In August 1977, a radio telescope operated by Ohio State University, and being used to support the search for extraterrestrial intelligence (SETI), detected a strong signal whose explanation has remained a mystery for nearly fifty years. Despite extensive searches, the signal never repeated, and nothing like it has ever been detected since. But some scientists poring over old data from the now-defunct Arecibo Telescope think they might have an answer.
What was the Wow! signal?
Ohio State’s Big Ear telescope conducted a search for extraterrestrial intelligence from 1973 to 1995. The assumption was that a strong radio signal at a specific frequency would be evidence of an intelligent origin. On the night of August 15, 1977, a very strong signal of very specific frequency was detected. A few days later, a volunteer working at the observatory to analyze the data discovered the signal on a computer printout. His circling of the signal’s intensity and his handwritten comment gave rise to the name.
https://upload.wikimedia.org/wikipedia/commons/d/d3/Wow_signal.jpg
What do those numbers and letters mean?
Think of the printout as a graph. Each vertical column is a narrow frequency channel. Each horizontal line represents a time interval twelve seconds wide. The numbers and letters represent intensity on a scale that runs from 1 to 9, then continuing from A through the alphabet. Here is another way to look at it that is a little easier to picture.
https://upload.wikimedia.org/wikipedia/commons/1/15/Wow_signal_profile.svg
The telescope swept the sky as it rotated with the Earth, so a source located at a precise spot would only be visible for a minute or so.
Why might this be considered a possible sign of intelligence?
For years, SETI scientists have postulated that a signal at a frequency of 1420 MHz would be a logical choice for interstellar communication. It lies in a quiet window with little interference or absorption from either galactic or terrestrial sources. That frequency is one specific to hydrogen, the most abundant element in the universe.
https://pages.uoregon.edu/jimbrau/BrauImNew/Chap28/6th/28_17Figure-F.jpg
Extraordinary claims require extraordinary evidence…
Any observation that cannot be replicated must remain in the “Who knows?” category. But in a preprint submitted in August, three scientists poring through data from the Arecibo Telescope found something they think might explain the Wow! signal.
A very brief and very rare source of radiation could, at least theoretically, cause a laser-like emission of radiation from a cloud of hydrogen. (Since the emission is of microwaves rather than light, the scrupulously correct term is maser.) The authors observed many signals similar to that observed in 1977, but of much lower intensity.
Here is a portion of the abstract from their submission. It has not yet been peer reviewed; the original can be found here.
- The methods, frequency, and bandwidth of these observations are similar to those used to detect the Wow! Signal. However, our observations are more sensitive, have better temporal resolution, and include polarization measurements. We report the detection of narrowband signals (∆ν ≤ 10 kHz) near the hydrogen line similar to the Wow! Signal, although two orders of magnitude less intense and in multiple locations. Despite the similarities, these signals are easily identifiable as due to interstellar clouds of cold hydrogen (HI) in the galaxy. We hypothesize that the Wow! Signal was caused by sudden brightening from stimulated emission of the hydrogen line due to a strong transient radiation source, such as a magnetar flare or a soft gamma repeater (SGR). These are very rare events that depend on special conditions and alignments, where these clouds might become much brighter for seconds to minutes. The original source or the cloud might not be detectable, depending on the sensitivity of the telescope or because the maximum brightness might arrive seconds later to the observer. The precise location of the Wow! Signal might be determined by searching for transient radio sources behind the cold hydrogen clouds in the corresponding region. Our hypothesis explains all observed properties of the Wow! Signal, proposes a new source of false positives in technosignature searches, and suggests that the Wow! Signal could be the first recorded event of an astronomical maser flare in the hydrogen line.
The epigraph for this section was popularized by Carl Sagan, one of the foremost proponents of SETI. It’s a version of Occam’s Razor, in that a simpler explanation is preferred to a more complicated one. Magnetars and masers may not seem simple to you, but they are far simpler than aliens attempting to communicate!
