Maybe we’re here only to say: house, bridge, well, gate, jug, olive-tree, window—at most, pillar, tower … but to say them, remember, oh to say them in a way that the things themselves never dreamed of existing so intensely.
—Rainer Maria Rilke
Flout ’em and scout ’em—and scout ’em and flout ’em; Thought is free.
—Shakespeare
The universe has four remarkable properties that encourage us to investigate whether we are alone in the universe.
The first is that space is transparent. A ray of starlight can speed unfettered through space for thousands of millions of years, bringing news of events long ago and far away, and the sailing is even clearer for radio waves. Natural radio noise—the chatter of hydrogen atoms adrift in space, the scream of electrons trapped in the magnetic fields of distant galaxies—can pass not only through the virtually perfect vacuum of interstellar space, but also though the clouds of gas and dust that clutter the disk of our galaxy and block the visible light of many stars beyond. These naturally occurring radio emanations need not be especially powerful for us to pick them up; all the energy gathered by all the world’s radio telescopes over the past thirty years amounts to less than the kinetic energy released by a snowflake falling gently to the ground. This suggests that artificial radio signals, too, could in principal be detected across interstellar distances, even if broadcast at modest levels of power. Radio telescopes in operation today could receive signals transmitted by similar instruments throughout much of our galaxy; a hundred billion stars and perhaps half a trillion planets lie within their range. And, since radio waves travel at the speed of light, three hundred thousand kilometers per second, their velocity of transmission is as fast as can be.
Second, the universe is uniform. Wherever we look, across millions of light years of space and eons of time, everything appears to be built out of the same chemical elements we find at home, functioning in accordance with the same natural laws. The carbon atoms of which diamonds and orchids are made are identical with the carbon atoms of the Pleiades star cluster. If life here on Earth arose through the operation of natural law—and there is no evidence to suggest otherwise—then it seems reasonable to suppose that life may have appeared elsewhere, too.
Third, the universe is isotropic, which is to say that on the large scale it looks pretty much the same in every direction. Every observer in the universe sees galaxies stretching off into all parts of the sky, just as we do. Contrary to what the ancient philosophers assumed, the earth does not sit at the center of the universe; indeed, there is no center of the universe. (Let a two-dimensional sheet represent three-dimensional cosmic space; bend it into a sphere, like the earth, as gravitation can curve space; there is no center to the universe, as there is no center to the surface of the earth.) Nor is there anything unique about the sun, which is one among many such stars in one of many galaxies. If nothing is strikingly special about our situation, then we have no particular reason to assume that the events that transpired early in the history of our planet—one of which was the advent of life—could not also have happened elsewhere.
Finally, the universe is abundant. Within the range of our telescopes lie perhaps one hundred billion galaxies, each home to a hundred billion or so stars. Astronomers estimate that at least half those stars have planets. If so, there are as many planets in the observable universe as there are grains of sand in all the beaches of the earth. In so rich a universe, many improbable things can happen: If intelligent life has arisen on but one planet in a billion, then fully ten thousand billion planets have given birth to intelligent species.
From such considerations has arisen the venturesome endeavor called SETI—the search for intelligent life in the universe.
Humans have long speculated about life on other worlds. Anaxagoras, Democritus, Aristotle, Epicurus, Philolaos, and Plutarch entertained the notion that the moon and planets were inhabited, as did Lucretius, Lambert, Locke, and Kant. Democritus’s student Metrodorus of Chios mused that “it would be strange if a single ear of corn grew in a large plain or there were only one world in the infinite.” Similar sentiments were expressed by the thirteenth-century Chinese philosopher Teng Mu, who wrote that “upon one tree there are many fruits, and in one kingdom many people. How unreasonable it would be to suppose that besides the heaven and earth which we can see there are no other heavens and no other earths.” None of these thinkers, of course, had any genuine evidence of extraterrestrial life, nor do we have any such evidence today. The difference is that SETI, rather than merely pondering the question, proposes to investigate it.
The modern SETI effort began in 1959, with a brief paper published in the British journal Nature. Titled “Searching for Interstellar Communications,” it was written by two scientists, Philip Morrison and Giuseppe Cocconi, who noted that beings capable of broadcasting and receiving radio signals could communicate all the way across the galaxy. Morrison and Cocconi reasoned that since interstellar signals can be transmitted without any great cost, and by means of relatively primitive technology, perhaps someone, somewhere, was doing it. If so, we might hear from them—provided we listen.
Though many scientists believe for various reasons that SETI is but a dream, there have from the outset been dreamers willing to give it a try. A few months after the Morrison-Cocconi paper appeared, the American astronomer Frank Drake pointed a radio telescope with a dish twenty-six meters in diameter at two sunlike stars and listened to them at a single frequency for a total of one hundred fifty hours. He heard nothing out of the ordinary, and the dish was returned to service in less speculative research endeavors, but the ice had been broken. SETI projects have proceeded in fits and starts ever since. By 1991 approximately fifty radio searches had been conducted, principally in the United States and the Soviet Union. Some, the “dedicated” searches, diverted radio telescope time to pure SETI work; others, called “parasitic,” sifted through data accumulated in normal astronomical observations, looking for unnatural patterns. Some enjoyed government support. Others were privately funded. A retired electronics technician stood a lonely SETI vigil for two years on the shores of Great Slave Lake in northern Canada, using swap-meet electronics hooked up to the military antennae of a decommissioned Distant Early Warning (DEW) line station built to warn of a Soviet missile attack. A Berkeley astronomer listened for signals with a 4.2-million-channel receiver financed in part by a contribution from his mother. A young Harvard professor named Paul Horowitz practiced what he called “suitcase SETI,” lugging around a portable receiver and hooking it up wherever he could beg or borrow a few hours on a radio telescope, before setting up shop with a more sophisticated receiver and an old antenna in Cambridge, Massachusetts, with the help of grants from The Planetary Society and movie director Steven Spielberg.
As typically occurs when hard information is lacking, the pendulum of opinion about whether SETI is worthwhile has swung back and forth wildly, with scientists expressing equally heartfelt if equally unsubstantiated sentiments pro and con. Soviet astronomers scanned the skies for years, then fell victim to frustration and threw in the towel. In the U.S., attempts by the National Aeronautics and Space Administration to fund a SETI project foundered in a wash of legislative scorn; one congressman read tabloid newspaper accounts of flying saucer landings into the Congressional Record, his lighthearted point being that “we don’t need to spend six million dollars this year to find evidence of these rascally creatures [when] … conclusive evidence of these crafty critters can be found at checkout counters from coast to coast.”
Not until 1991 did NASA get approval to proceed. Plans called for the Jet Propulsion Laboratory in Pasadena, California, to survey the entire sky with the three antennae of its worldwide Deep Space Network, while the NASA Ames Research Center would use larger, more sensitive antennae to scrutinize hundreds of sunlike stars. Though economical, as such things go—its budget ran a little over ten million dollars per year—the NASA project was technically impressive; it would rig radio telescopes with sophisticated spectrum analyzers capable of searching fifteen million separate radio frequencies at a time, sifting through endless natural radio noise in hopes of finding the coherent signal thought to be a signature of civilization.
Needless to say, no SETI project has yet detected a signal. (Had they succeeded, the newspapers would be full of little else.) But this was to be expected; given the enormous number of stars in the galaxy, plus the wide choice of possible radio frequencies at which a message might be received, one would expect a search to take many years before hitting paydirt, even if thousands of civilizations were beaming greetings our way. SETI advocates stress that substantial technological and scientific benefits may be reaped even if they fail to detect evidence of an extraterrestrial civilization: We might for instance identify coherent pulses produced by natural sources, a phenomenon as yet unknown in the universe. And in any event the challenge of building the data-reducing hardware and software required to analyze the avalanche of data produced by SETI observing runs could inspire quantum leaps in computer design. “Getting there is half the fun,” says Kent Cullers, a young physicist with the NASA SETI project. (Cullers, who is blind, designs signal-processing equipment for radio telescopes; like the cathedral builders of old, he does not expect success within his lifetime but says he likes his work anyway.)
Negative results cannot be interpreted as conclusive: It is always possible that we have examined the wrong stars, or guessed at the wrong combination of frequencies, or have in some way overlooked the obvious. The bittersweet truth is that we will never be able to prove that we are alone in the universe. SETI will either end in a pot of gold, or turn out to be an endless avenue.
SETI’s advocates are mostly astronomers and physicists. They marshal astronomically large numbers to argue on statistical grounds that intelligence probably occurs frequently on the panstellar scale. The SETI skeptics are mainly biologists. They use similar statistics to conclude that intelligence is unlikely to have evolved anywhere else, and that SETI is therefore a waste of time and money. Their debate revolves around how each camp views life and intelligence.
The basic case for SETI goes like this:
Life is a natural; it’s “in the cards.” The chemicals required to make living organisms—e.g., carbon and water—are abundant in the universe, suggesting that there are quite a few planets where conditions favor the appearance of life. And where the environment is right, it may not take long before the first organisms start wiggling in the ooze: Terrestrial life arose within the first billion years of the planet’s 4.5 billion year history. So prompt an origin implies that life appears more or less routinely, on earthlike planets at least. This hypothesis gains support from experiments in which conditions thought to replicate those widespread on the young Earth—a primitive atmosphere of methane, ammonia, water vapor, and molecular hydrogen, bathed in ultraviolet light and charged by electrical shocks like those produced by lightning—are reproduced in laboratories. These conditions, the experimenters find, lead readily to the formation of amino acids like glycine and alanine, the so-called “precursor” molecules on which life as we know it is based. So it seems reasonable to suppose, as a working hypothesis if nothing else, that there is life elsewhere in the universe.
As for intelligence, the standard argument is that while we don’t know how or why intelligence arose on Earth (something to do with the ice ages, perhaps), once it does appear on any given planet it may be expected to flourish, since it bestows considerable advantages upon the species endowed with it. “We say that because in the fossil record, there is only one category of thing that constantly improved and that is brain size, which we associate with intelligence,” Drake once told me. “There have been larger creatures in the past, higher flying birds, but the one thing that has consistently improved survival value has been intelligence.” The American astronomer Carl Sagan reasons similarly. “The adaptive value of intelligence and of manipulative ability is so great—at least until technical civilizations are developed—that if it is genetically feasible, natural selection seems likely to bring it forth,” he writes.
As a long-standing SETI enthusiast myself, I’m emotionally inclined to accept the conclusions of these arguments. I’m willing, in other words, to “believe” that there is life on other planets—though it makes not an iota of difference to the universe whether I choose to believe that it’s lively as a cloud forest or sterile as a surgeon’s scalpel. But I have to admit that the case for SETI, if evaluated as a scientific hypothesis, really doesn’t hold much water. Its weakness lies in the assumption that what we regard as intelligence will have been selected for in the course of biological evolution on other planets. Why should this be so?
The answer cannot be that we expect the anatomy of alien brains to resemble our own. As I will discuss later in this book, the brain is a ramshackle concatenation, slapped together through the course of millions of years of evolution in which many chance events, from the swift hammer-blows of meteor impacts to the slow advance and retreat of glaciers, appear to have played important roles. So unpredictable are all these twists and turns of fortune that our neuroanatomy almost certainly has been duplicated nowhere else in the universe. We are led, then, to speculate that intelligence is somehow universal even though the physical brain that gave rise to our intelligence is unique. By these lights, intelligence is akin to a computer program (“software”) that can run on many different sorts of computers (“hardware”). But who wrote the program, and how did He load it into our brains? This line of argument, I fear, is freighted with heavier theological implications than many of the scientists who employ it would feel comfortable supporting.
Suppose we try to avoid the problem by defining “intelligence” narrowly, as meaning nothing more than the ability to send radio signals across interstellar space. That seems fair enough—it reduces to a bare minimum the requisite overlap between alien minds and our own—but it leads to the curious conclusion that intelligence has existed on Earth for only sixty years. (The first radio telescope was built in 1931, by an engineer studying the effects of lightning on long-distance telephone lines.) Whereupon the same statistics that previously supported the SETI case suddenly turn against it: If there has been life on Earth for four billion years, and “intelligent” life for but sixty years, then how can we say that intelligence has been selected for in the course of biological evolution? One could just as readily argue that intelligence is not selected for, precisely because it has not appeared more often in terrestrial history.
Personally, I feel that there is something nonparochial about human intelligence—something cosmic about a brain that can chart the galaxies and fly itself to the moon. But I can’t prove it, and a hard lesson taught by science, as by life more generally, is that the broad emotional appeal of a hypothesis has nothing whatever to do with the likelihood that it is true. So I am forced to conclude that SETI, just as its critics maintain, has not been justified scientifically.
But if SETI is not yet a science, it may nevertheless be justifiable as a campaign of exploration.
The precepts of exploration are, after all, distinct from those of science. Science survives by making accurate predictions. Exploration does not; an explorer who could predict what his voyage of discovery would find would not be much of an explorer. Some of the most heroic voyages in human history were made for insupportable reasons: The ancient Chinese navigated the Pacific in search of the elixir of immortality, as did Ponce de León in Florida; and Columbus thought he could sail west all the way to the Indies, an impossibility, because he insisted against all evidence that the earth was a third of its true size. Explorers, like poets, often succeed by making fantastic leaps of the imagination, free from reason’s fetters. In that sense exploration is even more imaginative than science—which is to say that it is very imaginative indeed.
Shakespeare, who understood this perfectly well, had little use for science but was infatuated with exploration. The Tempest, his last play, was inspired by his reading of a contemporary account of a shipwreck that stranded one hundred fifty English seafarers in the mid-Atlantic. They were colonists bound for the New World, and the manuscript Shakespeare read had just been written by one of their number, the adventurer William Strachey. It told a stirring tale of how the flagship Sea Venture, her hull splitting apart in heavy seas and St. Elmo’s fire dancing through her rigging, was wedged onto the rocks of an uninhabited island in the Bermudas and miraculously prevented from sinking, just as those aboard were toasting one another’s fortunes in the next life. It detailed how they survived there for nearly ten months, from July 1609 through May 1610, during which time four men and a woman died, two babies were born (a boy, named Bermudas, and a girl, Bermuda), and a mutineer—one Henry Paine, who stole a sword, beat up a guard, and invited the governor to kiss his ass—was executed. Strachey’s memoir recounted how the colonists fashioned two makeshift longboats from the timbers of island cedars, christened them Deliverance and Patience, and sailed them across six hundred miles of open ocean to Jamestown, Virginia, only to find that their fellow colonists, near starvation in a fort surrounded by hostile Indians, were in worse shape than their shipwrecked confederates had been in Bermuda.
None of this, however, found its way into The Tempest. What caught Shakespeare’s eye was the alien mystery of the Bermudas, remote and unexplored and much feared, known in those days as the “Isles of Devils.” The islands lay beyond the firelight of the known, as the allegedly inhabited alien planets do today, and Shakespeare took full advantage of our love for the unknown. He peopled his fictional version of the island with fairies and beasts, and with a native, Caliban, who in a heartbreaking passage blurts out his regret at having been cajoled into trading his useful knowledge of the place for such trivia as the English names of the sun and moon:
… When thou camest first
Thou did strok’st me, and made much of me; wouldst give me
Water with berries in’t, and teach me how
To name the bigger light and how the less,
That burn by day and night; and then I lov’d thee
And show’d thee all the qualities o’th’ isle,
The fresh spring, brine-pits, barren place and fertile.
Cursed be I that did so! …
For I am all the subjects that you have,
Which first was mine own king; and here you sty me
In this hard rock, whiles you do keep from me
The rest o’th’ island.
To sail blue water in the seventeenth century was to venture into the unfamiliar at a level of personal risk such as has been attained by no subsequent campaign of exploration save spaceflight. Shipwrecks were so common, even among fishing boats operating within sight of the rocky coasts of the British Isles, as to have engendered a whole liturgy of loss at sea: Country parsons and high bishops alike routinely depicted lost seamen as “resting on the bosom of the deep,” and even landlubbers who died in bed were said to have taken a “voyage to the isles from whose borne no man returns.” It was this, the thrills and chills of contact with the unknown, that appealed to Shakespeare and has gripped his audiences ever since.
The great blue-water navigators were often on their knees, importuning God to save their frail barks amid heavy seas, and SETI, too, has something of the flavor of prayer. The point of prayer, in my view, is not that we know that God is listening, but that we do not know, and choose to pray anyway. In this regard, the young astronomer who spends years sifting the stars for an intelligent signal evinces the spirit of Alyosha in Dostoyevsky’s The Brothers Karamazov, who when stumbling outdoors after the death of the saintly Father Zossima broadcasts a prayer into the night sky:
The vault of heaven, full of soft, shining stars, stretched vast and fathomless above him. The Milky Way ran in two pale streams from the zenith to the horizon. The fresh, motionless, still night enfolded the earth…. Oh! in his rapture he was weeping even over those stars, which were shining to him from the abyss of space, and “he was not ashamed of that ecstasy.” There seemed to be threads from all those innumerable worlds of God, linking his soul to them, and it was trembling all over “in contact with other worlds.” He longed to forgive every one and for everything, and to beg forgiveness. Oh, not for himself, but for all men, for all and for everything. “And others are praying for me too,” echoed again in his soul.
We listen to the stars not because we know that we will hear something, but because we think we might—even while understanding that our ideas about life and intelligence in the universe may well be all wrong. The best argument for SETI is still the one with which Morrison and Cocconi concluded their original 1959 paper. “The probability of success is difficult to estimate,” they wrote, “but if we never search, the chance of success is zero.” Better to travel the endless avenue, knowing not where it leads, than to turn away from the stars because we think we know more than we do.