(Inside Science) -- It may the biggest and oldest question in science: Are we alone in the universe?
If the answer is no, a second question arises: Who else is out there?
Such questions have motivated a decades-long search for radio and light signals from intelligent beings on other planets. In a recent paper, Duncan Forgan, an astronomer at the University of St. Andrews in Scotland, analyzed how likely we are to intercept light beams sent between advanced civilizations in our galaxy. The short answer is: not very likely.
But, Forgan argues, alien civilizations may choose to send their signals in a way that makes them easier for a third party to spot. If so, astronomers can still increase their chances of detecting intelligent life by designing searches to intercept such communications.
Despite the recent discoveries of thousands of exoplanets orbiting far-off stars, nobody knows whether or how many other technological civilizations exist in the galaxy. But if any do, they may be far more advanced than we are. In that case they will probably have built telescopes powerful enough to detect life on other planets in the galaxy, and lasers powerful enough to send messages to them.
We, by contrast, are still at least decades away from being able to announce our presence in such an obvious way. Although electromagnetic waves from radio and television broadcasts have been leaking into space for almost a century, such undirected signals rapidly weaken and are unlikely to be detectable beyond a few stars in our immediate neighborhood. Even so, a more advanced civilization could have, in the past two billion years, detected chemical evidence of life in Earth's atmosphere and aimed a laser at the planet, essentially to say "hello." Human astronomers are also beginning to analyze light that has passed through exoplanet atmospheres for such signatures of life. But even a signature that indicates life would not necessarily imply that the life is intelligent.
So far, the optical search for extraterrestrial intelligence has focused mainly on the hope of receiving—and recognizing—an intentional, laser-encoded message. Researchers use dedicated telescopes or mine astronomical data collected for other purposes, like the Sloan Digital Sky Survey, to search for light pulses that could not be produced by any known object like a star. So far, no one has reported a light pattern that suggests an extraterrestrial intelligence.
But rather than look for light beamed directly at us, astronomers could also try to intercept signals sent between two distant civilizations. If advanced beings have existed for millions of years, they may well have found each other and started talking. Eventually many light beams would penetrate the intergalactic darkness, creating a criss-crossing network of communication beacons. As our solar system revolves around the galactic center, could we meander into the path of one of these beams?
In the new study, published October 27 on arXiv.org, on which papers are posted without formal peer review, Forgan sought to answer this question. He created a simulation in which technological civilizations were spread randomly throughout the part of the Milky Way known as the galactic habitable zone. While astronomers debate the exact dimensions of this zone, most agree it resembles a flattened doughnut around the galactic center with an outer radius large enough to encompass the sun's position at around 26,000 light-years from the center.
Forgan also assumed that beings advanced enough to announce their presence would actually want to do so. Each civilization in Forgan's simulation communicated with one other civilization by sending out a laser beam. He ran the simulation multiple times, varying the number of civilizations and the angle through which their communication beams spread out as they traveled. In each run, Forgan calculated the probability that a random star like our sun would move into the path of one of the beams during a 4.5-billion year period.
Unsurprisingly, he found that the chance of intercepting another civilization's messages increased as more civilizations joined the communications network. He also found that the interception probability increased dramatically as the angle through which the beams spread out increased.
But the probability of accidentally wandering through a beam remained small as long as the beams were narrow, or collimated, like a typical laser. The beams would have to spread out about 1,000 times more widely than a standard laser pointer—in other words, more like a flashlight beam—before we have a decent chance of intercepting them, Forgan says. Sending out such a wide beam would require far more energy than emitting a tightly collimated one.
Still, he says, it's worth designing optical extraterrestrial intelligence searches to look for weakly collimated signals. An advanced civilization with enough resources could design its beam to spread, increasing the chance a recipient will wander into it. Doing so would still require much less energy than sending out light or another type of electromagnetic radiation in every direction, and could give advanced civilizations the opportunity to stumble upon still-developing ones like us.
"This gives you quite a nice compromise," says Forgan. "It gives you a wider range of civilizations you can hit."
Forgan's analysis is "valuable," said astronomer Seth Shostak of the SETI Institute in Mountain View, California. "I'm glad he did it." With his mathematical model, Forgan has turned the possibility of intercepting extraterrestrial communication "from a hand-waving argument into a real simulation," Shostak added. While the results were similar to what most scientists in the field probably suspected, Shostak said the analysis "will make [researchers'] conversations a little bit more sober, because they have to count on aliens making a deliberate aim on our direction, it seems."