For several decades, astronomers have been aiming sensitive radio receivers toward the heavens hoping to eavesdrop on signals generated by beings on planets elsewhere in the galaxy. Nothing yet, of course, but now an international team of researchers is proposing to look for flashes from alien laser beams as well using gamma-ray telescopes.
Gamma-ray telescopes are designed to detect the highest-energy particles of light: photons from exploding stars and the like. But if their ultra-fast, ultra-sensitive cameras are tuned to the proper wavelength, they also can detect faint flashes of optical light of the sort that might come from lasers positioned thousands of light-years away. "There are 20 to 30 naturally occurring light flashes recorded every second" by gamma-ray telescopes around the world, says astrophysicist Joachim Rose of the University of Leeds in the U.K. The telescope software usually ignores the flashes because it is configured to reject "anything that it doesn't expect," he says.
But those flashes could be evidence of intelligent life among the stars, Rose says. As a start, he and his colleagues have analyzed about 1400 hours of archival observations collected since 1999 by the 10-meter Cerenkov telescope at the Whipple Observatory in Arizona. The data include observations of 129 sunlike stars in the Milky Way galaxy, which would be obvious places to look for alien life. Intriguingly, the data reveal a flash of light from such a star once every 6 hours on average, the researchers reported this week at a conference at the University of Surrey, U.K. Alas, non-sunlike stars flash at the same frequency. "Of course, it would have been even more interesting to find a [confirmed] signal, but we didn't see anything," Rose says.
The team will keep looking, however. The search will continue later this year using a new array of gamma-ray telescopes called VERITAS at the Whipple Observatory. The search also requires something definitive to look for. On that score, Rose thinks they've developed the right criteria: Incoming gamma-rays create spread-out and fuzzy-looking patterns in the detector data, he explains, while light from a laser beam creates a round, concentrated image.
Scanning the skies for alien light sources with VERITAS is a "great idea, but I still prefer the traditional radio search for extraterrestrials," says radio astronomer Peter Backus of the SETI Institute in Mountain View, California. For one thing, "if you're using an optical beacon you have to be more precise in your aiming" to light up a distant planet, he says, while sending a radio signal can cover a much wider area of a planetary system. So, the optical technique will spot aliens only if they happen to be beaming signals right at Earth.