Authors: Carl Sagan
Cosmos (46 page)
We know N
*
, the number of stars in the Milky Way Galaxy, fairly well, by careful counts of stars in small but representative regions of the sky. It is a few hundred billion; some recent estimates place it at 4 × 10
11
. Very few of these stars are of the massive short-lived variety that squander their reserves of thermonuclear fuel. The great majority have lifetimes of billions or more years in which they are shining stably, providing a suitable energy source for the origin and evolution of life on nearby planets.
There is evidence that planets are a frequent accompaniment of star formation: in the satellite systems of Jupiter, Saturn and Uranus, which are like miniature solar systems; in theories of the origin of the planets; in studies of double stars; in observations of accretion disks around stars; and in some preliminary investigations of gravitational perturbations of nearby stars. Many, perhaps even most, stars may have planets. We take the fraction of stars that have planets, f
p
, as roughly equal to ⅓. Then the total number
of planetary systems in the Galaxy would be N
*
f
p
≃ 1.3 × 10
11
(the symbol ≃ means “approximately equal to”). If each system were to have about ten planets, as ours does, the total number of worlds in the Galaxy would be more than a trillion, a vast arena for the cosmic drama.
In our own solar system there are several bodies that may be suitable for life of some sort: the Earth certainly, and perhaps Mars, Titan and Jupiter. Once life originates, it tends to be very adaptable and tenacious. There must be many different environments suitable for life in a given planetary system. But conservatively we choose n
e
= 2. Then the number of planets in the Galaxy suitable for life becomes N
*
f
p
n
e
≃ 3 × 10
11
.
Experiments show that under the most common cosmic conditions the molecular basis of life is readily made, the building blocks of molecules able to make copies of themselves. We are now on less certain ground; there may, for example, be impediments in the evolution of the genetic code, although I think this unlikely over billions of years of primeval chemistry. We choose f
1
≃ ⅓, implying a total number of planets in the Milky Way on which life has arisen at least once as N
*
f
p
n
e
f
1
≈ 1 × 10
11
, a hundred billion inhabited worlds. That in itself is a remarkable conclusion. But we are not yet finished.
The choices of f
i
and f
c
are more difficult. On the one hand, many individually unlikely steps had to occur in biological evolution and human history for our present intelligence and technology to develop. On the other hand, there must be many quite different pathways to an advanced civilization of specified capabilities. Considering the apparent difficulty in the evolution of large organisms represented by the Cambrian explosion, let us choose f
i
× f
c
= 1/100, meaning that only 1 percent of planets on which life arises eventually produce a technical civilization. This estimate represents some middle ground among the varying scientific opinions. Some think that the equivalent of the step from the emergence of trilobites to the domestication of fire goes like a shot in all planetary systems; others think that, even given ten or fifteen billion years, the evolution of technical civilizations is unlikely. This is not a subject on which we can do much experimentation as long as our investigations are limited to a single planet. Multiplying these factors together, we find N
*
f
p
n
e
f
l
f
i
f
c
≈ 1 × 10
9
, a billion planets on which technical civilizations have arisen at least once. But that is very different from saying that there are a billion planets on which technical civilizations now exist. For this, we must also estimate f
L
.
What percentage of the lifetime of a planet is marked by a technical civilization? The Earth has harbored a technical civilization characterized by radio astronomy for only a few decades out of a lifetime of a few billion years. So far, then, for our planet f
L
is less than 1/10
8
, a millionth of a percent. And it is hardly out of the question that we might destroy ourselves tomorrow. Suppose this were to be a typical case, and the destruction so complete that no other technical civilization—of the human or any other species—were able to emerge in the five or so billion years remaining before the Sun dies. Then N = N
*
f
p
f
l
f
i
f
c
f
L
≈ 10, and at any given time there would be only a tiny smattering, a handful, a pitiful few technical civilizations in the Galaxy, the steady state number maintained as emerging societies replace those recently self-immolated. The number N might even be as small as 1. If civilizations tend to destroy themselves soon after reaching a technological phase, there might be no one for us to talk with but ourselves. And that we do but poorly. Civilizations would take billions of years of tortuous evolution to arise, and then snuff themselves out in an instant of unforgivable neglect.
But consider the alternative, the prospect that at least some civilizations learn to live with high technology; that the contradictions posed by the vagaries of past brain evolution are consciously resolved and do not lead to self-destruction; or that, even if major disturbances do occur, they are reversed in the subsequent billions of years of biological evolution. Such societies might live to a prosperous old age, their lifetimes measured perhaps on geological or stellar evolutionary time scales. If 1 percent of civilizations can survive technological adolescence, take the proper fork at this critical historical branch point and achieve maturity, then f
L
≈ 1/100, N ≈ 10
7
, and the number of extant civilizations in the Galaxy is in the millions. Thus, for all our concern about the possible unreliability of our estimates of the early factors in the Drake equation, which involve astronomy, organic chemistry and
evolutionary biology, the principal uncertainty comes down to economics and politics and what, on Earth, we call human nature. It seems fairly clear that if self-destruction is not the overwhelmingly preponderant fate of galactic civilizations, then the sky is softly humming with messages from the stars.
These estimates are stirring. They suggest that the receipt of a message from space is, even before we decode it, a profoundly hopeful sign. It means that someone has learned to live with high technology; that it is possible to survive technological adolescence. This alone, quite apart from the contents of the message, provides a powerful justification for the search for other civilizations.
If there are millions of civilizations distributed more or less randomly through the Galaxy, the distance to the nearest is about two hundred light-years. Even at the speed of light it would take two centuries for a radio message to get from there to here. If we had initiated the dialogue, it would be as if the question had been asked by Johannes Kepler and the answer received by us. Especially because we, new to radio astronomy, must be comparatively backward, and the transmitting civilization advanced, it makes more sense for us to listen than to send. For a more advanced civilization, the positions are, of course, reversed.
We are at the earliest stages of our radio search for other civilizations in space. In an optical photograph of a dense star field, there are hundreds of thousands of stars. By our more optimistic estimates, one of them is the site of an advanced civilization. But which one? Toward which stars should we point our radio telescopes? Of the millions of stars that may mark the location of advanced civilizations, we have so far examined by radio no more than thousands. We have made about one-tenth of one percent of the required effort. But a serious, rigorous, systematic search will come soon. The preparatory steps are now underway, both in the United States and in the Soviet Union. It is comparatively inexpensive: the cost of a single naval vessel of intermediate size—a modern
destroyer, say—would pay for a decade-long program in the search for extraterrestrial intelligence.
Benevolent encounters have not been the rule in human history, where transcultural contacts have been direct and physical, quite different from the receipt of a radio signal, a contact as light as a kiss. Still, it is instructive to examine one or two cases from our past, if only to calibrate our expectations: Between the times of the American and the French Revolutions, Louis XVI of France outfitted an expedition to the Pacific Ocean, a voyage with scientific, geographic, economic and nationalistic objectives. The commander was the Count of La Pérouse, a noted explorer who had fought for the United States in its War of Independence. In July 1786, almost a year after setting sail, he reached the coast of Alaska, a place now called Lituya Bay. He was delighted with the harbor and wrote: “Not a port in the universe could afford more conveniences.” In this exemplary location, La Pérouse
perceived some savages, who made signs of friendship, by displaying and waving white mantles, and different skins. Several of the canoes of these Indians were fishing in the Bay.… [We were] continually surrounded by the anoes of the savages, who offered us fish, skins of otters and other animals, and different little articles of their dress in exchange for our iron. To our great surprise, they appeared well accustomed to traffic, and bargained with us with as much skill as any tradesman of Europe.
The Native Americans drove increasingly harder bargains. To La Pérouse’s annoyance, they also resorted to pilferage, largely of iron objects, but once of the uniforms of French naval officers hidden under their pillows as they were sleeping one night surrounded by armed guards—a feat worthy of Harry Houdini. La Pérouse followed his royal orders to behave peaceably but complained that the natives “believed our forbearance inexhaustible.” He was disdainful of their society. But no serious damage was done by either culture to the other. After reprovisioning his two ships La Pérouse sailed out of Lituya Bay, never to return. The expedition was lost in the South Pacific in 1788; La Pérouse and all but one of the members of his crew perished.
*
Exactly a century later Cowee, a chief of the Tlingit, related to the Canadian anthropologist G. T. Emmons a story of the first meeting of his ancestors with the white man, a narrative handed down by word of mouth only. The Tlingit possessed no written records, nor had Cowee ever heard of La Pérouse. This is a paraphrase of Cowee’s story:
Late one spring a large part of Tlingit ventured North to Yakutat to trade for copper. Iron was even more precious, but it was unobtainable. In entering Lituya Bay four canoes were swallowed by the waves. As the survivors made camp and mourned for their lost companions two strange objects entered the Bay. No one knew what they were. They seemed to be great black birds with immense white wings. The Tlingit believed the world had been created by a great bird which often assumed the form of a raven, a bird which had freed the Sun, the Moon, and the stars from boxes in which they had been imprisoned. To look upon the Raven was to be turned to stone. In their fright, the Tlingit fled into the forest and hid. But after a while, finding that no harm had come to them, a few more enterprising souls crept out and rolled leaves of the skunk cabbage into crude telescopes, believing that this would prevent being turned to stone. Through the skunk cabbage, it seemed that the great birds were folding their wings and that flocks of small black messengers arose from their bodies and crawled upon their feathers.
Now one nearly blind old warrior gathered the people together and announced that his life was far behind him; for the common good he would determine whether the Raven would turn his children into stone. Putting on his robe of sea otter fur, he entered his canoe and was paddled seaward to the Raven. He climbed upon it and heard strange voices. With his impaired vision he could barely make out the many black forms moving before him. Perhaps they were crows. When he returned safely to his people they crowded about him, surprised to see him alive. They touched him and smelled him to see if it was really he. After much thought the old man convinced himself that it was not the god-raven that he had visited, but rather a giant canoe made by men. The black figures were not crows but people of a different sort. He convinced the Tlingit, who then visited the ships and exchanged their furs for many strange articles, chiefly iron.