The Whirlpool Galaxy. Image credit: Hubble.
With this essay by Steven Soter, Astrobiology Magazine presents the first in our series of 'Gedanken', or thought, experiments - musings by noted scientists on scientific mysteries in a series of "what if" scenarios. Gedanken experiments, which have been used for hundreds of years by scientists and philosophers to ponder thorny problems, rely on the power of one's imagination to project these scenarios to logical conclusions. They do not involve lab equipment or, often, even experimental data. They can be thought of as focused daydreams. Yet, as in the famous case of Einstein's Gedanken experiments about what it would be like to hitch a ride on a light wave, they have often led to important scientific breakthroughs.
Soter is Scientist-in-Residence in the Center for Ancient Studies at New York University, where he teaches a seminar on Scientific Thinking and Speculation, and a Research Associate in the Department of Astrophysics at the American Museum of Natural History.
In this essay, Soter examines the Drake Equation, which asks how many technically advanced civilizations exist in our galaxy. He also looks at the Fermi Paradox, which questions why, if there are other technological civilizations nearby, we haven't heard from them.
If civilizations exist in our galaxy with levels of technology at least equal to our own, we might be able to detect some of them using radio telescopes. And if civilizations exist with technologies far in advance of our own, we might expect them to have colonized millions of habitable worlds in the Milky Way, and even to have visited our own planet. Yet there is no evidence in the astronomical, geological, archaeological, or historical records that extraterrestrial civilizations exist or that visitors from other worlds have ever been to Earth. Does that mean, as some have concluded, that ours is the only civilization in the galaxy? Or could there be a natural self-regulating mechanism that limits the intensive colonization of other worlds?
In 1961 radio astronomer Frank Drake devised an equation to express how the hypothetical number of observable civilizations in our galaxy should depend on a wide range of astronomical and biological factors, such as the number of habitable planets per star, and the fraction of inhabited worlds that give rise to intelligent life. The Drake Equation has led to serious studies and encouraged the search for extraterrestrial intelligence (SETI). It has also provoked ridicule and hostility. Novelist Michael Crichton recently denounced the equation as "literally meaningless," incapable of being tested, and therefore "not science." The Drake equation, he said, also opened the door to other forms of what he called "pernicious garbage" in the name of science, including the use of mathematical climate models to characterize global warming.
Crichton rightly pointed out that any numerical "answers" produced by the Drake Equation can be no more than guesses, since most of the terms in the equation are quantitatively unknown by many orders of magnitude. But he is utterly wrong to claim that the equation is "meaningless." An equation describes how the elements of a problem are logically related, whether or not we know their numerical values. Astronomers understand perfectly well that the Drake Equation cannot prove anything. Instead, we regard it as the most useful way to organize our ignorance of a difficult subject by breaking it down into manageable parts. This kind of analysis is standard, and a valued technique in scientific thinking. As new observations and insights emerge, the Drake Equation can be modified as needed or even replaced altogether. But it provides the necessary place to start.
When Drake first proposed his equation, we had no way to estimate any of its terms beyond the first one, representing the rate of star formation in our galaxy. Then in 1995, astronomers began to discover planets in orbits around other stars. These results now promise to sharpen our estimates for the second term in the equation, denoting the number of habitable worlds per star. Who knows what unforeseen discoveries will tell us about the other terms in the equation?
In Classical antiquity, when Aristarchus conceived the heliocentric view of the solar system and Democritus developed an atomic theory of matter, they had no possible way to test their ideas. The necessary observational tools and data would not exist for another two thousand years. Of course, the Crichtons of antiquity denounced such speculations as pernicious. But when the time finally came, the ancient ideas were still there, quietly waiting to inspire and encourage Copernicus and Galileo, and the pioneers of modern atomic theory, who took the first steps to test the theories. It may take centuries, but eventually the Drake Equation and all its elements will be testable.
We can express the Drake Equation in several ways, all of which are more or less equivalent. Here is one form:
N = Rs nh fl fi fc L
where N is the number of civilizations in our galaxy, expressed as the product of six factors: Rs is the rate of star formation, nh is the number of habitable worlds per star, fl is the fraction of habitable worlds on which life arises, fi is the fraction of inhabited worlds with intelligent life, fc is the fraction of intelligent life forms that produce civilizations, and L is the average lifetime of such civilizations.
The rate of star formation in our galaxy is roughly ten per year. We can define habitable worlds conservatively as those with liquid water on the surface. Many more worlds probably have liquid water only below the surface, but any subterranean life on such worlds would not be likely to produce an observable civilization. Recent discoveries of other planetary systems suggest that habitable worlds are common and that nh is at least one such planet in a hundred stars.
The remaining terms in the equation depend on the biology and social development of other worlds, and here we are profoundly ignorant. Our local experience may provide some guidance, however. We know that life on Earth arose almost as soon as conditions allowed - as soon as the crust cooled enough for liquid water to persist. This fact suggests that conditions for the origin of life on other habitable worlds are not restrictive, and that the value of fl is closer to one than to one in a thousand. But that is merely a guess. No one knows how life began on Earth, and we cannot generalize from a single case.
The conditions for intelligent life are probably more restrictive. On Earth this step first required the evolution of complex animals, which began about three billion years after the origin of life, and then the development of brains capable of abstract thought, which took another half billion years. Among the millions of animal species that have lived on Earth, probably only one ever had intelligence sufficient to understand the Drake Equation. This suggests that fi might be a small fraction.
The probability that intelligent life develops a civilization depends on the evolution of organs to manipulate the environment. On Earth, whales and dolphins may well have intelligence sufficient for abstract thought, but they lack the means to make tools. Humans, with dexterous hands, began making tools over a million years ago. Starting about ten thousand years ago, civilizations based on agriculture arose several times independently, in Mesopotamia, Egypt, China, Mexico, Peru, and New Guinea. This suggests that the value of fc is large, but again we should not generalize from the experience of only one intelligent and manipulative species.
We now come to the most intriguing term, the average lifetime L of a civilization. The Drake Equation assumes that, whatever the other factors, the number of civilizations presently in our galaxy is simply proportional to their average lifetime. The longer they live, the more civilizations exist at any given time. But what is the life expectancy of a civilization? On Earth, dozens of major civilizations have flourished and died within the last ten thousand years. Their average lifetime is about four centuries. Few if any civilizations on Earth have ever lasted as long as two thousand years.
History and archaeology show that the collapse of any given civilization causes only a temporary gap in the record of civilizations on Earth. Other civilizations eventually arise, either from the ruins of the collapsed one or independently and elsewhere. Those civilizations also eventually collapse, but new ones continue to emerge.
For example, in the eastern Mediterranean at the end of the Bronze Age, the prevailing Mycenaean civilization suffered widespread catastrophic collapse around 1100 BC. During a few centuries of "darkness" that followed, the population was illiterate, impoverished and relatively small -- but not extinct. Classical civilization gradually arose and flourished, and gave rise to the Roman Empire, which itself collapsed in the fifth century AD. Another period of impoverished Dark Ages followed, but eventually trade and literacy revived, leading to the Renaissance. Each revival of civilization was stimulated in part by the survival of relics from the past.
Our global technological civilization, with its roots in the Mediterranean Bronze Age, is now arguably headed for collapse. But that will not be the end of civilization on Earth -- not as long as the human species survives. And the biological lifetime of our species is likely to be several million years, even if we do our worst.
We should therefore distinguish between the longevity of a single occurrence of civilization and the aggregate lifetime of a sequence of civilizations. Almost all discussions of the Drake Equation have overlooked this distinction and therefore significantly underestimated L.
The proper value of L is not the average duration of a single episode of civilization on a planet, which for Earth is about 400 years. Rather, L is much larger, being the sum of recurrent episodes of civilization, and constitutes a substantial fraction of the biological lifetime of the intelligent species. The average species lifetime for mammals is a few million years. Suppose the human species lasts another million years and our descendants have recurrent episodes of civilization for more than 10 percent of that time. Then the average effective lifetime of civilization on Earth will exceed 100,000 years, or 250 times the duration of a single episode. Other factors being the same, this generally neglected consideration should increase the expected number of civilizations in our galaxy by at least a hundredfold.
While the aggregate lifetime of civilization on a planet may be only a hundred thousand years, we should allow the possibility that a small minority of intelligent life forms, say one in a thousand, has managed to use their intelligence and technology to survive for stellar evolutionary timescales -- that is, on the order of a billion years. In that case, the average effective lifetime of civilizations in our galaxy would be about a million years.
If we now insert numbers in the Drake Equation that represent the wide range of plausible estimates for the various terms, we find that the number N of civilizations in our galaxy could range anywhere from a few thousand to about one in ten thousand. The latter (pessimistic) case is equivalent to finding no more than one civilization in ten thousand galaxies, so that ours would be the only one in the Milky Way. In the former (optimistic) case, the nearest civilization might be close enough for us to detect its radio signals. Estimates for N thus range all over the map. While this exasperates critics who demand concrete answers from science, it does not invalidate the conceptual power of the Drake Equation.
If many civilizations have arisen in our galaxy, we might expect that some of them sent out colonies, and some of those colonies sent out still more colonies. The resulting waves of colonization would have spread out across the Milky Way in a time less than the age of our galaxy. So where are all those alien civilizations? Why haven't we seen them? The physicist Enrico Fermi first posed the question in 1950. Many answers have since been proposed, including (1) ours is the first and only civilization to arise in the Milky Way, (2) the aliens exist but are hiding, and (3) they have already been here and we are their descendants. In his book Where is Everybody? Stephen Webb considers fifty proposed solutions to the so-called "Fermi Paradox" but he leaves out the most thought-provoking explanation of all, one that I call the Cosmic Quarantine Hypothesis.
In 1981, cosmologist Edward Harrison suggested a powerful self-regulating mechanism that would neatly resolve the paradox. Any civilization bent on the intensive colonization of other worlds would be driven by an expansive territorial impulse. But such an aggressive nature would be unstable in combination with the immense technological powers required for interstellar travel. Such a civilization would self-destruct long before it could reach for the stars.
The unrestrained territorial drive that served biological evolution so well for millions of years becomes a severe liability for a species once it acquires powers more than sufficient for its self-destruction. The Milky Way may well contain civilizations more advanced than ours, but they must have passed through a filter of natural selection that eliminates, by war or other self-inflicted environmental catastrophes, those civilizations driven by aggressive expansion. That is, the acquisition of powerful technology ultimately selects for wisdom.
However, suppose an alien civilization somehow finds a way to launch the aggressive colonization of other planetary systems while avoiding self-destruction. It would only take one such case, and our galaxy would have been overrun by the reproducing colonies of the civilization. But Harrison proposed a plausible backup mechanism that comes into play in the event that the self-regulating control mechanism fails. The most evolved civilizations in the galaxy, he suggested, would notice any upstart world that showed signs of launching a campaign of galactic conquest, and they would nip it in the bud. Advanced intelligence might regard any prospect of the exponential diffusion throughout the Milky Way of self-replicating colonies very much as we regard the outbreak of a deadly viral epidemic. They would have good reason, and presumably the ability, to suppress it as a measure of galactic hygiene.
There may be many highly evolved civilizations in our galaxy, and some of them may even be the interstellar colonies of others. They may control technologies vastly more powerful than ours, applied to purposes we can scarcely imagine. But Harrison's regulatory mechanisms should preclude any relentless wave of colonization from overrunning and cannibalizing the Milky Way.
By most appearances, the dominant civilization on our planet is of the expansive territorial type, and is thus headed for self-destruction. Only if we can intelligently regulate our growth-obsessed and self-destructive tendencies is our civilization likely to survive long enough to achieve interstellar communication.
Steven Soter is Scientist-in-Residence in the Center for Ancient Studies at New York University, where he teaches a seminar on Scientific Thinking and Speculation, and a Research Associate in the Department of Astrophysics at the American Museum of Natural History.
