If extraterrestrials ever phoned us, they would be more likely to send narrowly directed bursts than constantly blaring signals more text-messaging than novel-writing, some scientists now suggest.
Such a strategy also would be more practical for anyone on Earth to reach out to aliens if one had at least a billion dollars to spend.
How cheap is talk?
For 50 years, people have watched the skies with radio telescopes, hoping to detect signals of intelligent alien life. So far, however, the efforts of SETI (the search for extraterrestrial intelligence) have proved fruitless.
Now some scientists suggest the kind of signals SETI has hoped to find for decades might not be what aliens would broadcast. In a way, the form of communication might come down to cost.
"Our grandfather used to say, 'Talk is cheap, but whiskey costs money, ' " said researcher Gregory Benford, an astrophysicist at the University of California, Irvine, and an award-winning science fiction novelist. "Whatever the life form, evolution selects for economy of resources. Broadcasting is expensive, and transmitting signals across light-years would require considerable resources."
Assuming that aliens would strive to optimize costs, limit waste and make their signaling technology more efficient, Benford and his twin, James a fellow physicist who specializes in high-powered microwave technology suggest the signals would not be steadily blasted out in all directions. Extraterrestrials would be more likely to send narrow "searchlight" beams delivered in pulses.
"This approach is more like Twitter and less like 'War and Peace,' " said James Benford, founder and president of Microwave Sciences Inc., in Lafayette, Calif. The Benford twins, along with James' son Dominic, a NASA scientist, detailed their findings in two studies appearing in the June issue of the journal Astrobiology.
The Benfords suggest a continuous signal blared at thousands of stars would simply cost too much energy. They say aliens might use short bursts say, anywhere from a second to an hour long and point these signals in narrow beams at one star and then another in a cycle involving up to thousands of stars that repeats over days or years.
For civilizations that constantly watch the skies, the bursts would convey enough data to be recognized as undeniably artificial. As observant civilizations concentrated on this simple beacon, other beacons could broadcast more complex data at lower power (assuming the aliens were still pursuing a frugal strategy).
The Benfords suggested looking at a broad range of radio signals in the 1- to 10-gigahertz range, where the travel of light is relatively unimpeded by interstellar matter. Currently SETI is focused on just the 1-to-2 gigahertz region, where components of water such as hydrogen and hydroxyl (a compound of hydrogen and oxygen) emit radio signals.
"The idea is actually based mostly on how all the electronics of the 1960s, when SETI was first starting out, only operated in the range of a few gigahertz," Gregory Benford explained. "Now they operate in a range of up to 100 or 200 gigahertz, so it's a reason to revisit our assumptions. Looking to 10 gigahertz makes sense, since it's cheaper by a factor of 10 to build a transmitter at 10 gigahertz than at 1 gigahertz."
The Benfords also said that instead of gazing at stars within 500 or so light-years, as most SETI efforts have done for decades, observers should point more toward our galaxy's center to distances up to 1,000 light-years from Earth, where 90 percent of the galaxy's stars are clustered.
"The stars there are a billion years older than our sun, which suggests a greater possibility of contact with an advanced civilization than does pointing SETI receivers outward to the newer and less crowded edge of our galaxy," Gregory Benford said.
Although the galactic center is home to many bursting stars whose explosions are expected to sterilize the space around them, "the vast bulk of stars in the 28,000 light-years between Earth and the galactic center are not in sterilizing environments," Benford told Space.com. "For an analogy, you wouldn't want to hang out in Times Square all the time, but if you live in New Jersey it's obvious that Manhattan is the place to look for the action."
"Will searching for distant messages work? Is there intelligent life out there? The SETI effort is worth continuing, but our common-sense beacons approach seems more likely to answer those questions," Benford said.
Have we seen a beacon?
One possibility of an extraterrestrial beacon is a puzzling transient radio source some 26,000 light-years from Earth that was discovered in 2002 in the direction of the galactic center. It sends out radio waves in bursts lasting up to 10 minutes in a 77-minute cycle.
Scientists have suggested the source, labeled GCRT J17445-3009, is a flare star, an extrasolar planet, a pulsar or a brown dwarf, but none of these explanations fits well, the Benfords said.
Based on the burst length and the short cycle, the Benfords doubt GCRT J17445-3009 is a beacon aimed at possible civilizations among a large crowd of stars. Still, if GCRT J17445-3009 is artificial in nature, it could be a signal that aliens pointed just at us, having detected signs of life from our planet. If that is the case, the Benfords said, we may want to pay closer attention to its signals to look for hidden details.
On the other hand, GCRT J17445-3009 could be one link in an interstellar communications network. If so, it would make sense to look in the opposite direction, to see if another beam was communicating at it.
"We studied GCRT not because we really think it's a beacon, but because it's an interesting way to look at similar bursting sources," Gregory Benford said. "There's the famous 'Wow' signal from 1977, for instance, that involved an enormous amount of power, and that there's still no good explanation for."
Instead of just searching for extraterrestrial intelligence, the Benfords also considered messaging to extraterrestrial intelligences, or METI. They calculated that a galactic-scale beacon, with an antenna roughly a half-mile wide with a range of a little more than 1,000 light-years, could be built for $1.3 billion. It would cost $200 million annually to operate. To work economically, it would use only narrow, high-power microwave beams and 35-second bursts aimed at each target star.
"Of course, if you want to send a message, first you have to find a billionaire for this," Gregory Benford told Space.com. He noted he has spoken with a number of billionaires, including former Microsoft chief technology officer Paul Allen and Amazon.com founder Jeff Bezos, "and everyone has the same remark that they would rather spend a billion dollars a different way."