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.
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