Probability 1

Probability 1

by Amir D. Aczel, Ph.D. Amir D.

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For thousands of years, it was the visionaries and writers who argued that we cannot be alone-that there is intellegent life in the universe. Now, with the discoveries of the Hubble Telescope, data emerging from Mars, and knowledge about life at the extremes, scientists are taking up where they left off. Amir Aczel, author of Fermat's Last Theorem, pulls together

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For thousands of years, it was the visionaries and writers who argued that we cannot be alone-that there is intellegent life in the universe. Now, with the discoveries of the Hubble Telescope, data emerging from Mars, and knowledge about life at the extremes, scientists are taking up where they left off. Amir Aczel, author of Fermat's Last Theorem, pulls together everyting science has discovered, and mixes in proabability theory, to argure the case for the existence of intelligent life beyond this planet. Probability 1 is an extraordinary tour de force in which the author draws on cosmology, math, and biology to tell the rollicking good story of scientists tackling important scientific questions that help answer this fundamental question. What is the probability of intelligent life in the universe? Read this book, and you'll be convinced, by the power of the argument and the excitement of the science.

Editorial Reviews

From the Publisher
"For one practiced in dealing with numbers, Amir Aczel certainly has a way with words. . . . Before reading Aczel's book, you might find it easy to shrug off his conclusions. After you've finished Probability 1, you may find it harder to do so."-Astronomy
"There are two reasons for recommending [Probability 1] to any person interested in the debate: It is clearly and gracefully written and it is up-to-date in its astronomical data."-Martin Gardner, Los Angeles Times Book Review
John Durant
...[A]s long as we're merely guessing, we should not dress up our interesting speculations as mathematical certainties.
The New York Times Book Review
Scientific American
Aczel's quest is for intelligent life anywhere else in the universe, not just in our galaxy. Here he is dealing with almost incomprehensibly big numbers.
William A. Dembski
...[O]ne of the most accessible popular science books that I have read...worth reading simply on that account....not a bad book!
Books & Culture: A Christian Review
The Economist
...[A] somewhat haphazard concoction of historical anecdote, personal experience and scientific exposition. Some parts of Mr Aczel's account are excellent, notably his account of the methods used to detect planets around other stars.
Scientific American
Aczel's quest is for intelligent life anywhere else in the universe, not just in our galaxy. Here he is dealing with almost incomprehensibly big numbers.
Kirkus Reviews
Life on other worlds, long a staple of sci-fi thrillers, is now the stuff of serious science. Aczel brings a statistician's tools to bear on the evidence. Enrico Fermi once asked, "Why, if there is intelligent life elsewhere, haven't they contacted us?" One response to this paradox is Frank Drake's equation for the number of civilizations that might exist capable of communicating with other civilizations. For in a universe billions of years old, nearly infinite in extent, it's inconceivable that our world is the only one to be inhabited. And yet, flying-saucer cultists aside, there is no credible evidence of Earth's being visited by aliens. (One likely reason for this lies in the daunting distance between stars, which makes expeditions beyond a race's home system too expensive for even a very advanced civilization to undertake.) Most of the other evidence argues that there are planets around other stars, where the chemical reactions necessary for life (notably, the spontaneous formation of DNA) can take place. We also have evidence (in the form of Martian meteorites) that the spread of these chemicals from one world to another is moderately commonplace. Aczel stops to consider the central philosophical question raised by Einstein and the quantum theorists (i.e., whether the universe is random or deterministic); he clears up a few misconceptions about probability before applying probability theory to an assessment of the value of Drake's equation. His conclusion, that at least one planet besides Earth almost certainly bears life, will seem inevitable to anyone who has followed his arguments. Lively writing and the ability to work scientific history smoothly into the narrative make thisa very useful addition to the growing body of work on a fascinating subject.

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Chapter One

Fermi's Paradox and
Drake's Equation

There are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number are borne far out into space. For those atoms have not been used up either on one world or on a limited number of worlds, not on all of the worlds which are alike, or on those which are different from these. So that there nowhere exists an obstacle to the infinite number of worlds. We must believe that in all worlds there are living creatures and plants and other things we see in this world.

Thus wrote Epicurus (341-270 B.C.) twenty-three hundred years ago. He had developed some of the ideas about extraterrestrial life put forward by the Greek philosophers Democritus and Leucippus, who lived two centuries earlier. Epicurus put down these thoughts about extraterrestrial life in a letter he wrote to Herodotus.

    For the ancient Greek philosophers, the "worlds" referred to here were not planets orbiting stars. The stars were considered part of the firmament of the heaven and were seen to orbit Earth at no greater a distance than those of the planets of our solar system. The "other worlds" were viewed as replications of Earth that could not be seen by observing the sky.

    Writing in the first century B.C., the Roman poet Lucretius (ca. 99-55 B.C.) further carried out the Epicurean philosophy of life in the universe. In On the Nature of the Universe, he wrote: "Granted, then, that empty space extends without limit in every direction and that seeds innumerable are rushing oncountless courses through an unfathomable universe. It is in the highest degree unlikely that this earth and sky is the only one to have been created."

    Attacks on these positions were common in antiquity. In his Timaeus, Plato (ca. 428-348 B.C.) asserts, "There is and ever will be one only-begotten and created heaven." And Aristotle (384-322 B.C.), the most influential of all Greek philosophers, wrote much about the uniqueness of Earth. It was, in fact, Aristotle's philosophy--which formed the basis for teachings at the universities and for the prevalent religious doctrine in Europe all the way up to the seventeenth century--that prevented the idea of the plurality of worlds from taking stronger hold earlier in history. Aristotle asserted that Earth was the center of the universe and that the Sun, a perfect bright circle in the sky, and the Moon, another unblemished circle, rotated around a stationary Earth together with all the stars in the firmament of the heavens. The theory of the perfection of space and the centrality of Earth to the entire universe was the biggest stumbling block on the road to acceptance of the ideas of Copernicus and Galileo, as well as those of the sixteenth-century Italian philosopher Giordano Bruno, who again suggested a plurality--in fact, an infinity--of worlds.

    A century later, Voltaire wrote Micromegas (1752), in which he described extraterrestrial life. His story centers on Micromegas, who is 120,000 feet tall and lives on a planet orbiting the star Sirius. Micromegas studies at the Jesuit college of his planet. He derives on his own all of the geometrical theorems of Euclid, and then proceeds to travel to other worlds, including Saturn and Earth. Micromegas has a thousand senses. He complains about life on Saturn, since its inhabitants have only seventy-two senses, which makes communicating with them somewhat less fulfilling than Micromegas would like.

    The extraterrestrial life debate reached an apex in the 1850s. In 1853 the British philosopher William Whewell sparked the debate when he published anonymously a book entitled Of the Plurality of Worlds: An Essay. The issues raised during the decade-long debate were very much like the advanced and specific elements of the modern argument for life outside Earth. These included the possible existence of planets orbiting stars like our Sun. The fact that variable stars like Algol were observed by astronomers, as well as stars dimmer or brighter than the Sun, led some participants in the international debate of the 1850s to conclude that Earth might be unique and that life might not exist anywhere else. This contention was bolstered by astronomers' inability to observe planets circling any star. These ideas simmered until our own century. The most powerful objection to the theory that life may exist outside Earth came right in the middle of the twentieth century, in the words of one of the most respected scientists of our time.

Fermi's Paradox

In 1950 the nuclear physicist Enrico Fermi dealt a blow to whatever remained of the belief that extraterrestrial life was possible when he asked his colleagues: "Where is everyone?" This question has come to embody what is by now known as Fermi's paradox. By Fermi's logic, if aliens existed, then, since the universe is so old and so large, surely a civilization would have existed that is vastly more advanced than our own and this civilization would have colonized our galaxy. In fact, judging from the experience we've had on Earth, the aliens would have dominated us and taken away Earth's natural resources for their own use, as colonial nations have done for centuries. With a 14-billion-year-old universe and with a galaxy of billions of stars, where are these aliens? Since we haven't seen them, Fermi's paradox suggests, they don't exist.

    Some scientists were convinced by Fermi's argument against the chances for the existence of intelligent life in outer space. Most astronomers devoted their research efforts to discovering the physical structure of the universe: how stars form, how they die, how galaxies develop and evolve, and whether the universe shows signs that it might expand forever, as observations from very distant galaxies now seem to imply. It became unfashionable, and in fact dangerous to the prospects for one's career, even to consider the problem of life beyond Earth. The majority of astronomers were not interested in looking for life or even for conditions that might favor life elsewhere in the universe--especially if they did not have permanent jobs or academic tenure. But not everyone felt this way.

    Frank Drake was a young doctoral student in astronomy at Harvard University. In 1957 he was working on his dissertation, observing stars through a radio telescope at the Oak Ridge Observatory. Drake had never heard of Fermi's paradox. Had he heard of it, he recently told me, he still would not have paid it any attention. Frank Drake was studying the Seven Sisters.

    Tucked behind the head and great horns of the bull of the constellation Taurus is a cluster of stars known as the Pleiades. These stars act as a marker for the Tropic of Cancer, which lies within one degree of their location in the northern skies. Although their number ranges from six to nine, depending on how one counts them, the Pleiades are known as the Seven Sisters. According to Greek mythology, the Seven Sisters were the daughters of Atlas and Pleione: Alcyone, Electra, Maia, Merope, Taygeta, Celaeno, and Sterope. The young girls were pursued by the great hunter, Orion, represented by the next constellation in the sky just west of Taurus. When the gods heard the screams of the distressed sisters, they protected them by placing them as doves in the sky, where they are often depicted as weeping--possibly for the loss of a sister who died, as scientists believe that another star once shone brightly in the region of the Pleiades and has since dimmed. The Pleiades are young stars: they have been forming from a large cloud of dust and gas, whose possible remnants may still be seen through a telescope as a mist enveloping them. The brightest sister is Alcyone, and the dimmest, hardly visible to the naked eye, is Sterope. Binoculars reveal dozens of new stars forming in this cluster, and a telescope reveals hundreds of them.

    As part of his doctoral dissertation project, the twenty-seven-year-old Frank Drake was studying the prevalence of hydrogen in the Pleiades, hoping to learn how new stars are born. "The Pleiades have a very distinct spectrum," Drake explained, "where the hydrogen lines are easily detected." He was looking at these hydrogen lines through his telescope for many weeks in early 1957, when the Seven Sisters were high in the night sky. The patterns of the spectrum were consistent from day to day, with no change, and Drake was performing various calculations that tell astronomers about the chemical compositions of stars. In this case he was trying to find out how much hydrogen is present in these young stars.

    One cold night in February, as Drake was looking at the screen of his radio telescope, observing the constant lines from the Seven Sisters, a signal flashed on his screen. This was odd given the usual routine of his observations. Drake straightened up in his swivel chair, his eyes fixed on the interloping signal. A shiver traveled the length of his spine as he understood that the strange signal could not have been caused by a natural phenomenon. Could someone in another civilization in the Pleiades, or beyond, be sending us a signal? Could he be the first person in history--all alone in the middle of nowhere with only his instruments as companions--to receive the call?

    A half hour had passed, and the signal was still there on his screen. Drake had a thought. What would happen if he were to shift his radio antenna away from its present direction--would the signal disappear? Very slowly, Drake turned the knob on the control panel of the great radio telescope and heard the motor whir as the dish moved slowly away from the direction of the Seven Sisters. The signal was still there. This, then, was not an alien signal, Drake now knew. He wasn't sure whether to be disappointed or relieved. The signal had to have originated on Earth, or else it would have been direction-sensitive and would have disappeared as the antenna's direction was changed. Frank Drake went home. He needed sleep.

    But as he awoke the next morning, Drake realized that the night's experience had left him with something. He could no longer escape the possibility that somewhere out there in space another technologically advanced civilization might be sending us signals. Would we hear their call? As the weeks passed, the notion of being beamed a message by fellow beings in the vastness of space and not hearing their call in the dark obsessed him. Drake became determined that we, as a civilization, must make every effort possible to listen to signals from space. But where should we begin? What was the logical starting point for this cosmic search? Drake kept pondering this question. In the meantime, he finished his dissertation and took his Ph.D. degree from Harvard. He then moved to Green Bank, West Virginia, where the government still maintains large radio telescopes used by a number of groups of astronomers.

    The same year, a young professor of astronomy at Cornell University was sitting one day at the concert hall on campus, listening to chamber music. But his mind was not on the music that evening--it was drifting into space. A few years earlier, Philip Morrison had received his Ph.D. degree from the University of California at Berkeley. There, he wrote a dissertation on quantum electrodynamics under the tutelage of the great American physicist Robert Oppenheimer. Listening to the music in 1957, Morrison also came to the conclusion that extraterrestrial beings might well be sending us messages from space, and that we on Earth must make every attempt to listen to such signals. But where should we begin? Both Morrison and Drake, thinking about the same problem independently, reached an identical conclusion: We should scan the microwave band of the electromagnetic spectrum. There, our chances of hearing signals from other civilizations would be the greatest. And we must look, no matter how small the odds, both scientists felt strongly. In 1959 Morrison coauthored an article with Giuseppe Cocconi in the prestigious British journal Nature. The article would have seemed like science fiction, except that it appeared not long after the Russians launched Sputnik and the great superpower competition had moved from Earth into space.

    The Cocconi and Morrison article suggested that a civilization on another planet in orbit about a faraway star might have concluded long ago that our Sun was a good candidate for the development of intelligent life on one of its planets. Such an extraterrestrial civilization could be patiently sending us radio signals and expecting that once our technology had reached the stage of being able to detect these signals, we would respond. In reaching their conclusion about which frequency to listen on, Cocconi and Morrison used a clever argument. The most abundant element in the universe is hydrogen, they reasoned, and hydrogen atoms, when excited, broadcast at a frequency of 1,420 megahertz (1,420 cycles per second). This frequency, within the microwave band, is separated from the frequencies where most "noise" of the background cosmic radiation occurs. If an intelligent society were to call us, the two concluded, they would be likely to use this particular frequency.

    Green Bank is an isolated valley in a remote region of the Allegheny Mountains. In this area, radio and television stations are few and far between, so that interference from human-made transmissions is minimized. Even cars are few here, so there is less electricity from spark plugs to interfere with the radio teleseope's dish. Soon after his arrival at Green Bank, Frank Drake confided in the center's director and managed to convince him to grant him observation time to look for extraterrestrial signals. Drake named the effort he began in 1958 Project Ozma, after the queen of Oz. Drake began by listening to a few radio frequencies from the directions of a number of nearby stars. First, Drake pointed his antenna to the nearby star Tau Ceti (in the constellation Cetus, the Sea Monster). The observations revealed nothing. When this star set behind the horizon, Drake aligned the antenna with another nearby star, Epsilon Eridani (a star lying at about the middle of the River, the constellation Eridanus, west of Orion).

    Within seconds Drake heard a strange noise in his earphones--something was pulsating at eight times a second. He felt great excitement, an adrenaline rush. But as a careful scientist, he made himself calm down and study the signal. Then, as suddenly as the signal appeared, it was gone. He would never hear this sound again. Weeks later it became clear that Project Ozma was not the only one to seek the seclusion of the Allegheny Mountains. Secret military communications experiments were being conducted in these mountains, and these, Drake concluded with no concrete evidence, might have been the source of the interference. Drake turned his attention to other stars. He did not find anything that would bear further scrutiny--all unidentified signals were eventually traced to earthly sources. But the electromagnetic spectrum was very, very wide. Was he looking for signals in the right frequency? Drake could not answer this question. Of all the frequency ranges in the spectrum, the one that seemed to be best was the microwave range. But there were so many others. Drake and his associates reached a conclusion similar to that which Morrison and Cocconi had suggested in their paper. They decided to monitor as many channels as they could within the microwave range, including the hydrogen frequency and those of other elements and molecules they believed were abundant in the universe.

    The 1950s and 1960s were exciting times for space exploration. Following both sides' initial successes in space, the United States and the Soviet Union were actively competing for new achievements. In 1961, the U.S. National Academy of Sciences was to hold a meeting at the Green Bank Observatory to discuss the new direction in the study of space: the search for signals from alien civilizations. The sole organizer of the meeting was to be Frank Drake. As soon as he was selected for this duty, Drake started preparing for the meeting. A successful conference, with fruitful, interesting discussions, could result in increased funding for Project Ozma. And the search for alien signals was in dire need of funds: money to pay more astronomers to search the skies, money to develop computing ability, and money to develop radio technology that could search the vastness of the universe more efficiently. Frank Drake felt he needed to come up with a strong agenda for the meeting.

Drake's Equation

Drake's agenda for the 1961 National Academy of Sciences meeting consisted of a single formula, now widely known as Drake's equation. It reads:

    N = [N.sub.*][f.sub.p][n.sub.e][f.sub.l][f.sub.i][f.sub.c]L

In this equation, N stands for the number of civilizations in the galaxy currently capable of communicating with other civilizations.

    According to Drake, this number depends on the values of the seven factors whose product forms the right-hand side of Drake's equation. What are these mysterious factors that, according to Frank Drake, can tell us the number of advanced civilizations in our galaxy with which we might hope to communicate?

[N.sub.*]: The Number of Stars in the Galaxy

The first term on the right side of the equation is [N.sub.*]. This stands for the number of stars in our galaxy. According to astronomers, the number of stars in the Milky Way may be as high as 300 billion (although some estimates are lower). This is a huge number. And remember that we are considering only our own galaxy. There are possibly as many as 100 billion other galaxies.

"Oh, Be A Fine Guy/Girl, Kiss Me!"

Let's go beyond Drake's equation. What we need is not just any star, but stars that--from what we know based on our experience on Earth--hold some promise of harboring life. We want to consider stars that are similar to our own Sun. We now know that our Sun is rare: only 5 percent of the stars in our galaxy are Sunlike. Stars on the main sequence (before they collapse and die, turning into white dwarfs or neutron stars or black holes) fall into seven categories of luminosity: O, B, A, F, G, K, and M. (Astronomy students learn the star-type order by memorizing the sentence "Oh, Be A Fine Guy/Girl, Kiss Me!") The Sun is a G-type star, shining yellow, while O and B are hot-burning blue stars; A and F are white, burning at lower temperatures than O and B; and K and M are orange and red, respectively, with less heat and light output than the Sun. The hotter-burning blue and white stars of the O to F categories have relatively short lifetimes, in the millions to a few billion years, while some G-type stars can last even up to 15 billion years or more. The orange and red K and M stars, which can last longer than our Sun, produce too little light and heat. In searching for extraterrestrial life, it is the G-type stars like the Sun that should be of most interest to us. In Drake's equation, therefore, we could multiply the number of stars in the galaxy by 0.05 to count only stars that are similar to our own Sun. This still leaves us many billions of stars.

[f.sub.p]: What Is the Percentage of Stars with Planets?

The next term in Drake's equation is [f.sub.p]. This is the fraction of stars with planets. When I asked Frank Drake what he thought about the exciting new discoveries of planets orbiting Sunlike stars (the story of which is told in the next chapter), he didn't mince words. "It confirmed in my mind what I already believed: that [f.sub.p] is equal to about 0.5." This estimate is based on the assumption that the present technology for detecting planets can be vastly improved. Currently, there is an inherent bias toward finding oddball planets: huge gas giants such as Saturn and Jupiter that--contrary to where we expect them to be based on experience in our own solar system--happen to orbit very close to their stars. If we can observe such seemingly unusual planets, what can be said about more "normal" planets, once we are able to detect them as well? The conclusion to be drawn from this seems to be that there are a lot of planets out there!

    Michel Mayor, the discoverer of the first extrasolar planet, told me he thought the fraction of stars with planets is 1.0. This means that every star has at least one planet. What would make someone make such a strong assertion? Mayor bases his belief on the way planets are thought to form. A star is born when clouds of dust and gas condense and merge because of their mutual gravitational pull. A large lump forms and heats up as it contracts, and nuclear reactions ignite the star; the burning of hydrogen into helium makes the star shine and keeps the matter from further collapse. But as this happens, matter that has not yet condensed remains in a disk that revolves around the star. As the matter in the disk coagulates, objects called planetesimals are formed. These hit against each other, eventually forming planets that continue to revolve around the star in the same direction as the original disk that formed them. If this indeed is the way planets form, there must be plenty of them, since the only way they would not form is if all the matter that condensed fell into the forming star-leaving nothing outside for the disk. This is unlikely to happen.

    But does life have to evolve only on a planet in orbit around a star? Recent research by Dr. Rudy Schild of the Harvard-Smithsonian Center for Astrophysics indicates that galaxies may contain quadrillions of "rogue planets"--planets that are not attached to any star and freely roam the interstellar space within a galaxy. Schild has reached his controversial conclusion using a technique called gravitational microlensing. The principle behind this method is the general relativity concept that light is bent around massive objects. Thus when light from a distant source passes near massive objects on its way to the observer, the light gets focused by the gravitational field of the masses it encounters. When Schild looked through his 1.2-meter telescope at light emanating from a distant quasar, he noticed the microlensing effect around the edges of the distant quasar galaxy. Calculations he then performed with the measurements he obtained led Schild to the belief that the gravitational microlensing was caused by planets. The frequency of the phenomenon then made him conclude that the number of these freely moving planets had to be immense.

    From all the information currently available, it is clear that there are at least some planets revolving around other stars. The proportion of such planets, the parameter [f.sub.p] in Drake's equation, may be high-from 0.5 to possibly close to 1.0. And planets may roam interstellar space as well. Such planets may be captured into orbit by stars they pass, or life may even develop on a roaming planet if it has its own source of internal heating from geological or radioactive sources. We cannot rule out this possibility. Our main concern, though, is with planets orbiting normal stars like the Sun, where life is more likely to develop.

[n.sub.e]: The Environmental Factor

The next factor in the equation is [n.sub.e]: the number of planets with environments favorable to the formation of life. This brings us to an important question: What is life? Based on our experience on Earth, we only know one set of conditions that can bring about the development of life. These conditions require abundant water as the solvent through which molecules can travel to form organic compounds leading to proteins. The basic element in these compounds is carbon, to which hydrogen, nitrogen, oxygen, and other elements attach themselves to form the large molecules that are the building blocks of proteins that make DNA and life. Oxygen is essential in the metabolic processes that produce the energy necessary for life. But are these elements the only ones that can lead to life? No one knows the answer to this question. Some scientists contend that life can only exist in the form we know on Earth and that it would require the same elements. Others, Drake included, believe that it may be possible for life to be based on a central element other than carbon: sulfur, for example. Others think that silicon, at some temperatures, may work similarly to carbon as we know it at Earth's temperatures.

    Assuming that water is essential for life brings us to the important concept of the habitable zone. Water is needed as a solvent, and this process only happens when water is in liquid form. So the basic assumption we will use is that planets that can develop and harbor life must have some liquid water on them. Let's look at our own solar system as an example. Here, of nine planets, only Earth is known to have liquid water in abundance. The recent discovery that Jupiter's moon Europa may contain an ocean of liquid water under its icy surface, allowing the possibility that marine life of some kind may have evolved there, may broaden the meaning of the habitable zone. What is striking, however, is that Jupiter and its moons are not usually considered to be within the Sun's habitable zone because, until recently, scientists believed that any water in that region of space had to be frozen solid since the Sun's radiation reaching Jupiter was so weak.

    For our solar system, the habitable zone is considered to extend from a point 5 percent closer to the Sun than Earth's orbit and up to a point 37 percent farther away from the Sun than Earth's orbit. Closer to the Sun, photodissociation will occur--the loss of gas and water from the planet into space. The extent of evaporation depends on the planet's mass: a small planet may not be able to hold on to its water even at distances above the lower limit. This effect apparently took place on Mars. Scientists now believe the planet Mars once had water but doesn't anymore because its small size did not allow it to hold on to its water and atmosphere for much longer than a billion years. The outer edge of the habitable zone is defined as that point beyond which water remains frozen all the time. In the case of our solar system, water remains frozen when a planet is farther away from the Sun by at least 37 percent more than Earth's orbit--but Europa, if it indeed has liquid water, would disprove this theory and show that the habitable zone is not as restrictive as scientists may have thought. The habitable zone for other stars may be defined differently from that of our own solar system. The reason for this is that other stars have different masses and energy outputs, and hence produce different temperatures at various distances.

    But [n.sub.e] in Drake's equation, the environmental factor, requires more of a planet than that it lie within the habitable zone. It also requires the existence of oxygen and organic compounds. Scientists believe that discovering oxygen anywhere in space will be a good indication that life may exist there. Oxygen is very reactive, and it usually is not found on its own but rather as a component of carbon dioxide or other compounds. Thus oxygen molecules or ozone ([O.sub.3]) are not likely to exist on their own unless they are produced by a metabolic process of life. Recent discoveries of oxygen on Saturn's moon Titan (though in small quantities) give hope for finding some form of life there.

    Some scientists believe the fraction of stars with the right environment for life is roughly 10 percent.

[f.sub.l]: The Fraction of Planets with Life

The term [f.sub.l] in Drake's equation stands for the actual proportion of planets on which life actually forms. Estimating this term is extremely difficult because we have absolutely no data on planets other than Earth that actually have life. Therefore, any statistically based estimation is impossible. The purpose of this book is to arrive at a probability for the existence of life on at least one extrasolar planet. Scientists involved in Drake's search for extraterrestrial life have conjectured that the value of the parameter [f.sub.l] is about 0.1 or 0.2. It is not clear how they arrived at these numbers.

[f.sub.i]: Intelligent Life

The term [f.sub.i] in Drake's equation stands for the proportion of planets on which intelligent life evolved. Again, there are no data for any statistical estimates. Later we will discuss the difference between life and intelligent life and review the argument as to whether intelligence is a fluke in the genetic development of life on Earth or whether it is a natural outcome of evolution. When scientists at the 1961 conference discussed Drake's equation, numbers ranging from 0.1 to 0.5 were offered as guesses for the value of this parameter.

[f.sub.c]: Communication

The term [f.sub.c] stands for the fraction of planets with civilizations able to communicate with others by radio transmission or other means. Since we have not yet received a single radio signal from outer space, we have no data for any statistical estimation of this parameter. Imagine an intelligent society similar to that of ancient Greece. This is a civilization that has achieved great knowledge, art, political thought, and everything we have come to consider as advanced--except for technology. Such a civilization would satisfy all the other term definitions of Drake's equation, and yet it would have no radio transmitters or receivers. On the other hand, imagine a civilization that is so advanced that it does not require radio-wave transmissions in order to communicate with its members: for example, members of this society may have fiber-optic networks that make radio transmission unnecessary. Part of the idea of the Search for Extraterrestrial Intelligence (the SETI project, the descendant of Project Ozma) is that intelligent extraterrestrial creatures may be detected not only by sending us radio signals intentionally, but also because they may be sending radio signals for their own communications. In both examples above, we have an advanced intelligent society that does not communicate by radio signals and will therefore not be detected by SETI or any other project that listens to radio waves from space.

L: Longevity

Finally, the term L in Drake's equation is the longevity of the civilization. The reasoning behind the inclusion of this last factor in the equation is that intelligent civilizations may eventually destroy themselves. Remember that the meeting took place during the height of the Cold War. The assumption on everyone's mind was that intelligent life does not last, since war--nuclear war--will eventually destroy it. This factor, the length of time of a civilization's existence, must be converted to a fraction, by dividing it by the length of time the Milky Way galaxy exists. What was aimed for here was the fraction of civilizations in our galaxy that are still alive, so that we can communicate with them. Of course, because of the tremendous distances involved, there is some uncertainty as to what we mean by this existence and what we mean by communication. If we get a signal from space that was sent fifteen thousand years ago, by the time we receive the signal, the civilization that sent it may no longer exist. A pessimistic, estimate of L might be from the time of the invention of the radio by Marconi to the time of Hiroshima. This is a very short period of time within the life of the universe. An optimistic estimate might be several million years. When converting L into a fraction, we get a small number. The idea is that the window of opportunity for communicating with extraterrestrial civilizations may not be very wide.

At the Green Bank meeting, Frank Drake and Philip Morrison met again. Drake had been a science student at Cornell, where Morrison taught. Now they met as the two prominent champions of the search for extraterrestrial intelligence. Morrison's coauthor, Cocconi, did not come to the meeting and, in fact, completely dropped out of the project and eventually became the director of the European Center for Nuclear Research (CERN). The meeting attracted many participants from around the world. Among them was Carl Sagan.

    Over the years, Drake's equation received wide acceptance in the scientific community. The ideas it expressed would inspire the search for extraterrestrial life for decades, and the thinking behind this equation would eventually result in the launching of the SETI project (depicted in Sagan's novel Contact and in the movie of the same name). The search began on October 12, 1992, exactly five hundred years after Columbus reached the New World, as the giant antenna at Arecibo, Puerto Rico, turned to the sky to look for radio transmissions from other civilizations.

    Frank Drake believes that the number of civilizations in our galaxy that can communicate with us is as large as 10,332. He bases this guess on his estimate of the various terms in his equation. The late Carl Sagan believed the number of such civilizations was about 1 million. In this book, we are not concerned with the number of extraterrestrial civilizations in our galaxy able to communicate with us. We are interested in estimating the probability of the existence of life, and of intelligent life, elsewhere in the universe. The first few terms in Drake's equation are very important to us, while some of the others are less so, since these latter conditions may be impossible to estimate in a meaningful, scientific way. Let's concentrate on the crucial terms, the ones we have some knowledge about, and summarize our knowledge about them thus far.

The Goldilocks Search

Once the existence of extrasolar planets is confirmed, the big question that remains is whether any of these planets are suitable for life as we know it to develop. A planet that orbits close to its star is too hot and cannot have liquid water. A planet that orbits far from its star is too cold and its water frozen. Scientists looking for the perfect planet have therefore named their endeavor the Goldilocks search: not too hot and not too cold. Once such a planet is found, other questions implicit in Drake's equation are whether the chemistry is right: Is there water on the planet? Is oxygen present? Are nitrogen and carbon present in the right amounts? These questions can be answered by science. We have methods of analyzing spectra of elements, and we can tell whether any element is present on a planet--if, that is, we can see light from the planet. Once the location of a star with a planet is ascertained, space-borne telescopes could be used to analyze the very dim light from such planets and tell us about the chemical composition of the planet. At present, the expected cost of such a space mission to analyze the spectra of extrasolar planets is about $2 billion, and it is not known whether Congress and the American taxpayers will agree to fund such a project.

    Assuming the three basic questions--Are there planets? Are they in the habitable zone? Are their chemistries right for life?--are answered in the affirmative, we are left with the final questions of whether life did indeed evolve and, if it did, whether such life is intelligent. But positive answers to the first three questions will already get us close to our goal. In light of new discoveries and the potential pitfalls of making rash decisions with little or no data, it might be a good idea to take a fresh look at Fermi's putative "paradox."

    In pre-Columbian times, a Native American sage could well have said that life on Earth existed only in the Americas. Surely, the logic would go, if life existed anywhere else on Earth, then--assuming there were other continents other than America--life would have developed intelligence, and one of these civilizations would have built large canoes and come here. Since we know of no one who has come here from beyond the ocean, we must be all alone on this "Earth." Well, at some point in time after this "paradox" might have been put forward, someone did come, whether Vikings or Vespucci or Columbus. When people landed on these shores, there was finally contact. The fact that there are time periods prior to the point of contact does not imply any paradox. There must be a first time for everything.

    It is no surprise that the SETI scientists like the analogy with Columbus. When asked why the SETI project has not yet produced a shred of evidence for any extraterrestrial civilization trying to contact us by radio waves, they point to the voyage of Columbus. "You wouldn't ask Columbus after his first five hundred miles whether he's found a new continent," they say. "We are still on our first five hundred miles."

    Unfortunately for the SETI project and its scientists, Congress terminated funding for the search in 1993. But Frank Drake, who heads the project, never gave up. He managed to convince wealthy private donors to contribute to the effort, and their donations now fund the project to the tune of $4 million a year. The sponsors Drake found were William Hewlett and David Packard of the Hewlett-Packard Company, Microsoft cofounder Paul Allen, and Intel cofounder Gordon Moore. Perhaps these donors have some designs for a Cosmic Wide Web.

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