by Carl Sagan
I would like to describe a famous case of the search for extraterrestrial intelligence-the search for beings more advanced than we-a case that failed. I want to explore why it failed, what lessons we can learn from this failure, and then move on to the modern search for extraterrestrial intelligence. I hope to stress where we have to be extremely careful, where we must demand the most stringent and rigorous standards of evidence precisely because we have profound emotional investments in the answer. Later I will attempt to use those skeptical strictures to apply more directly to the more conventional God hypothesis.
I suppose an equally good epigram for this subject is the following sentence said by John Adams, second president of the United States, but long before he was that. As a lawyer and advocate, he argued in defense of the British soldiers who were being tried at the Boston Massacre trials in December 1770. And he did this not because he was in favor of the British cause. He wasn't. He defended those he opposed because he believed that the truth should be pursued above all other considerations. He said, "Facts are stubborn things; and whatever may be our wishes, our inclinations, or the dictates of our passions, they cannot alter the state of facts and evidence." Well, sometimes they can, but we hope they can't.
The year is 1877, let us imagine. The motion of the Earth around the Sun and Mars around the Sun has brought Mars and the Earth close together, as they tend to be at intervals of roughly seventeen years.
An Italian astronomer named Giovanni Schiaparelli, looking through a newly completed and fairly large aperture telescope in Italy, was glancing at Mars and suddenly saw the surface of the planet reveal a profusion of intricate, fine, linear detail that a later observer described as being like the lines in a fine steel etching. Schiaparelli promptly called these lines canali, an Italian word meaning "channels" or "grooves." We can understand how it was translated into English as "canals," a word with a clear imputation of design, of intelligence, of vast engineering works constructed for a reason. The idea of canali on Mars was taken up by an American astronomer named Percival Lowell, a wealthy Bostonian. Lowell constructed a major observatory with funds out of his own pocket, near Flagstaff, Arizona, called, naturally, the Lowell Observatory, to study these markings.
Lowell was convinced that Schiaparelli was right, that the planet was covered by a network of intersecting single and double straight lines, that these lines passed over enormous distances and therefore could correspond only to engineering works on the most massive imaginable scale. Other observers also found the canals; that is, drew them. Photographing them was much more difficult. The argument was that atmospheric "seeing" was unreliable, due to the intrinsic turbulence and unsteadiness of the Earth's atmosphere, which generally prevent you from seeing the canals. But every now and then, by chance, the atmosphere steadies, the turbulent eddies of air are not in your line of sight to Mars, and just for a moment you can see the planet as it truly is with this network of straight lines. And then another bit of atmospheric turbulence comes by and the planetary image becomes shimmery and the details are lost. Lowell reasoned that a photograph, which involves a time exposure that adds up the rare moments of good seeing with the much more plentiful moments of bad seeing, would not reveal the canals. But the human eye can remember those instants of excellent seeing and reject the other moments, much more common, when the image is fading and blurring and distorting. And this is why, he argued, experienced observers skilled in drawing what they see at the telescope could obtain results that the photographic emulsion could not.
There were other astronomers who, for the life of them, couldn't see the straight lines, but there was a range of explanations: They were not in the best sites for their telescopes. They were not experienced observers. They were not adequate draftsmen. They were biased against the idea of canals on Mars.
Lowell and Schiaparelli were by no means the only astronomers who could find the canals. Astronomers all over the world saw them, drew them, mapped them, named them. And there were literally hundreds of individual canals that were named.
There was a point of view that said that the canals 'were not really on Mars, that they represented some sophisticated failure of the human hand-eye-brain combination, that Lowell and his confreres were too carried away by the power of the idea. Lowell, who was a superb popular expositor, dismissed these objections in various ways and pointed to the remarkable similarity of the maps that he had drawn to those that other independent observers had drawn, say, for example, W. H. Wright at the Lick Observatory. Lowell argued that this convergence by quite separate observers, with no prior collusion, onto the same pattern of straight lines could only be due to something on Mars, not to something on the Earth. Lowell deduced from these straight lines an ancient civilization on Mars more advanced than we, having to face a planetary drought of proportions unprecedented on Earth. And their solution was to construct a vast, globe-girdling network of canals to carry liquid water from the melting polar caps to the thirsty inhabitants of the equatorial cities. What's more, it was possible to conclude, Lowell thought, something of the politics of the Martians, because the network crossed the entire planet. Therefore there was a world government on Mars, at least as far as engineering detail went. And Lowell went so far as to be able to identify the capital of Mars, a particular spot on the surface called Solis Lacus, the Lake of the Sun, from which six or eight different canals seemed to emanate.
Now, this is a lovely story. It passed into the popular consciousness, into folk literature, was most powerfully impressed on the global consciousness through H. G. Wells's War of the Worlds, through a set of science-fiction novels by Edgar Rice Burroughs (the man who invented Tarzan), and then in 1958 by Orson Welles's "War of the Worlds," broadcast in America on the eve of the Nazi invasion of Europe, at a time when fears of a distinctly terrestrial, not extraterrestrial, invasion were in everybody's mind.
And yet there are no canals on Mars. Not one. The whole thing is wrong. It's a mistake. It is a failure of the human hand-eye-brain combination. Lowell's idea evoked a passion, I think a very understandable and humane passion. The vision of more advanced beings on a neighboring planet, with a world government, struggling to keep themselves alive, was a wonderful idea. It was so wonderful that the wish to believe it trumped the scrupulousness of the investigative process.
So what can we conclude from this? Well, we can conclude that in a sense Lowell was right, that the canals of Mars are a sign of intelligent life. The only question is which side of the telescope the intelligent life is on. And as we see, the intelligent life was on our end of the telescope. People staked their careers on an observable phenomenon, apparently reproducible by others in quite different parts of the world. A huge public concern and interest were generated. This was only one of several different arguments for intelligent life on Mars today, all of which are mistaken.
If scientists can be fooled on the question of the simple interpretation of straightforward data of the sort that they are routinely obtaining from other kinds of astronomical objects, when the stakes are high, when the emotional predispositions are working, what must be the situation where the evidence is much weaker, where the will to believe is much greater, where the skeptical scientific tradition has hardly made a toehold- namely, in the area of religion?
Let's think about the question of extraterrestrial intelligence. There are several approaches. There is one that says, well, it is a vast universe. There must be beings much smarter than we are. They must have capabilities vastly in excess of ours. Therefore they should be able to come here. If we are poking around in neighboring 'worlds in our planetary system, then should not intelligent beings elsewhere in our solar system, as Lowell thought, or in other planetary systems, of which we now know there are many, shouldn't they be visiting here? And that then takes us to the issue of unidentified flying objects and ancient astronauts, which we will get to. But here I would like to concentrate on what is now the mainstream scientific approach to the issue of extraterrestrial intel
ligence, one that I should say from the beginning I have been deeply involved in and support wholeheartedly. But at the same time I think it sheds light on this question of what is suitable evidence and what isn't.
At what moment do you say that the evidence is sufficient to deduce the presence of extraterrestrial intelligence? I believe that while the details are slightly different, the argument is not significantly different from the question, what would be convincing evidence of the existence of an angel or a demigod or a god? First off, there's the question, is it plausible? That is, whatever you do to search for extraterrestrial intelligence, it is going to cost some money. You want a plausibility argument first that it makes at least a little sense. Clearly, were we to find extraterrestrial intelligence, this would be a discovery of enormous importance scientifically, philosophically, and, I maintain, theologically. But you'd want to have some expectation of success, some argument to counter skeptics who might say, "There is no evidence that we have been visited; therefore it is a waste of time."
So what we would really like to know is how many sites of intelligent beings, more intelligent than we, there are in, say, the Milky Way Galaxy? And how far is it from here to the nearest one? If it turns out that the nearest one is some immense distance away-let's say, at the center of the Milky Way Galaxy, 30,000 light-years-then we might conclude that the prospects of contact are small. On the other hand, if it turns out that the nearest such civilization is relatively nearby-let's say, a few tens or even a few hundreds of light-years-then it might make sense in some way, which I'll go into, to try to search for it.
Now, a convenient approach to this issue (it is hardly precise) is what is called the Drake equation, after the astronomer Frank Drake, who has been a pioneer in the scientific approach to this question. And it goes roughly like this: There is a number, call it N, of technical civilizations in the Galaxy, civilizations with the technology to permit interstellar contact (that technology essentially is radio astronomy). That number is
N = R X fp X np X fl X fi X fc X L
the product of a set of factors, each of which I will define. (All that is involved in this equation is the idea that a collective probability is the product of the individual probabilities, quite like what we were talking about earlier on the probability that the right amino acid is in the first slot in the protein, and in the second slot, and in the third slot, and then you multiply those probabilities. The chance that you'll get heads in the first coin toss is one-half, the chance that you'll get heads in the second toss is one-half, the chance that you will get two consecutive heads is a quarter, three consecutive heads is an eighth, and so on.)
So the number of such civilizations depends on the rate of star formation, which we call R. The more stars that are formed, the more potential abodes for life there will be if they have planetary systems. That seems clear. Multiply that figure times fp, the fraction of stars that have planetary systems. But it's not good enough just to have planets; they have to be suitable for life. So multiply by n, the number of planets in an average system that are ecologically suitable for the origin of life, then times fl, the fraction of such worlds in which life actually arises, times fi, the fraction of such worlds in which over their lifetime intelligent life evolves, times fc, the fraction of such worlds in which the intelligent life develops a technical communicative capability, times L, the lifetimes of the technical civilization, because clearly if civilizations destroy themselves as soon as they are formed, everything else may go swimmingly well and yet there would be nobody for us to talk to.
So let me give my wild guesses about what these numbers are. I stress that we don't know these numbers very well, that our uncertainty progressively increases as we go from the leftmost to the rightmost factor. And that the largest uncertainty by far is in L, the lifetime of a technical civilization.
There are some hundred thousand million stars in the Milky Way Galaxy.
The lifetime of the Milky Way Galaxy is something like ten thousand million years, and therefore a modest average estimate of the rate of star formation is about ten stars per year. A very interesting number, that, by itself. Every year there are ten new suns that are born in the Milky Way Galaxy, and many of them, probably, with planetary systems. And billions of years from now, maybe they will have life.
On the question of the fraction of stars that have planets going around them, I previously talked about the burgeoning recent evidence from ground-based and space-based observatories for planetary systems, both those just forming and ones that are fully formed around nearby stars. The statistics are remarkable. The IRAS satellite data alone suggest that something like a quarter of nearby main sequence stars a little younger than the Sun have something like a solar nebula in the process of formation. It's an amazingly large number. And any of them that have fully formed planetary systems we can detect only in certain special cases. You would not expect that every star has a planetary system, but the number looks very large. Just for the sake of argument, I'll take the fraction fp to be something like a half. Now consider the number of planets per system that are in principle suitable for the origin of life. Well, certainly in our system, we know at least one, the Earth. And good arguments can be made that it is possible on other planets, on other bodies. We talked about Titan. There is an argument for Mars. Not to pretend any kind of accuracy, but just so that we can put in numbers that easily multiply each other, let us take that number, np, as two.
The fraction of ecologically suitable planets in which life actually does arise over a period of hundreds of millions or thousands of millions of years, I will take to be very high, on the basis of the sorts of arguments I made earlier, especially the speed with which the origin of life seemed to have happened on this planet. So I'll take fl to be around one.
And now we come to more difficult numbers. Life has arisen on a given planet, and you have thousands of millions of years in which the environment is somewhat stable. How likely is it that intelligence and technical civilizations arise? On the one hand, we might argue that there are a sequence of individually unlikely events that must happen for humans to evolve. For example, the dinosaurs had to be extinguished, because they were the dominant organisms on the planet and our ancestors in the times of the dinosaurs were furry, scurrying, burrowing creatures, about the size of mice. And it is only because of the extinction of the dinosaurs that our ancestors could get going. And the extinction of the dinosaurs seems to have been caused by an immense collision by an asteroid or cometary nucleus with the Earth some 65 million years ago at the end of the Cretaceous period. That is a statistical event, and if that had not happened, maybe I would be ten feet tall with green scales and sharp, pointy teeth, and you would be similarly tall and green and pointy-toothed. We would both likely consider ourselves extremely attractive. What handsome fellows we are. And how strange it would seem if I proposed that had things gone differently, then the little mice that bother us might have evolved to become the dominant organism, and the only remnants of us would be salamanders and crocodiles and birds. That's on the one hand.
On the other hand, there is no reason to think that there is only one path to intelligent life. The selective advantage of intelligence is clearly high. Other things being equal, if you can figure the world out, you have a better chance of survival. At least until the invention of nuclear weapons.
Human brains comprise a significant fraction of our body mass, more than for almost any other animal on the planet. And this then suggests a progressive development of brains to figure out the world. The more data processing, the more chances for survival we have. There is no reason to think that this is a peculiarly human situation, and it ought to be true on other planets as well.
So then we come to the question, if you have intelligent life, is it guaranteed to develop technical civilizations? Clearly not. The dolphins and whales are intelligent, based on many different anecdotal accounts and on the argument of brain-mass-to-body-mass ratio, and yet they have built nothing
, because they don't have hands and they live in a different environment than we do.
It is easy to imagine a world full of poets who do not build radio telescopes. They're very smart, but we don't hear from them. So not every intelligent life-form need be technological or communicative. What this product of fi X fc is, no one really knows. We can certainly point out that it took most of the history of the Earth before Ornithoides or Cetacea or primates developed. They all developed in the last few tens of millions of years. Why did it take so long? Well, there's probably a certain degree of complexity that is essential for being able to figure things out.
On the other hand, the Earth and the solar system have thousands of millions of years more ahead of them, as do other planets as well. A number for fi X fc that I believe to be modest is 1/100-1 percent. (I do not at all say that I know what these numbers are; these are merely rough estimates to collect the various uncertainties together. I do not claim this is holy writ.) If we multiply these numbers together, 10 X 1/2 X 2 x 1 X 1/100, the product is a tenth. So the number TV of technical civilizations in our galaxy would be one-tenth times their average life-time L in years. (L is in years because R was ten stars per year, and the product must not have any years in it, just the number of civilizations.)
So what is L? What is the lifetime of a technical civilization? We have had radio telescopes for only the last few decades. An argument could be made by reading the daily newspapers, among other things, that our civilization is in great peril. And therefore that, for the Earth at least, the lifetime of a technical civilization in this sense is a decade or a few decades. And if that number were typical for civilizations in general, then L would be, let's say, a decade, ten years. So let's call this the most pessimistic route. A tenth times ten is one, and the number of technical civilizations in the galaxy would be one. Where is it? It's us.