The How of the Wow

In 1977, the Big Ear radio telescope at Ohio State University [alma mater-mandated "Boo!"] picked up a burst of energy that was dubbed the "Wow! signal" thanks to researcher Jerry Ehman's note on the printout:

Big Ear

There's been a lot of speculation on possible sources – It was an exceptionally strong signal in a narrow bandwidth. On top of that, the frequency of the signal is associated with hydrogen's emission line, which had been identified by the first SETI paper in 1959 as a likely choice for extraterrestrial species to send messages. If you look closely at the image above, the circled bit reads "6EQUJ5". As this story got around, some people misunderstood this as a message in the signal, but it's actually the measurement of the signal-to-noise ratio in time: Each character is the SNR for a 10-second span, with letters A-Z representing SNRs of 10-35 – Quite high values!

A lot of the mystery surrounding this signal has been its transient nature. Followup searches found no recurrence of the signal, which ruled out a pulsar as the source, but also calls the ET explanation into doubt – Wouldn't a message like that be repeated? However, one of the arguments for the signal being artificial is its narrow bandwidth. Coincidentally, I've been listening to old episodes of the Futility Closet Podcast, and I came across an episode from almost 10 years ago about this signal. They liken the Big Ear telescope to a row of radios in a line, each tuned to a slightly different station – These correspond to the different columns in the printout above. One of the surprising things about the Wow! is that it only appears in one column. Most astrophysical signals would have more variation.

The signal is back in the news thanks to a new paper identifying its source. The authors were using the Arecibo Telescope (RIP) to search for similar signals to the Wow!, and were able to find several with the same spectral qualities, though never with the high intensity of the original signal. The hypothesis they arrived at was that an object like a magnetar or a soft gamma repeater released a burst of photons, which passed through a cloud of hydrogen, causing stimulated emission (the se in "laser") of photons at the detected frequency:

Figure 4

It might seem disappointing to have the possibility of a signal from aliens ruled out, but it's also an opportunity to understand the universe better, which in turn gives us better chances of finding a real signal in the future. Alternatively, we could end up making our own version of the Wow! for another species out there.