There may be other effective methods of communication that have substantial merit: interstellar spacecraft; optical or infrared lasers; pulsed neutrinos; modulated gravity waves; or some other kind of transmission that we will not discover for a thousand years. Advanced civilizations may have graduated far beyond radio for their own communications. But radio is powerful, cheap, fast and simple. They will know that a backward civilization like ours,, wishing to receive messages from the skies, is likely to turn first to radio technology. Perhaps they will have to wheel the radio telescopes out of the Museum of Ancient Technology. If we were to receive a radio message we would know that there would be at the very least one thing we could talk about: radio astronomy.
But is there anyone out there to talk to? With a third or half a trillion stars in our Milky Way Galaxy alone, could ours be the only one accompanied by an inhabited planet? How much more likely it is that technical civilizations are a cosmic commonplace, that the Galaxy is pulsing and humming with advanced societies, and, therefore, that the nearest such culture is not so very far away – perhaps transmitting from antennas established on a planet of a naked-eye star just next door. Perhaps when we look up at the sky at night, near one of those faint pinpoints of light is a world on which someone quite different from us is then glancing idly at a star we call the Sun and entertaining, for just a moment, an outrageous speculation.
It is very hard to be sure. There may be severe impediments to the evolution of a technical civilization. Planets may be rarer than we think. Perhaps the origin of life is not so easy as our laboratory experiments suggest. Perhaps the evolution of advanced life forms is improbable. Or it may be that complex life forms evolve readily, but intelligence and technical societies require an unlikely set of coincidences – just as the evolution of the human species depended on the demise of the dinosaurs and the ice-age recession of the forests in whose trees our ancestors screeched and dimly wondered. Or perhaps civilizations arise repeatedly, inexorably, on innumerable planets in the Milky Way, but are generally unstable; so all but a tiny fraction are unable to survive their technology and succumb to greed and ignorance, pollution and nuclear war.
It is possible to explore this great issue further and make a crude estimate of N, the number of advanced technical civilizations in the Galaxy. We define an advanced civilization as one capable of radio astronomy. This is, of course, a parochial if essential definition. There may be countless worlds on which the inhabitants are accomplished linguists or superb poets but indifferent radio astronomers. We will not hear from them. N can be written as the product or multiplication of a number of factors, each a kind of filter, every one of which must be sizable for there to be large number of civilizations:
N*, the number of stars in the Milky Way Galaxy;
fp, the fraction of stars that have planetary systems;
ne, the number of planets in a given system that are ecologically suitable for life;
fl, the fraction of otherwise suitable planets on which life actually arises;
fi, the fraction of inhabited planets on which an intelligent form of life evolves;
fc, the fraction of planets inhabited by intelligent beings on which a communicative technical civilization develops; and
fL, the fraction of a planetary lifetime graced by a technical civilization.
Written out, the equation reads N = N*fpneflfifcfL. All the f’s are fractions, having values between 0 and 1; they will pare down the large value of N*.
To derive N we must estimate each of these quantities. We know a fair amount about the early factors in the equation, the numbers of stars and planetary systems. We know very little about the later factors, concerning the evolution of intelligence or the lifetime of technical societies. In these cases our estimates will be little better than guesses. I invite you, if you disagree with my estimates below, to make your own choices and see what implications your alternative suggestions have for the number of advanced civilizations in the Galaxy. One of the great virtues of this equation, due originally to Frank Drake of Cornell, is that it involves subjects ranging from stellar and planetary astronomy to organic chemistry, evolutionary biology, history, politics and abnormal psychology. Much of the Cosmos is in the span of the Drake equation.
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 4x 1011. 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, fp, as roughly equal to 1/3 . Then the total number of planetary systems in the Galaxy would be N*fp » 1.3 x 1011 (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 ne = 2. Then the number of planets in the Galaxy suitable for life becomes N*fpne » 3 x 1011.
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 fl » 1/3, implying a total number of planets in the Milky Way on which life has arisen at least once as N*fpnefl » 1 x 1011, a hundred billion inhabited worlds. That in itself is a remarkable conclusion. But we are not yet finished.
The choices of fi and fc 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 fi x fc = 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*fpneflfifc » 1 x 109, 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 fL.
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 fL, is less than 1/108, 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*fpneflfifcfL » 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.