Chapter 19: Life in the Universe

A simulated view of the entire observable universe, approximately 93 billion light years (or 28 billion parsecs) in diameter.

If we were to apply simple statistics, the numbers alone would imply that life should be quite pervasive in the universe. There are over 10²¹ stars in the observable universe, and based on the statistics of searches for extrasolar planets, a similar number of planets! Is it possible that life only arose on one planet, Earth? To some, it may seem ridiculous to propose that we are alone in the universe. Yet we must remember that the principle of mediocrity is no more than an assumption. Life may indeed be a natural consequence of the evolution of stars and planets. However, it is equally possible that life, and in particular intelligence, has not taken place anywhere else in the universe. What if life is a "one-time deal" resulting from a set of unlikely chemical and biological events? Until we have more data, we can only speculate.

Dr. Drake, revisiting the variables of the Drake Equation, several decades after its inception

This search for extraterrestrial intelligence, known by its acronym "SETI" dictates that we first determine the likelihood that intelligent life exists in the universe. American astronomer Frank Drake is one of the pioneers of SETI. He introduced a method for organizing information that would result in the logical arrival of an estimate for the number of intelligent, communicable civilizations in just the Milky Way Galaxy. The result was a fairly straightforward mathematical equation that has come to be known as the Drake equation.

Image shows drake equation in higher quality. Drake equation is used to estimate chances of finding life on other planets.

The estimate achieved by the Drake equation is based on several factors such as the number of habitable planets expected to orbit a star. All of these factors are multiplied together to obtain an estimate of the number of civilizations with the capability for interstellar communication. If any single factor in the equation is completely unknown, no estimate can be made. Likewise, if any single factor is grossly over or underestimated, this will lead to an inaccurate estimate. Unfortunately, our ability to come up with estimated values for any of the factors is limited by our ignorance of the likelihood of life evolving to intelligence and technology. Our predictions are based on one and only one planet that we know has life and only one species that uses technology to communicate across interstellar space. We must recognize that any numbers that result from our use of the Drake equation cannot reliably be extrapolated; we have but one example.

Closeup front view of one antenna of the Allan Telescope Array, a radio telescope for combined radio astronomy and SETI (Search for Extraterrestrial Intelligence) research being built by the University of California at Berkeley, outside San Francisco.

There are many people that do not view SETI as a scientific enterprise. With no empirical evidence to show that the probabilities we have estimated are reasonable, we are left holding nothing but a bag of speculation. Nevertheless, we can still consider the logical products resulting from "optimistic" and "pessimistic" estimates. For example, let us just assume for a moment that intelligent civilizations are distributed randomly among the stars and planets. If the number of communicable civilizations is small, then the average distance between them will be considerably large. If the number of communicable civilizations is large, then the average distance between them will be relatively small. If we make an optimistic calculation of N, then the closest civilization might only be 10 to 15 light years away, practically a next-door neighbor from a cosmic perspective. Signals traveling at the speed of light could easily be exchanged within a single human generation. In contrast, if we make a pessimistic calculation of N, our civilization might be unique in the Milky Way Galaxy. Or civilizations may be so rare that we are operationally isolated in time and space. Two-way communication between two civilizations in separate galaxies could then take millions of years and might be vulnerable to the limited lifetimes of civilizations.