How would technical civilizations enter into communication with one another? Independent of the value of L, the Drake formula cited above implies that about one technical civilization arises every 10,000 years in the Milky Way Galaxy. Accordingly, it would be extraordinarily unlikely for humans to find a technical civilization as backward as Earth’s. The rate of technical advance on Earth in the past few hundred years makes it clear that no serious and reliable projection of future scientific and technical advances can be made. Advanced civilizations are expected to have techniques and sciences unknown to 21st-century people. Nevertheless, humanity is already capable of communication by radio over interstellar distances. If Earth’s largest radio telescope, the 500-meter- (1,600-foot-) diameter dish at the Five-hundred-meter Aperture Spherical Telescope in China and its receivers, is employed and if identical equipment is employed on some transmitting planet, how far apart could the transmitting and receiving planets be for intelligible signals to be passed? The rather astonishing answer is about 2,500 light-years. Within a volume centered on Earth with a radius of 2,500 light-years, there are more than 150,000,000 stars.

Problems would definitely surface in the establishment of such radio communication. The frequency, target star, longevity, and character of the message would all have to be selected by the transmitting planet so that the receiving planet would be able to deduce them without too much effort. None of these problems seems insuperable. One choice might be to listen to stars of approximately solar spectral type. Certain natural radio frequencies, such as the 1,420-megahertz (21-cm) line of neutral hydrogen, might also be used. In the absence of any symbols or language in common, messages that use the neutral hydrogen line might be the most appropriate for discerning intelligent origin and intellectual content from life-forms that do not share human evolutionary history. Very few anthropocentric assumptions would be needed.

Because Earth’s technologies are relatively new, it makes little sense to transmit messages to hypothetical planets of other stars. But it does make sense to listen for radio transmissions from planets of other stars. Other communication techniques include laser transmission and interstellar spaceflight, but these may not be feasible. American engineers Christopher Rose and Gregory Wright have argued that sending a physical artifact is a preferable communication technique because radio waves tend to disperse, whereas physical artifacts retain their information in compact form and are more likely to be readable when they arrive at their destination. However, such “messages in a bottle” would travel 1,000 times slower than light. If the measure of effectiveness is the amount of information communicated across a broad area per unit cost, then radio transmission is the method of choice.

A scientific search for intelligent extraterrestrial life that could communicate beyond its own celestial home was first called for in 1959 by Italian physicist Giuseppe Cocconi and American physicist Philip Morrison. Using the radio telescope at Green Bank, West Virginia, in 1960, Drake mounted the first (very brief) search, Project Ozma, which was oriented to two nearby stars, Epsilon Eridani and Tau Ceti. On the basis of the Drake equation, it would be very unlikely that success would greet an effort aimed at two stars only 12 light-years away. Not surprisingly, Project Ozma was unsuccessful. Related programs organized on a larger scale were mounted with great enthusiasm in the 1960s in the U.S.S.R.

After Project Ozma ended, various government and private projects continued the search for extraterrestrial intelligence (SETI). The Planetary Society, founded in 1980 by American astronomer Carl Sagan, planetary scientist Bruce Murray, and aerospace engineer Louis Friedman, has as one of its aims the bringing together of professionals and amateurs in support of SETI. Funding by American movie director Steven Spielberg permitted the society to start the first privately funded SETI project, the Megachannel Extraterrestrial Assay, in 1982.

Several searches for extraterrestrial signals that might indicate attempts of extraterrestrial life to communicate with itself are under way. Both radio and optical light transmissions are sought, with instruments receiving cosmic signals in both the Northern and Southern hemispheres. The involvement of amateurs is encouraged. Even the pooling of resources of home computers to analyze the prodigious amounts of data received from outer space helps in the effort. With the communication advantages of the World Wide Web, astronomers from all parts of the globe may aid in the effort. An array of radio dishes near Buenos Aires searches millions of channels for radio transmissions in the southern sky. Professionals and their amateur colleagues at Harvard University search for signals from the visible regions of the electromagnetic spectrum in the Optical SETI project at the Oak Ridge Observatory in Harvard, Massachusetts.

SETI is an extraordinary pursuit, in part because of the potential significance of success. SETI brings unity to a wide range of scientific disciplines as well. Astrobiology, which includes SETI, as the study of the origin and evolution of stars, planets, and life and of the evolution of intelligence and of technical civilizations, is arguably the most important science for understanding the human condition. Astrobiology includes the political problem of recognizing ourselves less as members of tribes and more as citizens of the universe. To pursue these studies, a number of modern methods—molecular evolution via computational proteomics and genomics, geochronological analyses, chemical element detections coupled with scanning electron microscopy, immunocytochemistry for study of protein dynamics, to name only a few—promise to refine definitions of life as well as detect life under extreme conditions on Earth and beyond.

Science fiction routinely depicts extraterrestrial beings as thinly disguised men and women. The unique circuitous one-way path of evolution on Earth makes it extremely unlikely that any mammal or flowering plant, to say nothing of a child, would have evolved on a moon of Jupiter or an extrasolar planet. In the words of Loren Eiseley (from The Immense Journey [1957]),

Lights come and go in the night sky. Men, troubled at last by the things they build, may toss in their sleep and dream bad dreams, or lie awake while the meteors whisper greenly overhead. But nowhere in all space or on a thousand worlds will there be men to share our loneliness. There may be wisdom; there may be power; somewhere across space great instruments, handled by strange, manipulative organs, may stare vainly at our floating cloud wrack, their owners yearning as we yearn. Nevertheless, in the nature of life and in principles of evolution we have had our answer. Of men [as are known on Earth] elsewhere, and beyond, there will be none forever.

Although there is only an infinitesimal possibility that humanlike beings will be discovered in outer space (to serve as a cosmic example of convergent evolution), the discovery of any other living matter anywhere else in the cosmos would be of the utmost scientific significance. Moreover, if no evidence at all for life beyond Earth is found after a significant search, this too would be of great scientific moment. The absence of the evolving matter-energy flow systems that are life would reinforce the awesome responsibility of protecting its diversity in this biosphere, which includes that precious, cosmically fragile, and recent growth form, human civilization.