An answer to the age-old question of whether or not we are alone in the universe may have come a step closer with the announcement that philanthropist Yuri Milner was pumping $100 million into SETI—the search for extraterrestrial intelligence. For 55 years astronomers have been sweeping the skies with radio telescopes in the hope of stumbling across a message from an alien civilization. So far, they have encountered only an eerie silence. But SETI has been hampered by an exceedingly tight budget. Now the program can be enormously ramped up.
How likely is it that other civilizations exist? Observations by the NASA satellite Kepler and other instruments have led to the discovery of hundreds of planets around nearby stars. A fraction of them will certainly resemble Earth. Some estimates suggest that the Milky Way alone may harbor billions of earth-like planets. Clearly there is no lack of real estate on which extraterrestrial life might flourish.
But just because a planet is habitable doesn’t mean it is inhabited. For a planet to host life, then life has to arise there. How likely is that? The short answer is, nobody knows. In his celebrated treatise Origin of Species, Charles Darwin gave a convincing account of how life on Earth has evolved over billions of years from simple microbes to the richness and complexity of the biosphere we observe today. But he pointedly left out how life got started in the first place. “One might as well speculate about the origin of matter,” he once quipped.
A century and a half later scientists are still largely in the dark about how non-living matter turns into life. Somehow a complex mix of chemicals was transformed into a primitive organism. If we knew the process that did it, we could have a go at estimating the odds. But you can’t work out the probability of an unknown process. Upbeat assessments of a universe teeming with life are based on little more than wishful thinking.
There are many other unknowns, too, such as the chances that a planet with life will evolve intelligence, and if it does, whether it will create technological civilizations. But at least we know the processes involved, namely, Darwinian evolution and social evolution. The probability (or improbability) of the origin of life remains by far the biggest unknown factor in determining whether SETI will ever succeed.
Another thorny problem is whether an alien civilization would pick out Earth to beam messages at. A sufficiently advanced technology, even many light years away, could in principle have deduced that there is intelligent life on Earth, for example, by imaging the pyramids of Egypt. But aliens would not know we have radio technology unless they were located within a few dozen light years of Earth, because that’s as far as our own radio signals have traveled. It would make no sense for them to message us until they knew we were on the air. And even SETI optimists think there is scant chance of a civilization located that close.
But we don’t have to actually receive a directed message to deduce that we are not alone in the universe. Any signature of alien technology would settle that. One idea championed by Gregory Benford at UC Irvine, is to look for beacons. Just as a lighthouse sweeps the horizon for the benefit of anyone who might be out there, so a super-civilization (perhaps one that long ago vanished) may have built a radio or optical beacon to sweep the plane of the Milky Way.
Detecting such a source requires a different search strategy. Traditionally, SETI astronomers hop from star to star listening for “narrow band” signals—those transmitted continuously at a fixed frequency, like terrestrial radio and TV channels. By contrast, a beacon would be a transient event—perhaps just a brief pulse or a series of pulses—that might repeat months or years later when the beam came around again. To spot it, astronomers would need to direct their instruments on the same patch of sky for months or years, which is an expensive undertaking.
Whatever strategy is used, searching for ET is still a huge shot in the dark. There may be no intelligent life out there, or even life of any sort. But to not even try would be hugely disappointing. Part of what makes us human is our sense of curiosity and adventure, and even the act of looking is a valuable exercise. As Frank Drake, the astronomer who began SETI on a shoestring budget in 1960, expresses it, SETI is really a search for ourselves, who we are and how we fit into the great cosmic scheme of things.