Conclusion

This book has provided a space age examination of the fundamental question: Is life, especially intelligent life, unique to Earth? Human interest in the origins of life and the possibility of life on other worlds extends back deep into antiquity. Today, as a result of space technology, the scope of those early perceptions has expanded well beyond the reaches of this solar system—to other stars of the Milky Way galaxy, to the vast interstellar clouds that serve as stellar nurseries, and beyond to numerous galaxies that populate the seemingly infinite expanse of outer space.

Astronomical evidence now suggests that planet formation is a natural part of stellar evolution. If life originates on "suitable" planets whenever it can (as many exobiologists currently hypothesize), then knowing how abundant such suitable planets are in the Milky Way galaxy would allow scientists here on Earth to make more credible guesses about where to search for extraterrestrial intelligence and what the basic chances are of finding intelligent life beyond this solar system.

One of the fundamental characteristics of human nature is our desire to communicate. In recent years, we have begun to respond to a deep cosmic yearning to reach beyond this solar system to other star systems— hoping not only that someone or something is out there but that "they" will eventually "hear us" and perhaps even return our message.

Because of the vast distances between even nearby stars, when scientists say "interstellar communication," they are not talking about communication in "real time." On Earth, radio-wave communication does not involve a very perceptible time lag—that is, messages and responses are normally received immediately after they are sent. In contrast, electromagnetic waves take many light-years to cross the interstellar void from star system to star system, so our initial attempts at interstellar communication actually have been more like putting a message in a bottle and tossing it into the "cosmic sea" or else placing a message in a type of interstellar time capsule for some future generation of human or alien beings to discover.

Positive, active attempts to communicate with any alien civilizations that might exist among the stars are often referred to as CETI—an acronym meaning communication with extraterrestrial intelligence. If, on the other hand, humankind passively scans the skies for signs of an advanced extraterrestrial civilization or patiently listens for their radio-wave messages, the process is often referred to as SETI—meaning the search for extraterrestrial intelligence. While the acronyms appear quite similar, the societal implications are quite different.

Starting in 1960, there have been several serious SETI efforts, the vast majority of which involved listening to selected portions of the microwave spectrum in hopes of detecting structured electromagnetic signals that were indicative of the existence of some intelligent extraterrestrial civilization among the stars. To date, none of these efforts has provided any positive evidence that such "intelligent" (that is, coherent and artificially produced versus naturally occurring) radio signals exist. However, SETI researchers have only examined a relatively small portion of the billions of stars in the Milky Way galaxy. Furthermore, they have generally only listened to rather narrow portions of the electromagnetic spectrum. Optimistic investigators, therefore, suggest that "the absence of evidence is not necessarily the evidence of absence." But detractors counter by suggesting that SETI is really an activity without a subject. Such detractors also regard the SETI researcher as a modern Don Quixote who is tilting with cosmic windmills.

But the Milky Way is a vast place, and it is only within the past few decades that the people of Earth have enjoyed radio, television, and other information technologies that are sophisticated enough to cross the threshold of some minimal interstellar communication technology horizons. A little more than a century ago, for example, Earth could have been "bombarded" quite literally with many alien radio-wave signals—but no one here had the technology capable of receiving or interpreting such hypothetical signals. After all, even the universe's most colossal natural signal, the remnant microwave background from the big bang was just detected and recognized in the mid-1960s. Yet, this interesting signal is detectable (as static) by any common television set that is connected to a receiving antenna.

Early in the space age, a few scientists did attempt boldy to communicate actively "over time" with alien civilizations. They did this by including carefully prepared messages on four robot spacecraft (Pioneer 10 and 11 and Voyager 1 and 2) that were scheduled to travel beyond the solar system and also by transmitting intentionally a very powerful radio message to a special group of stars, using the world's largest radio telescope, the Arecibo

Observatory. These technical deeds may be viewed as the human race's inaugural attempts at CETI.

It may come as a bit of a surprise to most readers that the people of Earth have also been leaking radio frequency signals unintentionally into the galaxy since the middle of the 20th century. Imagine the impact that some of our early television shows could have on an alien civilization that is capable of intercepting and reconstructing the television signals. The earliest of these TV broadcast signals are now about 60 light-years into the galaxy. Who knows "what alien society" is possibly examining the Ed Sullivan Show during which Elvis Presley made his initial television appearance. Should these speculative circumstances occur, a group of alien scientists, philosophers, and religious leaders might right now be engaged in a very heated debate concerning the true significance of the special message from Earth that proclaims: "You Ain't Nothing But a Hound Dog!"

Perhaps even more interesting is the fact that at some point later in this century, a coherent, "artificial" signal could be received and deciphered during a scientifically based SETI effort. This (for now) hypothetical event would provide the human race with a very interesting opportunity to turn SETI into CETI. Do the people of Earth respond to this alien message? If so, what might we say, and who should speak for Earth?

Chronology

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