Extraterrestrial message?

SETI Institute - The Allen Telescope Array

A strong narrowband radio signal was detected by Dr. Jerry R. Ehman on August 15, 1977, while working on a SETI project at The Big Ear radio telescope of Ohio State University. The signal bore expected hallmarks of potential non-terrestrial and non-solar system origin. It lasted for 72 seconds, the full duration Big Ear observed it, but has not been detected again. This signal generated a huge controversy.

Amazed at how closely the signal matched the expected signature of an interstellar signal in the antenna used, Ehman circled the signal on the computer printout and wrote the comment “Wow!” on its side. And so the signal became known as The Wow! Signal.

The highest value of intensity of this signal was also the highest ever detected by the telescope. The intensity in this case is the unitless signal-to-noise ratio, where noise was averaged for that band over the previous few minutes.

The bandwidth of the signal is less than 10 kHz (each column on the printout corresponds to a 10 kHz-wide channel; the signal is only present in one column). Two different values for its frequency have been given: 1420.356 MHz (J. D. Kraus) and 1420.456 MHz (J. R. Ehman), both within 50 kHz of the frequency of the hydrogen line, which is at 1420.406 MHz.

Determining a precise location in the sky was complicated by the fact that the Big Ear telescope used two feed horns to search for signals, each pointing to a slightly different direction in the sky following Earth’s rotation; the Wow! signal was detected in one of the horns but not in the other, although the data were processed in such a way that it is impossible to determine in which of the two horns the signal entered. This region of the sky lies in the constellation Sagittarius, roughly 2.5 degrees south of the fifth-magnitude star group Chi Sagittarii.

The Big Ear telescope was fixed and used the rotation of the Earth to scan the sky. At the speed of the Earth’s rotation, and given the width of the Big Ear’s observation “window”, the Big Ear could observe any given point for just 72 seconds. A continuous extraterrestrial signal, therefore, would be expected to register for exactly 72 seconds, and the recorded intensity of that signal would show a gradual peaking for the first 36 seconds—until the signal reached the center of Big Ear’s observation “window”— and then a gradual decrease.

Therefore, both the length of the Wow! signal, 72 seconds, and the shape of the intensity graph may correspond to a possible extraterrestrial origin.

The signal was expected to appear three minutes apart in each of the horns, but this did not happen. Ehman unsuccessfully looked for recurrences of the signal using Big Ear in the month after the detection.

Other researchers tried to capture other space signals similar to the Wow! Signal but did not detect anything like it.

Interstellar scintillation of a weaker continuous signal—similar, in effect, to atmospheric twinkling—could be a possible explanation, although this still would not exclude the possibility of the signal being artificial in its nature.

However, even by using the significantly more sensitive Very Large Array, such a signal could not be detected, and the probability that a signal below the Very Large Array level could be detected by the Big Ear radio telescope due to interstellar scintillation is low.

Other speculations include a rotating lighthouse-like source, a signal sweeping in frequency, or a one time burst. Some have also suggested it could have come from a moving space vehicle of extraterrestrial origin.


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