miraculous than the fact
which it endeavors to establish.
DAVID HUME,
HUMANITY HAS already achieved interstellar spaceflight. With a gravitational assist from the planet Jupiter, the Pioneer 10 and 11 and the Voyager 1 and 2 spacecraft have been boosted into trajectories that will leave the solar system for the realm of the stars. They are very slow-moving spacecraft despite the fact that they are the fastest objects ever launched by our species. They will take tens of thousands of years to travel typical interstellar distances. Unless some special effort is made to redirect them, they will never enter another planetary system in all the tens of billions of years of future history of the Milky Way Galaxy. The star-to-star distances are too large. They are doomed to wander forever in the dark between the stars. But even so, these spacecraft have messages attached to them for the remote contingency that at some future time, alien beings might intercept the spacecraft and wonder about the beings who launched them on these prodigious journeys. [3]
If we are capable of such constructions at our comparatively backward technological state, might not a civilization thousands or millions of years more advanced than ours, on a planet of another star, be capable of fast and directed interstellar travel? Interstellar spaceflight is time-consuming, difficult and expensive for us, and perhaps also for other civilizations with substantially greater resources than ours. But it surely would be unwise to contend that conceptually novel approaches to the physics or engineering of interstellar spaceflight will not be discovered by us sometime in the future. It is evident that for economy, efficiency and convenience, interstellar radio transmission is much superior to interstellar spaceflight, and this is the reason that our own efforts have concentrated strongly on radio communication. But radio communication is clearly inappropriate for contact with a pretechnological society or species. No matter how clever or powerful the transmission, no such radio message would have been received or understood on Earth before the present century. And there has been life on our planet for about four billion years, human beings for several million, and civilization for perhaps ten thousand.
It is not inconceivable that there is a kind of Galactic Survey, established by cooperating civilizations on many planets throughout the Milky Way Galaxy, which keeps an eye (or some equivalent organ) on emerging planets and seeks out undiscovered worlds. But the solar system is very far from the center of the Galaxy and could well have eluded such searches. Or survey ships may come here, but only every ten million years, say-with none having arrived during historical times. However, it is also possible that a few survey teams have arrived recently enough in human history for their presence to have been noted by our ancestors, or even for human history to have been affected by the contact.
The Soviet astrophysicist I. S. Shklovskii and I discussed this possibility in our book,
In the long litany of “ancient astronaut” pop archaeology, the cases of apparent interest have perfectly reasonable alternative explanations, or have been misreported, or are simple prevarications, hoaxes and distortions. This description applies to arguments about the Piri Reis map, the Easter Island monoliths, the heroic drawings on the plains of Nazca, and various artifacts from Mexico, Uzbekistan and China.
And yet, it would be so easy for an advanced extraterrestrial civilization to leave a completely unambiguous calling card of their visit. For example, many nuclear physicists believe that there is an “island of stability” of atomic nuclei, near a hypothetical superheavy atom with about 114 protons and about 184 neutrons. All chemical elements heavier than uranium (with 238 protons and neutrons in its nucleus) spontaneously decay in cosmically short periods of time. But there is reason to think that the binding between protons and neutrons is such that stable elements would be produced if nuclei having about 114 protons and 184 neutrons could be constructed. Such a construction is just beyond our present technology, and clearly beyond the technology of our ancestors. A metal artifact containing such elements would be unambiguous evidence of an advanced extraterrestrial civilization in our past. Or consider the element technetium, whose most stable form has 99 protons and neutrons. Half of it radioactively decays to other elements in about 200,000 years, half of the remainder is gone in another 200,000 years, and so on. As a result, any technetium formed by stars with the other elements billions of years ago must all be gone by now. Thus, terrestrial technetium can only be of artificial origin, as its very name indicates. A technetium artifact could have only one meaning. Similarly, there are common elements on Earth that are immiscible; for example, aluminum and lead. If you melt them together, the lead, being considerably heavier, sinks to the bottom. The aluminum floats to the top. However, in the zero g conditions of spaceflight there is no gravity in the melt to pull the heavier lead down, and exotic alloys such as AL/Pb can be produced. One of the objectives of NASA’s early Shuttle missions will be to test out such alloying techniques. Any message written on an aluminum/lead alloy and retrieved from an ancient civilization would certainly commend itself to our attention today.
It is also possible that the content rather than the material of the message would clearly point to a science or technology beyond the abilities of our ancestors: for example, a vector calculus rendition of Maxwell’s equations (with or without magnetic monopoles), or a graphical representation of the Planck black-body distribution for several different temperatures, or a derivation of the Lorentz transformation of special relativity. Even if the ancient civilization could not understand such writings, they might revere them as holy. But no cases of this sort have emerged-despite what is clearly a profitable market for tales of ancient or modern extraterrestrial astronauts. There have been debates on the purity of magnesium samples from purported crashed UFOs, but their purity was within the competence of American technology at the time of the incident. A supposed star map said to be retrieved (from memory) from the interior of a flying saucer does not, as alleged, resemble the relative positions of the nearest stars like the Sun; in fact, a close examination shows it to be not much better than the “star map” which would be produced if you took an old-fashioned quill pen and splattered a few blank pages with ink spots. With one apparent exception, there are no stories sufficiently detailed to dispose of other explanations and sufficiently accurate to portray correctly modern physics or astronomy to a prescientific or pretechnical people. The one exception is the remarkable mythology surrounding the star Sirius that is held by the Dogon people of the Republic of Mali.
There are at most a few hundred thousand Dogon alive today, and they have been studied intensively by anthropologists only since the 1930s. There are some elements of their mythology that are reminiscent of the legends of the ancient Egyptian civilization, and some anthropologists have assumed a weak Dogon cultural connection with ancient Egypt. The helical risings of Sirius were central to the Egyptian calendar and used to predict the inundations of the Nile. The most striking aspects of Dogon astronomy have been recounted by Marcel Griaule, a French anthropologist working in the 1930s and 1940s. While there is no reason to doubt Griaule’s account, it is important to note that there is no earlier Western record of these remarkable Dogon folk beliefs and that all the information has been funneled through Griaule. The story has recently been popularized by a British writer, R. K. G. Temple.
In contrast to almost all prescientific societies, the Dogon hold that the planets as well as the Earth rotate about their axes and revolve about the Sun. This is a conclusion that can, of course, be achieved without high technology, as Copernicus demonstrated, but it is a very rare insight among the peoples of the Earth. It was taught, however, in ancient Greece by Pythagoras and by Philolaus, who perhaps held, in Laplace’s words, “that the planets were inhabited and that the stars were suns, disseminated in space, being themselves centers of planetary systems.” Such teachings, among a wide variety of contradictory ideas, might be just an inspired lucky guess.
The ancient Greeks believed there were only four elements-earth, fire, water and air-from which all else was constructed. Among the pre-Socratic philosophers there were those who made special advocacy for each one of these elements. If it had later turned out that the universe was indeed made more of one of these elements than
