Does ET Exist? Lecture 1

INTRODUCTION

1.1 Are We Unique?

“There is, as I have said, no end; this truth speaks for itself, and blazes forth from the very depths of creation. Neither can we, seeing space reaching out in all directions, infinite and free, and seeds [atoms] infinite in number flying in eternal motion through the void, suppose that only this one Earth and sky of ours has been created, and that those teeming remote bodies of matter serve no other use but ours. Further, as this world was fashioned by Nature through the intrinsic motion, collision, and joining of seeds, thrown about at random without any design, at best accreting when thrown together by chance, so must all great things begin --- Earth, and sea and sky and races of living creatures. There must, I repeat, be such accretions of matter elsewhere, clasped like our Earth, in the embrace of infinite space.”

Lucretius, de Rerum Natura, 1^st Century, C.E.

Those of us fortunate enough at some time in our lives to have escaped our increasingly urbanized environment for the isolation of some remote mountain peak can’t help but to have looked up in awe at the grandeur of a night sky no longer polluted by the background glow of megawatts of city light. Instead of a few fuzzy-looking stars easily ignored under such conditions, thousands of them now accost your senses, impossible to ignore. Some are bright; some are dim; some are large; some are small; some are red; some are blue C and they all twinkle spectacularly. And through them all of them, twists a milky white, luminous ribbon, riddled with dark patches. A quick look through a simple pair of binoculars resolves this nebulous strand into thousands more of these beautiful stars. What this simple instrument has opened up for you is an edge-on view of a spiral ensemble of all the stars that make up a structure known as the Milky Way galaxy. This spiral-shaped structure is unimaginably large, more than 100,000 light years across and 1000 light years thick.^{^[1]} Its member stars, of which our Sun is one, number more than 400 billion. The visible universe contains a trillion more structures just like this one!

That's a lot of stars. It’s no wonder that most people fervently believe that we are not alone in the universe. Many stars resemble our Sun and many of those should have planets as well. Nowadays, we know that some of them do. Surely, some of those planets must be like Earth and some of those must serve as home to intelligent creatures more or less like us. And even if they don't resemble us, at least they ought to be intelligent like us, perhaps even more so. This belief seems to be embedded in the human psyche. Indeed, Stephen Spielberg, in his wonderful movie, ET the Extra-Terrestrial, struck a chord that resonated in all of us, with his delightful characterization of ET (Figure 1.1.2), that marvelous alien creature, so unlike us in superficial appearance, but so like us, or at least like our children, in compassion and sympathy --- those characteristics that define the very essence of our being.

Who of us that saw that movie failed to be deeply moved by the dying ET's woeful plea, "ET phone home" in hopes of contacting his distant comrades that they might come and take him back before he perished here on Earth?

How could we possibly be unique? Our graveyards are littered with the corpses of those who once asserted that Earth was the center of the universe and that its inhabitants were special creatures favored by god. For millennia, we believed that the Earth was a special place in the cosmos for it housed the unique human species. The Sun, Moon, planets and all the heavenly bodies played second fiddle to humanity's home C Earth --- the center of the visible universe. The Copernican revolution relieved us of that five thousand year old delusion. It turned out that the Earth was not the central cosmic drummer to whose tune marched the other members of the solar system. It played only third fiddle in a group of eight similarly disposed musicians engaged in playing the harmony of the spheres.

Failing to absorb the lessons of our past, we subsequently replaced the thesis of a central Earth with one centered on the Sun. Our brilliant, life-giving Sun assumed the role of the unique cosmological structure. If the Earth wasn't the center of the universe, then at least its Sun was. But increasingly sensitive examination of the cosmos with ever more powerful eyes, taught us that even the Sun was only one of many stars, and not the central one at that. It is a rather nondescript star, occupying a fairly insignificant spot about two-thirds of the way out from the center of 400 billion others of similar ilk that make up the Milky Way galaxy. And even worse, the Milky Way galaxy itself proved to be just one of many, insignificantly positioned somewhere in the universe at large.

Given the pitfalls of assigning an unwarranted uniqueness to humanity and its habitat, how could we possibly be so presumptuous that we again ignore the lessons history teaches us by asserting that we are alone as the reigning intelligence of the galaxy?

There are not an insignificant number of scientists who take this position, arguing convincingly that we might indeed be a very unique species. If the argument is correct, it gets us out of a conceptual dilemma known as Fermi's Paradox, paraphrased by “If they exist, then where are they?” Enrico Fermi’s response to the “... existence of intelligent extra-terrestrial life is plentiful” position held by most of his colleagues. We will discuss this dilemma in a later chapter but for now we will remark that there seems to be no other way out of it except to postulate the non-existence of ET or at least to postulate that if ET does exist then he lives a rather insignificant existence in only a few, remote and isolated locations in our galaxy. It's just possible that there are no others out there more advanced than we are. Such a notion makes us all just a little bit uncomfortable. No one likes to be alone and the possibility that, with the demise of humanity, intelligence and rationality could disappear, possibly forever, is a very sobering thought. But if it’s true, we ought to take care that we do not perish by folly for that would be the ultimate irony. It would bring to an end just what might be nature's grandest experiment of all and one that is not likely to ever be repeated again.

Regardless of any arguments that some might put forth about our uniqueness as an intelligent species, most of us hope, or at least have a lingering suspicion, that other intelligent life forms do exist somewhere in the galaxy. After all, you can never conclusively prove the nonexistence of anything or, as Martin Rees so succinctly stated, “... absence of evidence is not evidence of absence.” That's true enough. But conclusive proof of existence is another matter. Unequivocal discovery of even a single alien intelligence out there among the stars or, even more dramatically, floating in a saucer around the Earth, would do the job quite nicely. However, elephants might fly too and we could even estimate the probability that they might do so, but why bother? No one has ever seen one fly and this implies that the probability that they can is so low that the effort of attempting some estimation of that probability based on known scientific principles is likely to be viewed as an exercise in absurdity. Like the song says in that old Disney movie, “Ah done seen 'bout everything when ah sees an elephant fly…”

But if this admittedly weak argument --- that it can’t happen because no one has ever seen it happen --- is the best we can do, then we ought not be too shocked when ET actually lands his saucer on the White House Ellipse, emerges and introduces himself to the President of the United States. At that point, I’m sure that all the skeptics will get down on their hands and knees, beg forgiveness for their flippant skepticism and hum that old Disney tune in tribute to ET’s arrival.

It therefore behooves anyone, skeptic and non-skeptic alike, to try and honestly assess the probability of existence using the best science available. Simply arguing pro or con, with any effort less than that, is inexcusable given what we know in this day and age.

1.2 Why Address the Question Now?

We are the first species that is able to contemplate the nature of its own existence. Our churches are filled with parishioners searching for the meaning of life. Our universities and national laboratories are filled with scientists searching for the pathways that led to the emergence of life. Understanding how it all began --- and why --- is a fundamental issue that captures the attention of all of us. And we all worry about whether or not --- and how --- life might end. In part, we have offspring to insure that it won’t --- but we have too many of them which might insure that it will. Finding just one other ET somewhere in the galaxy would give us all hope that it is possible for a species to somehow sidestep those pitfalls that threaten its existence. Conversely, concluding that our own existence as an intelligent species is an unlikely fluke ought to sober us all up and force us to address the serious global issues that threaten our continued existence. Either way, our view of the world and our place in it will be profoundly altered.

Several recent events have rekindled our curiosity in the question, “Does ET exist?”

(i) Planets have been discovered around other stars.

(ii) Possible fossil evidence has been found in meteorites that came from Mars that indicate that life once existed there.

(iii) There is convincing evidence that an ocean of liquid water exists under the surface of Jupiter’s moon, Europa, that could harbor the conditions necessary for primitive life.

(iv) Hyperthermophilic bacteria have been found living in conditions thought to be too hostile for life thus opening up the possibility that its emergence might be more widespread than previously imagined.

(v) Complex organic molecules have been found in giant molecular clouds and in cometary dust and debris which argue that the seeds of life might be widely distributed throughout the galaxy.

None of these findings, suggestive though they are, answer the question definitively and it is unlikely that we will answer it soon. Even so, findings like the ones above give us hope that direct evidence of what might prove to be the primitive forerunner of an ET lies just around the corner and that it is now worth the effort to search for it with renewed vigor.

We are the first generation seriously equipped to attempt to answer the question, “Does ET exist?” We have acquired an almost complete understanding of the fundamental laws of physics that underlie the behavior of the natural world and we have accumulated a vast array of technological instruments to help us probe that behavior more deeply than ever before. Advances in science have led us to the brink of understanding how life began on Earth. We have learned that the scientific principles upon which that understanding is based are universally applicable and thus limit the kind of life that could emerge elsewhere in the galaxy.

The evolution of structure in the universe can be described as a process in which complexity emerged out of chaos as a consequence of physical laws. The emergence of life is one of those processes that can be described by the same universal laws that govern the emergence of any other complex structure. It is true that we do not yet have the complete picture of this process but we are closing in on one awfully fast.

We know that the universe began about 13 billion years ago in an intense primeval fireball called the Big Bang. We know that the fundamental laws of physics and the particles whose behavior they describe were born in its first instants. We know that all the light nuclei in the universe --- hydrogen, helium, and traces of lithium --- were created in the first few minutes. 300,000 years after the beginning, electrons combined with those nuclei to form atoms. Radiation decoupled from matter and the gravitational force caused some of it to clump together to form large scale structures like galaxies and stars. The first stars formed fused the hydrogen in their cores to make the elements of intermediate atomic weights. Upon exhausting their nuclear fuel, massive stars created even heavier elements in cataclysmic explosions known as supernovae. These heavier elements then mixed in with the hydrogen gas that permeated galactic structures and formed second generation stars with large amounts of dust around them that then condensed into planets. Our Sun is one of those second generation stars and the Earth is one of those planets. On some of those planets, the conditions were ripe for life to emerge.

We know that stars form out of clouds of gas and dust. The Hubble telescope has taken pictures of stellar nurseries where the formation process is in action.

Our theories of gravity, nuclear and atomic physics and thermodynamics tell us how stars live; how they generate energy by nuclear fusion and how their internal pressure supports their massive weight in a stable configuration. We also know the conditions they must satisfy if they are to steadily radiate energy into space for a time long enough that life can emerge on a planet that happens to orbit it at the right distance. We also know that all stars inevitably die and how they do it when their time comes.

In 1983, IRAS, the Infrared Astronomical Satellite discovered infrared emissions from several stars suggesting the presence of a disk composed of warm dust and gas swirling around them. In 1984, a direct image of the disk around the star, Beta Pictoris, was obtained.

These observations were direct evidence of the material that must exist around a star if planets are to form. In 1995, direct evidence of planets in orbit around other stars was found. Admittedly, these planets are all of the Jovian type; no Earth-like planets had yet been found --- not because they don’t exist but because our instruments weren’t yet good enough to detect them. If Jupiter-like planets exist, then Earth-like planets should too.

Recently, we’ve detected a planet about 5 times larger than Earth and about 3 times further from its Sun, a red dwarf star, than Earth is from our Sun. We now have the impetus to build the instruments sensitive enough to find Earth-like planets.

Spacecraft launched from Earth have visited most of the planets in our solar system and many more visits are planned for the near future.

The planetary science that we have derived from these visits has --- paradoxically --- taught us more about our own planet Earth than has millions of years of existence on its surface. We now have a better understanding of the environmental conditions that existed on primitive Earth and how those conditions proved hospitable for newly emerging life forms.

Continued advances in biochemistry and planetary science suggest that the environmental conditions that make life possible come in a wider variety of flavors than previously supposed. It seems that life is capable of drawing its energy from many more sources other than direct sunlight.

We do not know exactly where, or the details of precisely how, life originated on Earth but it now appears that it could have gained a foothold in any of a sizeable number ecological niches. We suspect that it could originate on other planets not necessarily in precisely the same way as it originated on Earth, but in a variety of other ways as well. If life is as robust and flexible as it appears to be, then it ought to be a rather widespread phenomenon and we ought to conduct a vigorous search for it.

The Earth is not unique if it is classified in a fairly non-restrictive way. Classes defined by only a few characteristics usually contain many members. Classes defined by a large number of characteristics contain only a few members and the Earth might be a quite unique member in a restrictive class; it might be one of a few places that harbors the special conditions necessary to nurture primitive life long enough for intelligent life to evolve! As we have indicated, it’s a big jump from primitive life to intelligent life and even though life might be prevalent throughout the universe, it might prove very difficult to hold a conversation with any of it! The key word here is intelligence; if we’re going to look for it, we better know what it is, otherwise we might not recognize it when we find it. We will argue that the scientific basis behind the emergence of intelligent life is restrictive enough that it is unlikely that unrecognizable intelligent life forms would emerge. The goal of this text is to present the science behind the search for extraterrestrial intelligence. It is essential that we decide whether or not it is worth searching and if it is, where we should look and how should the search be carried out. The key issue then is to estimate as best we can how many places there are in the galaxy, other than Earth, where intelligent life has emerged. If it likely happened in many places throughout the galaxy then we ought to be able to establish contact with some of its inhabitants --- but if it is likely to be a rare occurrence then maybe we should forget about a search until new facts come along that convince us otherwise. If none do, then we will have to reconcile ourselves with being alone.

1.3 Intelligence --- How Much Is One Plus One?

Does ET exist? Before plunging into a discussion of what we currently know about the probability of ET's existence, we need to be very clear about what we mean by intelligence. Learned academics could debate the nature of intelligence ad infinitum and I daresay never come to any agreement about what it is. But there is one definitive characteristic of intelligence that is an essential prerequisite for any ET who wishes to contact another:

An intelligent life form must know the fundamental laws of physics and it must have used that knowledge to have attained a certain level of technological expertise: namely, it must have achieved the ability to utilize some component of the electromagnetic spectrum in order to communicate across the void of space and time.

In order to make ET aware of our own existence, we had to achieve this ability, i.e., we had to acquire the capability of beaming radio waves into space.^{^[3]} A species has not become operationally intelligent unless it has developed this capability and we will argue that it cannot develop this capability by trial and error. It must first discover many of the fundamental laws of physics and be well on its way to discovering the rest. The human species did not reach such a stage until about 80 years ago when it invented radio.

Does this mean that dinosaurs weren’t intelligent during the 150 million years that they terrorized the other creatures that walked the Earth, including our likely forebears --- small, burrowing rodent-like mammals that were afraid to venture forth except during the dead of night? What about dolphins right now? Aren’t they intelligent? John C. Lilly, an eminent neuroscientist famous for his experiments in communication, argues convincingly that they are. And what about the human species prior to the 20^th century? Weren’t the ancient Babylonians intelligent when they developed a mathematics that allowed them to predict eclipses of the Sun and Moon? Weren’t the ancient Greeks intelligent when they began a method of intellectual inquiry that would serve as the foundation of modern western science? Could we possibly argue that Confucius wasn't intelligent when he put forth a philosophical system to which billions of people still adhere or that Sir Isaac Newton wasn't intelligent when he worked out the laws of mechanics that pretty much describe the motional behavior of the entire known universe? The answer is no --- none of them built radios; none of them built radio telescopes; none of them reached the stage of operational intelligence.

Even as you read these pages, the Amos n' Andy shows of yesteryear are spreading outward from Earth at the speed of light, passing by other star systems with each passing each year. These radio signals brand us potentially intelligent members of the galactic community of ET’s. Those signals that originated with our first radio broadcasts are now 80 LY away. Any species exposed to this radio flux might have already detected and deciphered the signals as you read these pages. That particular ET would know that they are no longer alone in the galaxy (although after deciphering our Amos n' Andy shows, they might wish they were). But the only ET’s that could be aware of our existence this way are those that lie within a sphere whose radius is 80 LY and those that have developed the ability to detect long wavelength electromagnetic waves, i.e., radio waves. And as you will soon see, that is likely to be precious few. The converse situation is not true. As soon as we turned them on, our radio receivers could have detected intelligent signals from ET’s potentially from any point in the Universe, at least within a few billion years after the Big Bang. Such ET’s would be ancient by our standards. For example, if an ET who transmits radio signals lives in a star system, say a thousand light years from us, they must have become operationally intelligent at least 1000 years ago and they must have been sending information for a reasonable fraction of the 80 years that our receivers have been operational; otherwise, we could never know of their existence. In any case, we could not have detected evidence of ET --- nor could they have detected evidence of us --- until about eighty years ago, Greek civilizations and Newtonian geniuses notwithstanding.

What Do You Think About That?

Radio waves are electromagnetic waves that travel at the speed of light. Are radio waves the same as light? Is light an electromagnetic wave?

Some of you might argue that such a definition of intelligence is far too restrictive and need not be a prerequisite for determining whether or not ET exists. Suppose ET actually comes here and makes its presence physically known to us, regardless of our state of intelligence. Some of you might believe that there is evidence that they might have done this already, perhaps thousands of years ago. Therefore, ET would discover us and we might discover ET before we became operationally intelligent. This is certainly a possible route of discovery, but in such a case, even though we had not yet reached the stage of operational intelligence, ET most assuredly had. If they could accomplish the prodigious feat of interstellar travel, then they must have discovered the laws of physics and in doing so had already become quite familiar with the electromagnetic spectrum and what could be done with it. But if ET found us thousands of years ago, they did so by accident. There is no way they could have known of our existence since we had not yet emitted any radio waves or any similarly coded form of electromagnetic energy. So, discounting such a serendipitous discovery by ET, we argue that intelligence in the strict operational sense is a necessary prerequisite if one ET is to establish contact with another.

So how do we search for life? Direct searches by probe, or human-manned missions, will be limited to the domain of our own solar system, at least in the near future. Any life discovered this way would most likely be of a primitive form. In fact, any extraterrestrial life to which we are first exposed by direct contact --- anywhere --- will most likely be of a primitive form. Our best bet for detecting intelligent life is via our large radio telescopes directed towards the stars, searching for the radio emissions that signal the presence of another operationally intelligent species somewhere else in the galaxy. Indeed, this is the goal of project SETI about which we will have much to say later in the text.^{^[4]}

1.4 Project Ozma

The radio astronomer, Frank Drake, had always dreamed about life on other planets and the forms that it might take. He never talked very much about his thoughts on the subject for fear of ridicule by scientists who took such notions about as seriously as the imagined aliens that romped through the pages of science fiction pulp. Drake, though, was very serious about it so he kept these thoughts to himself. In the fall of 1951, Drake was an undergraduate at Cornell University in Ithaca, New York. That year, the astrophysicist, Otto Struve, had been selected to give a set of talks as part of Cornell’s prestigious Messenger lecture series and Drake went to listen. Struve was an expert on the structure and evolution of stars. He studied stars by analyzing the spectra of their light.^{^[5]} Light can be thought of as a wave. It is characterized by its color which is defined by the length of the wave. The light from a star contains waves, or colors, of all types and varieties in amounts that vary in a fairly smooth or continuous way. The distribution of wavelengths in the mixture that makes up the light from any star is called its spectrum. A scientist who studies spectra of light is called a spectroscopist and the instrument used to carry out the study is called a spectrograph. A spectrograph is somewhat like a prism. It breaks up light according to its color, or wavelength, although in much finer detail. When light from a star is passed through a spectrograph and then photographed, the resulting picture exhibits a bewildering array of sharp bands of wavelengths that are almost completely missing. For example, the spectrum from our Sun is shown in the accompanying figure.

The pattern resembles the bar code on items in supermarket and, in fact, conveys the same kind of vital information about stars to an accomplished spectroscopist, that a bar code conveys to its scanner. The bar code tells the scanner what the item is and how much it costs. A spectrum tells the spectroscopist what the star is made of and how it is put together. The characteristic dark lines in the spectrum of the star are identical to the kind of dark lines that can be seen when white light is passed through a gas in a laboratory consisting of specific atomic elements. The lines in the laboratory spectrum uniquely signify the presence of hydrogen, helium, argon, or any other element that makes up the gas. The stellar spectrum tells us that stars are big balls of mostly very hot hydrogen and helium gas with traces of other elements that we find on Earth. This observation and others like it have rather profound implications: it tells us that stars, planets and everything else in the universe is made of the same basic building blocks that make up Earth and its inhabitants. It reinforced Drake’s view that life could be rather commonplace. But Drake knew all this.

It was Struve’s last lecture that really grabbed Drake’s attention. It was about light generated by rotating stars. When a source of light is moving toward or away from an observer, the received light is Doppler-shifted,^{^[6]} that is, the wavelength of the received light is either compressed towards the blue or stretched toward the red end of the light spectrum exactly like the pitch of the whistle of a train is raised or lowered by its motion either towards or away from the observer. The rotational motion of a star broadens its dark spectral lines because some of the light is emitted from points on the star moving toward us while some is emitted from points that are moving away. The width of the lines are like a speedometer --- the more rapid the rotation C the more the spectral lines are broadened. Struve showed data that indicated that large, hot, massive stars tended to be rapid spinners while small, cool, less massive stars like the Sun tended to spin slowly. Struve was sure that the distinction between the two types was a signature of the absence, or presence, of an unseen planetary accompaniment. The fast spinners spun alone; the slow spinners spun in the midst of a planetary system. He argued that stars form out of gravitationally collapsing clouds of gas and dust and they spin up as they do so, like an ice skater that pulls in her arms to dazzle the audience with a fast pirouette. But if a disk system forms with the stars from which planets spring forth, the stars give up their fast rotational motion by frictional interaction with the surrounding disk. Struve was convinced that, in addition to the nine planets that we already knew about, there were probably another 100 billion of them in orbit about the ten billion slowly rotating stars that existed in the Milky Way galaxy according to his estimate. This meant that life could, and probably did, exist in every nook and cranny in the galaxy. Drake was electrified by Struve’s conclusion. Suddenly, he was not alone. He had a powerful ally who believed strongly in the certainty of existence of extraterrestrial life C and the belief had a scientific foundation.

In 1959, Phillip Morrison and Giuseppe Cocconi published a paper in Nature suggesting that the newly emerging science of radio astronomy could be utilized to make a search for extraterrestrial intelligence. They argued convincingly that the radio portion of the electromagnetic spectrum would be the most likely choice that an intelligent civilization would make to try and advertise their existence across the void of space to other intelligent civilizations. They suggested a range of frequencies that would be the communication channels that intelligent aliens would most likely use. By then, Drake was a young radio astronomer fresh out of Harvard graduate school. He had just taken a position at the National Radio Astronomical Observatory (NRAO) at Green Bank, West Virginia Bank when he read the paper by Morrison and Cocconi. It excited him tremendously for he had only recently resumed thinking about the possibilities of alien life and he had become convinced that current technology was now good enough to make possible the detection of their existence --- if enough of them existed and if some of them had decided to broadcast evidence of their existence. Morrison and Cocconi’s reasons that a search should be initiated and their arguments about how to do it were almost identical to Drake’s own considerations.

Fortunately, Otto Struve had recently taken on the job of directing the research program at NRAO. Even before the appearance of Morrison and Cocconi’s article in Nature, Drake had quietly approached Struve about using the just-completed 85-foot radio dish at Green Bank to conduct a search for extraterrestrial intelligence along lines outlined by Morrison and Cocconi. Struve agreed that the time was ripe for such a search and Drake set up the plans and started putting together the necessary equipment. He called his project ---

Ozma --- after the queen of the fictional city of Oz. At approximately 3:00 a.m., on the morning of April 8, 1960, Drake climbed out to the focus of the giant dish and tuned the parametric amplifier that sensed and amplified the incident radio waves, a job he did not particularly relish. With the receiver set to scan a band of likely frequencies, he then hooked its output to a chart recorder and a loudspeaker and, along with two student assistants that he had hired for the observatory’s summer program, pointed the giant dish toward the star Tau Ceti, a nearby Sun-like star that he thought might have a decent chance of harboring intelligent life. He then turned on the electronic receiver and tracked Tau Ceti as it moved across the sky until it set in the west at noon --- nothing happened! Drake and the two students then swung the telescope towards Epsilon Eridani, another nearby Sun-like star, that was still above the horizon C and within five minutes, the loudspeaker erupted with strong, regularly-pulsed noise --- eight times a second! He looked at the chart recorder and it banged off-scale in synchronization with the loudspeaker blasts! Drake and his two students jumped up and down! Drake had never seen anything like it. Had they detected evidence of “little green men” in outer space?

A classic test that all astronomers make in such situations is to quickly point the telescope “off-source,” or in a direction in which no radio waves would be picked up if they were really came from a point in the sky. Drake swung the telescope away from the source and the signal went away just as it should if Epsilon Eridani was really the source. He then swung the telescope back on the source but the signal was no longer there. Either the “little green men” were incredibly perverse and had stopped transmitting or the signal had been a spurious one.

In order to check the validity of any future signal, Drake set up a simple radio horn antenna and tuned it to the same frequency as the big Green Bank dish receiver. If the giant radio telescope targeting a celestial source detected a signal that was caused by some local, terrestrial source such as a radio ham operator or some kind of military aircraft or radar, the small, but similarly tuned horn antenna would respond as well. If a real signal of extraterrestrial origin ever reappeared, the horn antenna would not pick it up. Drake and his team continued to scan Epsilon Eridani this way for the next few days. Five days later the signal erupted again C pulsing just like the last time --- eight times a second --- in both antennas! Green Bank had been selected for the NRAO site since it was secluded deep within the Allegheny Mountains, far from most sources of radio interference. The U.S. military had selected, for similar reasons, a nearby site in order to carry out secret radio communication experiments. Drake suspected it really was “little green men” who generated his spurious signals; unfortunately, these little green men were of the terrestrial variety.

Drake did not think of project Ozma as a failure. It was humanity’s first attempt to detect the existence of another extraterrestrial civilization and it removed such notions from the realm of science fiction. The experiment attracted a lot of attention. Mainstream science was now engaged. The seeds of the SETI program had been sown.

1.5 The Drake Equation

The early 1960's were the pioneering heyday of the United States space program. Many of its scientists had been brought up on a diet of pulp science fiction with its tales of aliens on strange worlds. A non-insignificant number of this group was intensely interested in the possibility of detecting intelligent alien life but most were afraid to view their opinions publicly since such speculation was still associated with the stuff of science fiction and UFO’s. Most mainstream scientists dismissed talk of the possibility of alien civilizations as scientific quackery. However, Peter Pearman of the Space Science Board of the National Academy of Sciences, one of our most respected scientific establishments, was not one of them. He heard about Ozma and contacted Drake, suggesting that a conference be held at Green Bank to discuss the possibility of conducting even more serious searches for extraterrestrial intelligence. Together, Drake and Pearman drew up a list of participants to invite to the conference and Drake convinced Otto Struve, the director of Green Bank, to act as its host and chair. Struve agreed and the first conference dealing with the possibilities of detecting intelligent alien life was held in Green Bank in 1961.

How does one assess the chance of contacting alien civilizations? Drake thought that this was the central question to be addressed. He had thought about it a lot and, for reasons that will eventually become apparent, he concluded that any search should focus only on locations within the Milky Way galaxy and not the universe at large.^{^[7]} He reasoned that the probability of establishing contact could be expressed as a product of the rate r at which civilizations that can and desire to communicate emerge in the galaxy times their lifetime L, i.e.,

N = r L

For example, suppose 10 intelligent civilizations are “born” each year somewhere in the galaxy and that their lifetime after becoming operationally intelligent is 100 years. At any instant the number of existing intelligent civilizations spread throughout the galaxy would be one thousand since all of them that were born more than one hundred years ago would have died, but during the most recent one hundred years, one thousand would have been born (see Focus Box 1).

Drake expressed the rate as a product of related factors

Each factor affects the overall probability that intelligent life might emerge somewhere in the galaxy. If all the relevant factors were known, one could estimate the number of intelligent civilizations in the galaxy that would currently be attempting to communicate with the others. Drake hoped that it would be a large number thus giving impetus to making a concerted effort to find one of them. Note, that such civilizations are the ones we have termed operationally intelligent. The number of such civilizations is equal to their rate of formation multiplied by their survival lifetime:

This equation has become known as the Drake equation. The factors in the equation represent the following:

· N the number of coexisting, operationally intelligent, civilizations in the galaxy

· the rate of formation of Sun-like stars in the galaxy

· f_p the fraction of those stars that have planets

· n_e the number of those planets that are AEarth-like@, or habitable

· f_l the fraction of those habitable planets upon which life emerges

· f_i the fraction of those that become intelligent

· f_c the fraction of those that develop the ability to communicate and attempt to do so

· L the lifetime of the intelligent civilization

Focus Box 1 Growth of Civilizations in the Galaxy

Frank Drake proposed that the number of civilizations in the galaxy that is capable and desirous of communicating with others is the product of a rate of birth r times a lifetime L. This condition is true only when civilizations are born and die in isolation, i.e., they do not multiply and spread. Under this condition, the number of civilizations in the galaxy will grow until the death rate equals the growth rate.

Mathematically, we can express this condition as

where is the number of civilizations that are either added to (due to birth) or subtracted from (due to death) the number of existing civilizations in the time interval . Starting from time t = 0, the number N will grow until the rate N/L at which civilizations die, equals the rate r at which new ones are born. If << L, the equation can be written as a differential equation

whose solution is

N grows until it reaches the value r L when the death rate equals the birth rate. This is illustrated by the red curve in the accompanying graph. The blue curve is a solution that includes statistical variations in the number of births and the number of deaths during a time interval taken to be 10 years, or 1/10 the lifetime L.

Note, that the human species does not grow this way. It has multiplied exponentially and spread from its place of origin somewhere on the plains of Africa until now it covers most of the Earth — and it shows no sign of slowing down its explosive rate of growth.

Drake knew that the value of some of the factors in his equation could be estimated reasonably well but he also knew that almost no one had any idea about the value of some of the others. However, he hoped that the equation would focus attention on those factors that were not known so well in order to see if there were any surprises that might pop up that would prove fatal for a favorable estimate for N. For example, it was known that there were about 10¹¹ stars in the galaxy and that about 1/10 of them were stars like the Sun. The galaxy had existed for about 10¹⁰ years, hence, sun-like stars have formed at the rate of about = 1 per year. All the other factors in the expression for the rate r were typically estimated to lie somewhere between 0.1 - 1. Drake, Carl Sagan and most of the conference participants were supreme optimists (that’s why they were there) who argued that these factors were close to one and thus Drake’s equation reduced to:

The final factor left, the average lifetime L of an intelligent civilization in years has been called the longevity factor. Its value was anyone’s guess. Drake and Sagan argued that it could be as large as a million years and hence the galaxy should be teeming with intelligent life.

On the other hand, extreme pessimists put many of the rate factors much lower than 1. For example, assume that the fraction of stars with planets f_p = 0.1; that the number of habitable planets n_e = 0.001; that the fraction where life evolves f_l = 10^-6; that the fraction where intelligence arises f_i = 0.01 and that the fraction that develop the ability to communicate and decide to do it f_c = 0.1. Then r = 10^-13 and Drake’s equation becomes:

In this case, intelligent civilizations would have to exist 100 times longer than the age of the galaxy in order for at least one to be in it. Obviously, our existence would be an incredible fluke.

Even today, the Drake equation leads to such a wide disparity between estimates of the number of intelligent civilizations in the galaxy that some take it to be almost worthless. It looks as though it is nothing more than a sophisticated, mathematical way of saying, “ ... your guess is as good as mine.” However, it serves to focus one’s attention on those factors that are subject to the most uncertainty of those Sun-like stars that have planets, what fraction of them will be habitable? On how many of those will life emerge? What is the probability that the life will become operationally intelligent? How long will such civilizations last? In this sense the equation is worth a lot. It draws our attention to the things we need to know and what we need to do if we want to intelligently assess the probability of whether or not we are alone in the universe.

And what will it mean for humankind if we find out one way or another? Positive evidence that another ET exists could be an extremely uplifting --- or humbling experience. It would certainly give us hope that as a species --- we can make it --- since, somehow, another species like us managed to avoid going extinct, either from an externally-induced or self-inflicted global cataclysm. On the other hand, lack of evidence, after a committed and exhaustive search for other ET’s, ought to make each and every one of us sit up and take notice that our very existence is an extremely precarious and precious thing --- and that we need to take great care and tread softly if we are to preserve this wonderful gift we call intelligent life for future galactic inhabitants.

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