Interstellar Signal from the 70s Continues to Puzzle Researchers

Dec. 05, 2002

by Seth Shostak - Senior Astronomer

Of the many "maybes" that SETI has turned up in its four-decade history, none is better known than the one that was discovered in August, 1977, in Columbus, Ohio. The famous Wow signal was found as part of a long-running sky survey conducted with Ohio State Universitys "Big Ear" radio telescope.

The Wow signals unusual nomenclature connotes both the surprise of the discovery and its sox-knocking strength (60 Janskys in a 10 KHz channel, which is more than 50 thousand times more incoming energy than the minimum signal that would register as a hit for todays Project Phoenix.)

But is the Wow signals notoriety merely the triumph of marketing over substance? Could this momentary cosmic burp have really been ET, or was it just random terrestrial interference dressed up with a sexy moniker? For a decade, Robert Gray, a long-time, independent SETI researcher from Chicago, has been trying to find out.

Gray, like many others, was attracted by an intriguing feature of the Wow signal: the manner in which it rose and fell over the course of 72 seconds. Why is this interesting? Just this: the Ohio State survey kept the telescope fixed, letting the Earths daily spin rotate the heavens through its narrow beam. The "beam," of course, was the elongated patch of sky to which the telescope was sensitive the direction from which it could pick up cosmic signals. The sensitivity was greatest at the center of the beam, falling off to either side. So as a celestial radio source passed by, it first rose in apparent intensity as Earths rotation brought it into the beam, reached a peak in the beam center, and then faded away. Given the size of the Ohio State beam, this rise and fall should take 72 seconds. And for the Wow signal, it did.

Now contrast this with what youd expect if the telescope had merely been briefly flooded by an interfering terrestrial signal. The intensity would suddenly switch full on, and then, sometime later, switch off. Even if the interference was due to a low-Earth orbit satellite, a source that might cause a rise and fall in intensity, you wouldnt expect it to fortuitously last for 72 seconds.

For these reasons, the Wow signal gets high marks for being a credible candidate for SETI.

On the other hand, there are some aspects of this seductive signal that nudge it toward a lower grade. The Ohio State telescope actually used two beams, situated side-by-side on the sky. Any cosmic source would therefore be seen first in one (for 72 seconds) and then roughly 3 minutes later in the other (also 72 seconds.) The Wow signal failed this simple test. It came on gangbusters in one beam, but was a no-show in the other: suspicious and disheartening.

But as Gray and others have realized, this odd, one-beam behavior could be caused by an alien transmission that simply went off the air during the 3 minutes between beams. Maybe ET went on vacation, or took an extended lunch break. If the putative aliens permanently shut down their transmitter, then theres no chance of ever hearing the Wow signal again. Like a single sighting of the Loch Ness monster, we would never be able to prove what it was. But if the signal is periodic if, for example, the aliens are using a rotating radio beacon that sweeps the star-studded strata of the Milky Way once every five minutes or every five hours then we could hope to find it by just looking again.

Robert Gray has looked again. And again. In the last decade, Gray and his colleagues have used the Harvard META SETI system and then the Very Large Array (VLA) to search for a reappearance of the Wow signal. The experiment at the VLA, in particular, was an impressive effort, as it was far more sensitive than the original Ohio State equipment and covered more of the band. Neither attempt succeeded in retrieving the signal, however.

Gray realized that he might be the victim of insufficient patience. The longest of his reobservations had been 22 minutes. What if the aliens beacon flashed less often than once every 22 minutes? What if their transmitter was fixed to the home planet, rotating (and flashing) once every 20 or 30 hours?

In the October 20 issue of The Astrophysical Journal, Gray and Simon Ellingsen, of Australias University of Tasmania, report on new observations (partially supported by the SETI Institute) designed to test this idea. Their new try was made at the 26-meter radio telescope in Hobart, Tasmania. This southern hemisphere instrument could continuously follow for most of a day the patch of sky (in the constellation of Sagittarius) where the "Big Ear" was pointing when it found the Wow signal. They made six 14-hour observations, and even though their telescope was rather smaller than the venerable Ohio State antenna, they still had sufficient sensitivity to find signals only 5% as strong as Wows 1977 intensity. They also covered five times as much of the radio dial as the original "Big Ear" telescope.

Bottom line? No dice. To quote from their article, "no signals resembling the Ohio State Wow were detected" Of course, if the signals repetition cycle were much longer than 14 hours, then even this careful experiment could have easily missed it. But as Gray and Ellingsen point out, if the signal were really this infrequent, then the chance to have found it in the first place was very slim.

So was the Wow signal our first detection of extraterrestrials? It might have been, but no scientist would make such a claim. Scientific experiment is inherently, and rightly, skeptical. This isnt just a sour attitude; its the only way to avoid routinely fooling yourself. So until and unless the cosmic beep measured in Ohio is found again, the Wow signal will remain a What signal.