What would E.T. do? It's an improbable question, but it's one Gregory Benford has been thinking about a lot lately. That's not entirely surprising, since Benford is an award-winning science-fiction writer. In this case, though, he's speaking in his capacity as a professor of physics at the University of California at Irvine. Along with his twin brother, James, and James' son Dominic, Benford has been rethinking the search for extraterrestrial intelligence, or SETI, now celebrating its 50th year.
The SETI project is, as its name suggests, a continuous hunt for sentient, otherworldly life. It involves pointing a radio receiver at a candidate star (one that is sunlike and not too far away) and listening for some sort of steady signal an alien radio broadcast, on all the time. Like Jodie Foster in the movie Contact (vaguely like that, anyway), SETI searchers would tune in for a while, and if they got nothing, they'd move on. "We've now looked out to about 500 light-years or so," says Gregory Benford, "and found no such transmissions."
The reason, the Benford clan claims in a set of papers in the journal Astrobiology, is that SETI scientists have been listening for the wrong thing. A continuous broadcast powerful enough to reach across interstellar space takes a huge amount of energy. The oft-repeated claim that our own TV and radio broadcasts could be leaking out and giving us away to alien listeners is a popular idea, but a highly dubious one. Yes, I Love Lucy shows have now traveled 50 light-years into space, but the transmissions are so incredibly weak that it doesn't matter.
Based on an exhaustive analysis, the Benfords have concluded that it would be far more cost-effective for aliens who wanted to be detected to send out short, powerful bursts every so often to signal their presence. "You send out a few pulses, then move on and come back every once and a while," Benford says. "That makes sense if aliens don't really know we're here." These so-called "Benford beacons" (a nickname bestowed by others in the community who are familiar with the idea) wouldn't necessarily show up when earthbound scientists happened to be listening, so it would be easy to miss them.
The solution, say the Benfords, is to monitor many stars continuously, and the direction to look is the center of the Milky Way, for three reasons. First, there are simply more stars there, where the galaxy gets more and more crowded. Second, those stars tend to be older, meaning civilizations would have a head start on developing technology. Third, a smart ET and we're assuming they'd be smart would point a beacon out along the precise radius of the galaxy, since such a clean and obvious direction is where an equally smart species (us, for instance) would aim its listening devices.
Unfortunately, just as continuous broadcasting takes up resources, so does continuous listening. "People assume that SETI searches are going on all the time," says Benford, "but if you add up the total observing time over last half century, it's a total of only a few months."
That's started to change with the debut of the Allen Telescope Array in California, run by the SETI Institute and devoted full-time to ET hunting, as opposed to all of the SETI work performed to date, which had to be scrounged from whatever telescopes were available. But for much broader coverage, the Benfords also want to enlist the SETI League, an association of amateurs who use small radio dishes including satellite-TV receivers to listen to the heavens. If they're right about what ET would do, so many people listening to so rich a region of the sky might even pick up the telltale signal of intelligent life on a distant world within a few years. Even the SETI folks agree that's possible and don't seem to mind being told that they've been going about things the wrong way.
"They make a good point that the aliens would use beacons, not continuous beams," says Seth Shostak, an astronomer at the SETI Institute. "The idea isn't new I think I have several papers published in the last five or more years with similar suggestions. But hey, these guys have worked out beacon parameters, and that's a good thing to do."
Of course, all the new work may be unnecessary, since it's just possible we've spotted ET already. Several times over the past 50 years, searchers have picked up radio signals that flashed once or twice, then disappeared. The best known of these is called the "Wow" signal, because that's what an astronomer who picked it up wrote on a printout from a radio telescope at Ohio State University in the 1970s. SETI searchers went back to the star in question immediately, but heard nothing. It may be well be, suggests Benford, that we detected extraterrestrials more than three decades ago and because we weren't taking into account what E.T. would do, failed to confirm it.
Lemonick is the senior science writer for Climate Central.