The Varieties of Scientific Experience: A Personal View of the Search for God

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 intelligence, 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 f_p X n_p X f_l X f_i X f_c 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 f_p, 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 f_l, the fraction of such worlds in which life actually arises, times f_i, the fraction of such worlds in which over their lifetime intelligent life evolves, times f_c, 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