What is the Wow! Signal?
The Wow! Signal is the most famous radio signal ever detected by a SETI experiment. It was a strong, narrowband radio signal observed by the Ohio State University “Big Ear” Radio Observatory on August 15, 1977. At the time, Big Ear was conducting a dedicated search for extraterrestrial intelligence. The signal closely matched the characteristics the project was designed to detect: narrow bandwidth, high intensity, and a time profile consistent with an astronomical source passing through the telescope beam. Despite extensive follow-up observations, the signal was never detected again. Its origin, therefore, remains uncertain.
Why is it called the Wow! Signal?
The SETI program at Big Ear was led by John D. Kraus, Robert S. Dixon, and Jerry R. Ehman. A few days after the detection, Jerry Ehman noticed the unusually strong signal in the computer printouts. Surprised by how closely it matched the expected profile of a candidate extraterrestrial signal, he circled it and wrote “Wow!” in the margin. After several years of observing, no other signal of comparable strength had been seen, making this detection especially remarkable.
Do we know what caused the Wow! Signal?
No definitive explanation has been confirmed. Over the decades, many hypotheses have been proposed, including radio interference, natural astrophysical sources, and an artificial extraterrestrial transmission. None has fully explained all observed properties of the signal or has been independently verified. In recent years, the Arecibo Wow! project has proposed a natural astrophysical explanation that is consistent with all its observed characteristics. However, confirmation will require detecting additional similar events.
What is the Arecibo Wow! project’s explanation?
The project proposes that the Wow! Signal may have been produced by a rare, short-lived enhancement of emission from a neutral hydrogen (HI) cloud. Hydrogen clouds are abundant in the Galaxy and naturally emit radiation at 1420 MHz, the hydrogen line, the same frequency as the Wow! Signal. Under very specific physical conditions, this emission might be temporarily amplified by coherent processes similar to a maser amplification, or superradiance emission. Both mechanisms can produce intense, narrowband, transient signals, although they involve different physical processes. Such events are expected to be extremely rare, which could explain why the signal has not repeated.
Have hydrogen masers or superradiance been observed in space?
Not conclusively. Astrophysical masers are well known for molecules such as OH, H₂O, and SiO. However, stable maser action in the hydrogen line is believed to be very difficult to sustain under natural astrophysical conditions. Hydrogen superradiance and related coherent emission processes have been demonstrated in laboratory experiments and explored theoretically, but they have not yet been firmly confirmed in astrophysical environments. A short-lived flaring event is considered more plausible than a long-lived maser. If the Wow! Signal originated from such a process; it could represent the first observational evidence of this phenomenon.
How confident are we that the signal was not terrestrial interference?
Confidence is high that the signal was not produced by human-made interference. Key reasons include: (1) the signal followed the telescope’s beam pattern as Earth’s rotation carried the source through the field of view. Its rise and fall in intensity matched the expected response for a fixed celestial source, (2) the Big Ear used active interference rejection techniques, (3) the observing frequency lies in a protected radio astronomy band, (4) no known satellites, transmitters, or reflected signals were consistent with the observation, and (5) the Moon was not in a position to reflect terrestrial emissions. Together, these factors strongly favor an astronomical origin. Note: This is analogous to how dispersion supports astrophysical origins in fast radio bursts, although the Wow! Signal itself was not dispersed.
Could the signal have been produced by comets?
No. Comets do contain neutral hydrogen, but they are not sufficiently compact, bright, or transient to produce a signal with the observed intensity and narrow bandwidth. They are more commonly associated with OH emission rather than strong 1420 MHz hydrogen bursts. Detailed analyses have shown that known comets cannot account for the Wow! Signal’s properties.
Has there been any recent update on the signal?
Yes. Recent reanalyses by the Arecibo Wow! project have refined several properties of the signal, including its estimated sky position, intensity, and frequency structure. These updates suggest that the source may have been slightly offset from earlier estimates and may have been more intense than originally reported. This could help explain why later searches failed to rediscover it. Under the proposed hydrogen-cloud interpretation, the event is likely stochastic and non-repeating. The same source is therefore unlikely to flare again, although similar events may occur in other regions.
Are there more Wow!-like signals?
Yes. The Arecibo Wow! project has identified additional candidate signals in archival Big Ear data spanning several decades. These events vary in strength and properties and may have multiple explanations, including instrumental effects, known astrophysical sources, and rare transient phenomena. They are currently being analyzed to determine how many are truly comparable to the original Wow! Signal.
Last update: February 2026