The fascinating mistery of the “Wow!” signal

I’ve always considered the “Wow!” signal one of the most exciting misteries of science of all time. I first came across the “Wow!” signal thanks to an article written by Paolo Attivissimo, who first presented me the topic. If you – like me – have a romantic perception of Science, then this story will almost certainly thrill you.

Why is it so exciting?

It’s august 15th, 1977 and Jerry Ehman is the volunteer who is monitoring the Big Ear Radio Observatory transcripts at Ohio State University, which is scanning sky towards the Sagittarius constellation. Suddenly, something happens.

Jerry marks with a red pen a bunch of unusual values and writes next to them: “Wow!”, amazed by the close matching with the expected signature of an interstellar signal.

As you can see in the image below, normal values in radio telescope transcripts were around 1 or 4 as the highest ones. But one column was simply stunning: not only numbers, but letters, too, meaning values higher than 9, up to 30 for “Q”.  Something incredible was probably happening: that column had the imprint of an artificial signal, coming from somewhere in the space.

"Wow!" signal

Part of the original transcript where Jerry Ehman wrote “Wow!”

The “Big Ear” is scanning the entire sky exploiting Earth rotation; consequently, it has a 72-second window until any object in this window comes out of it (see image below). A signal lasting more or less than 72 seconds would indicate a terrestrial origin of the signal, while a 72-seconds-lasting signal would be the evidence of a non-terrestrial origin of the signal, even if not necessarily artificial. Believe it or not, the signal lasts exactly 72 seconds.

The "Wow!" signal wave

The “Wow!” signal wave.
On x-axis there are time slices, on y-axis intensity values.

The “Wow!” signal seemed to have the non-terrestrial signals proper characteristic of intensifying during the first 36 seconds slice, when they reach their climax and deintensifying during the remaining slice of time. The signal was the most intense ever registered by the “Big Ear” and its bandwidth was below 10 KHz, meaning it was a well-defined source, lacking typical natural signals distortions.

The signal’s frequency was close to the fatidical 1420.406 MHz transmission frequency, considered to be the optimal frequency for interplanetary transmissions, as such waves travel through cosmic dust and would be the ideal candidates for an alien civilization seeking to reach mankind. Moreover, The frequency 1420 is significant for SETI searchers because, it is reasoned, hydrogen is the most common element in the universe, and hydrogen resonates at about 1420 MHz, thus extraterrestrials might use that frequency on which to transmit a strong signal [Wikipedia]. Though, that same frequency is prohibited to terrestrial transmissions.


Unluckily, the “Wow!” signal never repeated again. The “Big Ear” was made of two horns which both scanned the sky, the second horn with a 3-minute delay. The signal would have appear in the second horn after three minutes once been detected by the first horn, but strangely this didn’t happen. Several efforts were made in order to try to catch the signal again, even with the most recent radio telescopes, all without success. It seemed to be gone forever.

Maybe that signal was our only train to come in contact with a few-billion-light-year far civilization who sent us a weak radio beam originated at the time when Earth was populated uniquely by bacteria, a train we lost once for all and maybe we’ll never take again.

Speculations about the origin of the signal

Let’s clarify that still today nobody knows the nature of the “Wow!”. Some considerations were made about, some of which include the hypothesis that interstellar scintillation – that’s to say random fluctuations in the intensity of radio waves of celestial origin – be a possible explanation; however, even by using the significantly more sensitive Very Large Array, such a signal couldn’t be detected and the probability a signal weaker than the capacity of the VLA but stronger than the range of the “Big Ear” might be detected by the “Big Ear” radio telescope due to interstellar scintillation is very low.

Another speculation includes a source similar to a lighthouse, like a periodical sweeping signal or an explosion-like event.

At the beginning, Ehman excluded the possibility of an alien civilization attempt to come in contact with mankind, stating:

We should have seen it again when we looked for it 50 times. Something suggests it was an Earth-sourced signal that simply got reflected off a piece of space debris.”

but he later partly retired his skepticism due to the high unrealistic scenario given by such an accidental spaceborn reflector.

Via: Paolo AttivissimoWikipedia