The search for extraterrestrial intelligence (SETI), a research discipline that celebrates the 50th anniversary of its inaugural publication this week, has always sat at the edge of mainstream astronomy. This is partly because, no matter how scientifically rigorous its practitioners try to be, SETI can't escape an association with UFO believers and other such crackpots. But it is also because SETI is arguably not a falsifiable experiment.
Regardless of how exhaustively the Galaxy is searched, the null result of radio silence doesn't rule out the existence of alien civilizations. It means only that those civilizations might not be using radio to communicate. Indeed, SETI is marked by a hope, bordering on faith, that not only are there civilizations broadcasting out there, but that they are somehow intent on beaming their signals at Earth. An alien SETI project relying on a similar faith in Earth would be sorely disappointed. It's true that a random mix of radar and television signals has been expanding outwards from Earth at the speed of light for the past 70 years. But there have been only a few short-lived attempts to target radio messages at other stars — with each attempt arousing concerns over alien reprisals. Understandably, many scientists who support SETI in spirit have instead pursued astronomical targets more likely to offer positive data — and tenure. Governments have also been averse to funding an effort so likely to turn up nothing.
Nonetheless, a small SETI effort is well worth supporting, especially given the enormous implications if it did succeed. And happily, a handful of wealthy technologists and other private donors have proved willing to provide that support. This summer, the Allen Telescope Array, funded mainly by Microsoft billionaire Paul Allen, has begun to sweep the skies with its 42 dishes in the California high country (see page 324). The sophistication of this array, which it is hoped will grow even larger, shows just how far SETI has come. Whereas the first search in 1960 used a single radio channel, the Allen array can potentially monitor hundreds of millions of radio channels at once. With advances such as this, the speed of the searches — the rate at which star systems can be checked over multiple parts of the radio spectrum — has increased roughly as fast as the exponential growth in computer power described by a law named after yet another SETI supporter, Intel founder Gordon Moore. Moreover, the researchers at the Allen array are solving the immense computing challenges of operating large, multi-dish arrays, an expertise that will benefit the whole of radio astronomy when the time comes to build the huge arrays of the future.
Meanwhile, one of SETI's main missions — finding other worlds like our own — has become much more feasible than it was five decades ago. Very soon, probably within a few years, astronomers will find an Earth-sized planet orbiting another star. The next step will be to characterize it by studying spectroscopic signatures in the light from its atmosphere. Is there a fingerprint for life, in the form of oxygen or methane? How long is its day? What is its weather like? Does it have continents and oceans? For the 'Earths' orbiting nearby stars, answers to all of these questions should be within the reach of telescopes planned for the next decade.
Still, as momentous as it would be to find such indirect evidence for life elsewhere in the Universe, that would not be the same as finding other thinking creatures like ourselves. That will still require SETI in some form. Will we want to beam a message to these other Earths? That is a debate for another day. But we should at least train the Allen array on these worlds. Earth may decide it does not want to open its mouth, but it would be foolish to cover its ears.
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