. Assuming this is correct, quasars would have to be considerably closer, perhaps even within our Ga1axy,1482 because it is difficult to visualize the generation of galactic energies in a solar-system-size volume of space. And if quasars were really close, their computed total energy would drop to approximately of an oversized globular cluster -- perhaps interpretable as a consortium of advanced Type II civilizations.

Quasars exhibit rapid fluctuations in radio brightness, in optical bright ness, in polarization of visible light emissions, and so forth.1488,1486 Parts of quasars appear to be flying apart at velocities greater than the speed of light,1485,3166 although several explanations have been offered to avoid violations of Relativity theory.3165 Different spectral absorption lines often exhibit different redshifts.1556 A few quasars, such as the BL Lacertae objects, frequently display uncorrelated variations in the radio and visible spectrums (e.g., radio power increases while visible power decreases).3160 Still more surprising, "BL Lacs" and many other quasistellar objects emit radiation with the spectral characteristics of a nonthermal source (non-black-body), which implies "that the powerhouse inside must be generating radio power by a rather exotic mechanism.3151,3295

While such unusual behavior certainly is not "clearly artificial," to the best of my knowledge no serious studies have yet been done in which quasar emission signatures are analyzed for information content and periodicities of the sort which might be expected of the transmissions or electromagnetic "garbage" of advanced alien civilizations. Quasars, most likely are not extraterrestrial acquisition beacons, but still it is useful always to bear in mind the First Law of Serendipity: To find anything, one must first be looking for something.


* It might be possible to hide by arranging all waste heat to be radiated in a tight nonisotropic beam, straight up out of the Galactic Plane so that only a very few stars could ever hope to have any knowledge of the existence of the civilization by means of eavesdropping.

** For example, pulsar CP1919 in Vulpecula has a period of exactly 1.33730109 seconds.1384



24.2  Extraterrestrial Signaling

Let us assume that, for whatever reasons, an extraterrestrial civilization decides to signal rather than travel. What is the best way to do this?

The current majority opinion among xenologists is aptly summarized by Dr. Bernard M. Oliver of the Hewlett-Packard Corporation. According to Dr. Oliver, in order to receive information-laden transmissions we must at the very least be able to detect the presence or absence of a signal. To reliably pick up a pulse packet, the average number of particles in each such pulse must be great enough significantly to exceed the natural background noise in the communication channel used. In other words, signal must rise above noise.

Borrowing Drake’s Principle of Economy, Oliver then suggests that ETs will choose that signaling channel which best conserves transmitter power and which costs the least energy per bit to send. There are five criteria which may determine the particles of choice for frugal alien communicants:

1. The energy per particle should be as low as possible.

2. The velocity of transmission should be as high as possible.

3. The particles should be easy to generate, launch, and detect.

4. The particles should not be deflected by fields in space.

5. Absorption by interstellar matter should be as low as possible.

In his analysis for the Project Cyclops team, Oliver continues:

Except for photons, uncharged particles are difficult to accelerate, direct, and detect. Charged particles are deflected by magnetic fields, are absorbed by matter in space, and, except at very high energies, do not penetrate atmospheres. The total energy of a photon at 1420 MHz {radio} is one ten-billionth the kinetic energy of an electron traveling at half the speed. Photons are as fast as any known particle, are affected very little by the interstellar medium at low frequencies, and are the least energetic. Almost certainly electromagnetic waves of some frequency are the best means of interstellar communication.57,85

While many xenologists probably remain in basic agreement with this position,22 a few permit themselves the luxury of a nagging doubt. One of these persons is Carl Sagan, who, in an oft-quoted passage from his book The Cosmic Connection, suggests that:

We are like the inhabitants of an isolated valley in New Guinea who communicate with societies in neighboring valleys (quite different societies, I might add) by runner and by drum. When asked how a very advanced society will communicate, they might guess by an extremely rapid runner or by an improbably large drum. They might not guess a technology beyond their ken. And yet, all the while, a vast international cable and radio traffic passes over them, around them, and through them.. . .At this very moment the messages from another civilization may be wafting across space, driven by unimaginably advanced devices, there for us to detect them -- if only we knew how.15

The answer may lie right under our noses. Until this century it was widely believed that no material object could travel faster than the speed of sound. Yet the crack of a whip, involving the supersonic snap of the tip of the lash, had been known (but not understood) for thousands of years. Sagan, asserting that messages from advanced civilizations may lie in quite familiar circumstances, puts forth a fanciful suggestion:

Consider, for example, seashells. Everyone knows the "sound of the sea" to be heard when putting a seashell to one’s ear. It is really the greatly amplified sound of our own blood rushing, we are told. But is this really true? Has this been studied? Has anyone attempted to decode the message being sounded by the sea shell? I do not intend this example as literally true, but rather as an allegory. Somewhere on Earth there may be the equivalent of the seashell communications channel. The message from the stars may be here already. But where?15

Philip Morrison once noted that the really logical mode of interstellar communication may be by "Q" waves "that we are going to discover ten years from now."702 Others have proposed highly speculative modes of virtually instantaneous contact, including wormhole switchboards,2181 L- or T-fields and Universal Mind,2597 and psychic phenomena. Dr. Jack Sarfatti has attempted to invest ESP with scientific validity, using his theory of Superluminal Quantum Communication. (See Gardner,3145 Sarfatti,3147,3148 and Sarfatti, Wolfe, and Toben.3146) The essence of this theory is that it may be possible to transmit the "quality" of energy rather than the energy itself -- which Sarfatti describes as the "nondynamical transfer" of information. Robert Forward articulates a similar notion:

Do we need communication media? Communication is the transfer of information by modulation of some form of mass-energy or space/time. But information has the dimensions of negative entropy. It is not energy by itself. It is carried on energy. It might be possible to transfer information without using any form of mass/energy to transmit it. This is of interest because Special Relativity only limits the velocity of mass/energy, not information. (Some theorists will argue with this.) But still, this leads to a speculation that we might someday have faster-than-light information transfer even though mass/energy cannot go faster than c.2014

These and many other highly conjectural possibilities today remain only at the "idea" stage of technical development on this world.* Interestingly enough, however, there are at least four alternative signaling channels in which significant progress has been achieved in recent decades. The first two -- high energy particle and neutrino communications -- have, perhaps surprisingly, already reached the "practice" stage of engineering and development here on Earth. The other two techniques we shall discuss -- gravity wave and tachyon communications -- presently remain at the "theory" stage of technological development. They await verification and research before serious engineering efforts may begin.


* It will be recalled from Chapter 17 that technical progress and the realization of new technology normally proceeds in four distinct stages: Idea, Theory, Practice, and Profit.


24.2.1  Alternative Channels:  HEPs, Neutrinos, Gravitons and Tachyons

In 1977, D. M. Jones proposed using high energy particles (HEPs) in the attention-getting or acquisition mode of interstellar communication.2201 To avoid the disturbing effects of planetary magnetic fields, receivers and transmitters should be located in space or on nonmagnetic bodies such as Luna. Taking into account the problem of beam spreading, Jones calculates that to signal across a distance of 10 light-years ETs will need a 10 ampere transmitter beam consisting of protons, electrons, or ether HEPs. The beam energy should average about 1000 TeV (1015 eV).* The transmitter will require about 1011 watts of power, about two orders of magnitude higher than the largest accelerators constructed by humanity to date. The detector area should span an area of about 10 km2.

HEP beams, according to Jones, are so unusual that they must necessarily call attention to themselves as artificial in origin. Information may be impressed on these beams in at least three different ways. Most primitive is content modulation -- 10 minutes of electrons, then 5 minutes of protons, then 7 minutes of electrons, and so on. This would be technically difficult because of the large mass difference between protons and electrons (1836:1). The bit rate also would be low. Another possibility is energy modulation of the particle stream, which can be accomplished at the transmitter end quite accurately. But changing energy alters velocity. Packets will be delayed to varying degrees, so large pauses must be left between pulses. Receiver technology is complicated, and again the bit rate is low. Perhaps the third alternative -- pulse modulation -- is best. In this scheme, an homogeneous beam of particles with equal energy is pulsed in "dots" and "dashes" something like Morse code. If the beam is powerful enough, maximum bit rates may approach 1000 bits/second.

HEP beam communication has already been reduced to practice on Earth. In April 1972, Dr. Richard C. Arnold at the Argonne National Laboratory conducted an experiment in remote signaling using 0.012 TeV beams of muon particles.3112 Using Morse code, Arnold successfully transmitted a series of "V"s (dot-dot-dot-dash) via muon beam. The stream of information-bearing particles passed through a two-meter-thick wall and traveled 150 meters before reaching a "receiver" consisting of two coincidence counters. The message was encoded using pulse modulation by mechanically interposing a heavy brass barrier alternately to block or to pass muon packets emerging from the accelerator. This experiment is the first known use of a particle accelerator to transmit a message.3161

Dr. Arnold is of the opinion that muon-HEP beam communication systems are potentially competitive with radio and microwave over planetary distances. This is in part due to the fact that charged particles curve in a magnetic field and thus can be made to follow trajectories parallel to the surface of a world. Unfortunately, this very property proves to be the principle disadvantage of the HEP scheme in interstellar communications. As the author has pointed out, the Galactic magnetic field gives rise to unacceptable drifts in beam alignment even over fairly modest distances.2711 Solutions include increasing the beam energy to 10,000,000 TeV (suggested by Jones) or switching to uncharged HEPs such as neutrons.2712 Recent studies indicate that it may be possible to latch onto neutral particles and control their motion by coupling to the nonuniform positive and negative regions within their internal structure.2822

Another exciting possibility for interstellar communication is neutrino beams.2825 Neutrinos are extremely stable, neutral, massless particles which travel at the speed of light just like photons. They are ubiquitous throughout the universe, produced first at the time of the Big Bang of universal creation (power flux about equivalent to that of the photonic 3 K background radiation)2198 and later by the hot fusion reactions occurring in the interior of Sol and other stars.3114 The principal advantage of neutrinos is their tremendous penetrating power. A very high-energy photon can burrow a few centimeters into a chunk of solid lead before it is absorbed, but neutrinos can pass through about 50 light-years of lead before there is a 50% chance they will be captured. It is estimated that only one particle of every trillion passing through the Earth is absorbed by our planet.1557

Here, possibly, is the equivalent of the international cable traffic passing around Sagan’s New Guinea islanders. Each second many tens of thou sands of these phantom particles zip though our bodies without interaction. Could this be the "seashell communications channel" we are looking for?

First-generation neutrino detectors were understandably primitive in design. The earliest model was constructed 1500 meters below ground level in the Homestake gold mine in Lead, South Dakota in 1970 by Raymond Davis. It has since been used to detect neutrinos emitted by Sol. The subterranean location serves to provide a natural shield from cosmic ray "noise."

In the main experiment, a huge tank filled with 400,000 liters of perchloroethylene or C2Cl4 (commercial dry cleaning fluid) was placed at the bottom of the mine shaft. When a neutrino struck one of the chlorine atoms (of which there were about 1030 in the tank) it is captured. The chlorine transmutes into an atom of chemically inert but radioactive argon gas. Even though Sol gives off about 1038 neutrinos/second, the interaction of these particles with matter is so weak that only about one argon atom is produced in the tank each week. After a period of months, the experimenters flush the vat with helium to concentrate the argon and then carefully measure its concentration with delicate radiation counters.

Second-generation neutrino detectors have now been constructed which permit the sending of messages by neutrinos. Neutrino telecommunication has been reduced to practice on Earth today.3229 In 1977, A. W. Sáenz and his coworkers of the Naval Research Laboratory in Washington, D.C., established the world’s first neutrino communications link over a distance of 1 kilometer.3113

The 0.4 TeV proton accelerator at Fermilab, using 40 megawatts of power, was used to generate a pulsed beam of 1013 protons per pulse at the rate of one pulse every eight seconds. Each pulse lasted only 0.02 millisecond. The protons were sharply focused by a magnetic horn and directed onto an aluminum target. A variety of short-lived mesons were produced, flying off down a 400-meter tunnel wherein they decay into a 0.015 TeV neutrino beam with a spread of only 206 arcsec (about one-twentieth of a degree). There were about 1010 neutrinos in every pulse.

The beam was aimed at a liquid neon bubble chamber measuring 4 meters in diameter. It was located about one kilometer away from the source and served as the receiver for the neutrino transmissions. The particle beam generated by the Fermilab accelerator was sufficiently intense to produce one observable neutrino interaction per pulse in the 25 tons of detector material. The liquid neon receiver picked up a series of Morse "dots" transmitted to it from Fermilab. Communication was established.

When neutrinos interact with water, they emit a forward cone of decay products giving rise to what is called Cerenkov radiation. Cerenkov rays are photons emitted by any material object which is traveling faster than the speed of light in that medium. Sáenz and his colleagues suggest using Cerenkov counters to observe neutrino-induced interaction events in large bodies of water located 10,000 kilometers from the source (here assumed to be Fermi lab). Here’s how such a communications link might work.

Neutrinos generated by Fermilab would be directed thousands of kilometers straight through the mantle of the Earth to arrive in a large body of water at the surface containing 100 million tons of liquid. This might be a lake 1 kilometer wide and 100 meters deep. As the particles interact with the fluid, muons are produced. These travel about 50 meters in water, emitting along their path a 41°-wide Cerenkov cone consisting of about 200 photons per centimeter of path length in the visible wavelengths of light. These flashes of light would be picked up by photomulipliers and registered as information-carrying, communicative neutrino signals. Since a single counter can monitor water volumes of about 106 tons, only 100 Cerenkov counters are needed in the "receiving lake." About 2500 events/hour should be detectable, a bit less than one per second.

In order feasibly to use such a method for interstellar communications, substantial technological improvements must be made. If we optimistically assume a beam spread of only 10-6 arcsec using huge spaceborne transmitters, a beam energy of 1000 TeV, and require a bit rate of 1 bit/second across a range of 100 light-years, the receiver must consist of a giant sphere of water weighing 1015 tons and measuring about 124 kilometers in diameter. (This is about one-tenth of all the water stored in Earth’s polar icecaps.) About 109 Cerenkov counters would have to be deployed, a mission which a Type II society could probably handle with ease if it chose to do so. Going still further afield, imagine that a Type III civilization sets up a Library World near the Galactic Core and receives broadcasts from planets like Earth at a rate of 1013 bits/year using a water receiver as described above. The average distance to the Library will be 30,000 light-years, so detectors must monitor the equivalent of 50 Earth-oceans’ worth of water to ensure adequate reception. While this certainly is not impossible, and xenologists recognize that big problems require big solutions, the neutrino-transmitter/water-detector scheme does seem rather tendentious and inelegant.

What we need is more sensitive receivers. One oft-mentioned possibility for third-generation devices involves using gallium to capture a neutrino (which then transmutes into germanium). A large tank filled with liquid gallium metal could be about 100 times more sensitive than Davis’ chlorine/argon system. Unfortunately, 20 tons of material would be required.1987 This represents about 5 years of the current world production of the element,3114 and at the present price of $550/kilogram the receiver will not be cheap. A related proposal, in which indium metal is used to absorb a neutrino and change into tin, would require only 3 tons of the pure element because of its high natural isotopic purity.1562 An indium receiver of this size would be about three times more sensitive than the gallium detector.

Many other kinds of advanced receivers have been discussed in the literature. One such was elaborated by Dr. M. Subotowicz of Poland at the 3rd International CETI Review Meeting, held in Amsterdam on 4 October 1974 as part of the 25th International Astronautical Congress.1093 In his paper "The Use of Neutrinos in Interstellar Communication," Subotowicz discussed the technical problems involved in generating, modulating, collimating, transmitting and receiving neutrino beams, and he outlined some possible communication system designs. One of these designs involved a detector consisting of a single crystal of absolutely pure cobalt-60, weighing about 9 tons (a cubic meter in volume) and maintained at a constant cryogenic temperature of 0.01 K. Such a system would provide a favorable signal-to-noise ratio against the normal solar/stellar neutrino background and would permit extraterrestrial neutrino communications to be detected across interstellar distances. The receiver would be small enough, to fit in a modest-sized starship, and at least one writer has speculated that an advanced civilization might use such means to talk only with its peer races, to the deliberate exclusion of the humbler "peasant societies" using primitive radio wave systems.**1142

Xenologists can imagine still more sophisticated devices. In theory, to see a neutrino you need a receiver-target which is very rich in neutrons. The optimal detector configuration might possibly involve a free neutron gas (see Chapter 19). A specially shaped ultradense neutronium sheet might serve as a kind of lens to concentrate and to detect neutrino impulses. If magnetic monopole particle accelerators are available and neutron gas receivers are already set up, an advanced galactic neutrino telecommunications system may be as cheap to operate as an electromagnetic network -- and with competitive performance characteristics as well.

While both HEPs and neutrinos are in some sense "proven" or "practical" systems, the concept of gravity wave signaling remains at the theory stage of development.1345 Much as accelerated charged particles give rise to electromagnetic fields, General Relativity theory predicts the existence of a radiation of gravity generated by accelerated masses. Theorists are firmly convinced that gravitational radiation does exist, and there is now growing experimental confirmation of this supposition.3227,3228

Harnessing gravitational radiation will open up a whole new communication band from DC up to GHz frequencies and beyond -- but made up of gravity fields rather than electromagnetic fields. Gravity waves should have penetrating power even better than neutrinos. They experience very little attenuation when they pass through material objects, and they travel at the speed of light.

To set up a working communication system, we first must have a gravity wave transmitter. In principle, any mass which undergoes translational or circular acceleration emits gravitational radiation. But gravity energy is far weaker than electromagnetic energy. For example, a locomotive engine spinning so fast that it is just about to fly apart from centrifugal force (about 6 revolutions per second) would generate only 1032 watts of gravity wave energy.3116 The waves radiated by the whole Earth have an energy of only 10-3 watts, and the total emission from the entire solar system amounts to just a few hundred watts. This is insufficient for the purpose of inter stellar communication.

To make a good transmitter we need lots of mass concentrated in very dense clumps, and those clumps must be moved very rapidly (close to the speed of light) in order to generate significant quantities of gravitational radiation.2014 Hawking black holes (HBHs) probably would be ideal for this purpose. Once one has been captured or manufactured and an electrical charge imposed upon it, it may be manipulated by electric fields powered by the spontaneous evaporative energy thrown off by the HBH. For instance, a 1012 kilogram HBH spinning with a tangential velocity of 130 meter/second should emit 2 x 1012 watts of gravitational energy. This is equal to the spontaneous evaporative power output. (Such a system should be stable for about 1 eon.) Larry Niven has discussed a similar idea in his science fiction story "The Hole Man."686

At the end of every communications system there is a receiver. In a radio receiver, an electromagnetic wave accelerates free electrons in the antenna; in a gravity receiver, the passage of gravitational radiation will cause the physical deformation of the mass comprising the receiver. Pioneering work on such detectors was initiated by Joseph Weber at the University of Maryland, starting in the early 1960’s.658 Weber built a gravity wave detector consisting of a 1-ton aluminum cylinder which could be driven into oscillation by passing waves. These oscillations would then be picked up by piezoelectric strain sensors mounted on the surface of the cylindrical bar. The apparatus was supposed to be able to measure physical deformations on the order of 10-17 meter.

Second generation detectors, currently under construction or already in operation, aim for two orders of magnitude improvement in sensitivity -- down to about 10-19 meter.3115 At least six different schemes have been proposed.

The first approach is to take a Weber-type bar, make it more massive (say, 10 tons instead of 1 ton), and cool it down to liquid helium temperatures to reduce thermal noise. The system is further isolated from extraneous external vibrations by using superconductive magnetic levitation.3321,3319 A second approach involves using a dielectric monocrystal -- for example, a large, very pure, single crystal sapphire -- instead of a massive metal bar. The gain in the receiver is far higher using monocrystals, which compensates for the reduced mass. A third approach is to set up a laser interferometer with two arms laid perpendicular. The passage of gravity waves shortens one branch and causes a measurable visual fringe shift. The fourth detector, called the Braginsky notch capacitor, records the change in electrical capacitance of a metal bar with a notch cut in it, as a gravity wave passes and changes very slightly the length of the notch. Fifth, there is the Doppler shift tracking technique. The Doppler shift in signals emitted from a spacecraft are carefully monitored. When a gravity wave passed by, it jiggles the ship relative to Earth and thus produces a blip in the Doppler shift measurement,3316 Finally, there is the superconducting ring magnet detector. In this scheme, an electric current induced in the superconducting metal ring flows indefinitely. This persistent current generates a magnetic field perpendicular to the plane of the ring. When the shape of the ring is disturbed by the passage of a gravity wave, the magnetic flux through the ring changes and can be measured by a device called SQUID (Superconducting Quantum Interference Device).3117

Dr. Kip Thorne, theoretical physicist at CalTech, estimates that within a decade or so third generation devices will become available and extend sensitivity down to 10-21 meter.3115 This should be sensitive enough to map the entire "gravity wave sky."657 Assuming an HBH gravitational radiation transmitter as discussed above with a power output of 1012 watts and a directional gain of 108 (about 1 arcmin focus), Thorne’s proposed devices could probably receive communications from a sender located 100 light-years away. High gravitational antenna gains could be achieved by using an entire star as a lens.3118 Using a body the mass of Sol, a nearly parallel beam of gravity waves could be created, a beam with a diameter of 1000 kilometers that would not diverge appreciably out to distances on the order of 10,000 light-years. The focal length of such lenses would be of the same order as the dimensions of the solar system. Xenologists suspect that if the transmitter problem can be licked, gravity wave communication may be quite feasible across interstellar or even galactic distances.

The technology for tachyon signaling does not yet exist on Earth. Indeed we do not even know if tachyons exist at all. But the concept,, which has only just moved from the idea to the theory stage of development quite recently, offers the fascinating possibility of hyperoptic (faster-than-light) communications throughout the entire universe. (See especially Alväger and Kreisler,1479 Antippa,1495 Baltay et al,1498 Bers, Fox, Kuper and Lipson,1476 Bilaniuk, Deshpande and Sudarshan,1515 Bilaniuk and Sudarshan,1516 Bilaniuk et al,1517 Everett and Antippa,1477 Feinberg,648,1492 Fox,1504 Kreisler,1518 Mignani and Recami,1507,1519 Newton,645 Parmentola and Yee,1493 Raychaudhuri,1521 Recami,3252 Recami and Mignani,1511 Taylor,1190 Trefil,2026 and the short bibliography by Feldman complete through 1973.1514) As Stephen L. Brown at Stanford Research Institute points out:

Tachyons could be used for communication systems. Such systems would be useful only where ordinary electromagnetic radiation is too slow, as in interstellar communication. It would seem likely that any extraterrestrials with high technology would be aware of tachyons (if they exist) and would use them for communications instead of waiting centuries for replies at the speed of light. Perhaps the Project OZMA concept of monitoring electromagnetic radiation for intelligible patterns will turn out to have much less potential for interstellar contact than a tachyon monitoring system.1517

Tachyons may represent close to the most perfect signaling system we can imagine. The message carriers will always travel faster than light and in fact may go arbitrarily close to infinite velocity. The faster a tachyon goes the less energy it requires -- a transcendental tachyon moving at infinite speed has zero energy. So far as we know there is very little tachyonic background noise, and these particles (if they exist) should not interact appreciably with ordinary matter or galactic magnetic fields. The only requirement for optimal message carrier not yet satisfied is that message particles should be easy to generate, launch, and detect. But ETs may well possess the requisite technical knowledge to make the dream of hyperoptic signaling a reality.

The theoretical superiority of tachyons over photons rarely is fully appreciated. Dr. Martin Harwit of the Center for Radiophysics and Space Research at Cornell University has provided several mathematical formulae from which a simple calculation of the comparative bit rates of the two message channels may be made.3119 If bandwidth and detector size are held constant, the ratio of tachyon bit rate to photon bit rate is equal to (mlc/h)3/N, where m is tachyon mass, l is photon wavelength, N is tachyon velocity in units of c (speed of light), and h is Planck’s constant. This ratio may be called the Tachyon Advantage, and is tabulated in Table 24.1 above for various wavelengths of electromagnetic radiation, assuming tachyonic mass equal to the mass of the electron. Note the tremendous theoretical advantages of transmitting information with tachyons rather than with radio waves or photons of visible light. The tachyons are also traveling incredibly swiftly, yet another advantage over photonic signals.


Table 24.1 Theoretical Tachyon Advantage over Photons for Information Transmission,
using Tachyon Mass = Electron Mass and Equivalent Bandwidth and Detector Size

Tachyon Velocity

over Radio Waves

over Visible Photons

over X-Ray Photons


(n = 3 GHz, l = 0.1 meter)

(n = 6 x 1014 Hz, l = 5000 Angstroms)

(n = 3 x 1018 Hz, l = 1 Angstroms)


















What about energetic efficiency? Although the calculations are highly speculative, it would appear that the ratio of the theoretical tachyonic efficiency in bits/joule-sec to the theoretical photonic efficiency in bits/joule-sec is equal to (mlc/h)2. For tachyons having the mass of an electron, only high-energy x-rays (1020 Hz and higher) are more efficient. If proton-mass tachyons are transmitted, only powerful gamma-ray photons (1023 Hz and higher) should be more energy-efficient per bit.

If the existence of tachyons is finally verified,*** and if the generation, transmission and detection of these fleeting particles can be achieved with reasonable equipment., sentient ETs races may have at their disposal one of the cheapest and fastest communications systems imaginable. Perhaps someday human scientists may learn to tap this channel. The first tachyonic interstellar signals we receive may say: "Greetings! Welcome to the Galactic Club!"


* The largest machines on Earth today can achieve about 0.5 TeV, and 10 TeV machines are in the planning stage.

** According to the Conference Report, strange neutrino "signals" have already been detected on first- or second-generation equipment:

Professor G. Marx {Department of Physics, Budapest University, Hungary) was interested in Dr. Subotowicz’ theories of neutrino communication and reported that on 4 January 1974, a Philadelphia team had received bursts of neutrino pulses, where groups of pulses (approximately 1 microsecond pulse duration and 1000 pulses per burst) were received. Twelve such groups were received and then the counters saturated. Although obviously open to the wildest speculation, these signals were almost certainly due to the initial stages of supernovae explosion and stellar gravitational collapse.1093

*** Nuclear physicists have searched for evidence of tachyons for more than a decade. For the records of these experimental investigations, see Alväger and Kreisler,1479 Ashton et al,3120 Baltay et al,1498 Bartlett and Lahana,1500 Clay and Crouch,654 Danburg and Kalbfleisch,1502 Danburg et al,1496 Davis, Kreisler and Alväger,3121 Feinberg,648 Kreisler,1518 Murthy,1510 and Thomsen.646



24.2.2  Electromagnetic Waves and Frequency Selection

Based on the historical development of human communications technology, it is probably fair to say that electromagnetic radiation represents the most primitive technique for signaling across interstellar distances. Advanced cultures may possess particulate, neutrinic, gravitic or tachyonic communication channels, or they may have knowledge of only a few of these, but it is difficult to imagine any technical society in possession of any of such sophisticated technologies without at least having an awareness of photonic communication techniques. Electromagnetic waves are probably the easiest information markers physically to generate, and they are ubiquitous throughout the cosmos. Photons are most likely the most primitive communication technology available for interstellar discourse.

If this is true, which photons are the best to use? The electromagnetic communication band spans a useful frequency range of at least 16 orders of magnitude -- gamma rays, x-rays, ultraviolet, visible light, infrared, radio and so forth. Unless we can identify a preferential region of the spectrum, our search for intelligent alien signals will be frustrated by the enormous number of possibilities.

The most logical place to begin is with the question of energy efficiency. Following Drake’s Principle of Economy, we should expect ETs to select those photons which transmit the greatest number of bits per second for the least cost in joules of transmission energy. Unfortunately, both bit rate and photon energy are proportional to frequency, so when the two are divided the frequency dependence drops out. Hence, the criterion of energy efficiency is incapable of distinguishing between photons of different frequencies.

If we look back at Oliver’s criteria (Section 24.2), we see that there are really only two of them which are useful in making a choice between photons: Noise and absorption. Signals transmitted over channels which are too noisy are not detected. Messages transmitted through a medium which absorbs them do not reach the receiver.

Consider the graph in Figure 24.2. The vertical axis represents the total electromagnetic flux density in watts/meter2, assuming a detector which is looking at approximately 8% of the entire celestial sphere (1 square radian of sky, or "steradian"). Noise from the most important sky sources are plotted over the various frequency ranges in which they occur. Coverage stops at 107 Hz because electromagnetic waves with frequencies less than this are heavily absorbed by planetary ionospheres, and in any case, waves below 106 Hz are absorbed by the interstellar medium.


Figure 24.2 Integrated Flux Density of Background Radiation Likely to Obscure Interstellar Electromagnetic Communications3129

Flux Density


What conclusions may we draw from the data? First, it appears that the noisiest part of the electromagnetic spectrum is the region from 1011 Hz up to about 1016 Hz. Any signals sent by photons within this range must compete with starlight, the 3 K cosmic background, and a variety of other emissions and absorptions. Although interstellar communications seem least likely in this portion of the spectrum, a few xenologists have suggested making use of the laser’s ability to produce highly directional, extremely monochromatic beams. If the laser is tuned to emit in a stellar absorption band (a narrow frequency band where the sun is about an order of magnitude darker than normal), distant alien observers would observe an artificial spectral line winking on and off in a clearly intelligent pattern.1039

A better choice is the part of the spectrum which lies above 1016 Hz. X-rays and gamma rays may be useful in interstellar communications because they are not absorbed by the interstellar medium. (See Elliot,3144 Fabian,3137 and Kuiper and Morris.2608) But attempting to search the entire range from 1016-1023 Hz is hardly going to be easy. If we could check a 1 MHz band for ET signals every 10 seconds, the search time to cover the entire high frequency region would require a period of time on the order of the age of the universe. Somehow the possibilities must be narrowed. One way to do this is to look at "magic frequencies" derived from universal physical constants or defined by well-known physical phenomena.

The possibilities are endless.2608 For example, the Compton Wavelength* of the electron is 2.420 x 10-12 meter, corresponding to a frequency of 1.239 x 1020 Hz. This falls almost exactly into a noise minimum between hard x-rays and gamma rays on the flux density curve on the preceding page. The Compton Wavelengths of the proton and neutron, respectively, are 1.32134 x 10-15 meter and 1.31952 x 10-15 meter, which define a "narrow" 1020 Hz band between the frequencies 2.26885 x 1023 Hz and 2.27198 x 1023 Hz. This band conveniently lies in another local noise minimum on the flux density curve. Since it is defined by the two major particles from which all stable matter is constructed, this "baryon gap" may be the preferred region for interstellar communications using high-energy photons. Yet another approach -- one which fills in the third local noise minimum between soft and hard x-rays -- is somewhat biochauvinistic. It assumes that ETs will transmit signals between favored spectral lines of atoms or ions that are biologically important to them. For instance, silicon-based sentients may transmit between Si- Ka1 and Ka2 lines, located at 4.20736 x 1017 Hz and 4.20591 x 1017 Hz, defining sharply what might be called the "silicon hole" with a width of 1.45 x 1014 Hz in which signals might be detected. Similarly, we might define a "sulfur hole" between 5.58048 x 1017 Hz (Ka1 line) and 5.57758 x 1017 Hz (Ka2 line); a "chlorine hole" between 6.34108 x 1017 Hz (Ka1 line) and 6.33718 x 1017 Hz (Ka2 line) for chlorine-breathers; a "germanium hole" from 2.9464 x 1017 Hz to 2.9428 x 1017 Hz for germanium xenobionts; and so forth.

It is clear, however, that on the basis of noise/absorption criteria alone the low frequency end of the spectrum (below 1011 Hz) should be optimal for long-distance interstellar communications. The weight of xenological opinion today is that radio frequencies are the preferred mode of photonic information transmission between the stars.57,22

But exactly which radio frequencies are best? The graph in Figure 24.3 represents an expanded view of the quietest portion of the electromagnetic spectrum. The vertical axis is no longer energy density. Rather, intensity is expressed as sky brightness (blackbody) temperature which is what radio-astronomers actually measure.


Figure 24.3 Free Space Microwave Window


There are three fundamental sources of noise associated with all highly sensitive radio receivers. First there is "galactic noise," caused by synchrotron radiation arising from free electrons orbiting magnetic field lines in space. From the graph we see that this noise rises steeply below 1 GHz, depending very slightly upon the galactic latitude toward which we point our receiver. Second, there is "thermal noise," caused by the 3 K cosmic back ground, the relict radiation from the Big Bang. Third, there is "quantum noise" (spontaneous emission or shot noise), representing a fundamental quantum mechanical limitation on receiver sensitivity.

Above 1 GHz galactic noise falls below the isotropic cosmic background, and beyond about 60 GHz quantum noise exceeds the cosmic background and in creases indefinitely with frequency. (It is the dominant form of noise at optical wavelengths.) Thus from any point in interstellar space the sky is likely to be quietest from about 1-60 GHz. This is the microwave window out in free space.

Now look at the graph in Figure 24.4 on the Terrestrial Microwave Window. Atmospheric absorption must be added for receivers located on a planetary surface under a sea of air. If signals from the stars arrive at frequencies above 10 GHz, they will be strongly absorbed by water vapor molecules, oxygen molecules, and many other molecules not shown. These substances are likely to be present in the air of any terrestrial world that resembles Earth even remotely. We see that the Terrestrial Microwave Window is closed virtually for all radio frequencies save those few between 1-10 GHz. The great majority of xenologists agree that this is the range where alien electromagnetic signals most profitably may be sought.


Figure 24.4 Terrestrial Microwave Window


There is considerably less consensus on exactly where to search within the Window. Philip Morrison and Guiseppi Cocconi first suggested that the search should be made at or near the natural emission peak of neutral inter stellar hydrogen gas (the most abundant element in the universe). According to these early pioneers in SETI, the preferred frequency was 1.42 GHz:

On the most favored radio region there lies a unique, objective standard frequency which must be known to every observer in the universe: the outstanding radio emission line at 1420 Mc/sec of neutral hydrogen.1033

As the hydrogen line itself is rather noisy, a few scientists responded that searches ought to be made at integral multiples of 1.42 GHz.1054

Since 1959, the science of radioastronomy has made tremendous advances. It is now known that there are many other elements and molecules with emission lines within the Window. The hydroxyl (OH) radical has emission lines at 1.612, 1.665, 1.667, and 1.720 GHz. Spectral lines of molecular species are also quite popular in the speculative literature. For instance, some have proposed listening in at 4.83 GHz -- the natural formaldehyde emission line -- because it is comparatively less noisy than many other natural lines.3122

In the early 1970’s, interest turned to what is commonly called the "water hole." Much as with the x-ray bands described above, the water hole is the band of radio wave frequencies lying between the H and OH emission lines. Readers familiar with chemistry will recognize that H plus OH equals water, the basic solvent for all life as we know it on Earth. Dr. Bernard Oliver, who originated this idea in connection with his work on Project Cyclops (see below) in 1971, explains the rationale for the water hole in a particularly poetic fashion:

Nature has provided us with a rather narrow band in this best part of the spectrum that seems especially marked for interstellar con tact. It lies between the spectral lines of hydrogen and the hydroxyl radical. 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 for all species: the water hole. 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 signposts that taken together symbolize the medium in which all life we know began. Is it sensible not to heed such signposts? To say, in effect: I do not trust your message, it is too good to be true!3289,57

During the mid- and late-1970’s there has been an outpouring of new ideas and proposals for preferred frequencies in SETI, so the water hole concept today has a great deal of competition. Drake and Sagan suggest using an "average value" of the H and OH natural emission lines, obtaining a kind of "molecular center of mass" frequency of 1.65 GHz as the favored interstellar channel.3128 A related proposal is that in space, where the Free Space Microwave Window allows greater leeway, we should search the water line itself at 22 GHz2865 (or perhaps the ammonia line at 24 GHz, if we are looking for ammonia-based beings15).

We could adopt the "magic" frequency of 56 GHz, the point at which the blackbody 3 K background "thermal noise" curve intersects the "quantum noise" curve.22 Argue Drake and Sagan: "The 56 GHz channel has the provocative property of being determined simultaneously by quantum mechanics and cosmology."3128 Using another combination of basic physical constants, Kuiper and Morris have derived a "magic" frequency of 2.56 GHz.2608 Then there is the intriguing suggestion of Soviet SETI researcher P.V. Makoveskii of the Leningrad Institute of Aviation Instrument Manufacture, that the most probable frequencies for interstellar radio traffic will be the natural hydrogen line frequency alternatively multiplied and divided by such constants as p, 2p, and SQRT(2).3261 This identifies several "uniquely artificial" frequencies, including 0.23, 0.45, 1.0, 2.0, 4.5, and 9.0 GHz.

However reasonable they may seem, each of the above proposals rests on a plausibility argument whose conclusions perhaps are suggested but certainly are not compelled by the basic facts and assumptions of xenology. A few scientists have attempted to predict the optimum interstellar signaling frequency based solely upon fundamental physical laws and conditions expected to apply to all communicative civilizations in the Galaxy.

Using his Principle of Economy, Dr. Frank Drake points out that the best radio frequency is the one in which transmission power is minimized -- that is, where noise is lowest. This is customarily described in terms of the "brightness temperature" of the sky -- the temperature a black body would have to have in order to duplicate in brightness the observed radio radiation coming from a given spot in the sky. Following Drake, we write the noise temperature as a function of celestial right ascension a and declination d (the astronomers’ way of specifying sky position) as T(a,d), This temperature is not constant for all radio waves, but varies as a function of frequency n. Typically the variation follows a "power law" -- frequency raised to some variable exponent g -- of the form n-g. Since g is also a function of sky position, we shall write it as g(a,d). Both T(a,d) and g(a,d) can and have been measured very precisely by terrestrial radioastronomers for every point in the sky.

Finally, Drake derives the following equation for no the frequency of maximum economy (of maximum communication range):

where k is Boltzmann’s constant and h is Planck’s constant.3123

What does all this mean in plain English? Simply this: For each position in the astronomers’ sky there exists a unique frequency of minimum noise and maximum economy. Whatever direction you point your radiotelescope, range will be greatest if the radio frequency determined by the above equation is used. Best of all, the numbers that must be plugged into Drake’s formula are already known, so n0 theoretically may be computed today for any star system in the heavens with whom we may wish to enter into communication. Drake calculates that the range of frequencies of maximum economy span the Terrestrial Microwave Window from 3.75 GHz out to about 10 GHz.


* The Compton Wavelength is the change in wavelength corresponding to a loss of energy suffered by a photon whenever it collides with matter.



24.2.3  Acquisition and Artificiality Criteria

There are many different kinds of possible alien transmissions we might receive -- local radio traffic, beamed messages to regular correspondents, beacon signals designed to attract attention, and so on. We are most likely to pick up beacons first because these should be comparatively more powerful. But xenologists disagree on the exact nature of the beacon signals we may detect.3179 Artificiality criteria have not yet been worked out in full detail, but certainly not for want of trying. A variety of radioastronomical criteria have been proposed over the years which turn out to be insufficient: Small angular dimensions of the source, characteristic spectral density distribution of intensity, time-variability or flux pulsations in time, circular polarization, alternating left- and right-hand polarization pulses, corrected orbit-induced or planetary axial-rotation-induced Doppler shifts, and so forth. (See Kaplan,29 Konstantinov and Pekelis,25 and Tovmasyan.28) According to Vsevolod S. Troitskii, a well-known Russian radioastronomer, there do seem to be two basic requirements for all beacon signals that xenologists can agree upon:

1. The signal should not leave any doubt as to its artificial origin. The artificial signal should be distinctly different in its properties from natural radiations.

2. The signal should carry some information about the transmitting civilization.3124

The second of these criteria -- information content -- is highly significant. So far as we know, only the processes of life, intelligence and culture are able to impress large quantities of information onto packets of photons. Perhaps we should try to decide if there is some minimum information content than a given transmission must possess before we may regard it as artificial. As Philip Morrison points out:

A little more information content is needed than just the existence of a {stellar spectral} line of something rather rare or a very regular pulse, because if you look for a sufficiently long time, you are bound to find rare things just in the course of events. This is a very valuable lesson. Although rare things occur rarely, it is also true that rare things occur rarely.3127

Soviet astrophysicist Nikolai S. Kardashev believes that a signal should contain at least 10-100 bits of information before we may feel confident that it is artificial in origin.3126 Terry Winograd’s artificial intelligence computer program consists of about 106 bits, but Marvin Minsky estimates that a message with as few as 104-105 bits, properly situated, could be considered "intelligent."22

Hopefully the information will not be too difficult to extract. According to E.C. Shannon’s statistical theory of coding, the most efficient signals will appear indistinguishable from random thermal noise.3186,3187 That is, a maximally information-saturated message looks like noise -- unless the recipient happens to know the correct translation code. Xenologists suspect that ETs may not attempt to achieve maximum information content in interstellar beacon devices. Signals that look like noise don’t attract much attention. If such highly efficient beacon transmissions are used, they probably will be accompanied by special attention-getting messages. These are often referred to as "call signals," and may be expected to satisfy Troitskii’s first criterion on the previous page.*


* Specific search strategies have been proposed by Bihary,3158 Dixon,1266 Gray,3150 Haviland,1147 Morrison, Billingham and Wolfe,2865 Oliver and Billingham,57 Ridpath,3154,3257 Sagan,22 Shklovskii and Sagan,20 and Walker.158 Timing of the call is also of paramount importance -- we must know when to look as well as where and how. The interested reader is referred to the timing schemes offered by Gindilis,22 Makovetskii,3263 McLaughlin,2719 Pace and Walker,651 Tang,1613 and Tovmasyan.28



24.2.4  Alien Message Contents

Once a genuine extraterrestrial signal of some kind has been detected, the acquisition phase ends and the communicative phase may begin. The recipient must then be able to puzzle out the meaning of the alien messages he receives.

If the signals are being transmitted purposefully, then it is likely that the ETs at the other end will have done their level best to ensure easy decipherability of their messages by the intended recipients. Coding should be relatively simple and considerably redundant, full of clues enabling SETI scientists to achieve a full translation with high validity. Since this is exactly the opposite goal to that of the science of cryptography (secret codes), xenologists often refer to it as the Principle of Anticryptography.

According to the Principle, a beacon message transmitted from another world, prima impressionis, should be optimized for easy decoding by intelligent recipients.

The Principle of Anticryptography suggests the basic format of messages we may expect to receive from the stars. Consider a sequential transmission consisting of a string of symbols of some kind. If the message is very lengthy, and later parts are of greater complexity in reliance upon our understanding of earlier parts, then if we tune in near the middle or the end we probably won’t be able to understand anything at all. On the other hand, if the message is kept very brief and repetitive then, in the words of one radio-astronomer, it "bores us to tears for decades while we try to acknowledge."80 In keeping with the Principle, we might expect to find "nested messages," involving a frequently repeated call signal interspersed with short but complete "language lessons."22 Every so often a self-contained package of basic information would be substituted for the language lesson. On yet rarer occasions, the basic information package would be replaced with a more advanced information package, and so on to higher and higher levels of sophistication. Such a message format is highly redundant, repetitive, error-proof, informative, and so may be tapped into at any point in the transmission without loss of meaning.

What about message contents? Will we understand what ETs are trying to say to us? A few xenologists have proposed that the "universal language" of mathematics will provide the bridge of understanding between man and alien. (See Hogben,1112 Oakley,329 Pryor,99 and Sagan and Drake.3143) According to one scientist, it is difficult to imagine the existence of communicative beings unfamiliar with numbers and counting. Thus the earliest messages may consist of a series of irreducible prime numbers, say, 1, 3, 5, 7, 11, 13, 17,..., or the value of pi out to the first twenty digits or so.

Assuming for the moment the validity of this approach, can we do better than mere counting? Dr. Hans Freudenthal, professor of mathematics at the University of Utrecht in the Netherlands, has designed a purely mathematical language which conceivably could be used in interstellar discourse between alien races. Freudenthal originally devised his system of "lingua cosmica" -- Lincos for short -- as an exercise in logical linguistics, but he admits it easily might serve as the contact and communicative mode among ETs.3290

Lincos consists of a variety of mathematical terms and phrases, to be encoded into specific combinations of radio pulses and signal shapes and then beamed out into space. It is devised purely in terms of semantics and human logic, and accomplishes understanding by building from simple beginnings. Statements are arranged in words, sentences, and paragraphs.

Freudenthal’s program begins by establishing the meaning of the terms "plus" and "equals." He would, for instance, send signals something like "beep bloop beep tweet beep beep" (1 + 1 = 2), perhaps followed by "beep bloop beep beep tweet beep beep beep" (1 + 2 = 3) and so on. Eventually it should become clear that "bloop" represents addition and "tweet" signifies equality. In the first chapter of his book on Lincos, Freudenthal goes on similarly to introduce the concepts of subtraction, multiplication, division, basic symbolic logic ("and," "or," and "follows"), negatives, integers, decimals, fractions, and zero. In the second chapter the concept of time makes its appearance, including "seconds," duration, wavelength, frequency, "before," "after," and "occurs."

In chapter 3, Freudenthal develops concepts of correct and incorrect, right and wrong, good and had; to count, to search, to find, to describe, to prove, to change, to add or omit; to know, guess, understand, and mention; nearly and approximately, much and little, soon and long ago, age and now; necessary and possible, enable, be forced, allowed, forbidden; politely; conflict between necessity, duty, and desire; and so forth. For example, Freudenthal’s language lesson to teach the concepts "correct" and "incorrect" to ETs runs as follows:

*Ba Inq Hb•?x.100x=1010:

Human-a asks Human-b: If 4x = 10, how much is x?

Hb Inq Ha.1/10:

Human-b tells Human-a: x is 1/2.

Ha Inq Hb Mal:

Human-a tells Human-b: That is incorrect.

Hb Inq Ha.101/10:

Human-b tells Human-a: x is 5/2.

Ha Inq Rh Ben*

Human-a tells Human-b: That is correct. End of lesson.

As vocabulary slowly builds, ever more sophisticated statements become possible. In chapter 4 of Freudenthal’s book, mechanics and spatial extent are dealt with, including concepts of distance, position, length, growth, volume, motion, waves and oscillations, speed of light, mass, and astronomical concepts. Later chapters delve into geography, anatomy and physiology including the human reproductive process. By staging "plays" between symbolic human "actors," Lincos ultimately should be capable of portraying diverse facets of human behavior, emotions, social conventions, philosophies and religious rituals.

Unfortunately, there are many problems with the "universal mathematical language" approach. As we know, no system of logic is or can be universal. Gödel’s Theorem suggests that alien systems of mathematics, logic and philosophy necessarily must be at least somewhat incongruent. ETs may not understand our system of numbers, our Euclidean geometry, our Aristotelian bimodal logic, our astronomically-derived Newtonian physics, or our sequential Periodic Table of the Elements, simply because they view the universe through different sensors and thus reach different conclusions based on different theories. So aliens may not understand human-designed artificial languages like Lincos.

Still, the possibility of totally nonintersecting systems of knowledge is probably remote in most first contact situations. It is unlikely that man and alien will have absolutely nothing in common. In most cases, ETs evolving on Earth-like worlds should have much in common with us by virtue of the similarity of our native environments. To the extent the two paradigms do intersect, a basis for communication may be established from which the areas of nonintersection later can be cautiously explored. The most likely region of intersection in this case probably is in the hard sciences -- physics, chemistry, geology, meteorology, and so on.

Other approaches to the problem of extraterrestrial message anticryptography have been proposed. Perhaps the most popular of these is the interstellar pictogram, anticipated by H.W. and C. Wells Nieman back in 1920.170 The basic idea is that a string of radio pulses, coded as on/off, black/white, or 0/1 could he arranged in a rectangular raster pattern to form a two-dimensional pictorial image much like modern television systems (Figure 24.5).

Imagine you receive a string of 1271 zeros and ones on an appropriate interstellar frequency band. How might this be translated? The mathematically-inclined reader will recognize that 1271 is the product of two prime numbers, 31 and 41. This suggests that the data should be laid out sequentially on a gridwork either of 41 rows and 31 columns or 31 rows and 41 columns. As illustrated in Figure 24.5, the former of these makes little sense whereas the latter appears highly informative.1056


Figure 24.5 Sample Interstellar Pictogram

Suppose the above string of 250 pulses ("1") and 1021 pauses ("0") is received by a terrestrial radiotelescope during a scan of the Epsilon Eridani star system. There are a total of 1271 data bits. 1271 is the product of two prime numbers, 31 and 41. This suggests a two-dimensional message, in which the data are laid out sequentially in a rectangular grid pattern of rows and columns. There are two possible ways of doing this -- 41 rows or 31 rows. As we see at below left, the choice of 41 rows produces an evidently random arrangement of dots. But if the same data are set out in a grid with 31 rows, as shown at below right, the pattern is striking and informative.1056


It appears that the creatures depicted in the lower center part of the pictogram, presumably the species that sent the message, are beings having two arms, two legs, and an erect posture much like humans. Sexual cues suggest a mammalian physiology, with long-maturing offspring born one at a time and cared for during youth by pairs of bisexual parents. The crude circle and column of dots at the left suggest their sun and planets, and the being on the left is pointing at the fourth world which is evidently their home. The planets are numbered down the left-hand side in a ‘binary code" different from the one normally used by human computer scientists. The numbers one through eight are given in this new code. The creature on the right is pointing at the "binary number 1011, which is six in the alien code. Perhaps the ETs have 6 fingers on each hand. There is also a dimension line at far right. labeled with the "binary" number 11101. In the alien code, this is eleven, so we infer that the beings are eleven somethings tall. Since the only length we both know for certain is the wavelength of the radio waves upon which the message was transmitted (say, 10 centimeters, corresponding to 3 GHz), then the ETs must be 10 x 11 = 110 cm in height. (They are a race of pygmies.) The objects at the top of the pictogram represent atoms of hydrogen, carbon and oxygen, so their life chemistry is based on carbohydrates much like our own. The wavy line commencing at the third planet indicates it is a water world, and the fish-like form shows there is marine life there. Since the bipeds know this, they must have at least interplanetary space travel capability.


Variations may be imagined. For instance, we could use perfect squares rather than "rectangular primes." If the message has 1681 bits, which is 41 x 41 exactly, then there is only one way to lay the message out and the two-choice ambiguity is avoided. We might also receive a three-prime message, say with 2717 bits, which, when correctly arranged, permits the reconstruction of a spatial mode complete with height, width, and depth. If one of the three primes represents a time dimension, we will have the equivalent of an extra terrestrial Mickey Mouse cartoon. (The time-prime should be clearly distinguished from the spatial-primes for this purpose. The author suggests a message of 5819 bits, consisting of 11 sequential frames of 23 x 23 bits each.) Yet another twist, first proposed by Y.I. Kuznetzov of the Institute of Energetics in Moscow, is the possibility of transmitting a 3-D interstellar pictogram using variations in frequency, intensity, and pulse delay to build up each of the three physical dimensions of the image of a solid object.22 The pictogram mode of contact has appeared repeatedly in science fiction.70,1748

A number of chauvinisms associated with all pictogram schemes may render them significantly less universally interpretable.

First, the idea of laying data out in a grid-shaped raster seems logical enough to human scientists. Our TV sets work in essentially the same way. But aliens may have different ideas and technologies. Perhaps they use spiral scanning (either the Archimedean or logarithmic variety). At least one ancient human language was written in this format, and spiral tracing once was seriously proposed for use in Earthly television systems.1351 Aliens with spiral scanning video tubes may send messages of 1429 bits (an indivisible prime) to be laid out sequentially in a spiral pattern. Would we ever guess?

Another chauvinism is cultural in nature. According to Jan B. Deregowski, lecturer in psychology at the University of Aberdeen:

A picture is a pattern of lines and shaded areas on a flat surface that depicts some aspect of the real world. The ability to recognize objects in pictures is so common in most cultures that it is often taken for granted that such recognition is universal in man. Although most children do not learn to read until they are about 6 years old, they are able to recognize objects in pictures long before that; indeed, it has been shown that a 19-month-old child is capable of such recognition. If pictorial recognition is universal, do pictures offer us a lingua franca for intercultural communication? There is evidence that they do not: Cross-cultural studies have shown that there are persistent differences in the way pictorial information is interpreted by people of various cultures.66

Certain tribes in Africa, for example, are unable to recognize photographs as representations of the real world. They just don’t see things the way we do. And these are our fellow human beings. How much more difficult may be the problems of interpretation where ETs are involved?

A third and very serious chauvinism is the tacit assumption that all sentient alien creatures must necessarily be visually oriented. Consider a pelagic world inhabited by intelligent technological dolphins: These creatures have devised a special telescope which converts radio waves into acoustical signals which they can hear. One day their equipment is aimed at Earth and they receive the standard 31 x 41 pictogram, which they promptly arrange into an appropriate rectangular format of sonic pulses. But since the pictogram was assembled from the viewpoint of visual beings, the sentient dolphins cannot make head nor tail of our message. Reality just doesn’t sound that way to them.



24.2.5  SETI:  Yesterday and Today

As it has developed over the past two decades, the science of SETI -- Search for Extraterrestrial Intelligence -- today entails a passive listening strategy in hopes of detecting alien beacons or other transmissions. The older acronym CETI (Communication with Extraterrestrial Intelligence), still in use in the Soviet Union, implies a more active strategy in which humankind might actually converse with ETs. Because "communication" connotes a two-way exchange -- which may people fear -- NASA officials and American scientists engaged in this work on a professional basis prefer the isolationist connotations of "search." This, this believe, helps to avoid heated popular controversy and to reassure an anxious public that the intention is only to listen quietly, never to transmit.

Actually, SETI research of sorts has been underway since before the turn of the century. Nikola Tesla, native Croatian by birth and inventor of the induction motor, perhaps may be credited as the first SETI worker in history. Tesla believed that the Earth possessed a gigantic electric field that could be set aquiver if a sufficiently high voltage could be made to jump a large enough spark gap. In 1899, using money furnished by wealthy industrialist J.P. Morgan, Tesla erected a 70-meter-high transmission tower at a site in Colorado Springs, Colorado. One night when he was alone in his laboratory, the inventor observed "electrical actions" which he later interpreted as possible messages from Mars:

The changes I noted were taking place periodically, and with such a clear suggestion of number and order that they were not traceable to any cause then known to me. I was familiar, of course, with such electrical disturbances as are produced by the sun, Aurora Borealis and earth currents, and I was as sure as I could be of any fact that these variations were due to none of these causes.... It was some time thereafter when the thought flashed upon my mind that the disturbances I had observed might be due to intelligent control.....The feeling is constantly growing on me that I had been the first to hear the greeting of one planet to another.146

Italian physicist and inventor of the radio Guglielmo Marconi reported in 1920 that his company’s radio stations had been picking up mysterious signals for the better part of a decade. Some of these sounded like a code, but were otherwise meaningless. Occasionally Marconi heard three dots and a dash -- Morse code for the letter "V" -- at frequencies ten times lower than were then in common use in man-made transmitters. When asked if these signals might be messages from another planet, Marconi answered in the affirmative.3291

The next early SETI venturer was David Todd, chairman of the Astronomy Department at Amherst College in Massachusetts. Todd, excited by the prospect of the mysterious Marconi messages, tried to persuade all radio stations in the country to shut down during the closest approach of Mars to Earth in 1924 so he could listen for alien signals. While not entirely convincing, he managed to persuade the Chief of Naval Operations of the United States Navy to go along with his scheme. On 21 August 1924, the Chief sent word to the twenty most powerful military stations under his command to avoid unnecessary transmissions and to listen for strange signals from space. Army stations received similar orders, and in one instance a cryptologist from the Signal Corps was present to provide on-the-spot translations, if need be. But nothing substantive was detected, and the entire project died a quiet death. It was to be nearly 37 years before humanity once again would turn its ear to the stars.

It is useful to pause at this point and ask what is the likelihood of success of such a search. Many writers have pointed out that continuous transmissions into space would have a fairly low expectation of success in any given year, yet at the same time would be inordinately expensive. Also, first contact could bring unexpected trouble. Why announce our presence to the universe, these critics ask? Why invite danger and unnecessary risk? Hearing this, many SETI researchers have wondered out loud: "What if all alien civilizations were listening and no one was sending?"

At least three requirements should be satisfied if a society is to convert from a passive listening culture to an active transmitting one:

1. The civilization must have sufficient energy on hand that a 1015 watt continuous transmitter represents a negligible cost to the transmitting society;

2. The civilization should command a technology and energy re sources sufficiently powerful to defend itself against invasion or other military threats from any star system to which it directs its signals; and

3. The civilization should have great political and cultural longevity, in order to ensure continuity of operation and support of the beacon system.

It would appear that at least a Type II stellar culture may be necessary to fulfill all three preconditions of a transmitting society.1285,28

Using the Drake Equation (see Chapter 23) and assuming various values for cultural longevity, it is possible to estimate how deeply into space we must carry our search in order to achieve a specified probability of success. From the figures in Table 24.2, it appears that the most likely search ranges are from 100-1000 light years, a volume of space which encloses some 104-106 stars. If we spend just 1000 seconds examining each one, then the expected search time required to achieve a high probability of success on a modern, fully-dedicated radiotelescope with full computer guidance and tracking capability is on the order of several decades. Because of this somewhat encouraging result, growing numbers of xenologists are entering the SETI sweepstakes in hopes of becoming the first to detect messages from an extraterrestrial civilization.


Table 24.2 Probable Search Range Required for Success, as a Function of
Mean Longevity of Technological Civilizations2296

Mean Longevity of Technological 

Probable Search Range
to Ensure a 63% 
Chance of Success

Probable Search Range to Ensure a 95% Chance of Success


























The first scientist to attempt to pick up interstellar signals was Dr. Frank D. Drake.1619 Despite the many technical difficulties at the time, Drake initiated mankind’s first systematic search for messages from intelligent beings on other worlds in the late evening of 11 April 1960. His equipment had been set up at the newly built National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. Drake named his effort Project Ozma after the queen of the mythical land of Oz, a place very far away, difficult to reach, and populated by strange and exotic beings.1535

The Ozma apparatus was operated at the hydrogen frequency of 1.42 GHz with a narrow bandwidth of 100 Hz. Two receiving horns were placed close together at the focus of the 26-meter-wide radio dish, so that the target star and the adjacent empty space could be monitored simultaneously. By subtracting one signal from the other electronically, stray emissions and background noise could be eliminated. Drake pointed the receiver at each of two target stars -- Tau Ceti and Epsilon Eridani.

Early in the investigation a strong signal was picked up, while the equipment was pointing at Epsilon Eridani. A series of high-speed pulses, roughly eight per second, were detected "so regularly spaced that they could only be the product of intelligent beings."702 There was, as Drake later described the scene, "a moderate amount of pandemonium" in the control room. The strange signals were heard several more times over a period of several months. Eventually it was discovered that they were the product of a secret military experiment in radar countermeasures using airborne transmitters.

Listening continued through July 1960. Some 150 hours of total observational time were logged for the two target stars. Though Project Ozma did not detect any messages. from extraterrestrial civilizations, it cannot in fairness be reckoned a failure. Only two stars out of billions were sampled, and only in one narrow frequency band with only very poor (by modern standards) receiver sensitivity. The chance that alien signals had been overlooked was great. But perhaps the most significant achievement of Project Ozma was purely symbolic: Drake had proven that it was technically possible to perform scientifically valid SETI experiments.

Ozma was the first but certainly not the last. In the two decades since Drake’s initial pioneering effort nearly two dozen SETI searches have been conducted at observatories around the world (Table 24.3). Nearly 1000 nearby stars have now been at least briefly checked for intelligent transmissions, both in ultraviolet and in a variety of radio frequency bands. Individual whole galaxies have been monitored for evidence of emissions of the sort expected if Type III civilizations were present. Other searches have included simple sky surveys and sweeps of the entire sky for telltale pulsed signals. (Many highly unusual receiver systems have been proposed, including the Luneherg lens,1570 Siforov’s isotropic scanner,28 and Berger’s "celestial iconospherics" lenses.166)


Table 24.3 SETI Searches Through 1978


Inclusive Dates


Size and Type of Instrument





Objects Searched and Comments

F. D. Drake

May-July 1960


26-rn Dish



4 ×10-22


Project OZMA. 2 stars: t Ceti. e Eridani.

G. B. Sholomitskii








CTA-102 reported as possible alien signal; later found to be quasar.

Troitskii, Starodubtsev, 
 Gershtein, Rakhlin

Oct, Dec 1968 February 1969


15-rn Dish



2 ×1021


11 stars (e Eridani; r Ceti; 380,47, p' Ursa Majoris; o Coma Berenices; b Canis Venaticorum; h Boötis; i Persei; y5 Aurigae; h Herculis) and M31 {Gal. Andromeda}

Troitskii, Bondar, 

March-Nov 1970, 


Murmansk, Ussuri

Dipole Radiometer





All-sky omnidirectional search for sporadic pulsed signals, using cross correlation from 2 or 4 stations 8000 km apart in U.S.S.R. (50% continuous)

G. Verschuur

Oct-Nov 1971 June, Aug 1972


91-in Dish 
43-rn Dish


6900, 7200

5 ×10-24
1.7 ×10-23


3 stars: r Ceti, e Eridani, 61 Cygni A,B Project "OZMA") 
10 stars: Barnard’s Star, Wolf 359, Luyten 726-8, Lalande 21185, Ross 154, Ross 248, Epsilon Eridani, Ceti, 61 Cygni A.B, 70 Ophiuchi A,B.

P. Palmer,

B. Zuckerman

Nov 1972-Aug 1975, and 1976


91-rn Dish



1 ×10-23
3 ×10-24


Project OZMA II, Searched total of 659 stars. Used receiving system with 384 channels with system noise temperature 50 K.

Bowyer, Lampton, Welch, Langley,

Tarter, Despain



26-rn Dish

Various, microwave


1 ×10-21


Operates as a parasitic experiment on radio telescopes during conventional astronomical research. Precursor to SERENDIP (see below). Semirandom.

N. S. Kardashev

Sept-Oct 1972







All-sky search for pulses; 2 stations simultaneously in Pamir & Caucasus mountains.

N. S. Kardashev

Dec 1973- 
Feb 1974







All-sky search for pulses, using 2 stations in Caucasus & Kamchatka Peninsula

F. Dixon, D. M. Cole



53-rn Dish

1420.4 rel. to Gal. Center ±190 KHz 


1.5 ×10-21


Has operated full-time, 24 hr/day since 1973. Receiver tuned to hydrogen rest frequency relative to Galactic Center (as function of direction). All-sky area search for circular-polarized, modulated beacon, over 7°<d<48°.

Bridle, Feldman



46-m Dish





Has searched a total of 500 stars. Examined natural water maser frequency.

H. F. Wischnia

Nov 1974 
Summer-Fall 1975


Orbiting Ast. Observatory

Ultraviolet Radiation




1 star: e Eridani (Search conducted using NASA’s OAO-3 at 750 km 
2 stars: t Ceti, e Indi with high-precision stellar UV spectrometer.)

F. D. Drake. C. Sagan



305-rn Dish

1420, 1653, 1667 


8 × 10-25

4 × 10-25


4 galaxies searched: M33 (Great Spiral in Triangulum), M49 (Virgo), Leo I 
and Leo II. Examination of Local Group galaxies for Type II civilizations.

Univ. of Calif. Astron. Dept. staff



26-rn Dish



5 × 10-22


SERENDIP (Search for Extraterrestrial Radio Emission from Nearby Developed 
Intelligent Populations). Automated survey parasitic to radioastron. obser. 
(Bowyer, Welch, Lampton, Tarter, Freeman, Clawson, Turner, Langley, Despain)

Kardashev, Troitskii, 


RATAN-600 Radiotelescope in 
Northern Caucasus, U.S.S.R.





Ongoing search for Type II and Type III "supercivilizations" elsewhere in the Milky Way and in other galaxies.

T. A. Clark, D. C. Black, 
J. N. Cuzzi, J. C. Tarter



43-rn Dish 
91-rn Dish



2 ×10-24
4 ×10-25


4 stars. (VLBI high speed tape recorder combined with computer software 
201 stars, direct Fourier transformation to produce extreme frequency resolution with a 65,536-channel spectrum analyzer, but not in real time.)

F. D. Drake, M. A. Stull



305-m Dish.



8 ×10-26


28 sources searched, including 10 main sequence stars and several OH "natural masers." Broadband signal recorded on magnetic tape, rerecorded on photographic film, then Fourier-transformed w/optical processor; not real time.

N.R.A.O.--National Radio Astronomy Observatory, Green Bank, West Virginia, U.S.A. 
O.S.U.R.O.--Ohio State University Radio Observatory, Columbus, Ohio, U.S.A. 
A.R.O.--Algonquin Radio Observatory, Lake Traverse, Ontario, Canada 
N.A.I.C.--Arecibo Observatory, Arecibo, Puerto Rico (U.S.A.) 
H.C.R.O.--University of California Hat Creek Radio Observatory, Hat Creek, California, U.S.A. 
Zimenkie--Radioastronomy Station of the Scientific Research Institute of Radiophysics, U.S.S.R. 
E.N.I.C.R.--Eurasian Network, Institute for Cosmic Radiation, U.S.S.R.

INFORMATION COMPILED FROM: Belitsky,3168 Black et al,3129 Dixon and Cole,2102 Drake,3167 Lawton,1093 Ref. 3171, Macomber,234 Morrison, Billingham and Wolfe,2865 Murray, Gulkis and Edelson,3174 Troitskii,22 Sagan and Drake,3143 Ref. 1119, Ref. 2389, Sheaffer,3173,3172 Smith,2866 Troitskii et al,1316 and Verschuur.1315


So far, none of these searches has yielded unambiguously positive results. The most comprehensive search undertaken to date, completed in 1976 at Green Bank by Zuckerman and Palmer in a survey of 659 stars, turned up a number of peculiar signals. According to one writer:

Ten stars which had shown "glitches" (i.e., unexplained spikes of energy) were carefully resurveyed. Several of the glitches were traced to terrestrial sources of interference, such as aircraft, while others remained a mystery; however, since none of the spikes were repeated, they were unlikely to be due to beacon transmissions from other civilizations.3257

It is certainly possible that a beacon once trained on Sol, perhaps for years or even decades, has continued on in its signaling schedule to other stars and will not return to our direction for thousands of years hence. As Edward Fitzgerald once wrote: "The Moving Finger writes; and, having writ, moves on."

Although several searches are still in progress at the present time, many xenologists are convinced that SETI searches are doomed to failure so long as they depend solely upon individual initiative and random funding. What is needed, they argue, is a major long-term commitment to SETI in which major radiotelescope equipment is dedicated in part or in whole to the search for communicative societies. To implement such systematic, far-reaching schemes, a new generation of ultrasensitive apparatuses may be required.

In 1971 NASA/Ames Research Center and Stanford University conducted a joint summer study to design one such integrated system. Co-directed by Dr. John Billingham of NASA/Ames and Dr. Bernard M. Oliver, Project Cyclops was a design study for a giant array of radiotelescopes whose performance would imitate that of a single radio dish many kilometers in diameter (Figure 24.6).* The Cyclops system would consist of more than a thousand fully steerable paraboloid radio antennae, each 100 meters in diameter (about the size of the largest contemporary steerables). The output from each of the antennae would be carried into a large central computer facility using a sophisticated system of phased transmission lines. The computer would coordinate and synchronize the movements of the many radio dishes and would employ special signal enhancement techniques to try to dig alien messages out of the incoming data. From the air, the final Cyclops system "would be seen as a large central headquarters building surrounded by an orchard of antennas about 10 kilometers in diameter."57


Figure 24.6 Advanced SETI Systems: Project Cyclops

PROJECT CYCLOPS SYSTEM: At above left is an artist’s conception of a high aerial view of the entire Cyclops array of 100-meter radiotelescopes. Diameter of the entire antenna system is about 16 kilometers. At above right is as artist’s impression of the ground level view of the Project Cyclops installation, showing the central control and processing building nestled within the radiotelescope "orchard," and a small shuttlebus on tracks. (Photos courtesy of NASA/Ames Research Center)


The Cyclops array as originally proposed would be assembled piecemeal over a 25 year period, for a total cost of $10-25 billion. This is about the same level of commitment as was required for the Apollo Moon Program a decade ago. (Or, as one observer has wryly noted, 22 days of Defense Department spending would support Cyclops for a quarter-century of development.) The Project, covering 65 km2 in some remote desert wilderness area, would be sensitive enough to conduct searches for 1000 megawatt beacons out to 1000 light-years or to eavesdrop on the electromagnetic "garbage" of technical societies out to about 100 light-years.

The NASA SETI Advisory Panel recently considered a number of alternatives to the Cyclops system.2865 One major proposal under consideration is the concept of an orbital SETI installation (Figure 24.7). A giant spherical radio dish could be placed in Earth orbit at lunar distance, perhaps at one of the semistable Lagrangian points of Luna’s orbit. A shield would be erected to blot out all radio interference from transmitters on Earth. When fully implemented the system might measure 3 kilometers in diameter, but early test models could probably be flown with diameters of 30-300 meters.


Figure 24.7 Advanced SETI Systems: Orbital Telescope Facility

Artist’s impression of an intermediate size (300 meter) Space SETI system antenna showing relay satellite RFI shield and Shuttle type vehicle. Located in geosynchronous orbit.


This giant dish would sweep the entire sky once during each orbital period, so any target could be observed at least once every 28 days. Since the telescope is spherical rather than parabolic in shape, the feed horn at the focus may be moved to tune in on distant objects so that the entire dish does not have to be reoriented. Since the feed horn can move a full 180o across the surface of the collecting dish, this means that any target theoretically may be observed for a continuous 14-day period if so desired.

An additional advantage of an orbital SETI system is that multiple feed horns may be used without sacrifice of accuracy or sensitivity. This should permit the simultaneous observation of many different target stars within a given hemisphere of the celestial sphere. Finally, from a structural engineering aspect the orbital system would be much easier to build than a ground-based Cyclops network.3155

Lunar Farside systems (Figure 24.8) were also under serious consideration by the SETI specialists.3142 NASA has considered the relative merits of Cyclops-type arrays [many small dishes) and Arecibo-type arrays (few large dishes) if constructed on the far side of the Moon. Since this side always faces away from Earth, any SETI system located there would be entirely free of terrestrial radio interference because of the shielding effects of 3000 kilometers of solid rock. The abundance of lunar craters of all sizes makes an Arecibo-type array especially attractive, which one study found to be significantly cheaper than Cyclops type setups on the moon. (SETI enthusiasts often refer to such an installation as "Lunarcibo."3149) A large expense common to both proposals is the sizeable lunar colony which would be necessary to support the project, A countervailing advantage is that building materials should be close at hand.


Figure 24.8 Advanced SETI Systems: Lunar Farside Systems

Cyclops-type Array (at left): Artist’s impression of Lunar Cyclops array, showing central control and procuring building and lunar base in left middle distance. Arecibo-type Array (at right): A large dish could besited in a lunar crater on the far side of the Moon, to provide a large collection area with complete shielding from Earth.


During 1975-1976, NASA commissioned systems analysts at Stanford Research Institute to perform a cost-benefit analysis of sixteen different antenna concepts intended for use in the search for extraterrestrial signals of intelligent origin.3262 The study showed that the orbital SETI system compared quite favorably with a ground-based Cyclops network in terms of cost. Quite unexpectedly, the critical parameter turned out to be N, the total number of communicative civilizations in the Galaxy. If N is very high and we only need to search a few hundred light-years out into space to achieve success, then Cyclops is cheaper because of the lower levels of R&D required. But if we must listen out to 500 light-years and beyond (N is very small), then the space-based system is cheaper to build and to operate. The price of the Lunar Farside scheme was deemed significantly higher than either of these two options.3292

SETI systems, once constructed, should prove enormously useful in basic research in radioastronomy. SETI detection networks can make major contributions during off-time to the following areas of scientific investigation: Cosmology, external galaxies, quasars and radio galaxies, the intergalactic medium, kinematics and structure of the Milky Way, stellar evolution, super novae and pulsars, composition of the interstellar medium, and solar system studies including surface mapping of terrestrial planets, jovian moons, asteroids and meteorites.2865

There is growing interest in SETI in the astronomical community generally. A questionnaire sent to radioastronomy observatories around the world asked the question: "Have you ever engaged in any search for coherent or "intelligent" signals at your facilities?" Approximately 50% of the observatories responding answered in the affirmative.2865 Scientists in the United States and the Soviet Union are excited by the prospects.880 In America, a five-year program was proposed in 1978 that involves a search of specific stars by a team at NASA/Ames in Mountain View, California,3129 and a complementary search in the manner of a sky survey by a team at the Jet Propulsion Laboratory (JPL) in Pasadena, California.**3130 In Russia, two overlapping ten-year plans were advanced by SETI scientists in 1975 which offer a combination of ground-based and satellite listening posts capable of continuous monitoring of the entire sky, nearby galaxies, and other selected objects of interest.1480,3681

One final sticky issue remains for discussion. Will we -- should we -- transmit?120 At present there exist no laws in national or international legal systems to prevent this, so there is nothing to stop any private or public organization from doing it if they want. It is common knowledge among working astronomers that many minutes of valuable radiotelescope research time are sometimes diverted (between regularly scheduled astronomical observations) to take a quick listen to Tau Ceti or Epsilon Eridani from time to time. The temptation to transmit is equally great, and one writer about six years ago reported that "Tau Ceti has signals periodically transmitted to it by the Arecibo dish in Puerto Rico."1139 The writer later recanted, acknowledging that "I have now been informed that the information quoted was unofficial and that the big dish has not been used to transmit signals to Tau Ceti."1140 Nevertheless, it is likely that at least a small amount of unofficial broadcasting has occurred from time to time.

Further, we already know about one official "message to the stars" that has been sent out into space (Figure 24.9).


Figure 24.9 The Arecibo Pictogram Transmitted to M13 on 16 November 1974

ARECIBO MESSAGE IN BINARY CODE was transmitted in 1974 toward the Great Cluster in Hercules (M13) from the 1,000-foot antenna at Arecibo. The message is decoded by breaking up the characters into 73 consecutive groups of 23 characters each and arranging the groups in sequence one under the other, reading right to left and then top to bottom. The result In a visual message (see below) that can be more easily interpreted by making each 0 of binary code represent a white square and each 1 a black square.

ARECIBO MESSAGE IN PICTURES and accompanying translation shows the binary version of the message decoded. Each number that is used is marked with a label that indicates its start. When all the digits of a number cannot be fitted into one line, the digits for which there is no room are written under the least significant digit. (The message must be oriented in three different ways for all the numbers shown to be read.) The chemical formulas are those for the components of the DNA molecule: the phosphate group, the deoxyribose sugar and the organic bases thymine, adenine, guanine and cytosine. Both the height of the human being and the diameter of the telescope are given in units of the wavelength that is used to transmit the messages: 13.6 centimeters.


At 1700 GMT on 16 November 1974, the giant Arecibo radiotelescope (equivalent isotropic power output 2 x 1013 watts) was used to transmit mankind’s first deliberate radio message "for possible reception by other intelligent creatures."1571 The transmission was made at 2.38 GHz by Frank Drake and Carl Sagan, and consisted of 1679 bits of information arranged in a. 23 x 73 pictogram as shown in Figure 24.9. The simple message took 169 seconds to send, and was aimed at the globular cluster M13 which is 24,000 light-years away. At this distance the Arecibo beam just covers the 300,000 stars in the cluster. Theoretically we may receive a reply no sooner than 49,974 A.D. -- the round trip transit time at the speed of light. However, it is now reported that there are between 20-100 stars of the red giant and orange giant variety in the path between here and M13, and that literally thousands of stars may have been fanned by the Arecibo beam while it was being tuned up to send the actual message. Communicative civilizations at any of these sites may have an earlier chance to intercept the message or at least parts of it. We may get an answer sooner than we expect.***


* Cyclops appears to be the working out of ideas earlier put forward by Oliver. On 27 July 1965 at an AIAA Conference in San Francisco, Oliver suggested that by building between 1-10 thousand radiotelescopes, each about 30 meters in diameter, in a 6-kilometer-square of flat terrain in certain areas such as Texas, we could "detect the unintended radiation from another intelligent race."

** Jill Tarter at NASA/Ames has informed the author that the survey team would be happy to include any specific star in one of their SETI sky searches, upon request from scientists of from members of the general public.3149

*** Less than a year after Drake and Sagan sent the Arecibo signal, the following reply was received at Cornell University: "Message received. Help is on the way -- M13." This was printed out on a teletype machine that serves as a data hotline linking Cornell with the facilities at Arecibo. Since the response was 47,999 years too early, Drake suspects a somewhat closer source -- a staff member with a sense of humor.418



24.3  Extraterrestrial Starprobes and Artifacts

Surprisingly enough, the Arecibo radio pictogram was not mankind’s first intentional space message for ETs. Pioneer 10, a spacecraft, was first.

Pioneer 10 was launched toward Jupiter on 3 March 1972, loaded with scientific equipment designed to measure the radiation environment surrounding the giant planet. Traveling at 11,000 meter/second, the interplanetary probe encountered Jupiter on 4 December 1973 and swung around its massive bulk at close range. Assisted by a kind of "slingshot effect," the spacecraft rapidly accelerated up to 23,000 meter/second. In 1976 it reached the orbit of Saturn, and three years later the orbit of Uranus, where its radio signals finally became too weak for detection from Earth. In 1983 Pioneer 10 will cross the orbit of Pluto and head out into deep space at an interstellar cruising speed of 0.0043%c (13,000 meter/second).3136 It will become the first manmade object to leave the solar system, our first interstellar spacecraft.*

Attached to the exterior of the vehicle is a 6" x 9" gold-anodized aluminum plate engraved as shown in Figure 24.10. The message announces our existence to the cosmos. Any alien species which picks it up and deciphers its meaning can tell who built it, when and where it was built, how tall we are, our basic physiology, and our approximate technology at the time of launch.168


Figure 24.10 Messages from Interstellar Probes: The Pioneer 10 Plaque

Below is a drawing of the engraved gold-anodized aluminum plate which is now hurtling out into inter stellar space at about 0.0043%c, in the general direction of Aldebaran in the constellation Taurus.

The message begins at the top with a schematic representation of the "hyperfine transition" of neutral atomic hydrogen: In going from a state in which the spin of the electron and proton are aligned, to a state in which they are opposed, radio waves at 1.42 GHz are emitted. Since hydrogen is the most abundant element in the universe, and the "hyperfine transition" the most common change of state for hydrogen, it is expected that alien radioastronomers will early detect this radiation and thus understand the significance of the symbols engraved on the plaque. Since a frequency of 1.42 GHz is a wavelength of 21 centimeters, this is adopted as the unit of length by the binary digit designation "1" under the line connecting the hydrogen atom symbol. At the extreme right are depictions of human beings. Next to them are tote marks indicating the binary number eight. Eight times 21 centimeters is 1.68 meters, the average human height. The outline of the Pioneer craft behind them confirms this estimate. Where does the Pioneer craft come from, and when was it launched Curious ETs can answer these questions by carefully examining the plaque. At left of center is some object centered among 15 other objects. Each is labeled with a very long string of binary digits; the only thing that can be known to this accuracy is the period of a pulsar. Since pulsars slow down at known rates, scientists estimate that even 106-108 years after launch, the launch date of Pioneer can be pinpointed by ETs to the nearest century or millennium. The solid line to each pulsar represents its relative distance from Pioneer’s launch point. Using computer analysis, our solar system may be located to within 60 light-years from anywhere in our Galaxy. Our position is specified to 1 of 1000 possible Stars.

As a final aid in locating Sol, the Solar System is sketched along the bottom of the plaque. The path of the Pioneer spacecraft is also shown, emanating from the third planet, then whipping past the fifth and on out into inter stellar space. The distance of each world from the sun is given in binary digits. Finally, the radio dish points back at the third planet as it makes its exit from the system. Clearly, the intelligent beings are there.


A vastly more sophisticated message was affixed to each of the follow-up missions to the jovian worlds in the late 1970’s, called Voyagers 1 and 2.3131 Instead of a simple plaque, a phonograph record was the chosen medium. The Voyager phonograph record contents are too lengthy to reproduce here but are listed in Table 24.4.


Table 24.4 Complete Texts of the Voyager 1 and Voyager 2 Phonograph Records



Pictures (in electronic form).
President Carter's message (in electronic form).

U.N. Secretary General Waldheim's message ( spoken ).

Greetings in 60 languages. 

Sounds at Earth.


37 Family portrait, Nina Leen.
38. Diagram of continental drift. Jon Lomberg.

39. Structure of Earth, Jon Lomberg.

40. Heron Island (Great Barrier Reef of Australia), Dr Jay M. Pasachoff.

41. Seashore, Dick Smith.

42. Snake River and Grand Tetons, Ansel Adams.

43. Sand dunes, George Mobley.

44. Monument valley.

45. Forest scene with mushrooms, Bruce Dale.

46. Leaf, Arthur Herrick. 

47. Fallen leaves, Jodi Cobb. 

48. Sequoia, Josef Muench.

48. Snowflake, R. Sisson

49. Tree with daffodils, Gardens of Winterthur.

50. Flying insect with flowers, Borne On The Wind.

51. Diagram of vertebrate evolution, Jon Lomberg.

52. Seashell (Xaneidae).

53. Dolphins, Thomas Nebbia.

54. School of fish, David Doubilet.

55. Tree toad, Dave Wickstrom.

56. Crocodile, Peter Beard.

57. Eagle. Donona.

58. Waterhold, South African Tourist Corporation 

59. Jane Goodall and chimps, Vanne Morris-Goodall.

60. Sketch of bushmen. Jon Lomberg.

61. Bushmen hunters, R. Farbman.

62. Man from Guatemala, UN.

63. Dancer from Bali, Donna Grossenor.

64. Andean girls. Joseph Scherschel.

65. Thailand craftsman, Dean Conger.

66. Elephant. Peter Kunstadter.

67. Old man with beard and glasses (Turkey). Jonathon Blair.

68. Old man with dog and flowers, Bruce Bnumann. Mountain climber. Gnston Rebuffat.

70. Cathy Rigby. Philip Leonian.

71. Sprinters (Valeri Borzov of the U.S.S.R., in lead), The History of the Olympics.

72. Schoolroom, UN.

73. Children with globe.

74. Cotton harvest, Howell Walker. 

75. Grape picker. David Moore.

76. Supermarket, H. Eckelmann.

77. Underwater scene with diver and fish, Jerry Greenberg.

78. Fishing boat with nets, UN.

79. Cooking fish. Cooking of Spain and Portugal.

80. Chinese dinner party, Michael Rougier.

81. Demonstration of licking, eating and drinking, H. Eckelmann.

82. Great Wall of China. H. Edward Kim.

83. House construction (African), UN. 

84. Construction scene (Amish country), William Albert Allard.

85. House (Africa), UN.

86. House (New England). Robert Sisson.

87. Modern house (Cloudcroft, New Mexico) . Dr. Frank Drake.

88. House interior with artist and fire. Jim Amos.

89. Taj Mahal, David Carroll.

90. English city (Oxford), C. S. Lewis, Images of His World.

91. Boston, Ted Spiegel

92 UN Building Day. UN.

93. UN Building Night. UN.

94. Sydney Opera House, Mike Long.

95. Artisan with drill. Frank Hewlett.

96. Factory interior. Fred Ward.

97. Museum. David Cupp.

98. X-ray of hand. H. Eckelmann.

99. Woman with microscope. UN.

100. Street scene. Asia (Pakistan), UN.

101. Rush hour traffic, India. UN.

102. Modern highway (Ithaca), H. Eckelmann.

103. Golden Gate Bridge. Ansel Adams.

104. Train. Gordon Gahan.

105. Airplane in flight, Dr. Frank Drake.

106. Airport (Toronto), George Hunter.

107. Antarctic Expedition. Great Adventures with the National Geographic.

108. Radio telescope (Westerbork, Netherlands), James Blair.

109. Radio telescope (Arecibo), H. Eckelmann.

110. Page of book (Newton, System of the World).

111. Astronaut in space. NASA.

112. Titan Centaur Launch, NASA.

113. Sunset with birds, David Harvey.

114. Spring Quartet (Quartetto Italiano), Phillips Recordings.

115. Violin with music score (Cavotina).

(In Sequential Order)

1. Bach Brandenburg Concerto Number Two, First Movement. Karl Richter conducting the Munich Bach Orchestra.

2. "Kinds of Flowers" Javanese Court Gamelan, recorded in Java by Robert Brown, Nonesuch Explorer Record.

3. Senegalese Percussion, recorded by Charles Duvelle.

4. Pygmy girls initiation song, recorded by Colon Turnbull (Zaire).

5. Australian Horn and Totem song. Recorded in Australia by Sandra LeBurn Holmes. Barnumbirr-Morning Star Record.

6. "El Cascabel" Lorenzo Barcelata. The Mariachi Mexico.

7. "Johnny B. Goode", Chuck Berry.

8. New Guinea Men's House, recorded by Robert MacLennan.

9, "Depicting the Cranes in Their Nest" recorded by Coro Yamaguchi (Shakubachi).

10. Bach Partita Number Three for violin. Gavotte et Rondeaus, Arthur Gruminux, violin.

11. Mozart Magic Flute, Queen of the Night (Aria Number 14) Edda Moser. soprano.

12. Chakrulo. Georgian (USSR) folk chorus.

13. Peruvian Pan Pipes performed by Jose Maria Arguedas.

14. Melancholy Blues performed by Louis Armstrong, Columbia Records.

15. Azerbaijan Two Flutes. Recorded by Radio Moscow.

18. Stravinsky. Rite of Spring. Conclusion. Igor Stravinsky conducting the Columbia Symphony Orchestra.

17. Bach Prelude and Fugue, Number One in C Major from the Well Tempered Clavier, Book Two. Glenn Gould, piano.

18. Beethoven’s Fifth Symphony, First Movement. Otto Klem Klemperer conducting, Angel Recording.

19. Bulgarian Shepherdess Song. "Izlel Delyo hajdutin." sung by Valya Balkanska.

20, Navajo Indian Night Chant. Recorded by Williard Rhodes.

21. The Fairie Round from Pavans, Galliards, Almalus. Recorded by David Munrow.

22. Melanesian Pan Pipes. From the collection of the Solomon Islands Broad

casting Service.

23. Peruvian Women's Wedding Song. Recorded in Peru by John Cohen.

24. "Flowing Streams" -- Chinese Ch'in uu music. Performed by Kuan P'ing-Hu. 

25. "Jaat Kahan Ho"-- Indian Raga. Performed by Surshri Nt-tan Bai Kerkar.

26. "Dark Was the Night" performed by i Blind Willie Johnson. 

27. Beethoven String Quartet Number 13 "Cavatina", performed by Budapest String Quartet.


(Not In Sequential Order)






































Ila (Zambia)







Amoy (Min dialect)










(In Order of Sequence)

Planets (Music)


Mud Pots



Crickets, Frogs





Wild Dog

Footsteps and Heartbeats



Dogs, domestic

Herding sheep

Blacksmith shop




Morse Code


Horse and Cart
Horse and Carriage

Train Whistle



Auto gears


Lift-off Saturn 5 Rocket



Life signs -- EEG, EKG



    This Voyager spacecraft was constructed by the United States of America. We are a community of 240 million human beings among the more than 4 billion who inhabit the planet Earth. We human beings are still divided into nation states, but these states are rapidly becoming a single global civilization.
    We cast this message into the cosmos. It is likely to survive a billion years unto our future, when our civilization is profoundly altered and the surface of the Earth may be vastly changed. Of the 200 billion stars in five Milky Way galaxy, some -- perhaps many -- may have inhabited planets and spacefaring civilizations. If one such civilization intercepts Voyager and can understand these recorded contents, here is our message:

    "This is a present from a small distant world, a token of our sounds, our science, our images, our music, our thoughts and our feelings. We are attempting to survive our time so we may live into yours. We hope someday, having solved the problems we face, to join a community of galactic civilizations. This record represents our hope and our determination, and our good will in a vast and awesome universe."

Jimmy Carter, 
President of the United States of America.

The White House,
   June 16, 1977.


As the Secretary General of the United Nations, an organization of 147 member states who represent almost all of the human inhabitants of the planet Earth, I send greetings on behalf of the people of our planet. We step out of our solar system into the universe seeking only peace and friendship, to teach if we are called upon, to be taught if we are fortunate. We know full well that our Planet and all its inhabitants are but a small part of the immense universe that surrounds us and it is with humility and hope that we take this step.

Kurt Waldheim



1. Calibration circle, Jon Lomberg.
2. Solar location map, Dr. Frank Drake.

3. Mathematical definitions, Dr. Frank Drake.

4. Physical unit definitions, Dr. Frank Drake.

5. Solar system parameters, Dr. Frank Drake.

6. Solar system parameters, Dr. Frank Drake.

7. The Sun. Hale Observatories.

8. Solar spectrum. H. Ecklemann.

9. Mercury, NASA.

10. Mars, NASA.

11. Jupiter, NASA.

12. Earth. NASA.

13. Egypt. Red Sea, Sinai Peninsula and the Nile, NASA.

14. Chemical definitions, Dr. Frank Drake.

15. DNA Structure, Jon Lomberg.

16. DNA Structure magnified. Jon Lomberg.

17. Cells and cell division, Turtox/Cambosco.

18. Anatomy 1: Field Enterprises Educational Corp. and Row, Peterson & Co.

19. Anatomy 2: Do.

20. Anatomy 3: Do.

21. Anatomy 4: Do.

22. Anatomy 5: Do.

23. Anatomy 6: Do.

24. Anatomy 7: Do.

25. Anatomy 8: Do.

25a. Human sex organs. Life: Cells, Organisms, Populations.

26. Diagram of conception, Jon Lomberg.

27. Conception. Lennart Nilsson.

28. Fertilized ovum, Lennart Nilsson.

29. Fetus diagram, Jon Lomberg.

30. Fetus, Dr. Frank Allan.

31. Diagram of male and female, Jon Lomberg.

32. Birth, Wayne Miller.

33. Nursing mother, UN.

34. Father and daughter (Malasla), David Harvey.

35. Group of children, Ruby Mera.

36. Diagram of family ages. Jon Lomberg.


1 NASA Press Release 77-159, Aug. 1, 1977.


The record is a 12" copper disk to be played at 16 2/3 revolutions/minute using a ceramic cartridge and stylus enclosed for the purpose. Instructions for playback are written in pictorial sign language on the outside of the aluminum can holding the record. Remarks Carl Sagan optimistically: "If they’re able to tool around in interstellar space picking up stray, derelict spacecraft, they ought to be able to figure out our instructions.**


* According to computer projections, Pioneer 10 is creeping out into a relatively empty region of space. Estimates indicate that it should pass fairly close to the star Aldebaran (aTauri), which is 68 light-years from Earth, in the year 1,601,983 A.D.

** Computer projections by Michael B. Helton at JPL show that the Voyagers, like the Pioneers before them, will not closely encounter any alien solar systems. Voyager 1 will pass Pluto’s orbit late in 1987 and head out toward the constellation Ophiuchus (Declination 10.1º, Right Ascension 17h, 20m). Voyager 2, assuming it goes to Uranus but not Neptune, will exit the solar system in mid-1989 on the way to Capricornus (Declination -14.9°, Right Ascension 21h, 1m). In about 40,000 years both craft should coast to within 1.7 light-years (Voyager 1) or 1.1 light-years (Voyager 2) of AC+79 3888, a fourth magnitude star. Voyager 2 should pass a similar distance from another star (AC -24 2833-183) 100,000 years later in Sagittarius, and about 375,000 years after that, Voyager 1 will pass within 1.5 light-years of AM +21 652 in the constellation of Taurus.3207


24.3.1  Why Probes Are Better

While penurious planetary Type I societies may only be able to afford radio wave communications, we have already pointed out that technically sophisticated civilizations (whose technologies realize theoretical maxima in matter-energy systems) should view signals and probes as energetically indistinguishable alternatives for interstellar communication. Both may be used by advanced Type II or Type III cultures, for a variety of different purposes and functions. But energy efficiency cannot be used to distinguish the two choices when maximum technology is available. We must look elsewhere for distinguishability criteria.

The author believes that probes are probably the method of choice for technically advanced civilizations. In support of this position, he would like to offer several criteria which he believes argue persuasively for the inherent superiority of starprobes in interstellar communication.

First, there is the issue of communications feedback. A probe which discovers a garrulous inhabited world may engage in true conversation with the indigenes, an interchange and interweaving of cultures. Interactive exchanges may take fractions of a second between questions and answers. On-site starprobes, perhaps in orbit around the host’s sun or planet, can carry on real-time educational and linguistic functions with a precision no remote signaling system can match. As an added benefit, such intelligent devices would provide a noise-free channel of communication on any frequency of the contactee's own choosing. By comparison, the traditional beacon scenario appears little more than a sterile data swap requiring millennia for each cycle rather than milliseconds. With photonic transmissions, different sentient species cannot really converse.

Second, there is the question of acquisition efficiency. A beacon may radiate useful energy and information out into space for centuries, millennia, or even longer without getting any response. This energy, since it was detected by no receiver, essentially was wasted and constitutes pure economic loss for the sending society. Such a scheme necessarily assumes an inordinate (and possibly selectively disadvantageous) degree of generalized altruism on the part of the transmitting culture. Starprobes, on the other hand, become independent agents as soon as they are launched. There is no further need for energy expenditure by the transmitting society. Sophisticated messenger probes will be self-repairing, self-programming, and capable of refueling or recharging at every port of call. They can wait patiently in orbit for hundreds or even millions of years, waiting for the emergence of a communicative culture on suitable planets in the system; alternatively, they may hop from star to star until they find communicative lifeforms, and then enter into an exchange at no further cost to the original transmitting society. A subsidiary but nonetheless important benefit of starprobes is that they may function as "cosmic safety deposit boxes" for the cultural heritage of the contacting civilization. If the transmitting society is destroyed or the culture perishes for whatever reasons, the starprobes they sent to other worlds can still tell their story to any willing ears for perhaps geological time periods thereafter.

Third, there is the overwhelming advantage of military security for the transmitting race. Interstellar beacons are an invitation to disaster at the hands of unknown predatory alien civilizations. In any situation involving contact via signals, the transmitting society has given away the position of its home star system at great risk for mere speculative benefits. This terrible breach of military security may be remedied by using starprobes instead of signals. If local intelligent activity is detected by a probe in orbit around a target star, the machine may open contact with the indigenous technical species without ever having to disclose the whereabouts of its creators. If it is necessary for the starprobe to report what it has learned to the transmitting society from time to time, this easily may be accomplished in a manner which is virtually impossible to trace or to decode (e.g., by relaying trapdoor-function-encoded data through a series of widely dispersed and complexly organized repeater stations). In other words, starprobes can safeguard security in an exchange between alien societies.*

For these and many other reasons (see Chapter 17), more and more xenologists are beginning to view the interstellar messenger probe as the preferred mode of communication among extraterrestrial civilizations. (See Benford,3270 Betinus,3156 Bracewell,1041,1040,80 Clarke,3230 Forward,718 MacGowan and Ordway,600 and Niven.231)


* To guarantee the physical security of both races (host and visitor), perhaps each should send a probe to some common neutral meeting place far from the home star of either civilization. The two starcraft could then rapidly interrogate each other, interactively exchange information, and then move off and report back the results to their respective creators without risk. For the contact, a meeting site should be chosen that is known and accessible to both parties but which itself harbors no life (or only insignificant lower forms that pose no threat). Examples might include nearby O or B stars, local pulsars or black holes, white dwarfs, giant or supergiant suns, or very young solar systems located between the two potentially communicative alien civilizations.



24.3.2  Mission Profile

Assume that an advanced alien technical civilization institutes a major starprobe program and dispatches computerized messenger vehicles to spy out neighboring solar systems. One plausible "standard entry procedure" (Figure 24.11) might run as follows:

1. Preliminary acquisition -- 0.5 light-year away, traveling at 10%c, the starprobe makes its first high-resolution scan of the target and makes slight course adjustments to increase accuracy. Braking engines are activated.

2. Messenger searches for the star’s Zodiacal Light, stray sun light scattered from the dust and debris surrounding the star. All planets lie in this thin blanket of dust, so the probe corrects its course so as to enter the plane of the solar system. Speed has fallen to l%c or less.

3. Approach on cometary orbit until solar irradiance sensors indicate that the midpoint of the local habitable zone (for the desired biochemistry) has been reached -- say, just above the melting point of water. Drop into a circular, circum solar orbit.

4. Seek out and examine any planets in or near the ecosphere, and examine each for spectroscopic evidence of, say, water and oxygen in the atmosphere. Select the first planet having both in appropriate concentrations and move into orbit around it.

5. If the body is accompanied by a large natural satellite (such as Luna in the case of Earth), perturbations will seriously disrupt a simple global orbit. To negate the disturbing influence of the moon, settle into a relatively stable Trojan Point orbit. (For Earth, two of these points lie in the lunar orbital path both 60o ahead and behind.)

6. Activate search sensors to infeed data to look for any signs of intelligent life or technological activity. Collate and compile the information.

7. Record positional star map, including accurate fix on home world. Transmit to home world a preliminary report of sensor findings, including astronomical, biological and technical data gained by scanning and eavesdropping on the target planet.

8. Begin routine signaling or other activity to announce presence and to attract attention. If no intelligence is in evidence, enter dormant mode with specified wake/sleep schedule for periodic resampling of planetary environment and basic self-maintenance functions.


Figure 24.11 Synopsis of an Interstellar Probe Mission718

In 1998, an interstellar spacecraft, which has been drifting in space in the region beyond the Moon, launches itself toward the triple star system a Centauri. It thrusts at high acceleration, its engines running at a power level that is ten times the power of a Saturn V rocket. The exhaust of hot hydrogen plasma glows like a bright star, visible both night and day. After four months the probe has left the Solar System, and has reached 1/3 the velocity of light. It begins its long drift through interstellar space using its bulky first stage shell as a radiation barrier to protect it from the constant rain of high energy particles produced by its high speed through the interstellar hydrogen.

After drifting quiescently for 12 years (it has now covered four light years), it sheds its first stage, turns around, and begins deceleration. As the probe velocity drops well below relativistic speeds and it approaches its target, the spacecraft opens up from a compact mass into a 100-meter-diameter sphere. The sphere is a dense wire mesh embedded with arrays of tiny sensors and transmitters, close coupled to complex digital molecular circuitry, all held together and interconnected with high strength, one-dimensional superconductive fibers.

The array of sensors spaced over the hundred meter sphere collect the light and radiation from the three stars in the a Centauri system and correct the rocket thrust to zero in on Proxima Centauri. As it approaches the small red star, the probe searches for a planet. It is there, along with three others further out. The other three are cold, and probably lifeless, but they will be visited before the probe leaves Proxima to investigate the other two stars in the a Centauri system.

With its thruster at low power, the probe approaches the planet -- Proxima Centauri One -- constantly beaming pictures and sensor data back to Earth using a phased array of solid state lasers scattered densely over its mesh surface. Earth will not see the pictures of the new planet for 4.3 years, long after the probe has completed its survey and moved on to the other planets and stars in the a Centauri system.

Laser pulses from interstellar probe scanning Proxima Centauri One.

With no feedback possible from Earth, the computer circuits distributed through the sphere analyze for themselves the information its sensors collect as it approaches. The probe swings into a near polar orbit of the new planet and begins a survey. Wideband sensors sensitive to the entire electromagnetic spectrum produce imagery in the radio, microwave, infrared, visible, and ultraviolet bands. The one hundred meter size of the detector array gives the picture a resolution of less than 1 meter even from the 1000 km orbital altitude. Picosecond pulses of laser light beam down as a laser radar to measure the height variations of the topography. Certain regions that might have life are interrogated with selected laser wavelengths, and their return light analyzed to look for absorption or fluorescent bands characteristic of organic compounds. A few regions that have the most potential for life are selected. Small portions of the sensor mesh on the sphere detach from the main probe mesh and are driven down into the atmosphere with radiation pressure from the lasers on the probe. The small mesh sections drift down to the surface, collecting and storing images as they descend. The mesh settles on the surface where specialized chemically sensitive molecular circuits react to the various forms of chemical compounds found there. The orbiting probe interrogates the lander mesh with a laser beam, collects the images and chemical data it has stored, combines it with the other information that it has collected, and sends a detailed report back to Earth.

The probe then moves on to the next planet in the system, more slowly now, for it is no longer as lavishly supplied with fuel as it was at the start of its mission. It will not stop until it has made a complete survey of every planet in the three star system. This will take a long time, but the probe has a lot of time.

It will be at least 30 years before man will arrive to take over.


What sorts of things might the visiting probe look for to determine if intelligence exists on the planet? To be visible from space, the intelligence must manifest itself in artifacts. Direct photographic reconnaissance from the distance of lunar orbit could be very difficult. To resolve the artifacts of human civilization unambiguously would require a visible-light telescope with an effective diameter of many tens of meters. A neutrino detector to pick up evidence of fission of fusion power generation on the surface of the planet probably would be too massive. Atmospheric heat flow and composition analysis should be highly suggestive (e.g., fluorocarbon aerosols in the stratosphere probably cannot be generated naturally in an oxygenic carbon-aqueous biosphere), but may still be too ambiguous.

The two most critical parameters of technological civilizations -- energy usage and information flow rate -- frequently will be directly measurable from space. At night, the waste energy escaping the metropolitan regions of Earth can be measured from orbit, so the starprobe should be able to assemble a fair estimation of global power consumption. As for information flux, the silent alien craft could detect countless powerful radio stations whose emissions seem to wax and wane with the daylight. Assuming the ET spy is smart enough, it should be able to listen in on our transmissions, learn our languages and customs, and tap into our cultural and technological heritage. Before it makes overt contact, it will probably know a great deal about us.

What is the best way to attract attention and to initiate first contact with a planetbound Type I civilization such as our own? The orbiting starprobe could turn on a bright light or explode a bomb, but this would be inefficient, ambiguous, and might not even be seen at all by the intelligent planetary inhabitants. Contact lander craft or robot encounter vehicles could be soft-landed on the surface, but this is a rather tall order for a modest-sized automated starcraft. Ronald Bracewell, a Stanford University radioastronomer and an early advocate of interstellar messenger probes, has suggested that once intelligent radio emissions have been detected by the starprobe the selection of frequency and message is relatively easy:

What frequency will it use? In the case of a messenger probe, this is a nonproblem, since the probe can rest assured that on any frequency where a transmitter can be detected there will also be, somewhere, a receiver! The probe can choose any frequency which is already plainly in use. This automatically guarantees that someone is listening, because no one transmits if nobody is listening.. . . It is true that at least one receiver will be tuned in, and perhaps a large audience, but will they pay attention to an unwanted, interfering signal? If the probe gives out something that you don’t want, you will go away. So a simple procedure would be for the probe to amplify and transmit the same TV program or military communication it was receiving. Its signals would then have the appearance, to us, of echoes exhibiting delays of seconds to minutes depending upon its distance from Earth. For instance, if we were listening to the radio, each word would be heard twice, first by direct transmission from the station and then again a little later via the probe.85,80

In other words, the best frequency for a starprobe to use to attract attention is one which is already in use, since this guarantees a listening audience. The best message to send is a duplicate of whatever was transmitted, since this guarantees an interested audience.* The detailed contact plan of the "Bracewell probe," outlined in Figure 24.12, is one eminent xenologist’s view of the most likely way it may happen. Says Bracewell: "I believe we are on the eve of plugging into the galaxy-wide communication network."1040


Figure 24.12 Bracewell Probes: First Contact Scenario80

The Message

In my opinion, the message will be in television. Television is like sign language; although you and I may not speak the language, we can exchange through signs or pictures. Geometrical furnish a means whereby we learn each other's language. The words in dictionary that can defined by drawings probably run into the thousands and those that can be defined by animated drawings are many more. Not only nouns, but many adjectives, adverbs, and verbs can be depicted through television. Other words are harder, but if one had a dictionary and already knew a few thousand basic words, one could interpret some of the more difficult ones.

Until we set up a common language, television also permits us to ascertain quickly the answers to questions of basic importance, such as where the probe came from. The probe, too, would like us to know this without delay. To speculate a bit, I will assume that television is what we are going to see and that the first picture will be of a constellation of stars familiar to us, followed by a zooming in on the home star. At one time I thought the picture of the constellation would have one star winking on and off like an electric sign. but anyone who can make an animated movie to do that can just as readily simulate a zoom lens. The zoom lens technique is a very quick way for a foreign probe to tell us which star it came from without knowing our name for the star, or our coordinate systems, or anything about our language. You might also want to know how we are going to get our TV screen synchronized to its system of transmission. if the probe keeps running its program until we get the number of lines per frame and so on worked out and arrange a so on display it. then we can report our readiness by repeating the program back. Alternatively, since it has been listening in on cur TV transmissions, it might oblige us by adopting one of the numerous standards in use on earth.

Now that we know the home star, the probe will zoom in further until the star grows into a visible disc, perhaps with starspots. From their motion we will know the axis of rotation. The planetary system will then be displayed, and at last we will zoom in on Superon, time home planet Clearly a fantastic travelogue lies ahead, and it is well within the capacity of a modest probe to execute the simple steps required to display this information. After this brief preview there is some rather serious business to attend to, namely: the messenger probe must convey to us the schedule oh listening that is being observed on Superon and the frequency being used this rather tier urgent. Afterward the probe is dispensable but until the schedule and frequency are conveyed the probe's mission is incomplete. I believe this job can be accomplished with pictures, but I surmise the programmers on Superon will surprise us by the brevity and clarity of their particular method of conveying this vital information.

Language Barrier?

The nature of our direct transmission to Superon is a matter of taste. Obviously we can send zoom movies to match those the probe will have shown us, but much more interchange can take place with the probe while our direct transmission is being readied. I believe the probe can learn our language in printed form quite readily, working with an animated pictorial dictionary that we furnish for its computer memory. At first, its expressions might be quaint, but there is no reason why its compositions should not be televised back to it in corrected form. To be sure of getting a point across, the probe can do what we do -- say it again in different words and if we don't understand, we can question.

Knowledge of our language will enable the probe to tell us many fascinating things: the physics and chemistry of the next 100 years, wonders of astrophysics yet unknown to man, beautiful mathematics. After a while it may supply us with astounding breakthroughs in biology and medicine. But first we will have to tell it a lot more about our biological makeup. Perhaps it will write poetry or discuss philosophy. Perhaps the messenger knows how the universe started, whether it will end, and what will happen then. Maybe the probe knows what it all means, but I wonder...I think that is why Superon wants to consult us!

The Limit

In time, the probe's store of knowledge will be used up, as it is only a modest probe. Presumably the computing part need only be the size of a human head, which is, we know, large enough to store an immense amount of information. Meanwhile our transmission to Superon will have commenced. One might ask whether it would be better to use our language or Superese, which we could learn from the probe. Now while I think the probe could learn a functional form of our language. I don't think it would be practical to teach it to the people of Superon. The continual checks and confirmations, corrections and repetitions that are possible between ourselves and the probe resemble face-to-face encounters between humans meeting on language frontiers and would be ruled out by the round-trip delay to Superon and back. It is true that we could transmit our pictorial dictionary followed by text, and hope for the best: if the probe met an untimely demise that would be the only course. However, by the time the probe is well advanced in its mastery of our language, it will possess an on-board translation program that converts our language to Superese. Therefore we can copy its program and either transmit it to Superon or translate into Superese locally before transmission. Perhaps the probe would advise us as to our degree of success.

Mission Accomplished

From what has preceded we can see that the messenger probe scheme for contact overcomes dependence on terrestrial socioeconomic and political stability over centuries, circumvents the problem of determining the proper frequency, and is well adapted to its primary mission of detecting our existence and announcing the location of Superon. If the probe accomplishes no more than this, it has achieved the initial detection that seems so fraught with difficulty if attempted by direct radio; in fact it dos better by reporting to a home base which is ready and waiting to receive the report!

Spheres of exploration expanding around intelligent communities in the galaxy, Superon, our nearest superior neighbor, has just contacted the earth. Some time ago, is hound technical life at A, which is now making its own sphere of exploration.

The Galactic Club

Meanwhile, back on Superon, much time has elapsed since the messenger probe set out, so receipt of the probe's report is dependent on the very kind of political and economic stability that Superon should avoid relying on. But this is a relatively mild dependence for several reasons. First, the schedule and frequency of transmission as well as the report's direction of arrival are known, in fact, have been known well in advance. Second, the receiving equipment exists. Third, no financial outlay of great magnitude is required. And, fourth, the report will be interesting because of the new data it contains about another planetary system even if it does not carry the spectacular news that Superon is ultimately seeking.

As a protection against assorted mishaps, such as failure of report to reach Superon, the probe can announce to us the location of the other chain of communication. For there will likely be a galactic club hose members are experienced at finding developing communities such as ours and inducting them into the galactic community. Each will have responsibility in its own sphere of influence and will engage in an ongoing program of launching messenger probes, at an annual rate appropriate to local priorities, in an endeavor to comb their unexplored frontiers for new technical life. An idea of scale may be obtained by referring to the drawing above, where four superior communities -- A, B, C, and Superon -- are shown. The shaded circles, ranging in radius from 10 to 25 light-years represent the volumes of space around those communities that have been rather closely inspected by well-equipped expeditions; the larger circles show the limit now reached by the messenger probe program. Superon has reached the 100 light-year range, finding the earth. Some time ago it located technical life at A, which was inducted into the chain, is now a superior community, and is participating in the messenger probe program as indicated by the circle showing the progress of A's own exploration. Two other members of the chain are B, which is relatively old and has explored more space, and C, which was inducted by B long ago. Superon is in contact with C, not as a result of probe exploration but because Superon became independently aware of the existence of C when news of its discovery was relayed via links in the chain not shown in the diagram because they are not in the plane of the paper. Had this not been so, an interesting situation could have arisen in the lens-shaped region of overlap between Superon and C, where each could have had messenger probes in the field. (It is a whimsical thought to contemplate two automatons, both far from home in the reaches of space, exchanging notes about their builders, though I admit it is not a very likely encounter.)

Many localities remain unexplored in the vast crevices between the expanding spheres. For example by the time Superon expands its probed sphere from the 100-light-year radius to the 125-light-year radius indicated by the dashed line, it will have doubled the number of stars visited. Therefore even this apparently modest expansion will require much time. Of course, the exploration is helped as new communities such as A take over part of the work.

The Time Scale of Interstellar Dialogue

If the messenger probe plan is so good, why do scientific publications to extraterrestrial contact refer mainly to radio? I think the answer is that if a superior community averages one launching per year, 1,000 years will pass before enough probes have been launched to cover the likely stars within 100 light-years. When the first technical life is discovered, decades will elapse before the probe's report filters back. In addition, we have to consider the travel time of the probe to the star's environs. Depending upon the size of engine the probe uses, the travel time could be kept down to centuries or even decades. But a community that is prepared to wait it out for 1,000 years does not need to hurry. Perhaps 10,000 years travel time would be reasonable; it depends on a trade-off involving reliability in transit, cost per unit, number of launches per annum that can be afforded within the budget allocation, and the maximization of the probability of success. This is indeed a long-term project! (By the way, note that interruption of the launch program does not affect the chances of success of probes already launched; the plan is tolerant of diversion of resources to urgent priorities.) Of course, the human life span being what it is, we are reluctant to contemplate programs that stretch over many centuries; we have to realize that interstellar contact is not contact between individuals, but a contact between civilizations. This is a slightly depressing thought for action-oriented people. The arrival of a probe would be exciting. Nonetheless, the individual who directs the launching, be it a probe or a radio signal has to face the reality that it will not be he who receives the answer.


Near indeed -- is it possible that we have already detected a Bracewell probe? Duncan Lunan, former President of ASTRA (Assn. in Scotland for Technology and Research in Astronautics), once advocated that, just possibly, we have.

The story begins in 1927, during research then in progress on round-the-world radio echoes. (These echoes are propagated by reflections between the charged layers of the ionosphere and the Earth itself, and take about 1/7 second to circle the planet and return.) Taylor and Young in the United States reported hearing echoes they couldn’t explain, signals with delays of only hundredths of a second and coming from a point 2900-10,000 kilometers overhead. Today we surmise these came from the Van Allen radiation belt, discovered in 1958 by the Explorer I satellite, but in 1927 the effect was a real mystery.

Later that year Carl Størmer, a Norwegian mathematician, chanced to meet a telegraphist by the name of Jorgen Hals, who told him that the 10,000-kilometer delay was no so astounding since he, Hals, had heard echoes of three full seconds which he believed were coming from the moon. Størmer, fascinated by this, conducted his own experiments on the phenomenon with the assistance of Balthus van der Pol, a telecommunications specialist at Philips Radio, Eindhoven. In November and December of 1928, Størmer and van der Pol published two letters to the editor in the prestigious British science weekly Nature.211,210 In these letters they described their work which confirmed Hals’ claims. The first sequence reported by van der Pol, and confirmed by Størmer, consisted of pulse delays at the following intervals: 8, 11, 15, 8, 13, 3, 8, 8, 8, 12, 15, 13, 8, and 8 seconds. This was one of the first reports on the so-called Long Delayed Echoes (LDEs), and many more such sequences were later recorded by Størmer and van der Pol, and others.2869

Following the suggestion by Bracewell that LDEs are remarkably similar to the general kind of message we might expect to receive from an alien starprobe, Duncan Lunan3132 puzzled over the meaning of the delay times. He decided to start with the assumption that LDEs were signals from an extraterrestrial spacecraft, an evidentiary technique which once worked for Heinrich Schliemann in the discovery of the ancient city of Troy. The delay times, Lunan noted, were not the sequence of prime numbers that many had expected would accompany the first alien call signal. Yet if they were artificial they must have some meaning. Lunan decided to make a graph plotting delay time against sequence number. The result, to his surprise and delight, was the pictogram reproduced in Figure 24.13.


Figure 24.13 Space Probe from Epsilon Boötes?3132

First van der Pal sequence, evening of 11 October 1928 (tentatively identified as an incomplete map of Boötes).

This diagram can be interpreted as demanding an intelligent reply. By moving the 5th pulse (delayed 3 sec) to a position where it is delayed by 13 sec (marked X), the constellation Boötes is completed.

This is the required answer and if transmitted back the probe should transmit further information. Note the 8-second "barrier" dividing the diagram into 2 parts. The position of a Boötis -- "Arcturus" -- can be interpreted as tentatively identifying the map as compiled 13,000 years ago.

A tentative conclusion is that the probe arrived here from Epsilon Boötis 13,000 years ago.


According to Lunan’s original interpretation, the drawing represents a picture of the brightest stars in the constellation Boötes. The positions of the stars are shifted as they might have appeared 13,000 years ago (due to proper motion across the sky), so this may be when the probe first arrived in our system and compiled the original sky map. One star, Epsilon Boötis, is displaced to the left of the vertical line a distance equidistant from its true position on the right. To Lunan, the message seemed clear: The home star of the alien craft was Epsilon Boötis. Finally, since LDEs were known to follow the path of Luna’s orbit,1124 the starprobe presumably was located in Earth orbit at one of the two Trojan Points.

Lunan’s interpretation of LDEs was immediately questioned by the scientific community.172,1125 Occam’s Razor demands that the simplest explanation of phenomena should prevail, and experiments by the American researcher F.W. Crawford and his colleagues3133 and others1089 have led to the conclusion that LDEs probably are the result of shock wave propagation and amplification in the F-layer of the Earth’s ionosphere. Also, LDEs appear to occur seasonally, whereas an interstellar messenger would he expected to transmit on a continuous basis. A.T. Lawton and his coworkers in Great Britain have even beamed call signals at the trailing lunar Trojan Point in an attempt to elicit some response from the hypothetical starprobe, but no intelligent return signals were ever detected.

What is the latest word on Lunan’s theory? Lunan himself apparently abandoned his early hypothesis when it was discovered that the Epsilon Boötis double star system has suns much more massive than Sol, and thus probably too short-lived for life to have evolved. Lunan’s new hypothesis is that the starprobe’s point of origin was Tau Ceti. This has received support from others:

[Lunan] has since claimed that Epsilon Boötis would be a prime navigational reference for a starflight from Tau Ceti to the Sun; as seen from Tau Ceti, our Sun would lie in Boötis. In 1975 a Russian astronomer, A.V. Shpilevski, published an alternative interpretation of the 11 October 1928 LDEs in the Polish magazine Urania. By plotting the same dot series in a different way he found a star map of the constellation Cetus, with the star Tau Ceti indicated as the star of origin. If the star map has any reality, it shows us that the probe came from the nearby star Tau Ceti, from which no radio noise has ever been detected.3257


* Of course, the probe could simply broadcast high-powered radio interference on the appropriate frequencies. However, instant replays of local broadcasts would appear more friendly than indiscriminate pamming, and would also indicate that the starprobe was prepared to interact and communicate with the indigenous civilization.



24.3.3  The Nature of Alien Artifacts

Bracewell probes are just one kind of nonhuman artifact we may discover in our own solar system.3152 Foster,1136 Macvey,2724 and others have described a number of alternative possibilities:

Space Laboratories -- may be crewed by biological lifeforms, cyborgs, automata, robots, or other mechanical devices.3279 May also exist as wrecks, hulks, or otherwise in derelict condition.

Repeater Stations -- automatic and designed to operate for long periods of time unattended. Capable of receiving, sifting, organizing, and retransmitting signals across interstellar distances. May be part of a galactic communications network, relaying messages using radio waves, x-rays, neutrinos, tachyons, or whatever.3418

Telemetry Stations -- designed to observe, detect, and record changing local environmental characteristics., and perhaps to transmit these data, together with its own operational status, periodically to some agent or agency located outside the solar system. Interactive functions are not ruled out, but basic mission is observation.3293

Marker Buoys -- transmits navigational beams which future starships from the same visiting alien civilization can use to home in on Sol. Another function may be to tag fuel dumps, valuable local mineral deposits, databank storage facilities, unusual objects of special or potential interest, or caches of essential equipment left behind by a former expedition.

Monuments and Edifices -- serving to record or to symbolically identify past expeditions to a site; grave markers for deceased, unborn, or hibernating alien astronauts; nonfunctional obelisks or plaques.3256 Might also serve as informational or data repositories such as a Saunders Databank,2611 Edie’s organic message carriers in meteorites or comets,150,154 or something like the "Extraterrestrial Message Block" on display at the National Air and Space Museum in Washington, D.C.3157

Tools and Implements -- all sorts of equipment ranging from lost screwdrivers and wrenches (or the alien equivalent) to cast-off electronic components, sophisticated recording or sensing instruments, abandoned computers, and unserviceable nuclear reactors.

Refuse or Debris -- blocks or shards of metal, ceramic, or plastic scrap and other waste materials clearly of extraterrestrial manufacture or origin; undecomposed biological wastes or debris; deposits of industrial tailings; sites of chemical or radioactive contamination; alien corpses in spacesuits.3294

Environmental Evidence -- unnatural rearrangement of surface terrain; fused rock; inexplicable radioactive "hot spots"; severe paleomagnetic or geomagnetic anomalies; destruction of planetary bodies (our Asteroid Belt?); planetary orbital anomalies (our Pluto?); unusual geological or planetological phenomena (Saturn’s rings?). Another suggestion is that our DNA code may itself be an alien message left behind on a lifeless Earth eons ago by visiting ETs. Its ability to survive and to reproduce seems a perfect solution to the problem of durability over geological timescales.3178,2611

Are there any preferred locations in the Solar System when alien artifacts are most likely to be found? Many writers have suggested that there are four distinct places where physical evidence of the past presence of ETs may be found:

1. Objects in transient hyperbolic orbits around the Sun. (Single pass through our Solar System, very difficult to detect)

2. Objects in permanent orbit around Sol. (Orbit may be highly eccentric, or perfectly circular. Difficult to detect, but we have lots of time to look.)

3. Objects in orbit around planets, moons, or asteroids.

4. Objects located on or below the surfaces of planets, moons, or asteroids.

It would seem that (3) and (4) would offer the best prospects for easy detection, We have already mentioned (3) in connection with Bracewell probes. These devices might be parked in a stable synchronous orbit or in the Trojan Points if there’s a large natural satellite nearby. Unusual moons such as Iapetus or Titan of Saturn, Triton of Neptune, and Charon of Pluto similarly may be tagged with orbiting artifacts.

What about possibility (4), surface artifacts? The majority of planetary surfaces in our Solar System are unsuitable for the long-term preservation of objects soft-landed from space. Only a very small proportion of the total surface area of our System has any degree of permanence at all, and the forces of erosion which occur on any world with an appreciable atmosphere (or winds) or widespread geological plate tectonic (or volcanic) activity suffice to rule out many major planetary bodies. We must therefore exclude Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. This leaves the surfaces of Mercury, Pluto, most of the moons of planets, and all of the asteroids. Table 24.5 below shows the total "stabilized" surface area available for extremely long-tern storage of alien artifacts in our Solar System.


Table 24.5 Total "Stabilized" Surface Area Available in Solar System 
for Long-Term Deposition of Alien Artifacts (modified from Foster1136)

Celestial Body

Available "Stabilized" Surface Area

of Total

Satellites of Jupiter

2.0 ×1014


Satellites of Saturn

9.1 ×1013



7.5 ×1013


Satellites of Neptune

4.3 ×1013



3.8 ×1013



2.0 ×1013


Satellites of Uranus

1.0 ×1013



6.0 ×1012


Charon (Pluto’s moon)

2.0 ×1012


What if future astronauts from Earth stumble upon an artificial device or artifact clearly of alien manufacture, in the Solar System or elsewhere? Do we have the right to tamper with, and possibly destroy, property belonging to ETs? Or can we claim eminent domain or "salvage rights" and thus convert it to our own use, however we see fit? (After all, it is intruding in our solar system -- can’t we do what we like with it?) What if we mistakenly activate the mechanism, or use it incorrectly, and it causes us serious harm? Can we demand reparations from the extraterrestrial race that left it here, under some notion of "attractive nuisance"?

Certainly the aliens who planted the artifact are technologically far in advance of ourselves, assuming we find it in our Solar System before we have achieved interstellar travel capabilities. The mere fact that we have found the object thus suggests either that it was intended to be found or that it was not intended to be hidden (the ETs don’t care if we find their sandwich wrappers). Technically advanced aliens should be able adequately to hide an artifact from querulous human explorers. So such machines or devices probably will not be equipped with "death rays" or similar unpleasantries to ward off intruders.

If intended to be found, starprobes ought not be designed to be tamper-proof by curious sentients. In fact, in the interests of interstellar amity, the alien artifact should clearly demonstrate that it has nothing to hide (except the location of its home star); that it is a messenger of information, peace, and goodwill; and that it is not to be feared, suspected, or avoided. It should be easily disassemblable. The sacrifice even of a complex and expensive machine may well be worth the avoidance of interstellar hostilities between races.

If probes or artifacts are discovered by human astronauts, the event represents a variety of first contact and a tremendous opportunity for communication and technical progress. Duncan Lunan, summarizing the conclusions of an ASTRA study entitled "Man and the Stars,"1001 offers the following practical advice for handling alien devices of various types:

If [astronauts] stumbled on something accidentally, perhaps they should leave it alone until experts get there, but that is not practicable on interplanetary or interstellar missions. In the interplanetary situation, they should send data back to Earth but not touch, in the first instance -- the approach from there will be determined by what the thing appears to be and whether it still appears active. The next stage would be to try radio and light signals on it, with caution if we don’t know what it is: we may get a reaction as well as data. Before any close approach there would have to be tests for radioactivity etc., even if the object appears inactive.

A small enough object could be brought back to Earth for study, but that decision requires great caution -- the thing could be a container for radioactive or biological waste, or a weapon platform. It is entirely possible that something dumped, or launched, by someone else could be dangerous to us. If brought back for study, the object should be placed on the Moon or in Earth orbit rather than landed. If it is small enough to be brought back the odds are that we detected it by its emissions, therefore it is probably still active. We should be prepared for this situation to turn into a true Contact -- the object could be one of Professor Nonweiler’s hypothetical cryo-bio-packages, or an escape capsule with the occupant in suspended animation.

For a large, immobile object, detected for instance on a planetary surface by photographic survey, we would have to establish on-site investigation facilities. Even so the first checks should probably be made by Lunokhod-type vehicles, because of a reaction when we begin active study with radio, x-rays, etc. How far would we go in active examination -- would we take such a thing apart at any stage? Arthur C. Clarke’s "Sentinel" was meant to be broken, so that its makers would know intelligence here had mastered spaceflight and atomic energy. The black monolith, the version of the device depicted in 2001: A Space Odyssey, had only to be dug up: as soon as sunlight struck it, it was activated and began its study of us. In the former case, the destruction of the pyramid was to bring "Them" back; in the latter, where the monoliths were supposedly responsible for the development of our intelligence, we were directed to the third monolith which delivered the next part of the programmed learning course. The object we find in real life, however, may be more functional. If it is a navigational beacon, for example, interfering with it may bring a repair crew rather than a Contact team.

If we were leaving or launching an object that was intended to be found, we would put information or instructions on the outside {e.g., the Pioneer 10 plaque}. So if the object we find doesn’t have any, we should treat it with caution. But the absence of instructions may just mean that the makers did not expect intelligence to find it -- we don’t put external data on our interplanetary probes, arid probes from intelligence elsewhere in the Solar System (e.g. Jovian or Venus life) might be harmless even if unmarked. An intelligence from elsewhere, exploring the System before our time and anticipating the development of one or more intelligent lifeforms here, might be expected to leave unambiguous message-artifacts like Professor Bracewell’s hypothetical repeaters, and plant them in as many places as possible.1001



24.3.4  Project Daedalus

If technically advanced alien civilizations can build starprobes and send them to Sol, how long will it be before humanity can construct and launch interstellar messenger vehicles of its own? A small group of engineers and physicists, all members of the British Interplanetary Society (BIS), decided to find out. In February 1973 they initiated Project Daedalus, an impressive four-year feasibility study of a simple interstellar probe mission using only present-day technology or reasonable extrapolations to near-future capabilities. More than 10,000 man-hours were expended directly on the Project, which culminated in April 1977 with a prototype design and finally in 1978 with the publication of the final report. The following is a very brief summary of the design and mission specifications for Project Daedalus (Figure 24.14), the first comprehensive starship design study in the history of mankind.2953

The basic mission profile involves an unmanned and undecelerated starprobe which executes a flyby of Barnard’s Star at a distance of about 6 light-years from Earth. This particular target was chosen, not because of its inherent superiority to a Centauri (a closer and more likely system to harbor life3224), but rather because it lies near the midpoint of the expected maximum useful range of the Daedalus vehicle -- roughly 10 light-years.

The final design calls for a starship with a total initial mass of 54,000 tons, of which 50,000 tons is propellant in the form of deuterium/helium-3 frozen fuel pellets. The vehicle consists of a two-stage nuclear pulse rocket, a widely discussed conventional interstellar propulsion technique that has been described extensively in the literature. (See Chapter 17.) The trip to Barnard’s Star would require about 20 years of R&D effort (design, manufacture, and vehicle checkout), 50 more years of flight time at about 12%c, followed by another decade of data transmission from the probe relating to approach, encounter, and exit science. Therefore a basic funding commitment over at least the next 80 years would be required for implementation and successful completion of the mission.

As shown in the time-into-mission graph in Figure 24.14, the Daedalus starprobe would leave the Solar System probably from near-Jovian space. This is because the helium-3 needed for fuel is rare on Earth and must be harvested from the atmosphere of Jupiter using "aerostat factories" floating in the jovian air at medium altitudes. (This technology obviously requires at least a mature spacefaring Type I cultural level among humans, which should be attainable in the next century here on Earth.) The boost period, involving three propellant tank drops and a single stage separation, would last 3.8 years. At the end of these events, the starprobe would have achieved a cruising velocity of about 12%c.


Figure 24.14 Project Daedalus: Mission to the Stars2953

The Daedalus starship takes shape in orbit around Callisto, near Jupiter. A new frontier is about to open.



JOVIAN AEROSTAT FACTORY. Factory modules floating in the atmosphere of Jupiter harvest the isotope Helium-3 for use as fuel in the Daedalus starship. At far left is the overall scheme, with ascent vehicle docked. At near left is detail of the factory complex.












LEFT: At low thrust, the Daedalus starship leaves Callisto’s orbit. Soon it will have escaped from Jupiter and the Sun and head into interstellar space on its half-century flight to another star. RIGHT: Results of stellar target ranking out to 12 light years.



BASIC DAEDALUS STARSHIP MISSION PROFILE: Basic mission profile to Barnard’s Star, giving distance from the Solar System as a function of time into mission.






The main Daedalus vehicle, already well into the encounter with the planetary system of the target star, detects new phenomena on the star and deploys a high-velocity-gain subprobe to attempt a closer look.




Key to Figure: (1) Final checks, calibrations, and choice of trajectory; (2) Deploy inert materials and chemical tracers; (3) Final maneuver to pass to starward side of planet; (4) Deploy subprobes (one probe is lost at planet); (5) Begin active sounding with radar and laser; (6) Activate sub-probes, begin high-speed data acquisition, trigger chemical tracers; (7) Cease high-speed data acquisition, continue sounding, begin observation of tracer trails, begin probe interrogation and data dump to main bus.


During the flight out the payload remains active, making continuous measurements and constantly reporting data back to Earth. A wide variety of "coast phase" scientific investigations would be undertaken, including direct detection and observation of interstellar particles and fields and innumerable detailed astrometric very-long-baseline measurements of distances to other stars and of the size of the Galaxy. At the time of encounter with the Barnard’s Star system, a dispersible payload would be deployed much like the multiprobe Pioneer Venus (1978) spacecraft or the warheads of a MIRV’ed missile.

As the vehicle approaches Barnard’s Star, two onboard large space telescopes (Palomar-size 5-meter reflectors) swing into action, beginning the search for planets and an accurate determination of their orbits. Once these orbits are established, heavily instrumented subprobes would be launched on close-intercept trajectories for more detailed observations. The main ship carries 40 tons of extra fuel for this purpose, and the main propulsion system would be used for each maneuver. Throughout the encounter period the subprobes -- up to 18 in number -- would pass their data back to the mother ship, which receives the transmissions on each of eight 10-meter-diameter radio dishes studding the starprobe’s exterior. This information is processed and condensed by Daedalus’ semi-intelligent computer system, which is housed in a central core running through the payload. Later it is relayed back to Earth during the post-encounter period using the bowl of the dormant second-stage engine as a giant radio communications dish.

The total mission payload is about 500 tons, a large fraction of which is in the dispersible subprobes. A typical subprobe weighs more than 10 tons and measures 20 meters in length. Prior to deployment each is shaped like a narrow conical frustrum in order to facilitate radial packing into the cargo bay. Each subprobe’s communication channel, operating on a 1-kilowatt transmitter, can beam as much as 11 million bits/second of data back to the main vehicle in "video" mode. When talking to Earth, the starprobe uses a 1-megawatt radio transmitter operating at 2-3 GHz with a maximum bit rate of 864,000 bits/second. (See encounter scenario, Figure 24.14.)

The Daedalus starship design includes many necessarily innovative features too numerous to mention here. One example is the "wardens," created to help ensure the vehicle’s self-sufficiency:

The requirement for a high degree of reliability suggested that a system of "Self Test and Repair" philosophy should be adopted. Also, the payload effectiveness could be enhanced if it were possible to re-organize experiments when required en route. Further, in order to avoid the contaminated environment of the main vehicle it became desirable to place the particles and fields experiments a long way (several thousand kilometers) from the main vehicle. These requirements led to the concept of robot self-propelled vehicles carrying specialized tools and general manipulators. These vehicles would have a limited degree of data processing and machine intelligence, but any high level decision making would be carried out by the main mission computer on the main ship. Two of these "wardens" would be provided, each having a mass of about 5 tons. A total mass of spares of 15 tons would be available.2953

Starprobe Daedalus may never be built, but it is perhaps a primitive prototype design for the exploratory interstellar spacecraft of the coming century. It provides a firm basis for discussion of the plausibility of Bracewell probes and other artifacts that may find there way into our Solar System at the hands of alien adventurers. To those who remain skeptical of the ambitions of the BIS’s Project Daedalus, let them recall that it was the same British Interplanetary Society that conceived a model for a manned moon lander mission a mere 30 years before Armstrong and Aldrin first set foot on lunar soil.

Back to Contents