A new study suggests that cost-effective galactic radio transmissions would be at higher frequencies than SETI projects traditionally monitor, and ET’s attempts to make contact would be only few and far between.
“If ET was building cost-effective beacons, would our searches have detected them? The answer turns out to be no,” says James Benford, president of the company Microwave Sciences in Lafayette, California.
Aliens wishing to communicate would probably broadcast at frequencies between 1 and 10 gigahertz, where there is less astronomical background noise than in other wavebands. Most SETI projects tune in to the “cosmic water hole” waveband between 1.42 and 1.72 gigahertz. The reasoning goes that alien astronomers might expect earthly scientists to be looking there anyway as this is the frequency of radiation emitted by interstellar hydrogen and hydroxyl clouds.
But this fails to consider the cost to aliens. “Societies are always constrained by their resources,” Benford points out. “Why did cathedrals take centuries to build? Partly because they had only so many artisans, but also their capital was limited.”
Benford’s analysis of the economics of extraterrestrial beacons with his brother Gregory at the University of California, Irvine, and son Dominic at NASA’s Goddard Space Flight Center in Maryland suggests that aliens would choose to transmit at nearer to 10 gigahertz, as this makes it easier and cheaper to create a powerful beam.
Short pulses rather than a continuous signal would also enable frugal aliens to use small and cheap transmitters. Small transmitters can beam out powerful radiation using high voltages â€“ but only if they broadcast brief pulses that don’t give the electric fields time to discharge.
Benford concludes that frugal aliens would swing a pulsed microwave beam across the disc of the Milky Way, where most of our galaxy’s stars reside. “They wouldn’t want to target individual stars: there are far too many of them,” he says. “Instead, they’d build a powerful beacon, then swing that beacon around and repeat it.”
He calculates that aliens could use a dish antenna 0.9 kilometres wide to sweep a beam across the Milky Way’s disc once a year, broadcasting a single 35-second blast of microsecond pulses to all the stars within 1080 light years.
If aliens did follow that strategy, their signals would not repeat for many months. “Astronomers have seen some unexplained signals that lasted for tens of seconds then were never seen again,” says Benford. “Some of those could have been extraterrestrial beacons but there wasn’t enough observing time to wait for any repeats.” He urges astronomers to look through their archives for any signals that might fit the bill.