Mysteries of Wow

Of the many "maybe’s" 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 University’s "Big Ear" radio telescope.

The Wow signal’s 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 today’s Project Phoenix.)

But is the Wow signal’s 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.

SETI@home_skymap
Stellar Countdown locations. Skymap of Arecibo reobservations. Click image for larger view. SETI@home uses the idle time of over four million personal computers to sift through radio data for signals from extraterrestrial civilizations. The three main signal types of interest are:

Gaussians are the power curves produced when the Arecibo beam scans a steady celestial radio source. The signal is weak at first, strong when it is at the center of the beam, and then fades again. This produces a bell shaped power curve known as a gaussian.

Pulses represent any celestial radio signal of a fixed frequency that is distinguishable above the background noise.

Triplets are a sets of 3 equally spaced spikes. Whereas gaussians represent a constant signal from space, triplets may represent a series of pulses transmitted at fixed time intervals.
Credit: SETI@home

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 Earth’s 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 Earth’s 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 you’d 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 wouldn’t 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 there’s 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 Astrophysical Journal, Gray and Simon Ellingsen, of Australia’s 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 Wow’s 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 signal’s 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 isn’t just a sour attitude; it’s 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.


Related Web Pages

Terrestrial Planet Finder Home Page
What Does ET Look Like from 40 Light Years Away?
Anybody Out There? Part I
Anybody Out There? Part II
Search for Life in the Universe: Neil deGrasse Tyson Interview
Aliens Depend on Time to Grow Brains
Eddington Mission
Darwin Mission
Herschel Mission
Rare Earth? Are we so special?