Toward Distant Suns
by T. A. Heppenheimer
Copyright 1979, 2007 by T. A. Heppenheimer, reproduced with permission
Chapter 11: The Fermi Paradox
It is part of the lore of science that one evening at a dinner party in the year 1943, the physicist Enrico Fermi startled his companions by looking around and saying, "Where are they?" When asked "Who?" he replied, "The extraterrestrials." His point was that if advanced interstellar civilizations really exist, they should be readily detectable, and their denizens might reasonably be expected to make occasional visits. Then, the absence of such visits or of evidence for such civilizations could be regarded as a surprise, a paradox.
Or, to put it another way: If life is common in space, and if intelligent life is far from unknown, then in the fourteen-billion-year history of the Milky Way, one might imagine that some culture has swept across the Galaxy in the grand manner of Alexander, Genghis Khan, the conquistadores. It then is not entirely obvious why our interstellar surroundings should appear to be in a wild state, as untouched as a wilderness preserve.
In Chapter 2 there was an estimate of how many of our Galaxy's stars may be abodes for advanced forms of life, including intelligent life. We saw that the star cannot have any kind of large binary companion, or the formation of planets will most effectively be prevented. The star must evolve and grow brighter at the right rate, neither too slow nor too fast; a life-bearing planet must be of the right size and form its atmosphere neither too quickly nor with too much delay. In addition, of course, the planet must be at the right distance. From this point of view, a planet will support life only if it and its star actually turn out to be a very close replica of the Earth and Sun, a replica that is to be achieved purely by chance, by the random workings of nature. As also stated in Chapter 2 it is likely this happened only for one in every 227,000 stars in the Galaxy, there then being some 880,000 "good stars" in all.
If we seek the homes of extraterrestrial civilizations, these stars are the candidates. In order to understand more clearly what we may expect of them, it is appropriate to discuss the difference between "hang-ups" and "instabilities."
As previously stated, a hang-up is an obstacle that effectively prevents or long delays the rise of an intelligent civilization. The failure of a star to form planets would be one; so would the failure of any planet to possess conditions suitable for life. The long span of eons required for the evolution of advanced brains represents another hang-up. Still another may be posed by geography. A planet may be completely water covered. It could develop high forms of life such as whales and dolphins, but these would not be candidates for the founders of technical civilizations. There can be no fire in the sea, and fire is the art that first distinguished man from the beasts. Less anthropomorphically, an ocean environment provides little novelty or challenging variety, which appears to give the strongest evolutionary advantage to high intelligence. On such planets, the rise of civilizations may have to await the rise of continents emerging from the sea.
An instability, by contrast, is something that hastens the rise of civilization. We may think of a rock perched atop a mountain; if it begins to move due to its own instability, it will rapidly gather speed and may soon sweep all before it. It is a characteristic of instabilities that they advance in a relatively short time. For instance, planets may not form at all (hang-up) but if they do, their formation will take no more than a few tens of millions of years (instability). On Earth it took four billion years for life to evolve to the point where there were complex multicellular organisms with skeletons or hard body parts, but it took only the relatively brief hundred million years or so of the Cambrian epoch for life to diversify and flourish into very nearly the variety we know today. This flourishing of life was due to another instability, which operated when the buildup of an ozone layer in the atmosphere allowed life to emerge from the protection of the sea.
The rise of intelligence and civilization, if at all possible, certainly must count as an instability. Ten million years ago the smartest animal alive was a species of East African or Indian ape known as Ramapithecus; today there is the glory of you and I. (It is an exercise for the reader whether this indeed constitutes progress.) One million years ago the most advanced tools were the chipped quartz handaxes of the type known as Acheulian; today there is the Cray-1 supercomputer and much else. It has all happened very fast.
However, the evolution of the genus Homo took place against the backdrop of the great climatic upheavals known as the Pliocene Drought and the Ice Ages. Everyone has heard of the latter; the former was a ten-million-year time ending some two million years ago in which the African rainfall was one-third of present-day normal. It is too much to ask of coincidence that the ascent of man just happened to be then. It is much easier to believe that these climatic crises gave a survival value to intelligence, and that in a mild, equable climate its rise might have been delayed.
The density of stars in our cosmic neighborhood is about one per six hundred cubic light-years. This means that with one "good star" to every 227,000, the nearest star that has harbored intelligent species is about five hundred light-years away. The nearest abode of an intelligent civilization would be at least that far, and possibly a good deal farther.
Most such planets could not be expected still to be active locales for intelligence. They would offer fossil beds, artifacts strange as from Easter Island, dusty-dry records in unknown scripts that crumble at a touch. To speculate on how many intelligent species or civilizations exist today, we need to guess how long they last. If the experience of Earth's primates is typical, then the average lifetime of families of intelligent creatures (who do not necessarily have advanced technical societies) is some three million years. This means only about four hundred such species, assuming one to a planet, in the Galaxy today. The nearest one would be about five thousand light-years off.
As for the lifetimes of civilizations, there we cannot even guess. They might be as short as a few hundred years or as long as a billion. Even with lifetimes of hundreds of millions of years, there may be no more than a few dozen existing today. Like wildflowers blooming after a desert rain, they could flourish in their moments of glory while never knowing of each others' existence. But if civilizations overcome the stresses of immaturity and youth and go on to reach advanced ages, then the Galaxy may be planted with them as thick as a forest. If billion-year cultures are at all widespread, the nearest may be within a hundred light-years.
Another instability as well is influential here. Any sufficiently advanced and long-lived culture may surely learn the arts of space colonization and interstellar flight. To such a culture, the Galaxy would appear as did the island-studded Pacific to the first seafaring Polynesians. It would be a fertile milieu in which small bands of settlers might set out for a nearby star, colonize it, and let their numbers grow. In later centuries the descendants would produce new starfarers who would repeat the process.
Cultures may perish, species become extinct; but the presumed hundreds of thousands of civilizations must surely have had the opportunity to test all possible ways of life, all philosophies. Surely one of them, if not many more, may have succeeded in using its intelligence to devise those forms and practices that would permit long cultural life together with interstellar colonization. Such a culture could be to the Galaxy what the Western Europeans have been to Earth.
It has been ten thousand years since men laid the groundwork for civilization by learning to domesticate the native plants and animals. In the millennia since, myriad cultures and ways of life have arisen. Yet the Western Europeans came up with the conbination of technical and ethical principles that swept the world. Advanced technology, freedom and justice (including the Communist versions of these ideals), the urge to explore and expand—this was the winning combination. Few are the nations that have rejected or been untouched by at least a part of this combination; its influence and its people have penetrated the remotest corners of Earth. Is it too much to expect that somewhere in the Galaxy there has arisen a similarly influential culture, and from its ancestral stock have sprung far-flung nations that have left their touch on the planets of nearby stars and perhaps within our very Solar System itself?
We thus must come to grips with the question of the existence of starships and of stellar visits. The concept of the starship is an old one; like the Martians in Nigel Kneale's "Quatermass," it is a name "almost worn out before anything arose to claim it. " Yet it is quite a specific concept: a spacecraft capable of spanning the gulfs between the stars while supporting a community of onboard residents for the duration of the voyage. There has been much speculation about travel at close to the speed of light, as well as of keeping the travelers in suspended animation or freezing them for the duration. These speculations today are far from convincing. Nevertheless, there is a type of starship that even today can readily be envisioned.
This is simply a mobile space colony. The trend of space colonization may well lead to cosmic arks suitable for interstellar voyaging. Such an ark would be a colony in which self-sufficiency has been highly developed, allowing its people to live indefinitely by controlling the population and recycling materials. [Author's footnote: Which need not include the bodies of the dead. One indication of civilization is the respect shown the deceased; these could be cremated and the ashes stored in urns in a mausoleum. Even if ten thousand people journey through space for a thousand years, the amount of matter thus tied up would be only a few hundred tons, compared with the hundreds of thousands of tons that would be the mass of the starship itself.] For propulsion, thermonuclear power will be more than adequate as long as the speed is no more than a few percent that of light. If it speeds up to 5 percent of light speed and then decelerates at the destination, the total mission would require the energy obtained by converting 0.005 of the starship mass to energy, as in e = mc2. In fact, if the starship propellant tanks contain a mass of hydrogen equal to that of the ark, and if this hydrogen is fused into helium via the reaction that powers the Sun, the energy released would be 0.007 mc2.
This reaction actually proceeds so slowly as not to be practical or useful, but other thermonuclear reactions are speedier. These involve deuterium and tritium, the heavy isotopes of hydrogen, as well as the isotope helium 3. There is plenty of deuterium in the oceans—at least ten trillion tons of it—and it is readily extracted. A few million tons for a starship will never be missed. Helium 3 is rare on Earth, but that is merely because helium is rare. It is common in Jupiter and Saturn, and any culture that can build starships can certainly tap these sources.
Let us now imagine that some interstellar culture has done no more than reach this primitive level of technology that we envision today with our inadequate grasp of physics, but that we may well attain less than ten thousand years after inventing the wheel. It is well known in nature that species growth and extension continue as long as there are the resources to support further expansion. Indeed, one need only think of the world's small and remote volcanic islands—Hawaii, the Galapagos, Tristan da Cunha. All their native plants and animals (excepting those brought by man) are descended from ancestral colonists that survived the perilous voyage from the mainland: monkeys and other animals floating on natural rafts of tangled tree-trunks, insects borne on the hurricane, seeds brought by birds of passage. With far safer and more convenient modes of transport, and with the energy and resources to support very great expansions, it is easy to imagine a space culture doing the same.
Animal species do not increase their numbers beyond the carrying capacity of their environment, and we must certainly expect that a long-lived intelligent culture will practice population control. (They would hardly be intelligent if they didn't.) In any planetary system that it colonizes, including the home system, its population will not exceed some optimum level. Even so, Eric Jones, of Los Alamos Scientific Laboratory, has reached a remarkable conclusion: If a stellar civilization practices population control but devotes even a small effort to building starships, it will produce a wave of colonization that will sweep across the Galaxy in no more than one-hundredth the Galaxy's age.
Jones' calculations involve four quantities:
The emigration does not proceed uniformly toward stars in all directions. Star systems suitable for colonization are distributed randomly, and at first the colonists move along preferred directions toward the nearest good stars. Some stars relatively close to the home world are not visited till a branch of the colonization wave turns in their direction. Then an expedition might travel inward to reach a star closer to the home world than was the original star it settled.
For example, let the S.B.P. be one-half the O.P.; that is, starship-building is a low priority and is not undertaken till a population is well established, well along on its growth to what will be its ultimate extent. Let the E.R. be only one-millionth the G.R., which we take as 0.005 or .5 percent per year. This means that interstellar flight is in no way a means to reduce population pressures or to delay the approach to a stable population. At the risk of anthropomorphizing the starfarers' motives, one could say that starflight is a marginal activity engaged in by a small number that are its devotees and enthusiasts. Even so, there is then an expanding wave of settlement that sweeps across the Galaxy at 1.5 percent the speed of the starships. Since the Galaxy is some 100,000 light-years across, if a starship travels at 5 percent of light speed this wave of settlement will cross the Galaxy in some ninety million years. This figure is less than 1 percent the Galaxy's age, 2 percent the age of Earth.
Much faster results follow if the culture regards starfaring with at least the importance eighteenth-century Britain gave to the colonization of America. That colonization was largely the work of voluntary emigrants such as might be attracted to space. This contrasts with the much larger later waves of settlement in America, which were driven by the Irish potato famine and by the oppressions of czars and emperors as well as by the demands of the Industrial Revolution. Even so, between 1700 and 1790 some 350,000 Englishmen came to North America. With England's population at twelve million in 1750, the E.R. is 0.0003 per year.
For the G.R., again take 0.005 or .5 percent per year. Let us further assume that the S.B.P. is only 1 percent of the 0.P.; that is, a population waits only long enough to become well established in a new planetary system before it once again revives the founders' trade of starship-building. Since this culture takes starships seriously, let us also suppose that theirs are quite powerful and travel not at 5 but at a very fast 10 percent of light speed. Then the expansion wave goes at 16 percent of ship velocity. Within an evolutionary eye-blink of six million years, it has reached the farthest corners of the Galaxy.
We now must imagine that at some time in the past four billion years, some super-civilization (one of perhaps hundreds of thousands that have existed) launched just such an expanding wave of settlement. At some time along the way, like Fletcher Christian's Bounty mutineers discovering Pitcairn 's Island, the outriders of this expansion discovered our solar system. There, at a convenient ninety-three million miles from its star, was a lovely green planet, which like Pitcairn at a much later date, proved to be totally untenanted and truly a fine place to settle and colonize. What can we say about this?
To begin, while a colonizing visit to Earth must be a rare event, the geologic strata could preserve the evidence, just as it preserves the remains of meteor craters. Impacts by large meteroids are indeed rare, but the ancient granite of the Canadian Shield has faithfully recorded them. Any colonizing visit would quite likely leave abundant remains of roads, pavements, structures, machines and equipment, even the bodies of the deceased.
Next, it is appropriate to note that just such evidence exists for major events in human prehistory. The archaeological remains of ancient towns in the Middle East, and even of Viking explorations in Newfoundland, are examples. An even more telling case is a 300,000-year-old campsite on the French Riviera used by a small band of the near-men known as Homo erectus. The anthropologist Henry de Lumley has found evidence for their use of poles in constructing shelters or lean-tos. He has found a firepit and evidence for the existence of specialized craftsmen at that early date. (A small area, well away from the fire, contains chips from stone tools such as would have been made by an artisan.) He has even found the fossilized remains of feces.
Finally, we recall that geologists, archaeologists, anthropologists, and other diggers have been very busy over the past century and a half. We now have a very complete and consistent understanding of the fossil record for at least the last half-billion years, and a partial understanding of the geologic record for two or three billion years further back. In addition to the excavations of scientists in the field, there are canyons, mountains formed by upthrusting great rocky blocks along fault lines, and the diggings of those who have built roads and cities.
In short, the geologic strata would have been patiently collecting any evidence of settlers from the stars for billions of years. We are quite familiar with that strata and have traced in them the evidence of even very localized and temporary events. And in all that vast archival storehouse, that natural library of Earth's history, there is not a single artifact that is indubitably from a source not of Earth. Three billion years and more, and not a single stretch of concrete, or steel beam, or fragment of an abandoned set of scientific instruments. No evidence for settlement or even for visits for the purpose of exploration. No counterparts of our own Viking, Pioneer, Venera, Surveyor, Lunokhod, and Apollo spacecraft of exploration, now resting on the surfaces of Mars, Venus, the Moon. As Fermi asked, "Where are they?"
Eric Jones in his article, "Interstellar Colonization," has made this point somewhat more poetically:
In all fairness, one should not ignore the people who believe, with Erich von Daniken, that the cultures of prehistory, the civilizations of antiquity, could not have arisen without intervention, divine or extraterrestrial. However, most of the great works that are presented as proof fall into the categories of positional astronomy or monumental architecture. Regarding ancient astronomy, any high priest with a bent in that direction could have done quite a creditable job with even a crude transit instrument, particularly if his work was part of a centuries-old tradition. [Author's footnote: A transit instrument is a tube or pole along which one sights so as to view a star or planet. It is equipped with marked circles to measure the pointing directions. A sixth-grader today can make one and use it, and the ancient Near East priesthoods were a good deal smarter than that.] Regarding ancient architecture, there are people who forget (or don 't want to remember) that in those times there really were mighty kings with the power of life and death, who could cause great cities and monuments to rise in their names. In none of this is there convincing reason to think that humanity has ever had help from beyond the stars.
Yet the matter does not end here. It is quite possible for nonterrestrial visitors to come to Earth without leaving evidence in the geologic record. This possibility lies behind the idea that some of the UFOs are visitors from space.
There is a vast literature on unidentified flying objects, but very little of it is persuasive. It is far from rare for even experienced observers to see strange lights or flying objects, which turn out to be weather balloons or airliners, or planets such as Venus seen under unusual conditions. If all UFO sightings could be explained this way (and indeed most can), then there would be no more to the matter of UFOs than that people sometimes respond in strange ways to unfamiliar things. The problem is that when one filters through the reports, there is a hard core of good observations that cannot be explained.
A review of some of the most unusual UFO encounters will illustrate the problem. On July 17, 1957, an Air Force RB-47 reconnaissance aircraft was followed by a UFO for some ninety minutes and for over seven hundred miles as it flew across the southcentral U.S. The aircraft was equipped with electronic countermeasures gear and was manned by six officers, three of whom were radar specialists. At various times the cockpit crew saw the UFO as an intensely luminous light, and it was also detected by the plane's electronics equipment. It was also followed by ground radar. Several times the UFO disappeared and then reappeared, and these events were seen simultaneously by eye as well as by ground and air radar. At one time the UFO was seen at an altitude of twenty thousand feet when the RB-47 was at thirty-five thousand. The captain received permission to dive at it. As he approached twenty thousand feet the object blinked out, disappeared from the ground radar scope, and disappeared from an onboard radar monitor, all at the same time.
On August 13, 1956, a radar at Bentwaters, England, tracked something traveling at four thousand miles per hour westward, at four thousand feet altitude. At the same time, control-tower operators reported a bright light passing overhead toward the west, and the pilot of a C-47 aircraft, four thousand feet over the airfield, saw the bright light streak westward underneath him. Someone at Bentwaters station then phoned the second radar station at Lakenheath: "Do you have any targets on your scopes traveling at four thousand mph?" The Lakenheath station then detected a stationary target at twenty thousand feet altitude, which suddenly and with no buildup in speed went north at six hundred miles per hour. It made several sudden stops and turns, which were seen by two radar sets and three ground observers. After about forty minutes of this, an RAF Venom night fighter was vectored in for a close look. The pilot detected the UFO with his radar and locked his guns on it. Suddenly, with a quick circling movement, the UFO moved behind the Venom; the pilot took evasive action but could not shake it. All this was seen on ground-based radar as well. When the fighter flew off toward its base, the UFO followed only a short way. Then it once again became stationary before resuming its flight to the north.
On January 20, 1967, near Methuen, Massachusetts, three people were driving through a lonely area. Reaching the top of a hill they suddenly came upon a straight string of glowing bright red lights, apparently a few hundred feet away. The driver slowed the car and proceeded toward the lights. When almost broadside to them, the object to which they were attached swung around to reveal a new appearance. Four distinct lights formed a trapezoid, two red ones for the top and two white ones for the base. The driver pulled over to the side of the road directly broadside to the UFO, which now seemed to be lower and nearer; the engine was idling with lights and radio on. Then suddenly the engine, lights, and radio failed completely. The driver tried to start up the car but the engine gave only a low "moan." He shut off radio and lights and tried again, but still could not. The driver's side window was open, but there was no sound. Then the object began moving and then shot away at high speed. The driver then was able to start the car and the lights and radio worked perfectly.
In addition, UFO sightings extend back into history. An example has been reported by Carl Sagan in "Communication with Extraterrestrial Intelligence":
Such reports as these certainly sound like extraterrestrial visits. Yet such an explanation merely fits the spirit of the times; no more. We can conjecture that some UFOs are from outer space; we cannot readily prove they are not. Yet in earlier centuries the same would have been true for other explanations of UFOs, fitting the spirit of those times: angels' haloes, temptation of the devil, the spirits of departed kings, celestial chariots heralding the return of the Risen Christ, or the souls of the dead. Such explanations cannot be disproved any more readily than can the extraterrestrial conjecture.
What's more, this conjecture runs into inconsistencies. The idea that UFOs are observing us arose after World War II because of the peculiar resemblance of some of their activities to wartime reconnaissance aircraft. The similarity is really quite striking if we imagine, say, a flight of British "Mosquito" aircraft sent to reconnoiter a German target such as the battleship Tirpitz. The planes would fly in formation, slipping in and out of clouds, then venture to approach the Tirpitz while keeping a respectful distance. One pilot might swoop in quickly for a close encounter, to get the best photography. If he met too much anti-aircraft fire he would swiftly fly away; if the German defenses proved too strong, the whole formation would fly off at top speed. There are a number of UFO reports of this type.
Yet only twenty years after the sinking of the Tirpitz, the Air Force had the SR-71. Flying at eighty thousand feet and at two thousand miles per hour, it could scan the entire U.S. in only three passes with side-looking radar. Such performance is typical of reconnaissance today, yet is very untypical in the world of UFOs.
There is another and very telling problem with the UFOs; namely, they have not been detected by space surveillance. Since the 1950s the U.S. and its allies have set up an impressive array of radar networks to track objects in space. These include the Ballistic Missile Early Warning System, with powerful over-the-North-Pole radars, as well as the Air Force's Space Detection and Tracking System. This latter system routinely tracks objects as small as an astronaut's glove, which floated out of a Gemini spacecraft. In addition there are spaceborne reconnaissance satellites. One of them tracked a great meteor on August 10, 1972, which grazed Earth's atmosphere and barely missed impacting the surface. Had it hit, it would have struck with the force of the Hiroshima atom bomb. It was a most rare astronomical object; yet it was tracked from space. The same cannot be said of UFOs.
For a time in 1960 it appeared that our radar networks had indeed detected a visitor from space. On January 31 a Navy network known as Dark Fence detected two passes of an unknown space object. It was nineteen feet long by five wide, in a nearly polar orbit ranging from 134 to 1,074 miles. The commander of Dark Fence personally reported this to Admiral Arleigh Burke, Chief of Naval Operations, who took the news to President Eisenhower. The Soviets declared it was not one of theirs; U.S. specialists declared it was not one of theirs. It turned out that the intruder was an Air Force satellite, Discoverer V, launched the previous August. It had carried a retro-rocket to return a capsule to Earth, but the rocket fired in the wrong direction. The result was a higher, unpredicted orbit.
The fact that no similarly unknown objects have been tracked raises a real problem for UFOlogists, especially since UFOs have frequently been detected on radar. One could argue that alien spacecraft can make themselves radar-invisible. But why should they do that when they are out of the atmosphere, yet make themselves radar-visible when inside it?
Even very good UFO reports can be explained without resorting to the "we-are-being-visited" idea. Many apparent radar UFO sightings have actually been due to effects known as anomalous propagation, in which radar signals are channeled in the atmosphere to produce what are in effect radar mirages. In the words of one specialist in this field, "There is now abundant evidence that the atmosphere will effect radar propagation in almost unbelievable ways and produce virtual [apparent] targets which have apparently fantastic maneuverability. " Such images gave rise to a UFO flap in midsummer 1952 at Washington, D.C. In response to these strange radar echoes, interceptors from Andrews Air Force Base scrambled to seek the sources. Yet they were no more than false radar returns.
One well-explained UFO report could virtually have been taken from the movie Close Encounters of the Third Kind. A child, going to the bathroom in the middle of the night, turned on a light and woke his parents. Suddenly the light went out. The father got out of bed to investigate and happened to glance out a window. What he saw was a pulsating, reddish glow that moved irregularly over the sky, then faded out.
The explanation? A blown transformer had knocked out power just as the child turned on the light. The father, his eyes dark-adapted from sleep, caught the bright light full in his eyes. The result was an after-image, which drew his attention as he passed the window.
Some of the Close Encounter cases are more subtle. Imagine someone driving at night along a lonely road, his attention wandering. Suddenly he tops a hill and sees the rising full moon, copper-colored, hazily shining through distant mist. If he does not recognize what he sees, he may be startled, and if of a suggestible temperament he may even panic. Then he will likely say that the light is brighter, larger, nearer than it really is. This sort of thing is actually quite common in UFO lore, as when witnesses fail to recognize meteors blazing a hundred miles away and report a UFO that passed "just over the tree-tops."
What of the failure of auto ignitions, radios, and the like? I had a personal experience of that type once. I was driving along a nearly deserted stretch of Interstate 10 north of Tucson, Arizona. It was a bright, hot September day and my mind was only partly on the road. Soon my attention was diverted by a passing train. As I watched it, my car meandered over to the shoulder. Startled, I hit the brake and turned not knowing the car would go out of control. By the time I regained control the car had spun completely around and was coasting backwards; the ignition was dead. When I recovered my wits, I started up the engine and drove off, but in a far from calm state of mind.
Suppose it had been not a train that distracted me, but a coppery and unfamiliar-looking Moon at night (which vanished behind clouds a minute later). Were I of a suggestible turn of mind, the result might not have been just a case of my being a damn fool at the wheel. Instead, it could have been a classic Close Encounter of the Second Kind.
Still, such explanations leave a residuum of unexplained cases, of which Lakenheath and the RB-47 case are only two. What are we to say when multiple witnesses (possibly with instruments) calmly and with great care make mutually corroborating observations of a strange phenomenon in the sky? Here indeed we may have something new to be to be learned. It may take decades or even centuries, but the surprising explanation will indeed be found.
A very good case of just this type took place near sunset on June 18, 1178, near Canterbury, England. Here was not a UFO, but a strange event involving the Moon. As recorded in the medieval chronicles of Gervase of Canterbury,
What was it? To the lunar geologist Jack Hartung, the clue was the "flaming torch" which spewed out "fire, hot coals, and sparks." So might a meteoroid impact appear, flinging out white-hot rock to a great height above the Moon. The statement that "the upper horn split in two" means the impact could have occurred only in the region of lunar latitude 45° north, longitude 90° east. In fact, at 36° N by 103° E is a large, very bright, very fresh-looking crater, which from geological evidence appears to be among the most recently formed large craters on the Moon. It is named Giordano Bruno, after a sixteenth-century scientist who, ironically, argued in favor of the plurality of inhabited worlds. Had he lived a few centuries earlier, he would have had the unique honor of having his crater form after his death.
If even the best UFO reports are to fall before similar unusual explanations, then we can rule out the idea of interstellar visits in the present as well as of interstellar colonizations here in the remote past. Still, there is the possibility that advanced cultures engage in signaling or communication. In the past two decades the interest in SETI (Search for Interstellar Communications) has blossomed considerably. There may soon be a formal NASA program in this area, to be run by Dr. John Billingham of Ames Research Center. Thus far, Congress has voted no funds for the work, but the bureaucratic machinery is already in place. Billingham and his associates already have office space in Building 204 at Ames, and outside that building is clear evidence that SETI may soon be just another government project. There is a parking space marked CHIEF, SETI PROGRAMS.
The SETI experiments conducted or proposed to date have used the techniques of radio astronomy, which some people have suggested will turn out to be a primitive, quickly abandoned technology at the interstellar level. Advanced cultures could use powerful lasers, neutrino beams, gravity waves, or whatnot. However, any communications channel must compete for detection against the overwhelming power of the nearby star. It is by this comparison that radio transmission quite literally shines; it can easily be made far more powerful and readily detectable than the competing natural radio emissions. In any case, even if new communications modes come into use, the old ones may still be valued. Despite the efficiency of the telephone to transmit messages, we still also use a system that dates to the ancient Sumerians—the post office. Nor have we abandoned the institution that a century ago was as commonplace as sarsaparilla and mansard roofs, the telegraph. Quite the contrary; Western Union has moved with the times and has lately been building and launching its Westar communications satellites.
The fact that an extrasolar civilization might beam radio signals to us does not necessarily mean that the signals would carry information that we could decode. The most efficient way of encoding information is known as Shannon coding, after the theorist Claude Shannon of MIT. To one who lacks the means of decoding, a Shannon-coded signal is indistinguishable from random noise. An example of this is a hologram, which encodes vast quantities of information about an object's three-dimensional image. Such images can be reconstructed using laser light. Yet if one seeks the images by examining the hologram under the microscope, all that will be seen is a nearly random pattern of speckles resembling a bad case of snow on a TV screen. Still, there is a clear means of identifying artificial signals from space. Unlike natural signals, they will use only a very narrow range of radio frequencies. This is because for a given signal power, the transmission can be detected much more strongly and at greater distances if it is concentrated and not spread out in frequency.
Even the mere detection of an artificial signal, however devoid of content, would be the astronomical equivalent of the California gold rush. The first detected signals would play the role of nuggets of gold, which point to the existance of a rich mother lode. These early signals would be regarded as "acquisition signals" to direct our attention to the particular part of the sky. We then would dig down with receivers of increasing sensitivity, till we hit the lode: signals generated by that culture for its own purposes. These might be similar to our own TV transmissions and missile-detecting radars.
This does not mean tuning in the celestial equivalent of an "I Love Lucy" rerun; it takes ten thousand times better radio sensitivity to produce a good TV picture than merely to detect its carrier signal. Still, even without detecting any information content in such interstellar signals, careful tracking and study would tell us many things about the distant planet and its culture. Merely by studying such signals with standard techniques of radio astronomy, we could learn:
With such rich rewards in store, it is little wonder that a number of radio astronomers have devoted much effort to seeking the powerful acquisition signals. The first such search, Project Ozma in 1960, generated much publicity (including a cover story in Newsweek) but few results. More recent searches have proceeded with much less fanfare but have examined many more regions of the sky. As of 1978, the list of such surveys is already somewhat lengthy.
The accompanying chart shows that SETI is rapidly becoming an important activity in the community of radio astronomers. The trends have been to examine more and more stars, with better and better equipment. To date some fifteen hundred stars have been studied. The first searches used simple equipment, but the most recent ones have used such great radio telescopes as the one-thousand-foot Arecibo antenna and the three-hundred-foot transit telescope at the National Radio Astronomy Observatory. The most recent work has indeed been keyed to detecting signals that span a very narrow range of frequencies; the telescope then acts like a TV receiver with 65,536 channels. The chart's last column, "Sensitivity," is deceptive in its simplicity. The best sensitivity to date, 4 x 10-27 watts per square meter, would suffice to detect a forty-watt bulb at twenty times the distance to Pluto.
SUMMARY OF SETI EXPERIMENTS THROUGH 1978
Some of these surveys examine entire galaxies. This search strategy is akin to looking for life on Earth by observing the whole of North America at night with a telescope, and looking for the lights of cities. If any star in a galaxy boasts a civilization that has established a sufficiently powerful beacon, we could detect it and say there was intelligent life there. Such observations could well yield the first detection of artificial signals. The problem of SETI is to point a radio telescope in the right direction, at the right time, and to detect the right frequency with sufficient sensitivity. We already know how to scan simultaneously a broad range of frequencies so as not to miss the right one, and it should become possible soon to devote full time on a large instrument for this work. Pointing it at a nearby galaxy could also solve the aiming problem, since the signals would not be coming from just anywhere but rather from the limited region of the galaxy only.
Such galactic transmitters might be called Ozymandias method of signaling, after the poem by Shelley:
We can imagine that a culture wants to make its presence known across the Universe, as well as to achieve a kind of cosmic immortality by continuing to signal automatically long after the culture itself has gone to dust. Such motives are not unknown on Earth, and Ozymandias could be a type of pyramid-building on a scale never dreamed of by Cheops.
The interesting feature of Ozymandias is that it need not be the work of some mythical super-civilization that has tapped all the energy of its sun. Suppose that Ozymandias is to illuminate a distant galaxy with the power of 4 x 10-27 watts per square meter, the sensitivity limit of the best experiments to date. To be specific, let the target be a circle 100,000 light-years in diameter, the approximate size of the Milky Way. The nice feature of Ozymandias is that it then doesn't matter if the target galaxy is near or far. To signal to a galaxy 10 billion light-years off, the system merely focuses the beam to reduce its spreading.
The transmitting power of Ozymandias then is 2.8 10-15 watts. That's 100,000 times the level of a power satellite, but only ten-trillionths the power output of the Sun. If Ozymandias were powered by an enormous powersat, it would be a square twenty-seven hundred miles on a side. This is quite a large structure, but if we can seriously propose powersats the size of Manhattan Island, our descendants may well be able to envision one the size of the Moon.
If we were to build an Ozymandias transmitter, we could fulfill the suggestion of author Lewis Thomas when asked what message he would send for interstellar signaling: "I would vote for Bach, all of Bach, streamed out into space, over and over again. We would be bragging, of course, but we can tell the harder truths later.''
Ozymandias could indeed be a creation of Earth itself, in a century or so. Its cost would be close to a quadrillion dollars, which is a hundred times more than today's Gross World Product. Still, economic growth has its little ways of overcoming such matters. It is worth remembering that in the last century, one million dollars played roughly the same role as does one billion dollars today. The first billion-dollar federal budget came in 1889, and it will be surprising if the 1989 budget is much below a trillion dollars.
We have not yet detected anyone else's Ozymandias, though as yet we have not searched very hard. Still, the lack of success in all searches to date is sufficient to give one pause. In the past thirty years there have been any number of astrophysical phenomena predicted to exist, searched for, and found. There have even been important discoveries like quasars and pulsars, which were entirely accidental. (Pulsars were at first thought to be a cosmic time-standard broadcast service, but this theory was soon disproved.) In all this time there has been a predicted effect that has not occurred: transmissions from other civilizations.
This, then, is the Fermi paradox: We expect to find some clear evidence of the existence and pervasiveness of intelligence as an occurrence in astronomy, and we do not. Not in the rocks, nor in our skies, nor yet with radio telescopes.
We have not succeeded in resolving this paradox; therefore, we must look further. While doing so, one possible answer to this paradox should be kept in mind: We are alone; we are unique, there is none like unto us in this good green garden of our galaxy.