Another approach to the search for extrasolar life is the detection of radio signals sent by other civilizations. The modest attempts to pick up any such signals have generated much interest, speculation, and debate. It is true that the most powerful radio telescopes on Earth could receive signals from similar telescopes, aimed directly at Earth, from any other spot in the galaxy. Considerable thought has also been devoted to what wavelengths would be used for communication and what types of information might be sent. The laws of physics and radio propagation in the galaxy suggest that the best wavelengths are in what is known as the "water hole" near 20 centimeters. This activity is commonly referred to as SETI, the Search for ExtraTerrestrial Intelligence. In 1990, NASA funded a modest SETI effort, but its budget was cut after only a few years, before the program got seriously under way. Senator Proxmire gave SETI one of his famed Golden Fleece awards and fumed "not a penny for SETI." Others in influential positions were concerned about ridicule of a program that might run for decades or even millennia seeking faint radio signals from other civilizations, and public funding for SETI searches has been very limited. (Just as it is a problem on Earth, funding would probably be a critical factor on other worlds too. On Earth, in our most economically prosperous times, we cannot even listen. Sending the signals would be much more complex and costly.)
Unfortunately, it is very difficult to know if SETI is an effective use of resources. If the Rare Earth Hypothesis is correct, then it clearly is a futile ef fort. If life is common and it commonly leads to the evolution of intelligent creatures that have long, prosperous planetary tenures, then it is possible that enlightened aliens might be beaming signals off into space. A key factor in deciding whether SETI makes sense involves the lifetime of civilizations with radio technology. Does such a civilization last only centuries before nuclear war, starvation, or some other calamity causes its decline? Or does it last forever? In the most optimistic minds, "Star Trek" societies might populate the stars. But even if they do, it is a real question whether any of them would or could beam enormous amounts of radio power into space to potential audiences that are prevented by the vast interstellar distances from ever returning the message in a timely manner. There probably are other civilizations in the galaxy that have radio telescopes, but the vast numbers of stars and the vast distances involved are barriers that may always keep SETI more an experiment of the imagination than a large-scale scientific endeavor. An exception might be made for the limited number of nearby stars that have planetary systems. If some of these are found to have Earth-like planets with atmospheric compositions indicative of life, then the public might support either sending signals or listening. And, of course, even though we do not intentionally beam radio messages to nearby stars, Earth is a potent transmitter of radio power emanating from radar, television stations, and other sources.
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