The Drake Equation
"What do we need to know about to discover life in space?"
How can we estimate the number of technological civilizations that might exist among the stars? While working as a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia, Dr. Frank Drake conceived an approach to bound the terms involved in estimating the number of technological civilizations that may exist in our galaxy. The Drake Equation, as it has become known, was first presented by Drake in 1961 and identifies specific factors thought to play a role in the development of such civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used by the scientific community to examine these factors.
-- Frank Drake, 1961
Frank Drake at board
N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
R* = The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life bearing planets on which intelligent life emerges.
fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.
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. In each of its last four decadal reviews, the National Research Council has emphasized the relevance and importance of searching for evidence of the electromagnetic signature of distant civilizations.
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 realize how much we are a 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.
There is nothing in the universe that is more exact than mathematics itself and Dr. Frank Drake decided to take on the challenge. He created a mathematical equation that tells us an estimate of how many alien civilizations are at our level or higher in the Milky Way galaxy. He created this formula called the Drake Equation as a way to stimulate scientific discussion at the worlds first SETI meeting in Green Bank, West Virginia. Great right but lets get down to brass tacks. When the formula is completed it looks like this, N = 7 × 1 × 0.2 × 0.13 × 1 × 0.2 × 109 = 36.4 million (Wiki source). Holy @#$ Batman! That's a whopping number of planets out there that contain civilizations. Yes, because if N=36,000,000 N=the number of civilizations in our galaxy with which communication might be possible.
Of course some of them already know about this, but this raises a new question…why don't they reveal themselves to us? That my friend is easy. Some of these 36,400,000 planets with life have created space faring capabilities…let's say a very low estimate of 1% or 364,000. This number still blows me away, so lets say 1% of 1% has space technology. Thats still 3,640 space faring species!
When humanity is exploring space, usually its funded by governments and you know the red tape and rules they have about the simplest things. Well, aliens have rules about revealing themselves and it probably has serious implications if you are caught breaking their rules. Its a lot like on the TV show Star Trek and how the captain of the USS Enterprise doesn't want to interrupt in other cultures, but circumstance brings it to happen and sometimes it happens by accident. Same thing. They are trying to respect our way of life, and not make us dependent on them and their advancements.
I do believe that if enough messages are received from Earth from not one but may people, aliens civilizations will have to decide that Earth has come a long ways, and perhaps, just perhaps, they will deem humanity ready to meet them. How to send messages? Many ways, but most popular is by radio telescope or laser communicator. I sent hundreds of messages using radio telescopes around the globe, but most of them closed down or stopped allowing messages to be sent. SCW
The Odds Of Finding Intelligent Life
Searching for extraterrestrial life has become a hot topic among astronomers, biologists, and the general public. But not many remember how the subject was jump-started more than 40 years ago.
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published a landmark article in the British weekly journal Nature with the provocative title, "Searching for Interstellar Communications." Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 centimeters (1,420.4 megahertz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe. Other intelligences might see this as a logical landmark in the radio spectrum where searchers like us would think to look.
Seven months later, radio astronomer Frank Drake became the first person to start a systematic search for intelligent signals from the cosmos. Using the 25-meter dish of the National Radio Astronomy Observatory in Green Bank, West Virginia, Drake listened in on two nearby Sunlike stars: Epsilon Eridani and Tau Ceti. His Project Ozma (named for L. Frank Baum's story Ozma of Oz) slowly scanned frequencies close to the 21-cm wavelength for six hours a day from April to July 1960. The project was well designed, cheap, simple by today's standards, and unsuccessful.
Following the Ozma experience, Drake organized a meeting with a select group of scientists to discuss the prospects and pitfalls of the search for extraterrestrial intelligence — nowadays abbreviated SETI. In November 1961, ten radio technicians, astronomers, and biologists convened for two days at Green Bank. Young Carl Sagan was there, as was Berkeley chemist Melvin Calvin, who received news during the meeting that he had won the Nobel Prize in chemistry.
It was in preparing for this meeting that Drake came up with his famous equation:
N = R x fp x ne x fl x fi x fc x L
Today this string of letters and symbols can be found on T-shirts, coffee mugs, and bumper stickers. It is simpler than it looks. It expresses the number (N) of "observable civilizations" that currently exist in our Milky Way galaxy as a simple multiplication of several, more approachable unknowns.
R is the rate at which stars have been born in the Milky Way per year, fp is the fraction of these stars that have solar systems of planets, ne is the average number of "Earthlike" planets (potentially suitable for life) in the typical solar system, fl is the fraction of those planets on which life actually forms, fi is the fraction of life-bearing planets where intelligence evolves, fc is the fraction of intelligent species that produce interstellar radio communications, and L is the average lifetime of a communicating civilization in years.
The Drake equation is as straightforward as it is fascinating. By breaking down a great unknown into a series of smaller, more addressable questions, the formula made SETI a tangible effort and gave the question of life elsewhere a basis for scientific analysis.
Astronomers and biologists alike have tried to "solve" the equation ever since. At first sight, coming up with a reasonable estimate for the answer might seem fairly straightforward. But the number of communicating intelligences can't be judged so easily. Several of the variables in the equation have been firmed up since 1961. But at least three remain very unknown.
The rate of star formation in our galaxy is approximately one per year, R = 1. The next factor, fp, is probably smaller than one: not every star can have planets. On the other hand, if a star has a planetary system, it seems plausible that two or three of its planets and moons will have liquid water and be potentially suitable for the origin of life, so maybe the product of fp and ne is close to 1.
Optimists would argue that life will form wherever it can (fl = 1), that the Darwinian process of natural selection eventually favors the evolution of intelligence (fi = 1), and that no intelligent civilization would exist for long without discovering electricity and radio and feeling the urge to communicate (fc = 1). In this most optimistic case, the Drake equation boils down to the simple observation that N = L (the average lifetime of technological civilizations, in years). If L is, say, 100,000 years, there would be about 100,000 chatty civilizations in our galaxy. And that's assuming that only one arises during a planet's entire multi-billion-year lifetime.
That figure of 100,000 would mean there is one radio-emitting civilization right now per 4 million stars — reason enough to tune in on the heavens and start hunting for them. If they were scattered at random throughout the Milky Way, the nearest one would probably be about 500 light-years from us. A two-way conversation would require a time equal to a good fraction of recorded human history, but a one-way broadcast might be audible.
However, 40 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved enormously since the early 1960s. Granted, the "parameter space" of possible radio signals (the possible frequencies, locations on the sky, signal strengths, frequency drift rates, on-off duty cycles, etc.) is vastly larger than the tiny bit that has yet been searched. But we have discovered, at least, that our galaxy is not teeming with very powerful alien transmitters continuously broadcasting near the 21-centimeter hydrogen frequency. No one could say this in 1961.
Have we overestimated the values of one or more of the Drake parameters? Is the average lifetime of technological civilizations short? Or have astronomers overlooked some other, more subtle aspect?
Let's reevaluate the Drake equation by analyzing each term separately. R, the rate of star formation in the Milky Way per year, is indeed currently about 1 — astronomers are quite sure of that. In fact, astronomers have recently determined that stars formed at a higher rate several billion years ago, when the stars that might now bear intelligent life were being born. So a value of R = 3 is more realistic.
However, astronomers and biologists are much less certain about the subsequent terms in the equation.
How Many Planets? fp
The second variable is fp, the fraction of stars that have planetary systems. Recent discoveries that many or most young stars are surrounded by planet-forming disks, and detections of scores of actual planets orbiting nearby Sunlike stars since 1995, confirm what astronomers had already suspected: planets are common.
So-called "protoplanetary disks" are routinely detected by infrared observations and are seen directly in, for instance, Hubble Space Telescope photographs of the Orion Nebula, one of the most prolific star-forming regions in our part of the Milky Way. Submillimeter-wave observations have shown much more tenuous dust disks around many older stars, including Drake's first target, Epsilon Eridani. Many of these disks are doughnut shaped. According to many theorists, the central holes can only be swept clear by planets accreting gas and dust from the disk's inner portion. In addition, some of the disks (including Epsilon Eridani's) show distortions that may directly indicate a planet circling in their outer regions.
As for actual planet detections, extrasolar-planet searches find (as of June 2003) that about 12 percent of Sunlike stars have a giant planet orbiting within 5 astronomical units of the star (Jupiter's distance from the Sun). At face value, this might imply that about 12 percent of stars have planets, so fp would be 0.12.
However, this is only part of the story; the current search techniques are sensitive only to massive planets, especially those in small, fast orbits. Solar systems like ours cannot yet be recognized (though they should be in reach within a few years). Very likely the fraction of single Sunlike stars with planets of some kind is much higher than 12 percent. Reasonable guesses right now might be 20 to nearly 100 percent. (A September 2003 paper by Charles Lineweaver and Daniel Grether delves into this.)
So what do these new observations tell us about fp? Although we don't yet have a final value, it's now clear that fp is substantial and is not a bottleneck in the Drake equation.
How Many Good Planets? ne
There's less definite news when we turn to the equation's next term, ne. This factor represents the average number of worlds in a typical solar system that have environments suitable for the origin of life (the "e" stands for "Earthlike"). In his 1992 book Is Anyone Out There?, Drake recalls that the participants in the Green Bank meeting concluded that the minimum value of ne lay between one and five. In other words, every planetary system was expected to contain at least one minimally Earthlike place (defined as where liquid water is possible), and that there might easily be three, four or five hospitable worlds per system.
This optimistic view was based on the assumption that our own solar system is typical. Today Mars and Jupiter's moon Europa are being considered as possible sites of early biology, making three possible "Earths" (by the Drake-equation definition) in our solar system. However, the extrasolar planets found in the last few years have taught us a humbling lesson. Our solar system, with lots of worlds and moons in nice, circular, stable orbits, may be the exception rather than the rule. For all we know, Earthlike planets with long-term stable orbits and climates may be quite uncommon.
How Many Origins of Life? fl
In scientific circles there's much less concern now than in the past about the value of fl, the fraction of habitable planets on which life evolves. The molecular building blocks of life — complex organic compounds and even amino acids — are abundant in the universe. They have been discovered in meteorites, comets, and interstellar gas and dust. There are vastly more amounts of amino acids, for instance, in interstellar space than in the Earth's biosphere. Although hydrocarbons and amino acids are not living organisms, there's little doubt that a lot of prebiotic evolution is going on in the dark clouds between the stars.
Most significant are the recent discoveries that microorganisms appeared on Earth only moments (geologically speaking) after the last devastating, ocean-vaporizing impacts of the planet's youth some 3.9 billion years ago. There is clear evidence that bacteria were already around by 3.5 billion years ago, and more disputed evidence from 3.7 and 3.85 billion years ago. Apparently, given the right conditions, the origin of life is a rather straightforward process that happens easily — at least when given a planet-sized laboratory and millions of years for the experiment to run. If the process were rare or difficult, one would not expect it to have happened at the first possible opportunity on our home planet, but somewhat later in Earth's history instead. Biologists now discuss whether life may have arisen several times separately. There's every reason to think that all living things today have a common ancestry, but other, independent lines could have formed and been wiped out early. If life does form wherever it can, then presumably fl = 1.
That leaves us with three remaining unknowns. How likely is the evolution of intelligence (fi)? How confident can we be that at least some intelligent extraterrestrials will broadcast radio or other signals we can detect (fc)? And what is the average lifetime of radio-capable civilizations (L)? These biological and sociological factors in the Drake equation are subject to greater scientific debate and uncertainty than the astronomical ones.
According to many life scientists, it is naive to suppose that evolution on another planet should necessarily result in intelligence as we know it. In his bestseller Wonderful Life, the late paleontologist Stephen Jay Gould (Harvard University) asserts, "We probably owe our own existence to . . . good fortune. Homo sapiens is an entity, not a tendency." Evolution is unpredictable, undirected, and chaotic. Gould has pointed out again and again that if we could rewind the tape of biological evolution on Earth and start over, it is impossible that humans would again appear on the scene. We are the result of too long a chain of chance flukes and happenstance.
Others counter, of course, that humans are not what we are looking for. No one expects to find men among the stars (little green ones or otherwise). Rather, the issue is whether any species evolve enough symbol-based intelligence to use tools, store and manipulate information, and develop societies that grow large and complex enough to discover the principles of science and electronics. To optimists this seems like a difference only in degree, not in kind, from the levels of intelligence, tool use, and purposeful behavior that have emerged independently in widely divergent species of animals on Earth, from apes to octopi.
But Gould notes that there is no overall pattern in evolution, no preferred direction. If some recently evolved animals are bigger and smarter than earlier ones, that could just be a fluke. Human levels of planning and technology may be even more so.
To some biologists and SETI proponents, the phrase "survival of the fittest" implies that greater intelligence inevitably boosts a species' chance to survive and spread by natural selection. But the renowned biologist Ernst Mayr (retired from Harvard University) has argued that many astronomers and physicists are too optimistic concerning the emergence of intelligence. "Physicists still tend to think more deterministically than biologists," wrote Mayr in the May 1996 issue of The Planetary Report. "They tend to say that if life has originated somewhere, it will also develop intelligence in due time. The biologist, on the other hand, is impressed by the improbability of such a development."
Strangely enough, optimists and pessimists base their claims on the same key observation — namely that technology has appeared on this planet in 4 billion years. Pessimists (or realists, as they would prefer to be called) like Mayr see this as evidence of the unlikeliness of intelligence as an evolutionary given. For optimists, it strengthens their belief in the existence of extraterrestrial civilizations.
This divergence stems in part from different specialists' intellectual backgrounds. To a biologist, something that happened once in 4 billion years is terribly rare. Astronomers take a wider view: something that happened once in less than a single planet's lifetime seems reasonable for planets generally.
Optimists have pointed out that by some estimates, Earth has at least 1.2 billion good years ahead before it will get broiled by the expanding Sun. This is several times longer than the time since the first simple creatures crawled out of the sea onto land. If the emergence of intelligence were difficult and rare, the optimists argue, it would not have happened relatively early in the time available for it to do so on Earth. Given humanity's early arrival in the long era expected for land life, it seems likely that entirely different intelligent creatures will emerge a few more times in the coming geological ages (and will find our fossils). This argument parallels the point drawn from the rapid emergence of microorganisms on the young Earth.
Pessimists reply that we don't really know how long the Earth will remain clement. The Earth's seemingly stable climate may actually be the result of a long run of lucky flukes that could give out at any time, geologically speaking. If so, humans have arisen late in the total span of time available. Given the fact that we are here at all to ponder the question, a late emergence in the time span available would indicate that the birth of intelligence is an improbable event.
Contrary to popular belief, the fact that intelligence has arisen once tells us nothing whatsoever about how often it happens — for the simple reason that we ourselves are the one case! We are a self-selected sample. Even if intelligent life is so improbable that it appears just a single time in one remote corner of the universe, we will necessarily find ourselves right there in that corner observing it, because we are it.
Strangely enough, both camps accept the so-called Copernican principle, which claims that humankind enjoys no preferred position in time or space. Skeptics like Mayr say it is anthropocentric to believe that humanlike intelligence has appeared over and over again in the universe. Believers like Drake are unwilling to accept our uniqueness, because this would put us on a very un-Copernican pedestal.
Christopher Chyba, chair of the SETI Institute's Center for the Study of Life in the Universe, sums it up: "It's an argument that turns on the comparative importance of contingency versus convergence in evolution." In other words, how many evolutionary tendencies are random flukes, and how many repeatedly drive in a particular direction? "Are there data sets that we can analyze to actually help focus and quantify this argument?" Chyba continues. "The answer, it appears, is a resounding 'yes.' We don't have to guess about these questions, but can begin to quantitatively assess some of them using well-understood, quantifiable tools." The SETI Institute is assembling researchers to tackle this problem.
For now, however, fi is one of the most controversial factors in the Drake equation. Some scientists believe it is almost certainly next to zero; others are convinced it's close to one. There seems to be no middle ground.
Even if intelligence is a likely consequence of evolution, fi will probably be much lower than 1, based on recent insights into the stability of solar systems and planetary climates. Just because a planet starts out good for life doesn't mean it will stay that way forever.
Computer simulations by Fred Rasio and Eric Ford (Massachusetts Institute of Technology) among others show that Earthlike planets are probably unable to survive the gravitational tug-of-war in a system with two (or more) massive, Jupiterlike giants. They would be slung out of the system or sent careening into the central star.
Conversely, systems with no giant planets at all might also have dire consequences for life-bearing planets. Computer simulations by George Wetherill (Carnegie Institution of Washington) indicate that Jupiter acts as the solar system's gravitational vacuum cleaner, efficiently thinning out the population of hazardous comets that venture into Earth-crossing orbits. Without a Jupiter the current impact rate of comets would be about 1,000 times higher, says Wetherill, with truly catastrophic collisions (like the one that killed the dinosaurs 65 million years ago) happening about once every 100,000 years. This would surely frustrate any slow evolutionary progress from simple life forms to higher intelligences.
Also, dynamical studies by Jacques Laskar and Philip Robutel (Bureau des Longitudes, Paris) have shown that rocky, Earthlike planets show chaotic variations in orbital tilt that could lead to drastic climate changes. Fortunately, Earth's chaotic tendencies are damped by tidal interaction with the Moon. Without a relatively large satellite, Earth might have experienced variations in axial tilt similar to those of Mars, possibly as large as 20° to 60°. This would cause extreme variations in the patterns of the seasons. According to one analysis of planet formation, a world like Earth has only about a 1 in 12 chance of ending up with a nice, mild axial tilt that is safely stabilized by a large moon. (On the other hand a moonless Earth might have retained its original rapid spin, which would tend to stabilize its axis.)
It's anyone's guess how large axial swings would influence the evolution of life and the chance for the emergence of intelligence. Change and stress actually promote the emergence of new, versatile, adaptable species, biologists say. For instance, Paul F. Hoffman (Harvard University) and three colleagues proposed in 1998 that the series of intense global ice ages between 760 and 550 million years ago were the crisis that drove the remarkable "Precambrian explosion" of new life forms around or shortly after that time. The disastrous great extinctions later in Earth's geologic record were always followed by vigorous recoveries, eventually spawning more species than existed before. (Complete recovery from any great extinction, regardless of size, always seems to take about 10 million years.) Humanity's own emergence as a species during an unusual run of ice ages is sometimes cited as an example of stress-driven evolution leading to adaptability and intelligence. So a planet with a tippy axis might actually speed evolution along.
But planetary crises that are too extreme or frequent would kill off everything, or keep life beaten down to a low level. In any case, our existence here and now seems to be the accidental result of a number of astronomical coincidences that were unimagined in 1961.
Such coincidences are discussed in the book Rare Earth by Peter Ward and Donald Brownlee (Copernicus Books/ Springer, 2000). Ward and Brownlee argue that only very rarely will a good planet form and remain life-friendly for the billions of years that advanced creatures took to appear on Earth. Seth Shostak of the SETI Institute argues in a rebuttal essay that some of their points are overstated, that once life is established it is probably adaptable enough to thrive in un-Earthly conditions, and that it therefore need not require a planet with a narrowly Earthlike history.
Ward and Brownlee's associate Guillermo Gonzalez advocates the idea that there is only a narrow "habitable ring" in the Milky Way where conditions allow life-bearing planets. Closer to the galaxy's center, conditions are supposedly too violent; farther out there aren't enough heavy elements to make planets. This idea has been roundly criticized as a gross exaggeration. Heavy elements are in fact distributed widely throughout a galaxy's disk (the evidence is in plain view: dark dust clouds of carbon and silicates riddle most parts of most disks), and stars with a fairly wide range of heavy-element concentrations have been discovered to have planets. Dangerous radiation from an active galactic center would be blocked by a planet's thick atmosphere; that's why our own X- and gamma-ray telescopes have to be put in orbit. David Darling notes in his book Life Everywhere: The New Science of Astrobiology (Basic Books, 2001) that Gonzalez argues from his religious conviction, expressed in other writings, that God designed one world for one intelligent race, and that Gonzalez's astronomical views should be understood in this light.
How Would Aliens Communicate? fc
Suppose that extraterrestrial intelligences are rare but do exist. Could we expect them to communicate with us through radio signals? What fraction of civilizations are able — and motivated — to broadcast in a way we can detect? In other words: what is the value of fc? SETI advocates tend to believe that it is large: that sooner or later, any civilization curious and inventive enough to become technological at all will discover that radio is an efficient way to communicate over astronomical distances, and will choose to do so.
Might there be a naive form of anthropocentrism at play here? Is it reasonable to expect that wildly different beings on another planet, even if they are older, smarter, and more capable than us, will choose to build radio telescopes and send signals to the larger universe? Maybe we just don't appreciate the true diversity of biological evolution, or the uniqueness of humans' monkeylike curiosity. Or maybe radio is hopelessly primitive compared to something we have yet to discover.
With fi and fc completely undetermined, we're left with the last term of the Drake equation: L, the average lifetime of communicating civilizations. Here also, optimists and pessimists are far apart.
The optimists claim that a stable, intelligent society could last for tens of millions of years, if not forever. This would certainly mitigate the effect of any bottleneck earlier in the Drake equation. In addition, a long-lived species might have time to spread to many stars, multiplying its presence. The pessimists point out that humans invented radio technology only a century ago, and that the human race has been on the verge of destroying itself (through nuclear war or ecological overload) for much of that time. The same technological power that enables interstellar communication also enables rapid self-destruction.
But others have pointed out that the human animal (as opposed to human civilization) would be almost impossible to kill off completely at this point. People have become too widespread and too capable; a few pockets of individuals would find ways to survive almost any conceivable war or global catastrophe. These survivors would be enough to fully repopulate the Earth, to numbers in the billions, in just a few thousand years. And a second technological civilization would arise more readily than our first one has done, because there would be a precedent. Maybe this will happen many times.
Which brings up a little-noticed point. The value of L properly does not refer to the lifetime of one radio-transmitting civilization, but instead to the sum of all those that ever appear on a planet once it develops its first.
The long-term future of humanity and Earth's biosphere is explored in Peter Ward's book, Future Evolution.
The Fermi Paradox
Perhaps the pessimists' most telling argument in the 40-year SETI debate stems not from theory or conjecture but from an actual observation: Earth has not already been overrun with aliens (contrary to some popular opinion). This is a more profound observation than it might at first seem.
A civilization lasting for tens of millions of years would have plenty of time to travel anywhere in the galaxy, even at the slow speeds foreseeable with our own technology. The drive to fill up all available territory seems to be a universal trait of living things. And yet the Earth shows no sign in its fossil record of ever having been colonized by an alien high technology in its long history, much less today. This is known as the Fermi paradox, after the nuclear physicist Enrico Fermi, who as early as 1950 asked (in a lunchroom discussion about aliens at, ironically, a nuclear weapons lab), "Where is everybody?"
(UFO believers might reply that we are being overrun right now. But scientists and other careful investigators who have examined the UFO movement's claims conclude almost universally that nothing is going on here but human misperception, tale-telling, and willful folly. More than 50 years after it was born, UFOlogy remains barren of a single tangible result despite thousands of loud claims, which suggests that we can sit out the next 50 years of it and not miss anything.)
Optimists have replied to the Fermi paradox in many ways. Maybe any culture that is civilized enough not to destroy itself turns away from imperialism, or maybe the imperial drive runs out of steam after settling just a few thousand planets. Maybe we live in an uninteresting area of the galaxy — the equivalent of a backwoods area in the United States, a country that has been "completely settled" since the frontier was officially closed in 1890 but where you can find plenty of places where no one else is in sight. Or maybe aliens are thickly settled around us but obey, as in Star Trek, a prime directive "not to interfere" with living planets, which are kept off limits as nature preserves. This is the so-called zoo hypothesis. Or perhaps interstellar travel is really as expensive in effort and energy as it appears to us right now to be, and anyone capable of it has better things to do with the resources — such as investigating the universe by astronomy or radio.
A more sophisticated rejoinder to the Fermi paradox was published by William I. Newman and Carl Sagan in Icarus for September 1981. They analyzed how fast a spreading interstellar civilization would actually expand through the galaxy, based on mathematical models covering everything from the diffusion of molecules in a gas to the spread of animal species introduced into virgin territories on Earth. They found that how fast the galaxy fills up depends surprisingly little on the speed of interstellar travel; there are too many planets to be settled and populated along the way. "The expansion velocity of the colonization front is several orders of magnitude smaller than had been previously anticipated," they wrote; filling the galaxy might even take a time comparable to the age of the universe. Summing up, they quipped "Rome was not built in a day, although one can cross it on foot in a few hours."
But others have called their argument a stretch, because it assumes that population growth rates are always fairly low. In the end, the fact that aliens are not camped in your bedroom may truly mean that we are alone in the Milky Way. Perhaps almost every galaxy is either completely barren or settled in every inch. If so, we can expect the first radio signals we hear to come from beyond the Milky Way.
For more on this topic, see David Brin's influential 1983 essay "The Great Silence" (PDF format, 2.1 megabytes), Geoffrey Landis's analysis of partial, patchy galactic colonization based on percolation theory, and Stephen Webb's new book If the Universe Is Teeming with Aliens. . . Where Is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life (Copernicus Books, 2002).
"Success Can't Be Predicted"
Where does all this leave us? Can we still believe that N = L? Probably not. What about N = 0? To many people that extreme is inherently unacceptable, but of course the universe isn't obliged to live up to our hopes and expectations. Maybe there is some truth in the saying that nothing happens only once. Maybe alien civilizations are out there, and some are trying to announce themselves via radio transmissions. But their number could be very, very small.
In the preface to Is Anyone Out There? Frank Drake wrote that he wanted to "prepare thinking adults for the outcome of the present search activity — the imminent detection of signals from an extraterrestrial civilization. This discovery, which I fully expect to witness before the year 2000, will profoundly change the world." That was written in the heady days when NASA's now-aborted radio searches were about to get under way. In July 1996, at the fifth international bioastronomy conference in Capri, Italy, Drake confessed: "Maybe I was a little bit too optimistic. Success can't be predicted." Cocconi and Morrison already told him so in their 1959 Nature article: "The probability of success is difficult to estimate, but if we never search, the chance of success is zero."
Meanwhile, the Drake equation still stands as the best-known icon of one of the most forward-looking endeavors of the intelligent species here on Earth: the scientific search for coinhabitants of the dark emptiness of the cosmos, and for a wider, truer perspective on our place in space and time and on the meaning of our life. The "alien equation" has served this effort well by providing a rational basis for the search, by focusing our attention on the fundamental issues, and by defining a clearly visible goal.
We're a long way from that goal. The first term, R, has been known for decades, and we're now coming to grips with the second, fp. That leaves us with two medium-size question marks and three big ones — and a lot of speculation.
Moreover, the equation is showing its age, looking a little frayed around the edges by not explicitly treating newer issues that we now consider important, such as the rates of planetary catastrophes or the effects of slow, one-way changes in the universe itself that could either boost or diminish the abundance of aliens in our present era (see for instance the recent paper about this by Milan M. Cirkovic).
But maybe the Drake equation isn't to be solved after all. Its real value may lie in those thought-provoking question marks. Uncertainty and curiosity will keep the search going for years to come. Maybe the real payoff for SETI will not be to yield a yes-or-no result, at least not in our lifetimes, but to help us discover more about ourselves.
Alan M. MacRobert is a senior editor of Sky & Telescope. Govert Schilling is an astronomy writer in Utrecht, The Netherlands. His book Tweeling aarde: De speurtocht naar leven in andere planetenstelsels (Twin Earth: The search for life in other planetary systems) was published in 1997.
The Drake Equation and Alien Life Knowledge
"For all our feelings of self importance, we are only a kind of biological rust, clinging to the surface of our small planet, and weighing far less than the air that surrounds us"
"One of the distinctions and triumphs of the advance of science has been the deprovincialization of our world view"
-- Carl Sagan
A number of the implications regarding the possibility of extraterrestrial intelligence are discussed. The Drake equation is introduced and some values for the established parameters suggested, a further parameter is proposed. Some of the principle arguments for and against extraterrestrial life are discussed. It is acknowledged that we have already announced our existence to any nearby technological civilisations and that not to anticipate contact with an extraterrestrial intelligence would be unwise. It is suggested that extraterrestrial life is possibly quite abundant, however the prospect of life on Earth being unique is also recognised.
A note about astronomical distances
The scale of astronomical distances and other large numbers, such as the number of stars in a galaxy, is something which often causes confusion. The universe is so inconceivably enormous that this is hardly surprising.
If for example, the thickness of a sheet of paper, is used to represent the distance from the Earth to the Moon. At it's closest approach Venus, our nearest planetary neighbour, is about 45 million kilometres away, that distance would be represented by 112 pieces of paper. A sheaf about 370 pieces thick are necessary to represent the distance from the Earth to the Sun. To represent the distance to the next nearest star, a distance of around 4.5 light years, requires a stack of paper just over 11.5 kilometres high. (A light year is the distance light travels in a year, and to put that into perspective, light travels fast enough to get around the Earth about 7 times per second).
The Sun is an ordinary star, one of about 400 billion others in the galaxy which we refer to as the Milky Way. If each of those stars were represented by a piece of paper, that would be a stack 32,800 km high. This galaxy like many others, is a flat disc with a central region which is slightly thicker, it has spiral shaped arms extending outwards. (The whole thing is often likened to two fried eggs back to back, though the "yolks" would be a little too thick to accurately represent the central region). The galaxy is thought to be about 100,000 light years in diameter. To represent that diameter using the thickness of this page would require a pile of paper high enough to reach the Moon!
To represent the distance from our galaxy to another galaxy would need a pile of paper from here to the Sun and beyond. To begin to represent distances of wider significance, such as that to the edge of the observable universe, thought to be about 13 billion light years away, demands a pile of paper whose size can really only be appreciated by using astronomical distances to describe them. With this analogy in mind it is perhaps not surprising that comprehending scales and distances in astronomy causes much confusion.
Ever since man became aware of the cosmos in its widest sense, either as Ptolemy thought that we were at it's centre, or the contemporary view of us as a particularly small, and insignificant component of the universe, man has wondered about 'who' else might be out there.
Astronomy has taken a great many leaps forward since the Gallileo first turned a rudimentary telescope of the heavens, there are now optical telescopes capable of resolving incredibly fine detail, and techniques developed which permit the scrutiny of distant objects to the point where today's astronomers confidently predict the nature of the universe during the first two or three seconds of its existence some ten to twenty thousand million years ago. In recent years the existence a organic molecules including amino acids have been discovered amongst the stars, and in meteorites, plus an appreciation of how similar compounds may have been synthesised in the primordial environment which existing as the Earth began to evolve, these and an increasing knowledge of the dynamics of the universe has fuelled the eternal question of "are we alone"?
Since the possibility of the existence of life elsewhere in the universe was accepted as a distinct possibility, man has wondered at the possibility of contact and speculated on the consequences of contact with another intelligence. It can also be argued that searching for extraterrestrial life is a waste of time and resources, even pointless. Others have argued the opposite and won significant funding to undertake searches for indications of life elsewhere. Man has already announced his existence by virtue of the mass of radio transmissions emanating from planet Earth.
This report will summarise some of the significant aspects from the multitude of questions raised by the debate, "is there life elsewhere in the universe?", "how many advanced civilisations might there be?", or "are we alone?"
"Do there exist many worlds, or is there but a single world?
This is one of the most noble and exalted questions in the study of Nature".
St. Albertus Magnus (circa 1260 AD) 15
The belief in 'the plurality of worlds', is something Tipler 12 comments is generally linked to three beliefs, the first of which Lovejoy 19 has called the "principle of plenitude" which asserts that what can exist must exist somewhere - if a world like ours exists so must others, since no "genuine potentiality of being can remain unfulfilled". This principle of plenitude has become the more recent 'principle of mediocrity', that is the existence of intelligent life here is nothing out of the ordinary. A further belief is that of the cosmos being infinite, and the third, that believers in extraterrestrial life have tended not to have a 'sense of history' and they have failed to recognise how their ideas have merely been modern representations of old concepts.
(This representation of the original concept is something which occurs throughout the history of the subject, "many of the arguments pro and con are re-invented as a new generation of debaters take up their pens", and "extraterrestrial life believers have always been willing to suspend the physics of their day" Tipler 12.) Amongst the ancient Greeks and Romans reference to the 'world' implied a central Earth with it's Moon and Sun plus five planets and the fixed stars. Consequently the concept of the plurality of worlds implied each 'world' had it's own universe with a central and inhibited Earth. Both the Greek philosophers, Aristotle 16 and Plato 20 were against the concept of life elsewhere, Aristotle because of his belief that the other planets were of a completely different substance and in the finite nature of the universe. Plato because he believed the Earth to be unique.
St Thomas Aquinas 21, a pupil of Albertus Magnus argued against both the plurality of worlds and the principle of plenitude. His logic was simple, if God had made other worlds they would be either similar or dissimilar to this one. If similar, then they would be in vain and this would not be consistent with divine wisdom. If dissimilar, none of them could contain all things and therefore none would be perfect, and an imperfect world could not be the work of a perfect Creator.
To many people the shear size of the universe is so unimaginably big that they believe there must be other 'people', other 'life' somewhere out there. Popular science fiction, and science fantasy, seems to exist on the belief that where ever you might look, there will be lifeforms.
Certainly since the Renaissance almost every major scientific advance has confirmed the view of our mediocrity. We are not the centre of the solar system, Earth is one of many planets, and is vastly older than the human species. The Sun is merely an ordinary star, in an obscure location in our galaxy, the Milky Way, along with about 400 billion other stars. The Milky Way is just one typical galaxy, with perhaps hundreds of billions of similar galaxies grouped in clusters throughout the universe. It has recently been suggested that these groups of galaxies exist in 'strings' of a number of such clusters.
On planet Earth, we humans appear to have emerged from a common evolutionary origin as all the plants and other animals. "We do not possess any uniquely valid locale, epoch, velocity, acceleration, or means of measuring space and time", wrote Sagan and Newman 9. It has been suggested by Morrison 4 that once interstellar travel becomes a viable practicality at least one 'ceremonial voyage' would be undertaken, simply because the facility exists. After that it is debatable if long distance journeys would occur, intelligent species preferring what might in the contemporary jargon be termed 'virtual travel'. It must be recognised that as technological civilisations develop and become 'hyper-developed' to the point where they possess knowledge, (and possibly understanding), thousands or hundreds of thousands of years in advance of our development, that they will simply not have an interest in such primitive organisms as we humans.
Biochemistry of the young Earth
Oparin and Holdane 7 in the 1930's proposed that the atmosphere of the newly evolving Earth was similar to those of the outer planets of our solar system. In essence that the atmosphere was not rich in oxygen, as it is now, but contained large amounts of hydrogen and compounds such as methane and ammonia. This led in two scientists, Millar and Urey 7 in 1953 to undertake the first experiment to investigate the chemical reactions which are thought to have occurred in oceans and atmosphere of the primitive Earth. This now often repeated experiment consisted of heating water in a closed system of flasks and pipes forcing the vapour through a mock atmosphere of methane ammonia and hydrogen. This 'atmosphere' was exposed to continuous electrical discharge which simulated the effect of natural lightening causing the gasses to interact. The products of these reactions were passed through a condenser and dissolved in the water which represented a primitive ocean. The experiment was allowed to run continuously for several days, and analysis of the 'ocean' demonstrated that many amino acids were formed.
Some years after this initial experiment was performed, a meteorite fell near Murchison in Australia, and its subsequent examination showed it to contain a number of the same amino acids as Millar and Urey had synthesised in their primitive ocean. This coincidence led to support for the theory that such compounds are easily associated with the young Earth. Within a decade it had been established that nucleic acid bases could be obtained by the reaction of components known to have been present in the primeval conditions of the early evolution of the planet. What is more, the small molecules which have been identified as components of the these reactions, e.g. water, ammonia, formaldehyde, hydrogen, cyanogen and cyanoacetylene, have now been shown to be present in abundance in interstellar dust clouds - the regions where new stars form, so amply providing the evidence for the emergence of the chemical building blocks of life.
The Drake / Green Bank equation
The search for extraterrestrial life is the initial step towards a dialogue or contact with extraterrestrial life. Whether we should attempt communication with extraterrestrial life is, to all intents and purposes no longer a consideration. Since the first radio transmissions by Marconi the existence of a technological species here on Earth has been broadcast far and wide. Any attempts at hiding our existence are now practically impossible. Those early radio signals are rippling through space more than 90 light years away. Today thousands of gigawatts of radio energy radiate daily from the Earth broadcasting and highlighting our existence like a galactic lighthouse. As a result we should realise any nearby technologies might detect our (albeit) unintentional transmissions and return a message announcing their existence or despatch a probe to investigate our circumstances and degree of benevolence, (or threat).
In 1961 there was a now renown conference held at the National Radio Astronomy Observatory in Green Bank, West Virginia 3, to discuss the question of a 'search for extraterrestrial life' (SETI). That gathering brought together a worldwide array of prominent astronomers and 'exobiologists'. The conference set out with the intention of attempting to quantify, by theoretical means, the number of technically advanced extraterrestrial intelligence's within the galaxy. The solution was an equation, now known as the Green Bank equation, though also widely referred to as the 'Drake equation' after Frank Drake the astronomer who proposed the core of the expression. The equation seeks to quantify the number, N, of technical civilisations in the galaxy.
The equation has, N = R* fp ne fl fi fc L
R* = mean rate of star formation in the milky way, our local galaxy.
fp = the fraction of those stars which form planetary systems.
ne = the number of planets in those systems which are ecologically suitable for lifeforms to evolve.
fl = the number of those planets on which lifeforms actually develop.
fi = the number of those which evolve to an intelligent form.
fc = the number of advanced intelligent lifeforms which develop the capability of interstellar radio communication.
L = the lifetime of those advanced technically advanced civilisations.
Values for some of these parameters are, of course, open to considerable disagreement, something to which we shall return later, however a set of values is widely quoted. Most of these have not altered to any significant degree since that conference in 1961.
They are; R* = 10/yr, fp = 0.5, ne = 2, fl = 1, fi fc = 0.01, and L = 10.
The mean rate of star formation in the milky way, our local galaxy, and it's stellar population is well understood and this figure of 10 each year is widely held to be reasonable. The current theories of star formation accept the formation of an accompanying accretion ring which is expected to form the basis of planetary bodies. Although this is not universally accepted it has become possible in recent years to measure slight gravitational perturbations in the proper motion of stars. (Proper motion is the actual movement, of a star rather than it's apparent movement.) It has been found that a large proportion, around 50%, of the stars close enough to be subjected to this investigation have companion objects which affect their movement. These companions, which are too small or too dark to see, range from objects with mass a little smaller than Jupiter to a few tens that planet whose mass is 1.899 x 10^27 kg. Of course the most definite indication of the formation of planets is that of our own solar system with the nine planets and their satellites. Clearly the probable existence of objects affecting the movement of distant stars does not guarantee a viable ecology for life to exist, however the evidence does imply the possibility. In view of the limited observational data it seems reasonable to regard the solar system as a typical model, this suggests fpne equals one.
Because of the rapidity of the origins of life becoming established on Earth, as evidenced by the fossil record and experiments which reproduce early Earth biochemical environments, the likelihood of life evolving seems high. This may also be supported by the fact that many organisms survive in niche environments, at great ocean depths, else in remarkably inhospitable climatic conditions. These considerations support the contention that the value for fl being at least one.
The values attributed to fi and fc are somewhat more contentious, aspects of this argument are unable to draw on significant evidence, however many researchers of the topic agree 0.01 to be a 'fairly conservative' estimate. This seems reasonable given that intelligence has evolved about halfway through the expected lifetime of the Earth and Sun system.
Perhaps by far the most contentious issue of the entire equation is that of the lifetime of technically advanced extraterrestrial intelligence's within the galaxy. It is interesting to note that the 1961 conference proposed the rather pessimistic figure of 10 years before humans became unable to meet that description. At the time, when the 'cold war' was at it's most fierce, it was anticipated that man might not be able to manage the long term effects of the nuclear weapons then being amassed around the globe. Additionally the spread of nuclear power was something which many feared. A further aspect might have been the beginnings of the population explosion and the pressures which that and other environmental factors would put on the Earth. With these considerations in mind it appeared the most acceptable value for L would be 10. That being the case, the Drake/Green Bank equation can used to calculate N, giving the answer, 1. From that it appeared the only technically advanced civilisation in the galaxy was here on Earth. A result which some researchers, notably Tipler 11, has not overlooked, the main thrust of his argument is discussed below. Brin 2 has suggested arguments and values associated with the parameters are too simplistic and proposes the galaxy is rather sparsely populated.
Now, more than thirty years later, the world has seen a number of major changes, some affecting the Drake/Green Bank equation, a discussion of man's ability to control nuclear proliferation or world population is outside the scope of this discussion. However one aspect of the equation must be modified, and, in this writer's opinion, a further parameter added. The value of L can now safely be increased from 10 to at least 34, and an increase to 50 would not seem unreasonable. Incorporating 34 as the value for L into the equation gives N as 3.4, clearly any increase from the initially proposed figure of 10 implies a rather more hospitable galaxy. If technically advanced civilisations were to exist and have lifetimes of a few thousand years then a galactic community appears a distinct possibility. The Green Bank conference suggested that if technically advanced civilisations could avoid self annihilation then their lifetimes might be, by comparison to terrestrial geological time scales, very long indeed. The conference went on to suggest that if 1% of developing galactic civilisations made peace with themselves then our galactic neighbours would be only a few hundred light years away. Other researchers and working groups, for example Sagan 8 , have examined the question and concluded there could be 106 technologically advanced extraterrestrial civilisations in the galaxy.
The present writer considers an additional parameter in the equation is justified. Although the object of the original equation was to determine the number of technically advanced extraterrestrial civilisations, the debate has been centred on the search and eventual communication. A consideration is necessary to take account of the need for two communicating communities to exist simultaneously, or at least the searching community and it's potential target community. Given the age of the universe, currently believed to be some 10 to 20 thousand million years, Hawkins 5, and the elapsed lifetime of our evolution, it seems reasonable to consider there to be only a small chance, possibly 1%, that we might establish any communication with another community. With this factor in place the possibility of a dialogue is more remote but might easily be higher depending on other values entered into the equation.
Is there anyone there?
Many of the population would claim that extraterrestrials have been here in the preceding centuries pointing to many references to visitation by 'angels' and 'gods' in ancient texts and a wide variety of artifacts said to bear 'the finger prints of the gods'. These widely heard stories often have little basis in fact and are at best dubious. That many of these anecdotes exist in various sacred texts or are of other religious significance is used by their proponents to add weight to their case. In fact they have not stood the test of scientific scrutiny.
A large portion of the populous believe aliens to be regular visitors to Earth, citing the numerous reported sightings of unidentified flying objects (UFOs) and even abduction of witnesses as their evidence. The popular myth surrounding the UFO enigma is widely believed to be visitation by, mostly, benevolent aliens. There are hundreds of thousands of reported incidents where people have reported what have been labelled by the press as 'flying saucers'; alien craft piloted by intelligent humanoids. However a number of researchers having examined the evidence have reached a contrary conclusion. Haines, et al 4, have shown much evidence to illuminate the psychology of these experiences, the fallibility of witness testimony and some of the many sociological aspects of these phenomena. He has pointed to a considerable consistency amongst the reported features and correlates these findings with deep rooted expectations held by the population. The present writer and a colleague 13, have forwarded other evidence which contradicts the popular interpretation. Rather than 'spacemen' we have proposed a hypothesis to support the likely existence of a previously unrecognised form of atmospheric phenomenon. We have also found other possibilities which show an apparent link with the more exotic experiences such as those examined by Haines. These investigations have been well received by others working in the field and are held to represent a distinct advance in the subject. However, these events, which occur throughout most cultures and historical periods continue to be reported. Despite the protestations of eye witnesses, and the unproven assertions by others, there is no evidence to support the hypothesis that Earth is being visited by extraterrestrial entities.
Addressing the question of extraterrestrial life from our current position it is impossible to answer the question of whether a search for other life will be successful. The best which can be achieved from our limited knowledge and assessments is that the possibility for life elsewhere exists.
It has been argued that life in the universe is abundant, and therefore the possibility of communication with another civilisation is simply a matter of time. Whilst others have an opposing view that Earth is unique and therefore life is unique, that is we are alone in the vastness of the cosmos. There is however an intriguing alternative argument, that is although the number of civilisations may be considerable, we are never the less alone in space by virtue of our lifetime not coinciding with that of others.
Although there have been, and still are a number of observatories contributing to a search for deliberate or unintentional signals arising from an extraterrestrial civilisation, none have been detected. These searches have always been somewhat contentious, and their cost and funding being a major hurdle. In the last few decades the scientific research programs associated with the search for extraterrestrial life have benefited from conventional scientific funding. However, in 1993 the Sky Survey and Targeted Search aspect of NASA's High Resolution Microwave Survey, (HRMS), has had government funding withdrawn in favour of sourcing finance from private funds 17 . This search, now called Project Phoenix, will require around $3 million each year to complete the survey of about a thousand nearby F, G and K group stars. (These groups of stars are the most stable and are thought to be the most likely to harbour a suitable ecosystem in which life might evolve.) In an attempt to limit costs without affecting the integrity of the search, cuts have been imposed on the supporting conferences and collaborative ventures originally proposed.
Might we be alone?
There are prominent opponents to those who argument for there being significant colonies of intelligent species spread through the galaxy. Tipler 11 for example has claimed that extraterrestrial intelligent beings do not exist. He maintains that those who support the possibility of extraterrestrial life are mainly astronomers, and those who have argued against are largely biologists. He has made his camp firmly with the latter. His main thrust is simply, if they did exist, then they would have made themselves known to us, or we would have found evidence of their existence in the form of one or more of their exploratory probes. He proposes that any intelligent community capable of interstellar communications would, as a matter of course, go on to develop a means of interstellar travel. He goes further to claim that an automatic consequence of interstellar travel would be the exploration and/or colonisation of the galaxy. Tipler assumes that a developing technological species will eventually develop a "self replicating universal constructor with intelligence comparable with the human level - such a machine should be developed with a century", (this written in 1979). He goes on, "such a machine combined with present day rocket technology would make it possible to explore and/or colonise the galaxy in less than 300 billion years".
The machine Tipler proposes is the "von Neumann machine" 14 , a space probe sent by an emerging intelligence, equipped with an initial route plan to an inviting location - another planetary body, perhaps an inhabited one. Once there the von Neumann machine would use it's on board equipment to explore the new found environment and report back. From there the plan is that either the probe surveys the heavens for a second staging post, refuels itself and departs to repeat the process over again. Or, once at the first landfall, the von Neumann machine replicates itself, using the local materials to construct a clone (or improved version of itself), and then despatches one of the second generation probes to a further attractive destination. The first might remain at the first port of call and merely act as a production facility to begin a colony of von Neumann machines. The inbuilt instructions for their survival would include the means to prospect for and extract or synthesise whatever materials were needed for the subsequent generation, be that by mining planetary material, extracting the necessary elements from the local solar system or devising alternatives to suit whatever is available. As the fleet of probes explore ever further afield, their own capabilities might develop, but a built in quality control system could maintain the integrity of the original design concept and devise improvements for self implementation.
In this way it is suggested the galaxy is populated with intelligent probes in addition to the purely biological species which evolve. Tipler envisages situations where 'natural' biological species coexist with a peer group of von Neumann machines. This notion, bizarre though it may seem, would appear to be an inherent effect of the introduction of von Neumann machines. Perhaps it is because of this reason that Sagan and Newman 9 are opposed to the concept. Their stated objections include the obvious opposition that; given time these hungry and promiscuous constructions will overrun the entire galaxy and beyond. In fact it seems equally obvious that 'natural' intelligent species will expend a considerable amount of effort in ensuring that von Neumann machines are not permitted to exist. Unless stopped, these inventions would take over. That is their designed purpose. These intelligent entities, as they must become, would be determined and capable of whatever was necessary to guarantee their own survival. The danger is that once established, to attempt to destroy them it might become a destructive spiral with no survivors.
There is at least one alternative explanation of why we have not been contacted. Communication with extraterrestrial intelligence's, by definition, requires a discourse. Some, (for example Deardorff 3), have argued the possible existence of an embargo on contact until emerging civilisations have shown themselves to be capable of peaceful coexistence in the cosmos. This compellingly simple reason might be well founded. At the Green Bank conference the lifetime of our existence was postulated to be rather short. The proliferation of weapons of mass destruction, including the possibility of anarchic states, even individuals obtaining the nuclear potential is as Sagan accurately states "generally regarded as unstable". Even without that threat it seems entirely reasonable that if any existing extraterrestrial intelligence were monitoring our developmental progress they would not intervene until our global intentions became clear.
Deardorff also suggests that an advanced intelligence would not be likely to establish contact in a sudden or inconsiderate manner. Rather that initial contact may be made subtle means, perhaps more akin to a guiding hand or benevolent manipulation. He suggests that before any contact of genuine benevolence a period of acceptance of the concept of their existence would be necessary.
The history of mankind is liberally punctuated with wars and man made catastrophes, the assumption of mediocrity does not include a status which warrants sudden rescue from cataclysmic disaster.
If indeed we are part of the galactic population, then man will need to earn his place in that community. If not, then it might be that our passing goes unnoticed.
It appears certain that Earth is not currently under the direct scrutiny of an extraterrestrial intelligence. A considerable degree of difference currently separates the two schools of opinion; whether life as we know it is a natural and commonplace effect of evolution in the universe, or, if we genuinely are alone in time and space. In time the astronomers and biologists might agree a common scenario. However, it is not the sole domain of the scientist, the ordinary man and woman in the street is entitled to share the controversy, which at present is based as much on personal interpretation and opinion as scientific facts. The best science available to us cannot settle the question, it is impossible to state which side is correct, it is as much personal belief as it is proven knowledge.
It seems inconceivable that we will ever prove ourselves to be alone in the universe. The debate will only be resolved if, and when, the proponents of the former argument announce the most momentous discovery in the history of mankind.