Aliens: Can We Make Contact with Extraterrestrial Intelligence?by Andrew J. Clark, David H. Clark, Andrew Clark
The startling scientific answers to questions about advanced life on other planets.
If elementary life forms are common throughout the cosmos, could intelligent beings have evolved elsewhere, and are they seeking us out? A father-and-son team of scientists-both with research backgrounds in astronomy and physics-gives us the most up-to-date scientific answers about… See more details below
The startling scientific answers to questions about advanced life on other planets.
If elementary life forms are common throughout the cosmos, could intelligent beings have evolved elsewhere, and are they seeking us out? A father-and-son team of scientists-both with research backgrounds in astronomy and physics-gives us the most up-to-date scientific answers about extraterrestrial civilizations and our attempts to find them. If they exist, why haven't we been able to make contact? Could they be reluctant or unable to make themselves known? If aliens visited us before recorded history, are we now overdue for another visit? Even if we discount most UFO sightings as erroneous, how do we explain that more than four million Americans claim they have been abducted by aliens? Is there a case to be made for a future scientific study of UFOs? Here, in language requiring no prior specialized knowledge, the authors pull together the strands from all plausible scientific answers to present a unique merging of current astronomical findings with philosophical interpretation of the techniques used in the search for extraterrestrial intelligence.
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The Drake Equation
"It is a capital mistake to theorise before one has data."
It is widely believed in the world of publishing (although hopefully mistakenly so) that the inclusion of a single mathematical equation in a book for a general readership will slash the potential sales. Therefore, the inclusion in this book of a whole chapter on an equation might appear to be the ultimate literary folly. However, the so-called Drake equation is at the heart of the modern search for extraterrestrial intelligence (SETI), and hence an understanding of what the equation is about will prove extremely beneficial in addressing the issues surrounding the possible emergence of intelligent life in the cosmos. One really cannot understand SETI without trying to understand what the Drake equation is telling us. It is no good hiding it away in an appendix, or pretending it does not exist. If you want to appreciate the challenges of SETI, you have to understand what the Drake equation is all about. It is as simple as that. Hence, we have decided to be brave, and to put the equation right up front. No ambiguityonly clear explanations. No fudging the issuesjust straight talking. No lame excusesjust the hope that the conventional publishing wisdom about equations is wrong. Please stay with us; the effort will be worth it, we promise. Here goes.
The Drake equation has become one of the icons of modern science, almost on a par with Einstein's E = mc2. (You might even see it displayedon a T-shirt!) Don't be put off by the mathematics in what followsthe Drake equation is really just about common sense. It is not necessary to try to understand the mathematics fully. If you find equations confusing, read over them and concentrate on the words of explanation. We promise that the effort will have its rewards in understanding the challenges of SETI, and appreciating the likelihood that SETI will ultimately be successful.
An Imaginary Drake Equation
To understand the purpose of the Drake equation, let us imagine a situation somewhat more familiar to everyday experience than the search for ETI, namely, buying a pair of new shoes. We could have chosen just about anything that people frequently have to make choices about as a suitable illustration, but shoes seem to be suitably gender and age neutral. Suppose you saw a notice outside a shoe store grandly announcing, "Over 12,000 pairs of shoes to choose from!" If you were in need of a new pair of shoes, a claim of such a large stock would undoubtedly impress you. You might imagine that you would have ample choice from such an impressive number of pairs of shoes, on the assumption that the owner had established the stock evenly across all possible sizes and styles. Think again! The choice may not be as great as you at first suppose.
Of the 12,000 shoes, let us assume that half are for men and half are for women. Your choice is immediately reduced to 6,000. Let us then assume that shoes come in ten sizes. Now your choice is down to 600. Let us assume that there are three standard widths for each size. Now your choice is down to 200. We should then concentrate on the four basic shoe types available: leather upper and sole, lace-up; leather upper and sole, slip-on; leather upper and composite sole, lace-up; leather upper and composite sole, slip-on. By opting for one of these types, your choice of shoes is now down to 50. Then suppose there are five standard colors: black, dark brown, tan, gray, and white. Now the choice is down to 10. If you went into the shoe store expecting a very wide choice, you would soon realize otherwise: After you state your foot size, width, sole, and color preference, the store assistant may produce only ten pairs that you can then consider for style, comfort, and favorite brand name. This is not such a great selection after all, should you have been attracted initially by the sign signaling a massive stock. The shoe store owner had sought to attract your attention by appealing to large numbers12,000 shoes in stock. Essentially the unstated claim was "with so many shoes in stock, is it really conceivable that we won't have a pair suited to your needs?" Remember the assertion of Metrodorus: "To consider the Earth the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet only one grain will grow." The shoe store owner is saying, in effect, "to consider that we will not have your choice of shoes in such a large stock is as absurd as to assert that in an entire field sown with millet only one grain will grow."
Like the shoe store owner, some of the proponents of SETI love appealing to the large numbers readily available in astronomy to attract your attention: Is it really conceivable that in a universe containing billions of galaxieswith each galaxy containing billions of starshumans are the only intelligent life-form to have evolved? The answer may be no, but do not be fooled by the astronomically large numbers into expecting a selection of intelligent beings any greater than your disappointingly small choice of shoes from a massive stock.
To establish an equation to represent our choice of shoes, we need to introduce a form of mathematical shorthand. For the number of shoes we are likely to have available for a final choice, let us assign a symbol N. (Using symbols in mathematics just saves writing a lot of words: for "the number of shoes available for a final choice," merely think "N.") Suppose the total number of shoes in the store is N*. For the fraction of shoes available for either gender, we will use a symbol fg; the fraction of a particular size will be given the symbol fs; the fraction of a particular width will be fw; the fraction of a particular sole-upper combination will be fu; and the fraction of a particular color will be fc. We can then construct a very simple equation, where each symbol following the equal sign (=) is multiplied by the one that follows. The equation is as follows:
N = N*fgfsfwfufc
Each of the f symbols acts as a "filter" on our choice; we filter out the sizes we do not want, we filter out the widths we do not want, and so forth. For the values assumed in the above description, we would have the following:
N = 12,000 x 0.5 x 0.1 x 0.33 x 0.25 x 0.2 = 10
(The identification of gender means that the total number is multiplied by 0.5; the selection from ten sizes means we multiply again by 0.1; the selection from three widths means we multiply again by 0.33; the selection from four sole types means we multiply again by 0.25; and the selection from five colors means we multiply finally by 0.2, giving the final number of possibly suitable shoes as just 10 out of the stock of 12,000.)
Just for fun, we will call our equation "the Drake equation for shoes." Using a form of mathematical shorthand, we have started with the total number of shoes in the shop, and estimated how many might be suitable for purchase; not many, it turns out, from such an impressively large stock. Do not worry too much about the use of symbols in the "Drake equation for shoes"; just try to appreciate the method of estimation being used.
Here we are merely using the tools of mathematics to present a point of view in a logical and consistent way. But it is not the tools of mathematics that are important for our purpose here; it is the "common sense" that counts. And common sense (as well as everyday experience) soon leads to the conclusion that it is often difficult to find the shoes of your choice when you go shopping for them, regardless of how large the initial stock in any shop might be. Our imagined shoe store owner's appeal to large numbers has actually disguised a somewhat modest choice.
Our "Drake equation for shoes" is the product of "probabilities"; that is, what is the probability that a pair of shoes will be the correct size, what is the probability that they will be the correct width, what is the probability that they will be the right sole type, and what is the probability that they will be the desired color? In the imaginary situation we considered, the product of probabilities gives an overall probability that from a stock of 12,000 shoes, only about 10 would be of interest to any of us.
The overall probability of a SETI detection also depends on multiplying several individual (but interrelated) probabilities.
The True Drake Equation
The true Drake equation used for SETI has a similar form to our imaginary "Drake equation for shoes." But instead of being impressed by the sign outside the shoe store announcing "Over 12,000 pairs of shoes to choose from!" we are looking for life in a fabulously stocked Milky Way Galaxy where we have "Over 400 billion stars to choose from!" Again, as with the shoes, the choice looks enormously impressive. But again, as with the shoes, once we start applying filters to get rid of the stars and planetary systems that do not fit our requirements, we are left with a somewhat more modest range of planetary systems on which intelligent life might have evolved. We must resist the temptation to be overly impressed by any appeal to large numbers!
The true Drake equation comes in a variety of forms, a convenient one for comparison with our shoe equation being as follows:
N = N*fpneflfifcfL
Here N is the number of presently existing advanced civilizations in the Milky Way Galaxy able to communicate by radio transmissions, the number we would like to estimate (albeit with recognized uncertainty) to establish a basis for SETI. N* is the number of stars in the Milky Way Galaxy of the type that might survive long enough for life to evolve on any planetary system. The symbol fp is the fraction of those stars with planets. The symbol ne is the average number of planets per solar system whose environments are suitable for life. The symbol fl is the fraction of habitable planets on which life occurs. The symbol fi is the fraction of life-bearing planets on which intelligence evolves. The symbol fc is the fraction of planets with intelligence where radio communication potential develops. The symbol fL is the fraction of advanced civilizations existing at this time.
N*, fp, and ne are astronomical factorsit is the astronomers who can provide reasonable estimates of what these might be. By contrast, the terms fl and fi are the biological factors, where we require guidance from evolutionary biologists. And finally, the sociological factors fc and fL require some imaginative thinking about the social evolution of intelligent civilizations. The three astronomical factors lend themselves to observational investigation (and are likely to cause the least dispute). The two biological factors can reflect evolutionary theory (although here there is some dispute about this approach). However, for the two sociological factors we are into the realms of unbridled speculation, which does leave a feeling of unease.
It is fair to observe that, complicated as the Drake equation might already appear, many additional (albeit possibly less important) astronomical, biological, and sociological terms could be added to make it appear more robust to counterargument. For example, the impact of various terrestrial (for example, volcanoes) and extraterrestrial (for example, cometary impact) influences on the evolution of life on Earth could be considered. Later, we will see just how important such influences can be. A more rigorous Drake equation could accommodate many such additional factors, although this might divert attention from its primary purpose of focusing consideration on the principal factors that could determine the conditions for ETI. In Chapter 3 we will revisit this matter, and look at the effect of including plausible additional factors. We will ignore this potential complication until then.
The main appeal of the form of the Drake equation used here is clearly the way in which each successive term acts as a filter through which only those star systems we might potentially be interested in pass. We are then left with the residue of advanced civilizations, which in the case of contemporary SETI is usually understood to mean civilizations capable of interstellar communication.
The Start of It All
The SETI bandwagon was set rolling, at least in a public way, in a famous paper written by Giuseppe Cocconi and Philip Morrison, and published in the scientific journal Nature in 1959. In this classic work they argued that radio signals from ETI might be detectable with the radio technology then available. (Morrison, a gifted physicist and a great popularizer of science, was one of the Manhattan Project pioneers. His book reviews in Scientific American achieved cult status in recent decades. Cocconi was a student of the great Enrico Fermi.) In a rousing final paragraph to their fine paper "Searching for Interstellar Communications," the two Cornell University scientists wrote:
The reader may seek to consign these speculations wholly to the domain of science fiction. We submit, rather, that the foregoing line of argument demonstrates that the presence of interstellar signals is entirely consistent with all we now know, and that if signals are present the means of detecting them is now at hand. Few will deny the profound importance, practical and philosophical, which the detection of interstellar communications would have. We therefore feel that a discriminating search for signals deserves a considerable effort. The probability of success is difficult to estimate; but if we never search, the chance of success is zero.
Radio astronomy is at the heart of SETI. Radio waves were discovered by Heinrich Hertz in 1887. The best-known early pioneer of radio communication was Guglielmo Marconi. He predicted that radio would eventually be used for communicating with intelligence elsewhere in the cosmos. Radio astronomy had its beginnings in the experiments of a Bell Telephone Laboratories engineer, Karl Jansky, during the 1930s. Jansky was investigating the nature of radio noise, particularly that generated by thunderstorms that interfered with radio communication. In addition to noise of a terrestrial origin, he reported, "Radiations are received any time the antenna is directed towards some part of the Milky Way system, the greatest noise being obtained when the antenna points to the centre of the system." Radio astronomy was to prove of particular worth in understanding the violent nature of the cosmos. Until the advent of radio astronomy, the heavens were believed to be largely quiescent and unchanging. Radio astronomy was to change that reassuring picture.
The postwar reemergence of radio astronomy was led by a new breed of radio engineers, trained in the radar and radio direction-finding techniques of wartime, but so easily adapted to radio observations of the cosmos. The first discrete celestial radio source was identified in 1946 in the constellation Cygnus. By the late 1950s, catalogues were being produced of hundreds of objects in the radio sky, many of which could be identified with objects also detected in optical telescopes. Some radio sources were identified as lying within the Milky Way, and some were external galaxies.
Early in the history of radio astronomy, a very small fraction of galaxies were recognized to be particularly intense at radio wavelengths. They coincided with faint optical objects, and were given the name quasars. Although the optical objects looked almost star-like, it was demonstrated that they must in fact be galaxies at vast distances. Their radio brightness was such that the quasars had to be intrinsically at least 100 times brighter than any other known galaxy. A search of old photographic plates showed that quasars varied significantly in brightness over a period of just a few years. Nothing can travel faster than light. Hence, no object can coordinate its activity on its remote side with that on its near side in less time than it takes for light to travel across it. Thus, a change in intensity over, say, a 10-year period implies that such a quasar must be less than 10 light-years across, compared with, for example, the 100,000-light-year diameter of the Milky Way. So here was the dilemma: Not only were the quasars intrinsically brighter than any other galaxies, but also their energy was being radiated from a region a mere fraction of the size of a normal galaxy. Today, quasars are thought to be the highly active central nuclei of nascent galaxies at extreme distances. Early in the history of SETI a famous incorrect identification of a quasar led to a false claim of detection of ETI by Soviet scientists (more on that little saga in the next chapter).
Today, massive radio telescopes, with dimensions measured in kilometers, provide astronomers with "radio spectacles" through which they can "view" the heavens in radio waves in considerable detail. The picture revealed by radio astronomy of a universe undergoing violent upheaval and change is dramatically different from the apparently quiet universe observed since antiquity with the unaided eye. Radio observations have made a major contribution to the current understanding of cosmic objects and phenomena. And radio telescopes now provide the powerful "ears" for SETI.
A conventional radio telescope usually consists of a large parabolic bowl to collect the radio waves and bring them to a focus, in the same way as does the concave mirror of an optical telescope. The larger the collecting bowl, the fainter the radio signals that can be collected (for example, a 300-foot-diameter radio telescope could detect the signal from a cell phone at a distance of some 300 million miles). Large radio telescopes are used for SETI. There is a limit to the size of the collecting bowl that can be constructed physically, especially if it is to have the capability to be steered to point to different positions in the sky. Giant telescopes with a diameter of approximately 300 feet that can be steered with precision have been built (the famous Lovell telescope at Jodrell Bank has a diameter of 250 feet). But to achieve larger collecting areas, various novel techniques are used. The 1,000-foot-diameter telescope at Arecibo, used for SETI, is slung in a natural ravine, and although it depends on the rotation of the Earth to sweep across the sky, some clever technology does give it a limited capability to direct the receiving beam.
The size of a radio telescope does not only determine the faintness of signal that can be collected; it also determines the telescope's ability to resolve the structure within a radio source or to distinguish two nearby radio sources. This is referred to as resolving power. Resolving power can be increased by sophisticated techniques combining the outputs of two or more radio telescopes. The emission from most celestial radio sources does not vary over reasonable time scales of days or years (pulsating sources called pulsars being a rather notable exception), and an array of small radio telescopes can be used in a technique called aperture synthesis to mimic the performance of a radio telescope of massive size, as the rotation of the Earth allows the array of small telescopes to sweep out a large aperture. (A development of this technique known as very large baseline interferometry, VLBI, combines signals received at radio telescopes separated by hundreds or thousands of miles.) The aperture synthesis approach is fine when the radio emission is constant in strength and when one can afford the time for the telescopes to sweep out the larger aperture, but would not be appropriate for SETI where a rapid response to a varying signal is needed. It is the big parabolic radio telescopes that have offered the greatest hope of snaring the elusive prize of an ETI signal.
Radio waves are a form of electromagnetic radiation. Other forms of electromagnetic radiation are visible light, X-rays, gamma rays, and infrared and ultraviolet radiation. These various forms of radiation were discovered independently, before it was realized that they were all manifestations of the same physical phenomena. All forms of radiation travel at the speed of light, and are characterized by their wavelength, the distance between adjacent wave crests or troughs. The number of wave oscillations passing a particular fixed point each second is called the frequency. Frequency is measured in cycles per second, which is also called hertz, abbreviated as Hz. All forms of radiation are emitted from stars and stellar systems, and can be detected with suitable telescopes.
Optical and radio telescopes can operate on the Earth's surface (although large optical telescopes are usually placed in the clearer air of high mountain tops). Telescopes to detect X-rays, gamma rays, and infrared and ultraviolet radiation are sent into space because these radiations do not penetrate the Earth's atmosphere (although a little infrared can). There are also benefits in putting optical telescopes into space, to free them from the annoying distortions to optical images caused by the atmosphere, but the high cost of space missions means that the vast majority of professional optical telescopes remain Earth-bound.
Infrared radiation comes from comparatively cool objects and systems, for example, dust clouds and planets, with temperatures measured in hundreds of degrees. Visible light is characteristic of objects with temperatures of thousands of degrees, such as the surfaces of stars, while ultraviolet radiation suggests temperatures of tens of thousands of degrees, characteristic of the outer atmospheres of stars. For X-rays to be generated, objects must be at temperatures of millions of degrees. An intriguing category of X-ray objects involves binary stars (two stars orbiting one another), where one of the pair is a dense compact object and the other is a normal star. Gas from the outer extremes of the normal star is drawn by gravity onto the compact object, like water flowing down the plug hole of a bath with the tap at the other end kept running. The material does not flow directly, but via a swirling disc (an accretion disc) where the gas is heated to the extreme temperatures needed to emit X-rays.
The basic argument for SETI initially put forward by Cocconi and Morrison has a theoretical component, a practical component, a philosophical component, and an appeal to the priority of experiment expected in legitimate science (the ultimate challenge for science is "prove it"). All of these are captured in the stirring final paragraph of their paper. The theoretical component is captured in the phrase "the presence of interstellar signals is entirely consistent with all that we now know." The practical component is covered by the phrase "if the signals are present the means of detecting them is now at hand." The authors turn to philosophy with the claim "few will deny the profound importance, practical and philosophical, which the detection of interstellar communications would have." And finally there is the appeal to the priority of experiment: "The probability of success is difficult to estimate; but if we never search, the chance of success is zero." These various components are all intended to support the final conclusion that "a discriminating search for signals deserves a considerable effort."
The beautifully written final paragraph of Cocconi and Morrison's paper set the challenge for the SETI pioneersand still captures the spirit of the SETI programs presently being undertaken. As a clarion call for an area of scientific research, this final paragraph has had as profound an influence as any hundred words ever written in science.
There is a close relationship between the theoretical and practical components noted above. The real question is whether the existence of other radio-communicating civilizations in our own Galaxy is compatible with all we know. Actually, the most detailed current SETI project involves only the nearest 1,000 solar-type stars, this representing about one-hundred-thousandth of the Galaxy. Hence, what is really needed is a stronger theoretical commitment that ETI is indeed possible in such a relatively small population of stars. We need some indication that the mechanisms by which we believe intelligent life on Earth arose are sufficiently strong to have caused intelligent life also to be present, with a reasonable probability, somewhere in the one-hundred-thousandth or so of the Milky Way Galaxy that we hope to search in great detail in the foreseeable future. In the part of this book addressing the McCrea question, we will explore the basis for this stronger theoretical commitment.
The Green Bank Conference
A year prior to the publication of Cocconi and Morrison's paper, the young radio astronomer Frank Drake, working at the U.S. National Radio Astronomy Observatory in Green Bank, West Virginia, had already started planning just the sort of search proposed by the two Cornell physicists. By 1960, Drake's "Project Ozma" was under way. SETI was born into an uncertain world, of hope and vision more than matched by skepticism and funding uncertainties.
The simplest way to see the true function of the Drake equation in SETI is to consider the historical context at which it was first introduced, at an informal conference held at the National Radio Astronomy Observatory in Green Bank in November 1961. This conference was the first occasion on which the radio astronomers who were just beginning their search for extraterrestrial communications invited those from related disciplines to comment on their work. An interesting invitee was John C. Lily, who had just published his controversial book Man and Dolphin, in which he had claimed that dolphins were an intelligent language-using species. If anyone could comment on a plausible range of values for fl and fi, then surely Lily could. Another attendee, Melvin Calvin, a biochemist, was actually awarded during the course of the conference a Nobel Prize for his work on chemical pathways in photosynthesis. Bernard Oliver, an electronics expert from Hewlett-Packard and a remarkably influential presence in SETI history, was another key participant. Morrison attended, as did planet expert Carl Sagan, who was to become one of the great popularizers of astronomy, and of SETI in particular.
Trying to draw together scientists from diverse fields, Frank Drake, who was responsible for the scientific organization of the Green Bank conference, began by writing down the topics to be considered during the discussions. As he later recalled:
I took on the job of setting an agenda for the meeting. There was no one else to do it. So I sat down and thought "What do we need to know about to discover life in space?" Then I began listing the relevant points as they occurred to me....
I looked at my list, thinking to arrange it somehow, perhaps in the order of the relative importance of the topics. But each one seemed to carry just as much weight as another in assessing the likelihood of success for any future Project Ozma. Then it hit me: The topics were not only of equal importance, they were also utterly interdependent. Together they constituted a kind of formula for determining the number of advanced, communicative civilizations that existed in space.
I quickly gave each topic a symbol, mathematician style, and found I could reduce the whole agenda for the meeting to a single line.
Of course I didn't have real values for most of the factors. But I did have a compelling equation that summarized the topics to be discussed.
My agenda equation later became known as the Drake Equation.... I'm always surprised to find it viewed as one of the great icons of science, because it didn't take any deep intellectual effort or insight on my part. But then as now, it expressed a big idea in a form that a scientist, even a beginner, could assimilate.
How a conference agenda piece became one of the great icons of modern science is an intriguing bit of social history, reflecting the interest people have in the prospect of detecting intelligence elsewhere in the cosmos. Frank Drake became the "patron saint" of SETI, and it is fitting that the equation that captures the aspirations of SETI should carry his name.
Although the Drake equation has been at the heart of modern SETI, it has not been without its critics. The critics claim that we simply lack the theory to attach warranted values to most of the terms in the Drake equation. The equation has been criticized as falsely raising hopes of a detection of ETI, since it appears to suggest that detailed scientific methodology is being brought to bear to calculate a certain outcome. There are still critics aplenty, even after almost 40 years of the Drake equation being used to argue for (and against) the reality (or otherwise) of ETI. The Drake equation has survived all that the hostile critics have thrown at it, and come through with its honor intact.
The Drake equation and SETI are based on the premise that there is nothing special about planet Earth, that the conditions on Earth that allowed intelligent life to evolve could be repeated many times over elsewhere in the cosmos. Critics have attacked even this premise.
In defense of SETI, it is clear that critics may have read more into the Drake equation than was ever intended. The Drake equation simply does not play the role in SETI that is often claimed by many critics, as well as some supporters. The equation was introduced (at the 1961 Green Bank conference) as a conceptual tool for organizing discussion. Many fail to realize that what is presented as an equation need not function, fundamentally, as a calculating device. It must be conceded that it is not only the critics of SETI who fall into this trap; many of the scientists actually presenting arguments in support of SETI make a similar error. However, if we are looking at the best practice of SETI proponents, then the criticisms are not valid. The Drake equation serves to concentrate research on the issues that matterand its role in SETI therefore has been fundamental. What the Drake equation is really saying is, Here are the issueshere are the factors that we should really take account oflet's think about probabilities rather than certaintieslet's search! And that seems to be the sort of challenge that science should be prepared to face.
There are several aspects of the equation that, while they may not have been of paramount interest to Drake in organizing discussion at the Green Bank conference, are worthy of some discussion. In writing the probability symbols in the sequence he did, Drake made heavy evolutionary associations. The equation has dead planets progressing to life, then progressing to intelligent life, and finally progressing to technological civilizations able to transmit radio signals. The manner in which these evolutionary factors are considered in the Drake equation differs substantially from anything in the common run of evolutionary theory; so it is hardly surprising that conventional evolutionists have found little to please them in the Drake approach. Certainly evolutionary biologists have not attempted to develop an analogue to the Drake equation to consider the factors that led to the emergence of life, intelligence, and then technology here on Earth. And if the evolutionary biologists have not been able to develop a product of probabilities equation for the evolution of life on Earth, where we know with certainty the outcome, it does cause them to ponder the validity of such an approach in an alien environment where the outcome is unknown. Some of the most severe critics of the Drake equation have come from evolutionary biology, although we would argue that many of their criticisms are based on a misunderstanding of the true purpose of the Drake equation.
In introducing the various probability factors, Drake was distancing himself from a vast tradition of arguments for the existence of ETI based solely on large numbers; that is, Why have all those stars in the heavens and no one to live there? Many critics of the equation fail to realize how Drake abandoned such simplistic considerations.
In writing the equation as the product of probabilities, Drake was setting up SETI as potentially vulnerable on many fronts. It is a crucial aspect of the relationship between the probability factors involved that if only one of them turned out to be vanishingly small, for example, planets were exceedingly rare, or the origin of life was an extremely improbable event, then the whole project was undermined. It is probably this property that has led to the Drake equation being as popular with SETI's opponents as it is with SETI's proponents. Opponents have tried to demonstrate that one or other factor must be vanishingly small, thus invalidating the whole of SETI. Some have argued that even the mathematical formulation is overly simplistic: "all those multiplications, and not a single additionnature just isn't like that!"
Even at this point, the Drake equation is open to accusations of huge informational gaps, especially in the biological and social factors in the equation. While most astronomers recognize SETI as a legitimate endeavor, because they understand the nature of observations that can lead to satisfactory estimates of the astronomical factors, many scholars from other fields view the Drake equation with severe reservations.
Looking over these points, it is clear that the Drake equation does indeed embody a number of interesting commitments about ETI. These, rather than any sense in which it alone represents a strong computational tool that can justify the investment in SETI, are what have ensured its popularity. The history of SETI does little to undermine the interpretation that the main function of the Drake equation has been to focus investigation, as we will see in the next chapter.
A delightful summary of the equation was given by SETI pioneer Bernard Oliver, who described it as "a way of compressing a large amount of ignorance into a small space." This is a long way from the sense in which critics of SETI take it. The simple point that the Drake equation is a conceptual tool for organizing discussion and highlighting the areas in which the ETI hypothesis might be vulnerable counters many of the criticisms to which it has been subjected.
To support the idea that ETI will be sufficiently common to make SETI viable requires an argument that is best presented in two stages. The first stage involves the assumption that the solar-type stars that can now be observed should be relatively similar to our own Solar System at about the sort of stage when life started to develop on Earth, about 100 to 200 million years after the planet's surface cooled to the point where life could possibly become established. This is in many ways an amalgamation of Drake's factors fp, ne, and part of fl; essentially we are asking what needs to be in place so that biology can start doing its bit.
The second stage of the argument then requires that, given physical situations similar to those of the early Solar System, there is an evolutionary pathway that increases significantly the probability of a biological situation arising similar to that of the present Earth. Here is where the evolutionary biologists start to get particularly anxious!
There are significant scientific questions relating to both parts of this argument. We believe there are billions of solar-type stars in the Galaxy, and a similar number of situations that may have been like the early solar system. Hence, the preferred biological pathway would have to be available to about one in at least a million such systems to give more than a few thousand planetary systems with intelligent life, even allowing for an optimistic estimate of other uncertainties. This is probably the hopeful extreme of our practical SETI capability in even the long term. We are susceptible to the illusion of the generously stocked shoe store!
We need to look beyond reason via large numbers, and remember that it is wrong to treat the Drake equation as if it were a means of calculating anything. The Drake equation's great value is in allowing scientists to martial their thoughts. To quote from the SETI Institute's Web home page:
Within the limits of our existing technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. A search for extraterrestrial radio signals has long been considered the most promising approach by the majority of the scientific community. Besides illuminating the factors involved in such a search, the Drake Equation is a simple, effective tool for stimulating intellectual curiosity about the universe around us, for helping us to understand that life as we know it is the end product of a natural, cosmic evolution, and for making us realise how much we are part of that universe. A key goal of the SETI Institute is to further high quality research that will yield additional information related to any of the factors of this fascinating equation.
It is indeed a fascinating, and a beautiful equation. But beauty is in the eye of the beholder, and the beauty apparent to a majority of astronomers appears to be less evident to many evolutionary biologists and philosophersand some politicians who have fought the funding of SETI!
Before progressing further with our consideration of the Drake equation, we need to consider the birth, life, and death of stars, since the first term in the Drake equation is all about the stars. Our life story of stars will be somewhat brief, and we apologize for the fact that it leaves many questions unanswered. However, we are exploring the evolution of life, rather than the evolution of stars, so that our consideration of many fascinating facets of astronomy must be cursory.
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