Coming of Age in the Milky Way (56 page)

The countess has her doubts: “You have made the world so large,” she says, “that I know not where I am, or what will become of me. … I protest it is dreadful.”

“Dreadful, Madam?” Fontenelle replies. “I think it very pleasant. When the heavens were a little blue arch, stuck with stars, I thought the universe was too strait and close, I was almost stifled for want of air. But now it is enlarged in height and breadth…. I begin to breathe with more freedom, and think the universe to be incomparably more magnificent than it was before.”
⁸

Not until the latter half of the twentieth century did it become possible to actually begin searching for life on other worlds. One way to do this was to send spacecraft to other planets in the solar system. This endeavor began with the American Pioneer and Soviet Venera missions to Venus in the 1960s, and continued with unmanned
American missions to Mars and the Jovian planets in the following decades. The results of these and other preliminary reconnaissances were negative: Photographs taken by unmanned Soviet landers turned up no trace of life on Venus, which has a thick atmosphere but is hotter than Dante’s hell, and two landers dispatched to the surface of Mars by the American Viking project recorded no obvious sign of Martian life, either. But these, of course, were insufficient grounds upon which to reach a conclusion about the prevalence of extraterrestrial life in general, since the solar system harbors fewer than one ten-billionth of the total number of planets estimated to exist in our galaxy alone.

One might search for life beyond the solar system by traveling to the stars, but to do so within any reasonable amount of time is a very tall order indeed. The stars are just too far away: A spacecraft capable of traveling a million miles per hour—and this would be a stunningly fast ship, one that could fly from Earth to Mars in less than two days—would take nearly
three thousand years
to reach Alpha Centauri, the nearest star. If the expeditionaries proceeded to the next promising star—Delta Pavonis, spectral class F8, would be a reasonable choice—and then hastened on to, say, Beta Hydri, and then kept going to Zeta Tucanae before stopping for a well-earned rest, they would have succeeded in visiting about one one-hundred billionth of the stars in the galaxy—a sample statistically less significant than attempting to understand all Shakespeare’s writings by examining only two letters from one of his sonnets. What is more, the trip would have taken over thirty thousand years, which is a very long time; thirty thousand years ago, our Paleolithic ancestors were carving the world’s first wooden drums.

There is, however, a better method of searching for intelligent life beyond the solar system. It is to employ radiotelescopes to listen for electromagnetic signals—radio or television transmissions—beamed into space by alien civilizations. Such a signal, transmitted using only a few cents’ worth of electricity, travels at the speed of light and could be intercepted by radiotelescopes on Earth across distances of many thousands of light-years. This was the realization behind what came to be called SETI—the search for extraterrestrial intelligence.

SETI was first proposed in 1959 by two scientists, Giuseppe Cocconi and Philip Morrison. “The probability of success is difficult to estimate,” they noted, “but if we never search, the chance of
success is zero.”
⁹
The first SETI experiment, Project Ozma, was conducted in the early 1970s by the American astronomer Frank Drake. Drake observed a total of 659 stars over a three-year period, listening at but a single frequency with radio dish antennae of 300 and 140 feet in diameter. He detected no artificial extraterrestrial signals, but was not disappointed, given that the total number of stars in the Milky Way is so large; even if there were, say, a thousand civilizations beaming signals our way at exactly the listened-for frequency, the odds against Project Ozma’s having detected one of them would have been nearly a million to one. When one factored in the many other uncertainties—guessing at the right frequency, allowing for Doppler shifts introduced by the motion of the sun and earth, and so forth—the chances became even slimmer. If SETI were to succeed, it would have to be a continuous, long-term endeavor.

In the meantime, however, the very existence of even a few modest SETI projects in the United States and the Soviet Union spurred fresh reaction against the extraterrestrial-life hypothesis. When NASA proposed spending two million dollars a year to divert a small amount of radiotelescope time to SETI, Senator William Proxmire of Wisconsin scotched the idea, bestowing upon it his “Golden Fleece” award for wasteful public spending and declaring that extraterrestrial civilizations, “even if they once existed, are now dead and gone.”
¹⁰
Thus ridiculed, SETI had to proceed in fits and starts, and by the mid-1980s only a few thousand hours of listening time had been accumulated on major radiotelescopes.

The opponents to SETI marshaled two central arguments. One was probablistic in nature, and like many statistical arguments was dogged by confusion. The other, called Fermi’s question, raised important issues that, if they did not dismiss the SETI case, did hold interesting implications for its search strategy.

The probabilistic argument consisted of adding up all the conditions thought to have been necessary for intelligent life to have evolved on Earth, then calculating that it was highly unlikely that the same thing had happened elsewhere. Its proponents began with the size of the earth’s orbit—were the earth slightly closer to the sun all its water would boil, and were it slightly farther away all the water would freeze—and tallied all the twists and turns in evolutionary history thought to have led to the emergence of
Homo
sapiens
. Were all these variables ascribed to chance (as both sides presumed they should be) the result was a vanishing small likelihood of intelligent life appearing
anywhere
. There are more than one million species of life on Earth today, the evolution of each of which is estimated to have involved perhaps a thousand unsuccessful mutations that led nowhere; the probability, therefore, that another planet has evolved a similar biology can be estimated at a thousand million to one. Allow for all the cultural and biological variables involved in the advent of human beings and their civilization, and the odds go up to perhaps 10
¹⁵
or 10
¹⁸
to one—a number that exceeds the likely total of all the planets in the galaxy. Therefore, went the argument, we are almost certainly alone.

This line of thought resembled the old argument from design, as succinctly summarized by Bertrand Russell: “You all know the argument from design: Everything in the world is made just so that we can manage to live in the world, and if the world was ever so little different, we could not manage to live in it. This is the argument from design.”
¹¹
The poverty of this argument is that one cannot reliably calculate the odds of a particular thing having happened unless one either understands the process—that is, can properly identify and quantify all the variables involved—or has an adequate experimental data base from which to draw phenomenological information about it. If, for instance, we want to predict how close an intercontinental ballistic missile will land to its intended target, we can calculate all the variables—the flight characteristics of the missile, the influences of environment on its navigational system, etc.—or we can test real missiles, as often as possible, in order to generate a data base about how they perform. In practice one does both, since both approaches may err. But when the question involves intelligent life arising on other planets, we can with confidence do neither, since we have only a rudimentary understanding of the variables involved, and none whatsoever regarding the statistics. To reason probabilities without them is to fall victim to the post hoc fallacy, by the lights of which almost every event may be calculated to be unique. If, for instance, we were to ask how likely it is for you to be reading this page at this moment, we might add up all the twists and turns of your life and mine, beginning with our births and running down through a billion variables to the circumstances in which I wrote these words and
you read them, and conclude that the thing is so nearly impossible that it would almost certainly never have happened, anywhere in the universe. Yet here we are.

In fairness, though, sentience might, indeed, be but an improbable accident, in which case we humans with our poor little radiotelescopes represent the highest form of intelligence in the universe. A SETI project could never prove that this were so, but if it went on listening for many decades and found nothing, the implication surely would be that we have few if any cosmic compatriots. But until that happens, as Michelangelo used to say, permit me to doubt.

The other argument is credited to Fermi, who is said to have asked, at Los Alamos one day in the late 1940s, “Where are they?” His reasoning was that if technically advanced extraterrestrial civilizations are prevalent, some of them ought to be able to migrate to other star systems, colonizing new planets as they go; the colonists subsequently could launch additional interstellar missions, until virtually every star system in the galaxy was occupied. Yet they are not here. Therefore, they do not exist.

On the face of it, Fermi’s question is easy enough to answer: Advanced extraterrestrials are not here because, for one reason or another, they are not able or do not want to be here. Perhaps interstellar travel is too expensive and time-consuming an enterprise for them to bother making the trip to other stars, or at least to
our
star, a yellow dwarf in the galactic suburbs orbited by a small blue planet the atmosphere of which is contaminated by the notoriously poisonous gas oxygen. Or perhaps they know we are here, but scruple not to interfere with our development (the “zoo hypothesis”). One can think of many such explanations; the point is, as they say, that absence of evidence is not evidence of absence. Lobster exist on earth, but I can set a place at my dinner table and wait a very long time before a lobster will come in the front door and climb up onto my plate; it’s just not worth the lobster’s trouble.

Fermi’s question, however, returns with far greater force when applied to the question of whether the solar system has been visited, not by a gigantic ark full of intrepid aliens, but by an automated, instrumented, self-replicating probe. Suppose that a nonhuman society were to send ten such machines to ten stars, and that each, upon arrival, were to mine an asteroid or planet in the system for materials and fuel from which to construct and dispatch ten more
probes, while itself staying behind to keep an eye on things. In this fashion it would be possible for an advanced civilization to establish a remote-sensing presence in many star systems. If, for instance, the average time required for a probe to reach a new star, replicate, and send forth ten new probes were ten thousand years, there would be probes in orbit around half the stars in the galaxy within one hundred thousand years after the project began. The galaxy is more than ten billion years old, so presumably there has been plenty of time for someone to have tried such a thing. The cost is manageable, the return in data enormous. So where are the probes?

One obvious answer is that they may already be here. The first rule of interstellar space-flight is to make everything as light as possible, in order to save fuel. Consequently we would expect a probe dispatched to the solar system from another star to be small. How small? A study conducted at NASA’s Jet Propulsion Laboratory in 1980 concluded that an interstellar spacecraft with sensing devices and an antenna capable of calling home could be encapsulated, using near-future technology, in a package weighing less than three hundred pounds. The example of the human brain suggests that it should be possible to miniaturize considerable sensory and data-processing power into an even smaller package, perhaps the size of a grapefruit and weighing only a few pounds. A probe
that
small could be in orbit around the sun right now—adrift in space or sitting on one of the millions of asteroids orbiting the sun or on a satellite of Mars, or Jupiter, or Saturn, quietly watching and transmitting its findings back home—and we would not know it. We have as yet no means of detecting such a probe, unless it had been programmed to make itself conspicuous, and there are good reasons for it not to call attention to itself, the foremost among which is that if we discovered its presence we might very well go get it and take it apart. The further exploration of the solar system might, therefore, eventually turn up evidence of intelligent extraterrestrial life. This of course was the thesis of Arthur C. Clarke’s science fiction tale
2001
, in which an astronaut investigates an alien probe discovered in orbit around Jupiter, the sun’s largest planet.

The possibility of automation also raises an intriguing prospect with regard to the search for intelligent interstellar radio signals: It suggests that the first signal acquired by a SETI receiver might very well have been dispatched, not by the inhabitants of another planet, but by an intelligent machine. To see how this could be
so, we need only to consider the practical exigencies that an advanced civilization would encounter once it had been in the interstellar communications business for a while.

Suppose that yours is one of among a hundred and one worlds in the Milky Way galaxy that have established radio communication with one another. You now have a minimum of one hundred antennae in action, each maintaining contact with a different planet thousands of light-years away. This arrangement has two drawbacks. First, it is inefficient; for the sake of economy, you would prefer to be using as few antennae as possible. Second, far more serious, is the Q and A time; if you ask a question it takes thousands of years to get an answer.

The way to alleviate both problems is to network the system. You install a single, automated station in space to handle all the radio traffic, and you link it to your planet via a single antenna system. Getting out your galactic map, you then determine strategic locations for siting other such automated stations, and you transmit an appeal to the worlds located at those junctures to please build them. Soon—meaning in a matter of a few dozen millennia or so —everybody is sending and receiving data to and from all the other worlds through local junction terminals, which may be in their own star system or in one next door. This way they need not employ separate antennae for each planet with which they communicate, any more than Earthlings maintain a separate telephone for every person they call.