7 April 2010

The Wow! signal

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The most famous signal in SETI history was detected on the night of 15 August 1977 at the Ohio State University Big Ear Observatory. Has anything happened since?

As on every other night, while Big Ear was searching the skies for an alien signal, its observations were being recorded on a printout sheet.

A long list of letters and numbers was continuously being churned out, one long string for every one of the 50 channels scanned by the telescope.

A series of characters appeared recording an unusual transmission at the frequency of channel 2: “6EQUJ5″ the list read. This startled Big Ear volunteer Jerry Ehman, a professor at Franklin University in Columbus, who was monitoring the readings that night.

He circled the code for later reference and added a single comment in the margins” “Wow!” The signal entered SETI lore as the “Wow!” signal.

The series “6EQUJ5″ described the strength of the received signal over a short time-span. In the system used at the time at Big Ear, each number from 1 to 9 represented the signal level above the background noise.

In order to extend the scale, the staff added letters, with each one from A to Z representing increasingly stronger signal levels. 6EQUJ5 represented a signal that grew in strength to level “U,” and then gradually subsides.

In more familiar notation, the signal increased from zero to level 30 “sigmas” above the background noise, and then decreased again to zero, all in the span of 37 seconds.

Two aspects of this signal immediately caught the attention of Ehman and project director John Kraus, who saw the results the following morning.

First of all, 37 seconds was precisely the time it takes the Big Ear scanning beam to survey a given point in the heavens.

Because of this, any signal coming from space would follow precisely the “Wow!” signal’s pattern – increasing and then decreasing over 37 seconds. This practically ruled out the possibility that the signal was the result of Earthly radio interference.

Secondly, the signal was not continuous, but intermittent. Kraus and Ehman knew that, because Big Ear has two separate beams that scan the same area of the sky in succession, several minutes apart. But the signal appeared on only one of the beams and not on the other, indicating that it had been ‘turned off’ between the two scans. A strong, focused, and intermittent signal coming from outer space: could it be that Big Ear had detected an alien signal?

For a month following the discovery the Big Ear crew tried repeatedly to relocate the signal, but to no avail. In 1987 and again in 1989 Robert Gray led Wow! signal searches using the 26-metre radio telescope of the Planetary Society-funded META array at the Oak Ridge Observatory in Massachusetts, but found nothing.

Gray also managed to secure the services of the entire Very Large Array (VLA) in New Mexico, composed of 27 25-metre dishes. This, according to Gray, was a first: “Contrary to popular belief since the movie Contact,” he emphasizes, “the prestigious $80 million telescope hardly ever listens for broadcasts from the stars.”

During two observing sessions in 1995 and 1996, Gray and his colleague, Kevin B. Marvel, used their telescope time to investigate several scenarios.

One possibility was that the Wow! signal in fact represents a weak but steady transmission that momentarily gained in strength due to interstellar scintillation.

The high sensitivity of the VLA guaranteed that such a source would be easily detected by Gray’s survey. But despite identifying several radio sources hundreds of times weaker than the Wow! signal in the vicinity, nothing resembling a steady transmission was found.

Another scenario assumed that the Wow! signal was a brief powerful signal designed to attract attention to a weaker continuous one. Such a strategy would be more energy efficient than sending a continuous powerful beacon.

But again, the VLA could detect no signal even 1,000 times weaker than the Wow! signal.

Finally there is the possibility that the signal is there, but is only broadcast intermittently. Because of their limited telescope time, Gray and Marvel could only devote less than an hour to any given position. It could be that the signal is on at other times, when no one is listening.

The problem is unavoidable from any location in the Northern Hemisphere, since the Wow! signal locale is below the Northern horizon during most of the day.

To account for that possibility, in 1998 Gray joined forces with Simon Ellingsen of the University of Tasmania in Australia, who made 6 observations, tracking the area for 14 hours at a time. But despite the rigorous hunt, the Wow! signal remains as enigmatic as ever.

Find out more in the latest issue of Cosmos magazine, with a 39-page special on SETI!

Amir Alexander is a historian of mathematics and an editor at the The Planetary Report, in which a longer version of this article first appeared.
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