ET's transmitter, image from Phonographia
What is the most likely radio frequency range that ETs might use to contact us across space? A group of researchers from UC Irvine, NASA, and a private company Microwave Sciences suggest that it may be mostly outside the range we've been listening to. Apparently, it's expensive (on Earth, anyway) to generate the desired power density at long range in the "cosmic water hole" waveband. So in two new papers published in the scientific journal Astrobiology, the researchers ask "What would SETI beacon transmitters be like if built by civilizations that had a variety of motives but cared about cost?" and how could one be built. From New Scientist:
(Microwave Sciences president James) 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.
"Stingy aliens may call us on cheap rates only" (New Scientist)
"Searching for Cost-Optimized Interstellar Beacons (Astrobiology)
"Messaging with Cost-Optimized Interstellar Beacons" (Astrobiology)
