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

star has a planetary system, but the number looks very large. Just for the sake of argument, I'll take the fraction f_p 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, n_p, 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 f_l 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 f_i X f_c 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 f_i X f_c 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.

So there's nobody to talk to except ourselves, and we hardly do that very well. In that case, if you believe that argument, it would be foolish to make a massive or expensive search for extraterrestrial intelligence because even if this number L were a few decades, the number of civilizations would be only a few, and therefore the distance to the nearest one would be enormously far away.

Now let's take another route, the optimistic one. And that is, it seems perfectly possible that we are able to solve the issues of technological adolescence that confront us. And even if there's only a small chance of doing that, say, 1 percent, 1 percent of all those civilizations in the Galaxy living for very long periods of time implies a very large number. Suppose that 1 percent of civilizations lived for evolutionary or geological or stellar evolutionary timescales-say, billions of years. If there's only 1 percent that do that, then the average lifetime would be 1 percent of 10⁹ which is 10⁷, so 10 million years would be the value for L. Multiply that by a tenth and the answer would be a million, 10⁶ civilizations in the galaxy, a vastly different story.

So you can see that while there are significant uncertainties for each of these factors, by far the largest uncertainty, the place we have the least experience (none whatever, as a matter of fact) is in the average lifetime of a technical civilization. And it is this connection of L with the number of civilizations and the distance to the nearest one that in a remarkable way binds up this quite outre subject of extraterrestrial intelligence with the most pressing human concerns. Because it means that the receipt of a message, never mind being able to decode it, from elsewhere would say that L is probably a large number, that someone has been able to survive technological adolescence. It would be knowledge very much worth having.

If there are a million technical civilizations in the Galaxy, then you can readily calculate to first order, just extracting a cube root, the distance to the nearest civilization. If they are randomly distributed through the Galaxy, and we know how many stars there are in a galaxy, how far to the nearest one? And the answer is, it's just a few hundred light-years away. It's next door. It's not next door as far as visiting, but it's next door as far as radio communication.

Now, even a few hundred light-years away means that we must not imagine much in the way of dialogue. It's more monologue. They talk, we listen, because otherwise they would say, let us imagine, 'Hello. How are you?' and we would say, 'Fine, thank you, and you?' and that exchange would take, say, six hundred years. It's not what you might call a snappy conversation.

On the other hand, it's very clear that one-way transmission of information is something that can be enormously valuable. Aristotle talks to us. We do not, except for spiritualists, talk to Aristotle. And I have grave doubts about the spiritualists. (Although Aristotle is almost never on their list of contacts.)

Now, let's therefore say a few more words about this idea of radio communication. What we imagine is that beings on a planet of another star know that emerging civilizations will stumble upon radio. It's part of the electromagnetic spectrum; it is, as I will show you in a moment, a clear channel through the Galaxy. The technology is relatively simple and inexpensive. Radio waves travel at the speed of light, faster than which nothing can go, so far as we know. The information that can be transmitted is enormous, not just 'Hello, how are you?' Put another way, if an identical system were at the center of the Galaxy and we were here using our present detection technology, we could pick up that signal coming from thousands of light-years away. It gives you an idea of the remarkable power of this technology, which has in fact been only lately brought up to its actual capabilities.

There is a question of frequency. What channel would you listen on? There's an enormous number of possible radio frequencies. We have here the radio frequency spectrum in gigahertz, thousands of millions of cycles per second, against the noise background from various sources in degrees absolute. And what we see is that at the low frequencies there is a background from charged particles in magnetic fields in the Galaxy, the galactic background. It's noise. And it gets to be very substantial noise.