The Great Game

IT WAS 37 seconds long and came from outer space. On 15 August 1977 it caused astronomer Jerry Ehman, then of Ohio State University in Columbus, to scrawl "Wow!" on the printout from Big Ear, Ohio State's radio telescope in Delaware. And 28 years later no one knows what created the signal. "I am still waiting for a definitive explanation that makes sense," Ehman says.

Coming from the direction of Sagittarius, the pulse of radiation was confined to a narrow range of radio frequencies around 1420 megahertz. This frequency is in a part of the radio spectrum in which all transmissions are prohibited by international agreement. Natural sources of radiation, such as the thermal emissions from planets, usually cover a much broader sweep of frequencies. So what caused it?

The nearest star in that direction is 220 light years away. If that is where is came from, it would have had to be a pretty powerful astronomical event - or an advanced alien civilisation using an astonishingly large and powerful transmitter.

The fact that hundreds of sweeps over the same patch of sky have found nothing like the Wow signal doesn't mean it's not aliens. When you consider the fact that the Big Ear telescope covers only one-millionth of the sky at any time, and an alien transmitter would also likely beam out over the same fraction of sky, the chances of spotting the signal again are remote, to say the least.

Others think there must be a mundane explanation. Dan Wertheimer, chief scientist for the SETI@home project, says the Wow signal was almost certainly pollution: radio-frequency interference from Earth-based transmissions. "We've seen many signals like this, and these sorts of signals have always turned out to be interference," he says. The debate continues.